SYNTHESIS AND HYDROSOLUBILITY STUDIES OF AMPHIPHILIC … - Andrea Carletto... · solubilizzazione...

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POLITECNICO DI MILANO

Facoltà di Ingegneria dei Processi Industriali

Dipartimento di Chimica, Materiali e Ingegneria Chimica

“Giulio Natta”

Corso di Laurea in Ingegneria dei Materiali

SYNTHESIS AND HYDROSOLUBILITY STUDIES OF

AMPHIPHILIC CORE-SHELL POLYMERS

Tutor: Prof. Stefano TURRI

Supervisor: Prof. Rinaldo POLI

Master Thesis of :

Andrea CARLETTO

Academic Year 2009-2010

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To my family

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<<Science is the belief in the ignorance of the experts>>

Richard P. Feynman

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AKNOWLEGMENTS

First of all I want to thank Prof. Rinaldo Poli , for giving me the chance to do my project in his

research group at the CNRS-LCC laboratories in Toulouse, under his supervision. Thank you for

the teaching received, the continued and sincere support and the patience and time you always gave

me during these months.

I would like to thank Prof. Stefano Turri, and Prof. Marinella Levi for giving me the possibility to

complete my master thesis here in Milan at the ChIPS Lab.

A special thank goes to Andrés Fernando Cardozo Perez (PhD student, CNRS-LCC, ENSIACET-

LGC), a very good friend of mine.

Thanks to Dr. Eric Manoury for the help given by his extensive knowledge in organic chemistry.

I would like to thank everyone who is working and worked in the Equipe G and at the LCC

laboratories. I want to thank in particular Dr. Cristina Tiozzo, Dr. Pavel Dub, Aurélien Béthegnies,

Aurélie Morin, Dr. Nadia Vujkovic, Dr. Dominique Augustin, Dr. Zhigang Xue, Romain Adcock,

Dr. Regis Laurent., Cyrille Rebout.

Special thanks go to Miss Katie Smart, for the support and love given. Thank you.

I’m grateful to all people who helped me in this hard task.

My parents also deserve special thanks. Thank you for the love, support, chances and trust you

accorded me, day after day.

Last but not the least, a very very special thank goes to Andrea Carletto

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CONTENTS

ABSTRACT ................................................................................................................................................................................................7

CHAPTER 1 ............................................................................................................................................................................................ 11

1. INTRODUCTION: SYNTHESIS OF WATER-SOLUBLE STAR-BLOCK LIKE COPOLYMERS BY “ARM-FIRST” APPROACH

USING ATRP.......................................................................................................................................................................................... 11

REFERENCES ......................................................................................................................................................................................... 13

CHAPTER 2 ............................................................................................................................................................................................ 14

2. PRINCIPLES OF ATOM TRANSFER RADICAL POLYMERIZATION (ATRP) AND DISCUSSION OF COMPLEX

MACROMOLECULAR ARCHITECTURES ........................................................................................................................................... 14

2.1 THE ATOM TRANSFER RADICAL POLYMERIZATION (ATRP) .............................................................................................................. 14

2.2 STAR POLYMERS ............................................................................................................................................................................ 17 2.2.1 Core first: ........................................................................................................................................................................... 19

2.2.2 Arm first:............................................................................................................................................................................ 21

2.2.3 Mikto-arm star copolymers:........................................................................................................................................... 23

REFERENCES ......................................................................................................................................................................................... 24

CHAPTER 3 ........................................................................................................................................................................................... 25

3. MICELLIZATION AND WATER-SOLUBLE POLYMERS ................................................................................................................ 25

3.1 MICELLIZATION: AN INTRODUCTION ............................................................................................................................................... 25 3.1.1 Different Amphiphile Systems........................................................................................................................................ 27

3.1.2 Micelles Formation at the Critical Micelle Concentration (CMC) ............................................................................ 29

3.2 POLYMERS IN SOLUTION AND THE CHOICE OF MONOMERS.............................................................................................................. 30

3.2.1 Various Classes of Water-Soluble Polymers ................................................................................................................ 31 3.2.2 Polyelectrolytes as Charged Polymers .......................................................................................................................... 32

REFERENCES ......................................................................................................................................................................................... 34

CHAPTER 4 ............................................................................................................................................................................................ 35

4. ANALYTICAL INSTRUMENTS : THEORY AND BASIC PRINCIPLES ........................................................................................... 35

4.1 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (NMR).............................................................................................................. 35

4.1.1 Introduction ...................................................................................................................................................................... 35

4.1.2 NMR spectrometers and experiments .......................................................................................................................... 36 4.1.3 Chemical shifts and spin–spin interactions ................................................................................................................. 39

4.1.4 Magic-angle spinning, dipolar decoupling and cross polarization .......................................................................... 41

4.1.5 Spin diffusion .................................................................................................................................................................... 42

4.2 SIZE EXCLUSION CHROMATOGRAPHY (SEC) .................................................................................................................................... 44 4.2.1 Separation System ........................................................................................................................................................... 44

4.2.2 Plate Theory ...................................................................................................................................................................... 46

4.2.3 Partitioning of Polymer with a Pore ............................................................................................................................. 48 4.2.4 SEC With an On-Line Light-Scattering Detector.......................................................................................................... 49

4.2.5 The Universal Calibration ............................................................................................................................................... 51

4.3 ATOMIC FORCE MICROSCOPY (AFM)............................................................................................................................................. 52

4.3.1 Introduction ...................................................................................................................................................................... 52 4.3.2 Imaging and Analysis Principles .................................................................................................................................... 53

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4.3.3 Contact Mode ................................................................................................................................................................... 55

4.3.4 Tapping Mode.................................................................................................................................................................. 57

4.4 GAS CHROMATOGRAPHY (GC) ....................................................................................................................................................... 58

REFERENCES ......................................................................................................................................................................................... 60

CHAPTER 5 ............................................................................................................................................................................................ 61

5. POLYMERS SYNTHESIS : EXPERIMENTAL PROCEDURES AND RESUL TS............................................................................... 61

5.1 VINYL-PYRIDINE (4VP) AND POLY(4-VINYL-PYRIDINE) .................................................................................................................... 63 5.1.1 Poly(4-vinyl-pyridine) synthesis ..................................................................................................................................... 64

In figure 5.8 the 1HNMR spectra of sample P4VP(1) performed in CDCl3 is rapresented.............................................. 67

5.1.2 Copper Catalyst Removal in Poly(4 vinyl pyridine) ATRP .......................................................................................... 67

5.1.3 P4VP-b-PS and PS-b-P4VP blockcopolymers .............................................................................................................. 70 5.1.4 Competitive Side Reactions in ATRP Synthesis Involving 4VP .................................................................................. 75

5.2 POLY(TERT-BUTYL METHACRYLATE) AND POLY(TERT-BUTYL ACRYLATE)............................................................................................ 76

5.2.1 Poly(tert-butyl methacrylate) synthesis ....................................................................................................................... 77

5.2.2 PtBMA-b-PS blockcopolymers....................................................................................................................................... 82 5.2.3 Poly(tert-butyl acrylate) synthesis ................................................................................................................................ 86

5.3 STAR POLYMERS SYNTHESIS: CROSSLINKING................................................................................................................................... 91

5.4 POLYMER HYDROLYSIS AND SOLUBILIZATION.................................................................................................................................. 94

REFERENCES ......................................................................................................................................................................................... 98

CHAPTER 6 ............................................................................................................................................................................................ 99

6. STAR POLYMERS CHARACTERIZATION....................................................................................................................................... 99

6.1 SIZE EXCLUSION CHROMATOGRAPHY (SEC) ................................................................................................................................. 100 6.2 PROTON NUCLEAR MAGNETIC RESONANCE (

1H NMR)................................................................................................................. 102

6.3 ATOMIC FORCE MICROSCOPY (AFM)........................................................................................................................................... 104

REFERENCES ....................................................................................................................................................................................... 107

CHAPTER 7 .......................................................................................................................................................................................... 108

7. CONCLUSION ................................................................................................................................................................................. 108

ANNEXES ............................................................................................................................................................................................. 110

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ABSTRACT

Questo progetto è finalizzato alla realizzazione di polimeri a stella “star polymers” di tipo “core-

shell” che possano servire da precursori per la sintesi di architetture macromolecolari complesse

utilizzabili successivamente come siti di catalisi in fase acquosa, grazie all’introduzioni di opportuni

monomeri funzionali. Ci si propone di sintetizzare polimeri formati da un guscio (shell) esterno

composto da catene lineari a duplice carattere (idrofilico/idrofobico), in modo da rendere l’intero

sistema solubile in acqua e un “core” interno (composto da un segmento lineare interno e da un

nano gel reticolato) a carattere idrofobico, le catene lineari e flessibili si estendono in maniera

radiale, dando all’intera architettura macromolecolare le sembianze di una stella, dalla quale la

denominazione in inglese di “star polymer” . La costruzione di ta li macromolecole non è

immediata e richiede l’utilizzo di tecniche di polimerizzazione radicaliche controllate, più

specificatamente nel caso in esame l’ATRP e approcci di sintesi differenti, come i metodi "arm-

first" e "core-first" normalmente utilizzati per l'assemblaggio di tali molecole. Una breve

introduzione alla tecnica ATRP e agli approcci "arm-first" e "core-first" viene riportata, per

maggiore chiarezza di seguito.

La polimerizzazione ATRP (atom transfer radical polymerization), consente la sintesi di polimeri

con un’alta definzione di peso molecolare e polidispersità basse, fornendo numerosi vantaggi

potenziali nel design macromolecolare, parametro fondamentale per l’ottenimento di star

polymer con proprietà ben definite e riproducibili.

Figure 1 – Schema di reazione di una generica ATRP

L’immagine qui sopra, illustra il principio di funzionamento di una sintesi di ATRP, un iniziatore

alogenato (Cl o Br), in presenza dell’alogeno del metallo di transizione (in questo caso Cu),

consente il passaggio continuo dell’ equilibrio verso la specii attive a destra (aggiunta di unità

monomerica) ed il conseguente ritorno a sinistra (specii dormenti) e quindi assicurare un processo

di polimerizzazione controllata .

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Come anticipato, esitono due approcci principali alla realizzazione di polimeri che possiedono le

qualità ed i requisiti strutturali accennati all’inizio (struttura core-shell), questi due metodi

prendono il nome di arm-first e core-first. Il primo, che è quello usato in questo progetto,

consente la realizzazione del polimero a stella, partendo dai segmenti lineari esterni (arm=braccio)

che costituiscono il guscio reso in seguito idrofilico. Una volta sintetizzati polimeri lineari, si

faranno reagire con un agente reticolante bifunzionale (divinlibenzene) che assembla l’insieme

conferendo alla struttura una conformazione detta a stella, data la presenza delle catene lineari

crescenti radialmente verso l'esterno della struttura. La sintesi viene condotta in condizioni di

ATRP.

Br

BrBr

Br

Br

BrBr

Br

Cross Linked Core

Br

Br

Br

Br

Br

Br

Br

Br

ATRP

Approccio ARM FIRST

Figure 2 – Procedura arm-first , le catene lineari reticolano formando una struttura a stella

Il “core-first” è l’approccio esattamente l'inverso, in questo caso è il nucleo costruito prima via

ATRP facendo reticolare l’agente bifunzionale, che preserva i terminali alogenati , questi nuclei di

nano gel reticolati vengono poi usati nella seconda fase come macroiniziatori per consentire

catene lineari di crescere verso l'esterno e formano il polimero a stella analogamente al primo

caso.

Cross Linked Core

Br

Br

BrBr

Br

Br

BrBr

Cross Linked Core

Br

Br

Br

Br

Br

Br

Br

Br

ATRP

Approccio CORE FIRST

Figure 3 – Il “core-fisrt” prevede la sintesi iniziale del nano gel , seguita dalla polimerizzazione delle catene lineari

Nel corso di questo lavoro, differenti monomeri per la realizzazione della “shell” esterna sono stati

presi in considerazione, in particolare rispetto alle proprietà che il polimero corrispondente

poteva offrire, in termini di idrofilicità, considerando l’eventualità di renderlo idrofilico, e quindi

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solubile in acqua, in un secondo momento. I primi quattro mesi di lavoro sono stati focalizzati

sull’impiego di un monomero molto interessante per gli aspetti sopracitati, la 4-vinil-piridina (4VP).

La polimerizzazione ATRP della 4VP si è svolta prevalentemente in solventi protici (2-propanolo),

mediante utlizzo di ligandi amminici tetra-dentati (Me6TREN), per la coordinazione e

solubilizzazione nel solvente organico dei catalizzatori alogenati (CuCl e CuBr). D iversi polimeri

sono stati ottenuti con tale metodo, e una serie di caratterizzazioni (SEC, 1HNMR) sono state

eseguite a scopo di indagine sulle proprietà di questi ultimi. La poli(4-vinil-piridina) si è rivelata da

subito un polimero avente diverse problematiche , dapprima per la difficoltà riscontrata nella fase

di rimozione del catalizzatore, infatti il forte potere coordinante del polimero, fa si che

un’importante frazione di rame rimanga intrappolata nel solido risultante dalla polimerizzazione,

inficiando i risultati sperimentali ottenuti. Sono state inoltre effettuate delle prove di

copolimerizzazione del segmento lineare di polistirene, utilizzando la P4VP-X come macro

iniziatore (sintesi diblocco lineare esterno idrofilico/idrofobico). Le reazioni di copolimerizzazione

del secondo blocco si sono svolte in solvente comune per la P4VP e i monomeri s tirenici (nel mio

caso specifico (DMF), con le stesse condizioni usate nel primo caso e a 100°C di temperatura .

Le sintesi non hanno dato esito positivo; conversioni calcolate tramite tecnica gas cromatografica

hanno evidenziato conversioni scarse o nel peggiore dei casi nulle. Diversi parametri operativi

sono stati variati (rapporti stechiometrici, solventi), confermando i risultati poco convincenti e di

scarso interesse ottenuti in principio. Una possibile spiegazione di ciò, può essere associata alla

presenza di reazioni parassite (ciclizzazioni di catena), che riducono notevolmente, l’attività dei

terminali di catena alogenati non permettendo l’accrescimento controllato del polimero.

Si è pensato quindi, di abbandonare l’ipotesi di un blocco esterno realizzato in P4VP (con

eventuale possibilità di renderla solubile in acqua mediante quaternizzazione dell’atomo di azoto

piridinico), e si è considerata la possibilità di usare polimeri acrilici, poli(tert-butil metacrilato)

PtBMA e poli(tert-butil acrilato) PtBA, per realizzare il blocco esterno, reso idrofilico in un secondo

momento mediante idrolisi acida per dare gli acidi acrilici corrispondenti. Non sono stati riscontrati

problemi nella sintesi dei macroiniziatori lineari, PtBMA-X e PtBA-X, cosi come nella sintesi del

secondo blocco di polistirene (confermato da analisi GC, SEC e 1HNMR) , verificando cosi la

fattibilità di questa classe di polimeri. Le polimerizzazioni ATRP, in questo caso, sono state

condotte in toluene o in anisolo utilizzando ligandi amminici tridentati (PMDETA).

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L’intera gamma di copolimeri lineari sintetizzati è stata impiegata (secondo metodologia arm-first),

come macroiniziatori, nella fase di reticolazione, e una serie di polimeri a stella sono stati ottenuti

e caratterizzati mediante le tecniche sperimentali già accennate prima.

CH3

OO

CH3CH3CH3

m

OO

CH3CH3CH3

m

PtBMA

[Ext Block]

Polystyrene

N

m

P4VP

n

PtBA

Figure 4 – Struttura generale del polimero a stella, e segmenti esterni (ExtBlock = P4VP,PtBMA,PtMA)

Sui polimeri reticolati ottenuti e sui copolimeri lineari di tipo (PtBMA-b-PS o PtBA-b-PS), sono state

condotte delle prove di idrolisi in presenza di acido trifluoroacetico (CF3COOH), e i poli acidi

corrispondenti sono stati sintetizzati (acido polimetacrilico e poliacrilico). La solubilizzazione

completa dei polimeri, è stata ottenuta in soluzioni acquose e in soluzioni acquose a pH basico

(K2CO3 0.4 M). La natura morfologica dei polimeri a stella è stata in seguito indagata, mediante

tecniche microscopiche a forza atomica (AFM).

Una trattazione più approfondita, riguardante i risultati, le metodologie di polimerizzazione e le

caratterizzazioni sperimentali compiute sui polimeri, è fornita negli ultimi capitoli di questo lavoro.

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Chapter 1

1.INTRODUCTION: SYNTHESIS OF WATER-SOLUBLE STAR-BLOCK

LIKE COPOLYMERS BY“ARM-FIRST”APPROACH USING ATRP

There is interest in developing new ways to confine homogeneous catalysts, leading to a more

efficient catalyst recovering and recycling with minimum losses. One interesting concept is that of

micellar catalysis, where the catalyst is covalently liked to amphiphilic molecules that self-

assemble in the form of micelles above the micellar critical concentration in water, and further

applications under organic/water biphasic conditions. Two problems associated with micellar

catalysis are the equilibrium with the free amphiphile, which places itself at the interface leading

to catalyst losses, and the formation of stable emulsions. These problems may be eliminated by

covalently linking the micelle core into a nanogel structure. We therefore propose to prepare

suitably functionalized core-shell polymers with a hydrophilic shell and a hydrophobic cross -linked

core. Core cross-linked core-shell polymers have already been prepared, but the application of

these objects to biphasic catalysis has not yet been explored. The technology to develop the

desired polymer molecules, through controlled polymerization techniques and specifically through

atom transfer radical polymerization (ATRP), is now well established. The project will consist of the

synthesis of such molecules through the arm-first approach. The first step is the polymerization of

flexible hydrophilic arms (linear chains) which will constitute the external shell, it follows the

generation of second linear segment of polystyrene grown by ATRP copolymerization. For catalytic

applications, this second linear segment should contain a functionalised styrene co-monomer

distributed sadistically among the regular styrene monomer. This functionalised monomer

contains the ligands needed to link the catalytic metal in a further step. The last step is the

creation of a nanogel by polymerization of a cross-linking monomer such as divinylbenzene or

EGDMA. The flexibility of the ligand-functionalized arms insures “homogeneous” conditions and

facile metal accessibility for the catalytic application1. However, the generation of this ligand-

functionalized polymer will occur only in a second phase within the research group. The

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contribution of my thesis was restricted to the optimization of the synthetic protocol for the

generation of suitable amphiphilic core-shell polymer scaffolds that are not ligand-functionalized,

and to determine their solubility in water. Several polymers differing in a variety of controlled

parameters will be synthesized and characterized. Once this phase will be optimized, the statistical

incorporation of ligand-functionalized monomers in the hydrophobic cores can be studied and the

resulting ligand polymer can be complexed to the catalytic metal and applied to biphasic cata lysis.

This project consists in the synthesis of star shaped macromolecules through the arm-first

approach by atom transfer radical polymerization (ATRP), which could be used for confined

catalysis application, by utilizing functional monomers instead of polystyrene (used in this work),

as soluble nano sites in water media. The following chapters will highlight, basic concepts of star-

shaped polymers (star block copolymer in this specific context) and atom transfer radical

polymerization in order to clarify these notions to the reader and to rationalize the choice of the

materials (monomers and catalytic metal complexes) used in the project.

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References

1) H. F. Gao, K. Matyjaszewski, Macromolecules 2008, 41

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Chapter 2

2. PRINCIPLES OF ATOM TRANSFER RADICAL POLYMERIZATION

(ATRP) AND DISCUSSION OF COMPLEX MACROMOLECULAR

ARCHITECTURES

2.1 The Atom Transfer Radical Polymerization (ATRP)

The field of radical polymerization has exploded within the past few years with the advent of

controlled radical polymerization processes. These methods provide the ability to synthesize

polymers with predictable molecular weights and block copolymers with high blocking efficiency

and to prepare new chain architectures that have not been successful using ionic polymerization.

One such process is atom transfer radical polymerization (ATRP or transition metal-mediated living

radical polymerization), this particular kind of radical polymerization was independently

discovered by Mitsuo Sawamoto and by Krzysztof Matyjaszewski and Jin-Shan Wang in 1995. ATRP

requires reactivation of the first formed alkyl halide adduct with the unsaturated compound

(monomer) and the further reaction of the intermittently formed radical with additional monomer

units (propagation). The "livingness" of this polymerization can be underlined by a linear first-

order kinetic plot, with a consequent linear growth of the polymer molecular weight with

conversion, with the value of the number-average degree of polymerization (DPn) determined by

the ratio of reacted monomer to introduced initiator at the beginning (i.e., DPn = D[M]/[RX]0) 2.

Figure 2.1 – ATRP reaction scheme

http://en.wikipedia.org/wiki/Krzysztof_Matyjaszewski

http://en.wikipedia.org/wiki/Jin-Shan_Wang

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The normal schematic of the ATRP equilibrium, which emphasizes the repetitive nature of the

activation and deactivation phenomena and the need to push the equilibrium to the left side,

thereby forming a high mole fraction of dormant chains, is shown below in figure 2.2.

Figure 2.2 – ATRP equilibrium scheme

Mechanistically, ATRP is based on an inner sphere electron transfer process, which involves a

reversible homolytic (pseudo)halogen transfer between a dormant species, an added initiator or

dormant propagating chain end (R-X or R-Pn-X) and a transition metal complex in its lower

oxidation state (Mtm/Ln), resulting in the formation of propagating radicals (R.) and the metal

complex in the higher oxidation state with a coordinated halide ligand (e.g. X-Mtm+1/Ln). The active

radicals form with a measurable activation rate constant kact, subsequently propagate with a rate

constant kp and are reversibly deactivated kdeact, but, since ATRP is a radical based process, the

active species can also terminate with a rate constant kt. However, the incidence of the

terminations (that are bimolecular in radicals) is reduced by the reversible activation equilibrium

(strongly shifted towards the dormant species , kact << kdeact) which keeps the free radical

concentration very low. Furthermore as the reaction progresses radical termination is less and

less, as a result of the persistent radical effect (PRE), increased chain length, as well as conversion

and viscosity.

The higher oxidation state transition metal (complex), the equivalent of the persistent radical in an

ATRP, can be added directly to a reaction prior to initiation to increase the efficiency of initiation,

by reducing the fraction of low molecular weight termination reactions initially required to

generate the PRE, or can be formed in situ by reaction with dissolved oxygen. Addition of the

persistent radical, (Mtm+1 in the case of ATRP) is of particular utility when conducting a "grafting

from" reaction with multifunctional initiators or grafting from a surface. It is also strongly

recommended when ATRP is carried out in protic, particularly homogeneous aqueous media4.

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ATRP is in many ways a complex chemical process, since the polymerization includes one or more

(co)monomers, a transition metal complex in two or more oxidation states, which can comprise

various counter ions and ligands, an initiator with one or more radically transferable atoms or

groups and an optional solvent, suspending media or various additives. All of the components

present in the reaction medium could, affect the interactions between the reagents that

constitute the ATRP equilibrium. The initiator is most frequently an alkyl (pseudo)halide which can

be either a low or high molar mass compound or even a part of an insoluble material, such as

when initiators are tethered to the surface of modified particles, flat wafers, or even fibers, etc 5.

In addition to copper and ruthenium, heavily used by the two pioneers of the technique

Sawamoto and Matyjaszewski, a wide range of other metals have been employed in an ATRP

including titanium, molybdenum, iron, ruthenium, osmium, and palladium but in this particular

work and in most of the following discussion we will use copper as an exemplary transition metal.

Copper is the transition metal of choice, as determined by the successful application of a spectrum

of copper complexes as catalysts, for the ATRP of a broad range of monomers in divers e media.

However, iron may eventually be considered as an interesting the transition metal to be used, for

environmental reasons unless industrially viable procedures for internal reuse of the copper

complexes are adopted. It should also be noted that Ru and Os have certain advantages as a

consequence of their high halidophilicity that may make them a good choice for use in protic

media. Polymers prepared by other polymerization processes can be functionalized at the chain

endings or along the backbone and incorporated into an ATRP as a macromonomer or

macroinitiator, or at the same time by using both a macroinitiator and a macromonomer to

improve incorporation of the macromonomer into the polymer, leading to the preparation of well

defined block and graft copolymers. There may be multiple initiating sites in either a small

molecule or a macroinitiator, leading to chain growth in several directions 6.

The transition metal complex should be at least partially soluble in the reaction medium and

reactions can be run under homogeneous or heterogeneous conditions. In general, the former

provides better control since the concentration of activator and deactivator can be controlled.

Reaction temperatures typically range from room temperature to 150 0C, but can be

correspondingly altered. The reaction can be run under vacuum or pressure. Reactions can not

only be conducted in the presence of moisture but even in the presence of water under

homogeneous or heterogeneous (micro-emulsion → suspension) conditions7.

http://www.cmu.edu/maty/chem/fundamentals-atrp/protic-media.html#ATRP Protic

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Oxygen should be removed from the reaction medium, but a limited amount of oxygen can be

tolerated particularly in the presence of an added reducing agent e.g. Cu(0), Sn(EH2), ascorbic acid,

reducing sugars or amines. The order of addition of reagents may vary but generally the initiator,

or catalyst activator, is the last reagent to be added to a previously prepared solution of the

catalyst in the monomer/solvent. An important parameter may be the addition or formatio n of a

small amount of the Cu(II) species at the beginning of the reaction, since it enables the

deactivation process to occur immediately without requiring its spontaneous formation by

termination reactions, thereby providing both higher initiator efficiency, reduced cost and

instantaneous control. Understanding, and controlling the equilibrium, and hence the dynamics of

the atom transfer process, are basic prerequisites for running a successful ATRP. Therefore, the

correlation of reactivity related to the structure of each of the involved reagents, the oxidation

states of the transition metal complex, radicals and dormant species, in addition to solvent effects

and reaction temperature, remains an important objective, in order to achieve a fundamental

understanding of the reaction kinetics required for the selection of optimal conditions to conduct

the desired reaction8.

2.2 Star Polymers

Star polymers consist of several linear polymer chains linked to one point or to a central core. Star

molecules prepared by anionic polymerization were examined prior to the discover and

development of controlled radical polymerization (CRP). However due to the limitations of ionic

polymerization the composition and functionality of the materials was limited but their compact

structure and globular shape provide them with a range of unique properties, such as low solution

viscosity, and the core shell architecture enabled several potential applications, from

thermoplastic elastomer's to drug carriers9.

Figure 2.3 – Star Polymers and mikto-arm structure

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Based on the chemical compositions of the arm species, star polymers can be classified into two

categories: homo-arm (or regular) star polymers or mikto-arm (or heteroarm) star copolymers.

Homo-arm star polymers consist of a symmetrical structure comprising radiating arms with

identical chemical composition and similar molecular weight. In contrast, a miktoarm star

molecule contains two or more arm species with different chemical compositions and/or

molecular weights and/or different peripheral functionalities. There are several approaches that

can be employed for synthesis of star copolymers 10.

Figure 2.4 – Core-first and arm first procedures

As illustrated in the previous image (figure 2.4 ), star polymers can be synthesized by variations on

one of three methods:

The "core-first" approach, where the controlled polymerization is conducted from either a

well defined initiator with a known number of initiating groups or a less well defined

multifunctional macromolecule.

An approach that received much attention is the "coupling onto" approach where a

functional linear molecule reacts with a presynthetised core molecule containing

complementary functionality. In order to improve the coupling efficiency, a highly ef ficient

organic coupling reaction is required, such as click reactions. The preparation of stars using

click chemistry could be applied to almost any of the s trategies.

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There are two approaches to the "arm-first" synthesis of star polymers. One is where a

linear "living" copolymer chain, or added macroinitiator, is linked by continuing the

copolymerization of the mono-functional macroinitiator with a divinyl monomer forming a

cross linked core. The other, is the direct copolymerization of a macromonomer with a

divinyl monomer in the presence of a low molecular weight initiator. A combination of

"arm-first" and "core-first" methods is particularly useful for synthesis of miktoarm star

copolymers. One employs the retained initiating functionality in the formed "arm first"

core to initiate the polymerization of a second monomer in a "grafting out" or a "grafting

from" copolymerization.

2.2.1 Core first:

Here below some examples ( “grafting form”, “in situ generated core”) of core -first procedure will

be given, in order to show how this particular approach can be used.

Low molecular weight multi-functional molecule is used in a "grafting from" reaction to form star

macromolecules with a well defined number of arms. An example is the use of hexakis

(chloromethyl)benzene as a well defined multifunctional initiator for the polymerization of styrene

provided the first multi-arm polymer prepared using ATRP. The composition of the core and arms

are quickly expanded (figure 2.5).

Figure 2.5 – Grafting form core-first approach

Since the tethered chains in a grafting from reaction retain their terminal functionality they could

be chain extended to form star block copolymers and/or the radically transferable atoms on the

chain ends (X) could be converted to other functional groups (F) suitable for post-polymerization

functionalization reactions.

20

Less well-defined multi arm star structures are prepared by grafting from a soluble multifunctional

hyperbranched core. The core is usually prepared by polymerizing a molecule containing both a

reactive group suitable to initiate a CRP and a polymerizable double bond, an initiator/monomer

or inimer. Self-condensing vinyl polymerization using ATRP has been applied to inimers, i.e.

monomers that also contain activated halogen atoms, such as α-bromoesters and benzyl halides. A

simple sequential polymerization of a crosslinker followed by polymerization of a monomer

provides a largely applicable approach to star copolymers. This new method, termed "star from in

situ generated core", belongs to the broader category of "core-first" methodology and presents an

alternative strategy for star synthesis when compared to the traditional "arm-first" method, in

which monomer is polymerized first followed by formation of the core by (co)polymerization of a

cross-linker11.

This new concept of star polymers synthesis using controlled radical polymerization techniques,

was applied to the ATRP homopolymerization of a crosslinker, such as ethylene glycol diacrylate

(EGDA) to generate a multifunctional crosslinked core (nanogel). Monovinyl monomers are added

to the reaction mixture at high conversion of the crosslinker and were polymerized from the

polyEGDA nanogel macroinitiator (MI) to form the star arms. A spectrum of different acrylate

monomers could be polymerized from the first formed core providing star polymers with different

compositions for the arms.

Figure 2.6 – in situ generated core core-first approach

Several parameters affect the structure of the star, such as the initial concentration of the

crosslinker, the molar ratio of crosslinker to initiator, and the moment of addition of the

monovinyl monomer to the ongoing polymerization reaction. The star polymers preserve the

initiating sites at the chain ends, and they are further used as star MIs for arm extension by

polymerization of a second monovinyl monomer to form a star block copolymer (figure 2.6).

21

Indeed this approach of preparing well defined polymers by copolymerization of a crosslinker and

a monomer can be expanded to provide access to a full range of stars/gels/networks12.

2.2.2 Arm first:

The "arm first" approach forms the core of the star macromolecule by coupling monofunctional

"living" polymeric chains with a bifunctional reagent. A similar approach via ATRP was utilized

during the present work. Initially the simple chain extension of a linear macroinitiator with a

crosslinker provided star macromolecules with broad polydispersity as a result of star-star

coupling reactions.

There are several parameters in an ATRP that should be carefully controlled in order to maximize

the yield of stars and prevent/reduce star-star coupling reactions. Some detailed studies have

been carried out on the coupling of monofunctional polystyrenes and polyacrylates with

divinylbenzene (DVB) to prepare star polymers and the following guidelines were developed13:

The ratio of bifunctional crosslinker to growing chains seems to be optimal in the range of

10-20.

Monomer conversion (or reaction time) has to be carefully controlled and stopped before

star-star coupling occurs. It seems that ~5% of arms cannot be incorporated into the star

macromolecules under typical one pot conditions.

Higher yields of stars are observed for polyacrylates than for polystyrenes. This may be

attributed to a higher proportion of terminated chains, in styrene polymerization under

"standard" ATRP conditions.

The choice of the bifunctional reagent is important and reactivity should be similar to, or

lower than that of the arm-building monomers.

Halogen exchange slightly improves the efficiency of star formation.

Solvent, temperature and catalyst concentration should be also optimized.

Figure 2.7 – Arm-first synthesis sequence

22

A range of initiators, monomers and cross linkers , shown in figure 2.8, could be implied for the

preparation of star molecules with peripheral functionality and cores of differing phobicity.

Figure 2.8 – Suitable initiators, monomers and bifunctional monomers for ATRP

An alternate approach for the arm-first sequence, is a copolymerization of macromonomers with a

divinyl crosslinking monomer (figure 2.9).

Figure 2.9 – arm-first approach using a divinyl monomer

The sequential addition of additional initiator and cross linker to the reaction increases the

number of macromonomer units incorporated into each star.

23

2.2.3 Mikto-arm star copolymers:

Both approaches to form star molecules using the arm first approach retain the initiating

functionality within the core of the star and therefore provide a simple approach to form mikto-

arm stars by conducting a controlled polymerization of a second monomer from the accessible

initiator functionality in the core14.

Figure 2.10 – In-out approach in mikto-arm synthesis procedure

As noted above one arm-first procedure involves the synthesis of a linear mono-functional

polymer chain polyA, which is used as a macroinitiator (MI) in a subsequent cross -linking reaction

using a divinyl compound to produce a (polyA)n-polyX star polymer, where polyX represents the

core of the star polymer and n is the number of polyA arms. The initiating sites are preserved

within the core of the star polymer (i.e., alkyl halide groups in ATRP) and the (polyA) n-polyX star

polymer can be used as a multifunctional star initiator in a chain extension reaction with a

different monomer, B, to yield a miktoarm star copolymer, (polyA)n-polyX-(polyB)m. This

combination method for synthesizing miktoarm star copolymers was termed the "in-out" method

(figure 2.10).

The efficiency of initiation of the second set of arms is dependent on the compactness of the first

formed core with less densely cross linked cores providing more efficient initiation i.e., a greater

fraction of the encapsulated initiator sites, for the grafting out polymerization.

Normally, one seeks to, or has to, form a chemically stable core. However, it is possible to select a

crosslinking agent with a degradable link between the two functional crosslinking groups and

prepare a material with a degradable core.

24

References

1) H. F. Gao; Matyjaszewski K., Prog. Polym. Sci. 2009, 34.

2) Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117.

3) Pintauer, T.; Matyjaszewski, K. Chem. Soc. Rev. 2008, 37, 1087-1097.

4) Matyjaszewski, K.; Coca S.; Gaynor, S. G.; Wei, M.; Woodworth, B. E. Macromolecules 1998, 31.

5) Braunecker, W. A.; Matyjaszewski, K. Progress in Polymer Science 2007, 32.

6) Tsarevsky, N. V.; Braunecker, W. A.; Matyjaszewski, K. Journal of Organometallic Chemistry 2007.

7) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101.

8) Xia, J.; Zhang, X.; Matyjaszewski K., Macromolecules 1999, 32,

9) Zhang, X.; Xia, J.; Matyjaszewski K., Macromolecules 2000, 33.

10) Baek, K.-Y.; Kamigaito, M.; Sawamoto M. ,Macromolecules 2001, 34.

11) Gao, H. ; Matyjaszewski K. ,Macromolecules 2009, 39.

12) Baek, K.-Y.; Kamigaito, M.; Sawamoto, M. ,Macromolecules 2001, 34.

13) Gao, H.; Ohno, S.; Matyjaszewski, K. ,Journal of the American Chemical Society 2006,

14) Gao, H.; Matyjaszewski K. ,Macromolecular Symposia 2010, 291.

25

Chapter 3

3. MICELLIZATION AND WATER-SOLUBLE POLYMERS

3.1 Micellization: an Introduction

The present work, as we discussed in chapter 2, is focused on the synthesis and characterization

of complex macromolecular architectures, such as core-shell polymers, which may be used to

confine homogeneous catalysts in aqueous solution. Core-shell polymers are particular

macromolecular structures formed (in this particular case) by linear amphiphilic chains connected

into a crosslinked core, that can be considered as micelles, with similar shape, dimensions, and

behaviour in aqueous environment. The name amphiphile is sometimes used synonymously with

surfactant. The word is derived from the Greek word amphi, meaning both, and the term relates

to the fact that all surfactant molecules consist of at least two parts, one of which is soluble in a

specific fluid (the lyophilic part) whereas the other one is insoluble (the lyophobic part). When the

fluid is water one usually talks about the hydrophilic and hydrophobic part, respectively. A

fundamental property of surfactants (amphiphiles), is that the free unassociated surfactants

molecules (called “unimers”) tend to form aggregates in solution, so-called micelles. The micelles

formation, or micellization, can be viewed as an alternative mechanism to adsorption at the

interfaces for removing hydrophobic groups from contact with water, thereby lowering the system

free energy. It is an important phenomenon since surfactant molecules behave very differently

when aggregate in micelles than as free unimers in solution. Only surfactant unimers contribute to

surface and interfacial tension lowering dynamic phenomena, such as wetting and foaming, are

governed by their concentration. The micelles may be seen as a reservoir for surfactant unimers.

The exchange rate of a surfactant molecule between micelle and bulk solution may vary by many

orders of magnitude depending on the size and structure of the surfactant. Micelles are already

generated at very low surfactant concentrations in water. The concentration at which micelles

start to form is called the critical micelle concentration or CMC, and is an important characteristic

of a surfactant. A CMC of 1mM, a reasonable value for an ionic surfactant, means that the unimers

concentration will never exceed this value, regardless of the amount of surfactant added to the

solution.

26

In a micelle the surfactant hydrophobic groups are directed toward the interior of the cluster and

the polar ones are directed towards the solvent. The micelle, therefore, is a polar aggregate of

high water solubility and without much surface activity. When a surfactant adsorbs from aqueous

solution at a hydrophobic surface, it normally orients hydrophobic groups towards the surface and

exposes its polar groups to water. The surface has become hydrophilic and, as a result, the

interfacial tension between the surface and water has been reduced. The polar part of the

surfactant may be ionic or non-ionic and the choice of polar group determines the properties to a

large extent. For a non-ionic surfactants the size of the head group can be varied at will; for the

ionic ones, the size is more or less a fixed parameter. In general, the relative size of the

hydrophobic and polar groups, not the absolute size of either of the two, is decisive in determining

the physicochemical behaviour of a surfactant in water1. A spherical micelle of dodecyl sulphate is

shown in figure 3.1 below.

Figure 3.1 – Spherical micelle of dodecyl sulphate

27

3.1.1 Different Amphiphile Systems

Surfactants self-assembly leads to a range of different structures, of which a few are shown in the

following page (figure 3.3). Systems containing amphiphile are best classified into homogeneous

(or single phase) systems and heterogeneous systems of two or more phases. The single-phase

systems can in turn be divided into isotropic solutions, solid phases and liquid crystalline phases.

The solid crystalline phases have, as do crystals in general, both long-range and short-range order,

but the degree of short-range order varies between different phases. Isotropic solution phases are

characterized by disorder over both short and long distances, while liquid crystalline phases or

mesophases have a short-range disorder but some distinct order over larger distances. In both

isotropic solutions and liquid crystals, the state of the amphiphile alkyl chains can be denoted as

“liquid like”. In crystals, formed below the chain melting temperature , the state is more or less

”solid like”. The more important amphiphile systems can be classified as shown in figure 3.2

Figure 3.2 – Principal amphiphile systems

28

Figure 3.3 – Representation of principal micelle structures

Figure 3.3 above shows how surfactant self assembly leads to a range of different

structures for which few are shown:

a) Spherical micelles with hydrocarbon chains at the interior and polar chains outside.

b) Cylindrical micelles with an interior composed of the hydrocarbon chains and polar

groups facing water.

c) Surfactant bilayers which build up lamellar liquid crystals.

d) Reversed micelles with by the surfactant polar groups at the core, surrounded by

the hydrophobic tails.

e) Bicontinous structures with surfactants aggregated into connected films by two

curvatures of opposite sign.

f) Vesicles are built from bilayers similar to those of the lamellar phase, and

characterized by two distinct water compartments.

29

3.1.2 Micelles Formation at the Critical Micelle Concentration (CMC)

In measuring the different physicochemical properties of an aqueous solution containing a

surfactant, many peculiarities should be encountered as illustrated here below (Figure 3.4)

Figure 3.4 – Influence of concentration on the main properties

At low concentrations most properties are similar to those of a simple electrolyte. One notable

exception is the surface tension which decreases rapidly with surfactant concentration. At some

higher concentration, which is different for different surfactants, unusual changes are noticeable.

For instance, the surface tension, as well as the osmotic pressure, takes on an approximately

constant value, while light scattering starts to increase and self-diffusion starts to decrease. All of

the observation suggest and are consistent with a switch from a solution containing single

surfactant molecules (unimers) to a system where the surfactant occurs more and more in a self -

assembled or self-associated state. We will consider the structures formed, as well as the

underlying mechanism, and will only note two general features. The concentration for the onset of

self-assembly is quite well defined and becomes more so the longer the alkyl chain of the

surfactant. The first-formed aggregates are generally approximately spherical in shape (micelles)

and the concentration where they start to form is known as the critical micelle concentration

(CMC), as already defined in the previous paragraph. The CMC is the single most important

characteristic of a surfactant, useful, inter alia in consideration of the most practical uses of

surfactants. It depends on a large number of different of different parameters, but it is mostly

affected by the chemical structure and properties of the surfactants, the temperature and

30

presence of cosoluted species. The two most common and general techniques used to measure

the CMC are surface tension and solubilization , i.e. the solubility of an otherwise insoluble

compound. For an ionic amphiphile, the conductivity offers a convenient approach to obtain the

CMC value. However, as a very large number of physicochemical properties are sensitive to

surfactant micellization, there are numerous other possibilities, such as self-diffusion

measurement, NMR and fluorescence spectroscopy. Moreover, CMC is not an exactly defined

quantity, which causes difficulties in its determination. For long-chain amphiphiles, an accurate

determination is straightforward and different techniques give the s ame results2.

3.2 Polymers in Solution and the Choice of Monomers

Synthetic polymers are obtained by polymerization reaction of monomers. The monomers in the

polymerization constitute the repeat units, for example acrylic acid is polymerized into poly(acrylic

acid). A polymer can either be linear, branched or cross linked. It s classify polymers according to

their solution behavior, which can differ considerably. The polarity of the monomer units is a

convenient basis to classify non-biological polymers:

i. Non-polar polymers such as polystyrene and polyethylene

ii. Polar, but water-insoluble, polymers, such as poly(methyl methacrylate) and poly(vinyl

acetate)

iii. Water soluble polymers, such as polyoxyethylene and poly(vinyl alcohol)

iv. Ionizable polymers, or polyelectrolytes, such as poly(acrylic acid)

The configuration of a polymer in solution depends on the proportion of a segments to another

one. In general terms, a polymer can form a random coil, an extended configuration or a helix. For

synthetic polymers, the random coil is the most common configuration. Polyelectrolytes, where

the monomer units are charged, can under certain circumstances form stiff rods. Dissolution of a

polymer can sometimes be a problem, not only in the laboratory scale but also on an industria l

one. The dissolution process starts with the solvent, which is more mobile than the polymer

chains, penetrating into, and hence swelling the polymer. The polymer solution then becomes

highly viscous and sticky . Subsequently the polymer chains disentangle from the gel and start to

diffuse into the solvent. This is a slow process since the polymer chain dynamics, which are

dependent on the polymer molecular weight, are rate determining. The polymer stuck to the

container walls, has a rather small exposure area to the solvent and hence the dissolution maybe

31

quite time-consuming. Dissolving Poly(vinyl-pyridine) into water at neutral pH, for example, takes

at least 24h. Technically, the problem of dissolving a polymer can be circumvented by avoiding to

obtain the solvent-free material and by only handling polymers that are already dissolved at some

high concentration in a solvent. When the pure polymer is in the form of a powder other tricks

may be used. Let us take the poly(vinyl-pyridine) example as an illustration. At high pH the

polymer is not soluble in water and the polymer powder can therefore be easily dispersed at pHs

above 8 at high stirring rates. The pH is then quickly lowered to about 3-4 under vigorous stirring.

The polymer particles will then not agglomerate but rather start to dissolve. Now the dissolution

process will be orders of magnitude faster since the total area of the polymer particles is much

larger than that of a polymer adhering to the container walls.

3.2.1 Various Classes of Water-Soluble Polymers

Some example of water-soluble polymers are given here, together with their properties and uses.

We shall first consider non-ionic water-soluble polymers with an oxygen or nitrogen in the

polymer backbone. Out of the polyoxyalkylenes only polyoxyethylene (POE) is water soluble.

Polyoxymethylene is not water soluble despite the fact that it contains a higher portion of oxygen

than polyoxyethylene. These polymers can be synthesized with molecular weights up to millions.

POEs are used in various applications such as cosmetics and pharmaceutical formulations, ceramic

binders, etc. For polyoxypropylene (POPs) only oligomers are water soluble whereas longer

polymer chains sometimes serve as hydrophobic entities in surfactants. If one changes the oxyg en

atom in POE for nitrogen, we then obtain polyethyleneimine (PEI) another water soluble

macromolecule. Commercial samples of this polymer are normally branched and the ratio

between the secondary, tertiary and quaternary amine groups is normally 1:2:1. Another family of

water-soluble polymers are those containing an acrylic group, the first examples are poly(acrylic

acid) (PAA) and poly(methacrylic acid) (PMAA) large use of which was made in this work. Strangely

enough, the latter polymer shows higher water solubility than the former. The reason is that the

latter material forms a helical configuration, thus burying the hydrophobic backbone inside the

helix. These polymers are water soluble even in the non dissociated form, but can also dissociate

to generate polyelectrolytes (see next section). PAA and POE form a complex in aqueous solution

where the hydrogen atoms in the PAA are H-bonded to the oxygen atoms on the POE. Another

example of a water-soluble acrylic polymer is polyacrylamide (PAAm). This is a very hydrophilic

polymer which is insensitive to the addition of salts. This polymer is used as a flocculent since it

has a high affinity to surfaces due to its cationic nature at lower pH values. Thirdly we consider

32

water-soluble non-ionic polymers containing a vinyl group. The first example is poly(vinyl alcohol)

(PVAI), which is synthesized by the hydrolysis of poly(vinyl acetate). Polymers with a degree of

hydrolysis over ca. 86% are soluble in water. If the degree of hydrolysis is higher than 90%, the

system needs to be heated in order to be dissolved completely. Once dissolved in hot water, the

polymer remains in solution when cooled. This apparent irreversibility is due to the formation of

internal hydrogen bonds in the solid polymer. Another example of a highly water soluble polymer

is poly(vinyl pyrrolidone) (PVP). This polymer usually associates with anionic surfactants, in

aqueous solution. Aqueous solutions of PVP are used in pharmacy, cosmetics and medicine due to

its low toxicity and high water solubility, PVP is also used in detergent formulations where its role

is to prevent re-deposition of soil on fibers. In the fourth and final group, we can classify polymers

that have natural origin, such as the cellulose derivatives (32andomiz- and carboxy-celluloses) and

the polysaccharides (used in food industry as gelants)3.

3.2.2 Polyelectrolytes as Charged Polymers

Polyelectrolytes in solution have many applications and are used technically as thickeners,

dispersant, flocculation aids, etc. The word “polyelectrolyte” is sometimes used for all types of

aggregates which carry a high charge. In some of the literature it is reserved for charged polymers

which is the family of polymers considered in this section. A flexible polymer often achieves a net

charge from carboxylate or sulfate groups (-COO- , -SO4-)or from ammonium groups or protonated

amines. Polyelectrolytes are often classified as strong (quenched) or weak: the net charge of the

latter changes with pH. If a polyelectrolyte carries only one type of 32andomiza monomer unit, for

example, acrylic acid, one can easily define the degree of ionization α, as the fraction of

32andomiza groups that are ionized. This property is, of course, dependent on the pH and the

relationship can be written as follows (eq.3.1):

Equation 3.1

where pK is the acidity (or basicity) constant of the monomer unit. This constant is not a constant

in a strict sense since it depends on the degree of ionization.

This is accounted for in more elaborated description of polyelectrolyte behavior in aqueous

solution. The degree of expansion of the polyelectrolyte increases with the degree of ionization, α,

33

due to repulsion between the ionized groups. The radius of gyration of a polyelectrolyte,

poly(methacrylic acid), varies as a function of the degree of ionization, as shown in figure 3.5. To a

first approximation, the radius of gyration reaches a saturation value at a degree of ionization of

about 0.3. Thus , if elongated polyelectrolyte molecules are required there is no point in charging

the polyelectrolyte beyond this value since the dimensions will not change much once ca. 30% of

the ionizable groups have become charged. This correspond to ca. half a unit from the pK value of

the polymer.

Figure 3.5 – Gyration radius of poly methacrylic acid as a function of the degree of ionization

The low solubility of polymers and related phenomena are due to the low entropy of the polymer

when compared to free monomers. It is also clear that the different solution behavior of neutral

polymers and polyelectrolytes systems is largely due to the counterion entropy and only indirectly

to electrostatic interactions4.

34

References

1) Teraoka I. , Polymer Solutions : An Introduction to Physical Properties, 2nd Edition, Wiley-

Intersciences 2002.

2) MacCormick C.L. ; Lowe A.B. , Enc. of Polym. Science and Tech. Vol.12, Water Soluble

Polymers, John Wiley and Sons 2001.

3) Holmberg K.; Jonsson B.; Kronberg B. , Surfactants and Polymers in Aqueous Solution, 2nd

Edition, Wiley-Intersciences 2002.

4) Davis F.J. , Polymer Chemistry : a Practical Approach,1st Edition, Oxford University Press

2004.

35

Chapter 4

4. ANALYTICAL INSTRUMENTS : THEORY AND BASIC PRINCIPLES

4.1 Nuclear Magnetic Resonance Spectroscopy (NMR)

4.1.1 Introduction

Nuclear magnetic resonance spectroscopy can give a large amount of information about both the

structure of polymers and the nature of the molecular motions taking place within them. As for

infrared and Raman spectroscopy, the spectra are due to transitions between quantized energy

levels caused by the interaction of the material with electromagnetic radiation. Although the

energy levels are of a quite different type from those involved in vibrational spectroscopy, it is the

influence of the structure and internal motions of the polymer on the precise form of the

spectrum that provides the information about the structure and motions, just as for vibrational

spectroscopy. Most NMR studies, including those on polymers, are carried out on solutions. The

spectra are then greatly simplified, and invaluable for the elucidation of chemical structure. The

method relies on the fact that some atomic nuclei have a spin angular momentum and an

associated magnetic moment. For many nuclei the spin is ½, as for the proton and the 13C nucleus,

both of which are particularly important for the NMR spectroscopy of polymers. When such a

nucleus is placed in a magnetic field Bo the spin can take up two orientations with respect to the

field, ‘parallel’ and ‘anti-parallel’. According to quantum mechanics this means that although the

magnitude of the spin magnetic moment µ is:

Equation 4.1

36

where is the magnetogyric ratio, the value of its component parallel or anti-parallel to Bo is only

and only the square of the component in the perpendicular direction is defined. The

interaction energy with the field is thus

, depending on whether the spin component

along the direction of the field is parallel or anti-parallel to Bo. The semi-classical interpretation of

the undetermined individual components of the magnetic moment perpendicular to Bo is that l

processes around Bo with an angular velocity , as shown in figure 4.1 , which also shows

the energy-level diagram. The angular frequency is equal to the frequency of the radiation

necessary to cause the transition from the lower to the upper state and is called the resonance

frequency.

Figure 4.1 – Angular velocity perpendicular to B0

If all the nuclei of each element present in a sample were isolated the corresponding spectrum

would consist of a series of sharp lines, one for each type of nucleus that has a non-zero spin. In a

material each nucleus is actually under magnetic influences from the electrons surrounding it,

including those binding the atom to other atoms, and from the dipoles due to the nuclei of other

atoms. These influences cause splittings, broadenings and shifts of the lines in the spectrum, from

which information is obtained about the structure and internal motions of the material. In the

simplest kinds of experiments the contribution that each spin of a given type makes to the

magnitude of the NMR signal is, however, independent of its environment, which makes

quantitative interpretation more straightforward than for IR or Raman spectroscopy1.

4.1.2 NMR spectrometers and experiments

The values of Bo used in NMR are in the range 1–12 T, corresponding to values of for

protons in the radio-frequency (rf) range 40–500 MHz. The earliest form of NMR spectroscopy

used the continous-wave (CW) method, in which an rf field of small amplitude is applied to the

37

sample continuously and either the frequency of this field or the value of Bo is varied to scan the

spectrum. The transitions are usually detected by a separate coil surrounding the sample. Very

little NMR work now uses the CW method, which has largely been replaced by the use of short

high-power pulses of rf radiation to disturb the distribution of nuclei in the various energy levels

from their values in thermal equilibrium. Fourier-transform (FT) methods are then used to extract

spectral information from the signals induced in a detector coil as the distribution relaxes after the

perturbing pulses. The pulsed FT method has many advantages over the earlier method, including

speed of acquisition of spectra, easier determination of relaxation times and the possibility of

using many specialized sequences of pulses to extract various kinds of information about the

sample. It is important to realize that, at thermal equilibrium, the difference between the number

of spins parallel and anti-parallel to Bo is very small because the difference in energy is very

small compared with kT. This means that the NMR signals are very weak and require sensitive

detectors. Figure 4.2 shows a block diagram of the essential features of a spectrometer for pulsed

NMR studies. The sample sits within the probe which is surrounded by the magnet that produces

the field Bo, which is usually a superconducting electromagnet. The probe contains a coil

surrounding the sample that is used to produce the rf field and to detect the induced signal. It also

contains mechanisms for rotating the sample and for varying the temperature.

Figure 4.2 – Essential scheme of an NMR spectrometer

38

The simplest experiment uses a single pulse of rf radiation lasting for a time of the order of

microseconds. Any pulse of radiation of length Δt contains a range of frequencies of about 1/Δt,

so that a 1 µs rf pulse ‘of frequency 250MHz’ actually contains a range of frequencies spread over

about 1 MHz and is thus only defined to about one part in 250, or about 4000 parts per million

(ppm). For a constant value of Bo the values of the resonance frequencies for a given type of

nucleus vary with their environment by only a few hundred parts per million, so that such a pulse

can excite all the resonances equally, to a good approximation. A so-called 90° pulse is frequently

used. To understand what this means it is useful to consider a rotating frame of reference.

According to the semi-classical picture, each spin precesses around Bo at the frequency

,because the magnetic moment µ is not parallel to the field, which therefore exerts a torque on it

perpendicular to both µ and Bo. This torque continuously changes the direction of the spin

angular-momentum vector and hence of µ, so causing the precession. For a set of isolated spins of

the same kind, has the same value for each spin, but the phases of the precessions are

random, as illustrated in figure 4.3. If a rf field B1 is applied at right angles to Bo in such a way that

its magnitude remains constant but it rotates with angular velocity in the same direction as

the precession, an additional torque is applied to each spin, which changes its precessional

motion.

Figure 4.3 – Random phases of precession

Imagine now a frame of reference rotating at exactly . Each spin would, in the absence of B1

and of any interactions between the spins, remain stationary in this frame of reference, so that in

this frame Bo has effectively been reduced to zero. Switching on B1, which remains in a fixed

direction in this new frame, causes all the spins to precess around B1. Their phases in this

precession are not random, because they all start to precess around this new direction at the

same time, so it is easier to visualize what is happening by considering the behavior of the total

39

magnetization Mo. Before application of B1, the magnetization Mo is parallel to Bo, because the

components of l in the direction of Bo for all spins add together, whereas the components

perpendicular to Bo cancel out. On application of B1, Mo starts to precess around the fixed

direction of B1 in the rotating frame. If B1 is applied for just the right length of time, this

precession will rotate the magnetization M through 90° into the plane perpendicular to Bo,

maintaining its magnitude Mo. This constitutes the application of a 90° pulse. Immediately after a

90° pulse has been applied and B1 has been returned to zero, all the spins (and thus M) are

precessing around Bo in the laboratory frame. This is not, however, an equilibrium state, because

at equilibrium the magnetization is Mo parallel to Bo, with all the constituent spins precessing with

random phases around Bo. Over time the spin distribution relaxes back to this equilibrium state.

Two things must happen for this state to be reached. One is that the spins must on average lose

energy to regain their original components parallel to the field which add up to give the final value

Mo for the component of magnetization parallel to Bo. This energy is lost to the thermal motions

of the polymer. The process is called spin–lattice relaxation or longitudinal relaxation and a

characteristic relaxation time, T1, is associated with it. In addition, the net component M┴ of the

magnetization perpendicular to Bo, which is equal to Mo immediately after application of the

pulse, must fall to zero, corresponding to the randomization of the phases of the precessions of

the spins around Bo. This relaxation process, caused by the mutual interaction of the spins, is

called the spin–spin or transverse relaxation and has a characteristic relaxation time T2 different

from the spin–lattice relaxation time T1. In solids T2 is generally very much smaller than T1. The

transverse relaxation of the spin distribution causes the magnitude of the signal induced in the

detector by the rotation of M┴ around Bo to decay with the characteristic time T2. The observed

signal is therefore called the free-induction decay (FID), from which the required spectral

information is obtained by Fourier transformation. The single 90° pulse is only one of a vast range

of pulse sequences that are used in modern NMR spectroscopy2.

4.1.3 Chemical shifts and spin–spin interactions

In a molecule of any kind a particular nuclear spin is surrounded by a large number of electrons

and may interact with other nuclear spins in the molecule. The presence of the electrons leads to a

shielding of the externally applied magnetic field that is different for the nuclei of atoms in

different chemical environments, so that, for example, the frequency in a particular external

magnetic field for a 13C atom that is part of a —CH3 group is different from that of a 13C atom in

the backbone of a molecule. The difference between the frequency observed for a spin in a

40

particular molecular environment and that which would be observed for the free spin is called the

chemical shift. The sensitivity to chemical environment is in fact such that atoms several chemical

bonds away can influence the precise frequency of resonance. In the solid state, however, the

resonances are broadened by several effects and overlap. The first broadening effect is that the

shielding that gives rise to the chemical shift is anisotropic and is in fact describable by a second-

rank tensor. This means that the chemical shifts for corresponding nuclei in polymer segments

with different orientations with respect to Bo are different. A second effect is that the interactions

with the surrounding nuclear-spin dipoles also broaden the resonances. If information about

chemical shifts is sought, this broadening is a nuisance. On the other hand, the precise nature of

the broadening can provide structural information about the material. A third broadening effect

that is particularly important for polymers is due to the disorder of the structure that is always

present. In solution the rapid tumbling and flexing of the molecules causes all these interactions to

be averaged out, so that the resonances remain sharp and the spectra can be used to elucidate

chemical structure. The interaction, or coupling, of the spins of different nuclei can be understood

to a first approximation as follows. Imagine two spins S1 and S2 separated by a distance r and let

the line joining them make an angle θ with Bo. The spin S1 has a dipole moment with component

µB1 parallel to Bo of magnitude , where represents the spin quantum number for the

component of spin parallel to Bo and is equal to ± ½ for nuclei with spin ½. The component of the

dipole perpendicular to Bo is undefined. It follows from the standard expressions for the field due

to a dipole that the dipole S1 produces a magnetic field at S2 with a component BS1 parallel to Bo of

magnitude

Equation 4.2

The net field experienced by the spin S2 is thus Bo + BS1 and the energy of the spin S2 is therefore

changed from what it would be in the absence of S1 by the amount

Equation 4.3

In a solid polymer sample each nucleus is usually surrounded by a large number of different spins

with different values of r and θ, which leads to a broadening of the resonance, as already

41

mentioned. Because different spins experience different net fields parallel to Bo and have

correspondingly different resonant frequencies, they precess around the direction of Bo at

different rates and this is one of the reasons for the randomization of the phases of their

precessions after the application of a 90° pulse, i.e. for the transverse relaxation associated with

the relaxation time T2. The greater the spread of resonance frequencies, the smaller T2 value. The

next section explains how the effects of the anisotropy of the chemical shift and of this interaction

between spins can be removed.

4.1.4 Magic-angle spinning, dipolar decoupling and cross polarization

The effects of the anisotropy of the chemical shift and of the spin–spin interactions in the spectra

of solids can be eliminated by the use of a technique called magic-angle spinning (MAS). The

frequency of the transition for a particular spin S2 interacting with another spin S1 can be written:

Equation 4.4

where A is a constant for given types of spins,

is the second-order

Legendre polynomial in cosθ and θ is the angle between the line joining S1 and S2 and the

direction of Bo. The value of always lies between - 0.5 and 1, so that the width of the

broadened line is of order A. Imagine now that the sample is spun at high angular frequency

about an axis making the angle s with Bo. It is possible to show from the properties of the

Legendre polynomials that the time-average value of is given by

, where is the angle between the line joining S1 and S2 and the axis of spin. If is

chosen equal to 54.7°, the so called magic angle, , so that the time-average value

of is equal to , the unperturbed value. The value of oscillates about this time average,

which leads to the existence of spinning side-bands separated from by integral multiples of .

For a sufficiently large value of , i.e. ˃˃ A, the amplitudes of the spinning side-bands fall to

zero and the signal that remains has only the frequency ,. Although the above discussion shows

only that magic-angle spinning can remove the effects of spin–spin interactions, it can be shown

that it also removes the effects of the anisotropy of the shielding that gives rise to chemical shifts.

Another method that is used to remove the spin–spin coupling in experiments on 13C NMR is

called dipolar decoupling (DD). The natural abundance of 13C nuclei is very low, so the spins of

such nuclei are well separated from each other and do not generally interact. They can, however,

42

interact with the spins of nearby protons. This interaction can be effectively suppressed by

irradiating the sample during the FID with a rf field of the correct frequency to cause transitions

between the proton-spin levels and a high enough amplitude to cause upward and downward

transitions with a frequency much greater than the precessional frequency, so that their mean

components parallel to Bo are zero. The corresponding pulse sequence is illustrated in figure 4.4.

In any pulsed NMR experiment the pulse sequence is repeated many times and the results are

added to increase the signal-to-noise ratio.

Figure 4.4 – Pulse sequence representation

The repetition must not take place until thermal equilibrium has been restored, which is usually

regarded as implying a clear time of about 3T1 between sequences. The value of T1 for 13C is often

very long, so that the simple DD method cannot be used. In these cases a more complicated

method called cross polarisation (CP) can be used. Essentially the method involves the excitation

of the 1H spins and the transfer of the excitation to the 13C spins. The FID of the 13C spins is then

observed under DD conditions. Because of the transfer of excitation from 1H to 13C spins, the rate

at which the sequence can be repeated is determined by the value of T1 for 1H, which is much

shorter than T1 for 13C. This means that the pulse sequence can be repeated more frequently. The

efficiency of excitation of the 13C spins is also greater in this method, but the simple

proportionality of the signals to the numbers of spins in different environments is lost. The CP

technique is often combined with MAS to remove the anisotropic chemical shift broadening and

the combined technique is called CPMAS3.

4.1.5 Spin diffusion

In considering the interaction between spins, the component of the dipole of spin S1 perpendicular

to Bo was considered to be undefined. In the semi-classical picture, however, the spin is precessing

around Bo and therefore produces a rotating component of magnetic field perpendicular to Bo. If

S2 is a spin of the same kind as S1 this is at just the right frequency to induce a transition between

the two possible energy states of spin S2. This action is, however, mutual to the two spins, so that

43

a flip–flop or spin exchange takes place, with no net change of energy. If there is a region within

the material where there are initially more spins with a positive component of magnetization

along the direction of the field than there are in an immediately surrounding region, the effect of

these spin-flips will be to cause this excess magnetization to diffuse outwards. This phenomenon

of spin diffusion is of great use in studying structural inhomogeneities in polymers. The spin

exchange also contributes to the dephasing of the precessions of the spins and thus reduces T2 for

the interaction of like spins4.

44

4.2 Size Exclusion Chromatography (SEC)

4.2.1 Separation System

Size exclusion chromatography (SEC) has been widely used since its introduction during the 1960s.

It offers a simple yet unbiased method to characterize the molecular weight distribution of a

polymer. Although it uses a flow system, the separation principle and the analysis are based on a

static property of the polymer molecules in solution. We briefly look at the separation system here

before learning the principle. Figure 4.5 illustrates the separation system. A high-pressure liquid

pump draws a solvent called a mobile phase from the reservoir and pumps it into a column or a

series of columns at a constant flow rate. At one time, a small amount of a dilute solution of

polymer dissolved in the same solvent is injected into the stream from a sample loop by changing

the position of the injection valve. The column is packed with porous materials, typically polymeric

beads with many tiny through holes (pores). The polymer molecules are partitioned between the

small confines of the pore, called the stationary phase, and the interstitial space between the

beads (mobile phase). Polymer molecules with a dimension smaller than the pore size enter the

pore more easily than larger polymer molecules do. As the injected polymer solution is

transported along the column, low-molecular-weight components are frequently partitioned to

the stationary phase, whereas high-molecular-weight components remain mostly in the mobile

phase (see figure 4.6). Therefore, it takes a longer time for the low-molecular-weight components

to reach the column outlet. The band of the polymer in the mobile phase is narrow when injected

but spreads according to the molecular weight distribution as the solution moves along the

column.

Figure 4.5 – SEC apparatus

45

The liquid that comes off the column is called the eluent. A detector with a flow cell is placed

downstream to measure the concentration (mass volume) of the polymer in the eluent. A

differential refractometer is most commonly used to measure the difference in the refractive index

between the eluent and the pure solvent. The difference is proportional to the concentration, with

dn/dc being the proportionality constant. If the polymer has an ultraviolet absorption but the

solvent does not, one can use an ultraviolet detector. The absorbance is proportional to the

concentration by Beer’s law.

Figure 4.6 – SEC column operating principle

Figure 4.7 shows the signal intensity of the detector plotted as a function of retention time (tR), the

time measured from the injection of the polymer solution. The retention volume (VR), the

cumulative volume of the fluid out of the column since the injection, can also be used for the

abscissa. The curve is called a retention curve or a chromatogram. The height of a point on the

curve above the baseline is proportional to the concentration at a given retention time. The signal

maximizes at the peak retention time (tp).

Figure 4.7 – Example of size exclusion chromatogram

46

The integral of the curve is proportional to the total amount of the polymer injected. The spread

of the polymer band by the column is translated into a broadened chromatogram. Because high-

molecular-weight components elute earlier, the time axis can be regarded as a reversed molecular

weight axis.

SEC has other names. When the mobile phase is an organic solvent, SEC is also called gel

permeation chromatography (GPC). When it is aqueous, SEC is also called gel filtration

chromatography (GFC) or aqueous GPC5.

4.2.2 Plate Theory

Plate theory is useful to explain the band broadening during the transport of polymer molecules

along the column. In the theory, the whole length of the column is divided into Npl plates of an

equal height. Each plate consists of the mobile phase and the stationary phase. Figure 4.8 explains

what is supposed to occur in the plates. In each plate, the polymer molecules are partitioned

between the two phases. The mobile phase moves to the next plate in a given time t1 (plate

height/linear velocity of the mobile phase), whereas the stationary phase does not. The moved

mobile phase establishes concentration equilibrium with the stationary phase in the next plate.

Equilibration and transport of the mobile phase are repeated in all of the plates each time. As a

result, a completely excluded polymer (too large to enter the pore) requires a time of t1Npl to

reach the outlet. A lower-molecular-weight polymer molecule needs a longer time to come out of

the column. When equilibrium is reached in the plate, the polymer concentration is cS in the

stationary phase and cM in the mobile phase. Their ratio is called the partition coefficient K:

Equation 4.5

Figure 4.8 – Plate theory resuming scheme

47

When the concentration is sufficiently low (cM « c*, overlap concentration), K does not depend on

cM but depends on the ratio of the chain dimension to the pore size. The partition ratio k’ is

defined as the ratio in the number of molecules between the two phases and given as

Equation 4.6

where VS and VM are the volumes of the two phases. The polymer molecules are partitioned with

a probability of k’/(1 + k’) to the stationary phase and with a probability of 1/(1 + k’) to the mobile

phase. Partitioning in each occurs independently of the other plates and of the equilibration at

other times. If the retention time of a particular polymer molecule is tR = t1(Npl + Nex), then this

polymer molecule has been partitioned Nex times to the stationary phase and Npl times to the

mobile phase before it reaches the outlet. Then,

Equation 4.7

From equations 4.6 and 4.7, we find that K depends linearly on tR by

Equation 4.8

as seen in Figure 4.8, where t1Npl is the retention time for a completely excluded component.

48

4.2.3 Partitioning of Polymer with a Pore

Figure 4.9 illustrates the polymer molecules partition equilibrium between the pore space

(stationary phase) and the surrounding fluid (mobile phase). The concentration equilibrium is

reached when the chemical potential of the polymer molecule in the two phases is equal. At low

concentrations, the solution is ideal.

Figure 4.9 – Macromolecule partition equilibrium

The chemical potential of the polymer molecule (µM) in the mobile phase of concentration (cM) is

given by:

Equation 4.9

where µ° is the chemical potential in a reference state of concentration c° in the ideal solution.

When the polymer molecule is brought into the stationary phase, its entropy changes by ΔS and

its enthalpy by ΔH. The entropy change is related to the decrease in the available space the

centroid of the molecule can reach as well as the decrease in the total number of conformations.

Because of these geometrical restrictions, ΔS < 0. The enthalpy change is due to interactions of

the polymer molecule with the pore surface and can be positive or negative. When the polymer

chain enters the pore, surface–monomer contacts replace some of the monomer–solvent

contacts, resulting in the enthalpy change. The chemical potential in the stationary phase (µS) of

concentration (cS) is then given by

Equation 4.10

The concentration equilibrium is dictated by µS = µM:

Equation 4.11

49

which gives the partition coefficient K = cS/cM:

Equation 4.12

Because of the specific nature in the three-way interactions between polymer, surface, and

solvent, there is hardly a universal method that allows us to predict ΔH for a given combination of

the polymer, surface, and solvent. In contrast, ΔS is universal because it is determined by the

geometrical confinement of the polymer molecule by the pore. In ideal SEC, the stationary phase

is designed to provide purely entropic effects for any combination of polymer and solvent as long

as the solvent is good to the polymer. Then, ΔH = 0 and

Equation 4.13

Because ΔS < 0, K < 1. With eq. 4.13 , we then find that tR ranges between t1Npl and t1Npl(1 + VS/

VM). In a different mode of chromatography, S is rather suppressed and the differences in ΔH

between different polymers are utilized to analyze the chemical composition of the polymer. If ΔH

> 0, the pore wall repels the polymer. Otherwise, it adsorbs the polymer. Recall that a polymer

chain is described by a thin thread in the crudest approximation. This geometrical object interacts

with the pore, another geometrical object. The confinement effect is manifested in the partition

coefficient and the change in the chain conformation. We can expect an interesting relationship

between the chain and the geometry of the pore. However, the geometry in the porous medium

used in SEC is far from simple. The pore is rather highly tortuous 6.

4.2.4 SEC With an On-Line Light-Scattering Detector

Since approximately 1990, on-line light-scattering detectors have been increasingly used in SEC,

providing more detailed information on the polymer chain conformation in the solution state. The

detector has a flow cell with a small cell volume and measures the scattering intensities at

different angles. The advantage of this scheme for the characterization of polymer in solution is

obvious. As the column separates the polymer according to molecular weight, each fraction is led

to the light-scattering detector for instantaneous measurement of the scattering intensities

[Iex(θ)], as illustrated in figure 4.10 . The concentration detector such as a refractive index detector

50

and an ultraviolet absorption detector connected in series gives the estimate of the polymer

concentration c.

Then with the preinput data of dn/dc, a Zimm plot is prepared for each fraction. The plot is for one

concentration only, but it is sufficiently low because of the band broadening (further dilution) by

the SEC column of the already dilute injected solution.

Figure 4.10 – Scattering measurement

Because the measurement is instantaneous, injection of a broad-distribution polymer sample

results in a plot of the molecular weight M and the radius of gyration Rg as a function of the

retention time. Thus we can obtain a plot of Rg as a function of M. This method eliminates the

need to fractionate the polydisperse polymer on a preparative scale and run the tedious light-

scattering measurements for each fraction. figure

4.11 here on the right shows an example of SEC

chromatograms for branched polyethylene. The

first graph shows the refractive index signal Δn,

which is proportional to the concentration, and

the light-scattering intensity Iex at 90°. Because

Iex is proportional to cM, the peak of Iex appears

ahead of the peak in Δn. The second graph shows

the molecular weight M, and the last one plots

Rg. At both ends of the chromatogram, the

concentration is low. The uncertainty in the

estimates of M and Rg are larger at both ends. Furthermore, an on-line viscosity detector can be

connected in tandem to the concentration detector (and the light-scattering detector). As it is easy

Figure 4.11 – Light scattering and refractive index signals

51

to understand, the solution viscosity gives an important piece of information on the state of

polymer molecules in solution7.

4.2.5 The Universal Calibration

Beginning with the Mark–Houwink–Sakurada relationship,

, it is easy to show

that the average molecular size is given by:

Equation 4.14

where represents the root-mean-square end-to-end distance of the polymer chain. The right-

hand side is proportional to the polymer’s hydrodynamic volume. A new aspect of SEC calibration

arises from the recognition that a polymer’s hydrodynamic volume might form the basis for

molecular weight determination. Since SEC depends on the hydrodynamic volume rather than its

molecular weight per se, a new calibration method is suggested. This is the “universal calibration,”

which calls for a plot of [η]M versus elution volume.

Figure 4.12 - Universal calibration procedure for poly(vinyl acetate) and polystyrene

Figure 4.12 illustrates the universal calibration procedure for poly(vinyl acetate) and polystyrene.

Note that the two sets of data lie on the same straight line. The universal calibration is valid for a

range of topologies and chemical compositions. However, it cannot be used for highly branched

materials or polyelectrolytes, which have different or varying hydrodynamic volume relationships.

The universal calibration procedure is especially useful for estimating the molecular weight of new

polymers, since the intrinsic viscosity is usually easy to obtain. The procedure also tends to correct

for differences in the hydrodynamic relationships when several polymers are compared, and only

one of them (e.g., polystyrene) is used as the calibration material8.

52

4.3 Atomic Force Microscopy (AFM)

4.3.1 Introduction

Development of new polymers for use as stand-alone materials, polymer solutions, blends, or

composites is at the core of macromolecular science. The atomic force microscope (AFM) enables

characterization of these materials and therefore the development of more new materials. The

microscope is an invaluable tool to the materials scientist. There are two quantities that enable

microscopy: contrast and resolution. Sensitivity is not inherently an issue in microscopy: signal

level is not limiting because it is now possible to count single photons and electrons. Contrast and

resolution determine one’s ability to see at all scales. Contrast is the ability to measure changes in

signal with a detector. The detector can be your eye, a CCD camera, or an electronic amplifier. The

contrast of the signal can be from intensity changes, spectral changes, phase differences,

scattering of electrons or ions, transmission of tunneling electrons, or force on an atom from a

probe, amongst others. Without contrast there can never be resolving power. Resolution is

defined as the smallest distance between two points in a sample, that one can detect as a change

in signal. There are two modes to operate a microscope. The difference between far-field

detection mode and near-field detection mode is that in the former the detector is “far” away

from the signal source. When the source-to-detector distance is several times the wavelength of

the signal, the system displays its wave character and is therefore subject to the diffraction limit of

light. This is a fundamental limit and is a result of Heisenberg’s uncertainty principle, which is of

course a result of one of the postulates of quantum mechanics. Near-field detection does not

require wave propagation of the signal. Therefore, resolution is determined by the size of the

probe or pinhole detecting the signal. To increase the resolving power of the microscope, the tip

of the probe needs to be smaller. This is the concept that drives the maturing area of scanning

probe microscopy and is what makes the scanning tunneling microscope and atomic force

microscope so powerful. In figure 4.13 the AFM scheme is illustrated. Microscopy of polymer

surfaces and their high spatial resolution analysis is accomplished typically with the atomic force

microscope (afm) used in various modes of operation. The contrast mechanism of the afm is the

interatomic forces between the scanning probe and the sample’s surface. The forces are

measured easily and result from the mean pairwise potential energies of the interacting atoms.

Extracting the exact form of the potential energy versus tip-to-sample position is difficult with

regard to the microscopic detail. However, it is generally described by a steep attractive

interaction followed by a very steep repulsive interaction at closer separation. It is the shortness

53

of range of this interaction energy that enables the near atomic resolution commonly provided by

the afm9.

Figure 4.13 – Atomic force microscopy scanning representation

4.3.2 Imaging and Analysis Principles

Synthesis and processing are integrally important when designing for the properties and

performance of a polymer system. Bulk characterization of the system is often not enough to

predict, control, and maintain desired system performance when surface interactions and particle

incorporation are significantly involved. The discovery, development, and optimization of new

polymeric systems (eg, composites, films, or fibers) often require ensembles of analytical

investigation; no one technique is enough. The cohort of surface analytical techniques is

voluminous and not appropriately reviewed here. It is, however, important to realize that the

spatial resolution of an analysis is both technique- and sample-dependent. Atomic force

microscopes are versatile analytical tools. They excel at topographic characterization of surfaces.

Moreover, they can be used to probe the identity of chemical constituents at a surface and the

mechanical properties of the near-surface region, both with high spatial resolution. Mechanical

design and control of the afm is now mature. Early in its development there were some design

constraints similar to all models. Notably, reduction of environmental noise, sharpness of the tip-

probe asperity, and sensitivity of the force transducer are the most critical design elements. By far,

the most commercially successful afm design is based on the laser-diode optical lever coupled to

an integrated micromachined Si or SiN cantilever and tip as illustrated in figure 4.14. There are

several limitations to the resolving power of an afm; noise being one of the more benign and easily

removed. Since the tip is not infinitely sharp and the tip or samples are not infinitely hard (ie, they

have measurable Young’s moduli), the resultant micrograph will be a convolution of the chemical,

physical, and topographic properties of the entire system. This is a very important point since an

artifact is only realized in hindsight. For the idealized case of hard materials, an extremely

54

small/sharp tip and a relatively flat sample, the principal concern is tip–sample convolution

artifacts.

Figure 4.14 – Laser diode coupled to the cantilever

Typical, commercially available afm probes have tips with radii of curvature of the order of 30 nm

and with varying aspect ratios. As the topographical relief of the surface increases so must the

aspect ratio of the tip. A common artifact found in samples of high relief is that the sample is

actually imaging the shape of the tip instead of the converse. Since the quality of a tip varies with

manufacturability and with use, overinterpretation of image structure without knowledge of tip-

shape is ill-advised. A second common artifact, for the idealized sample, is observed when there is

significant thermal drift, and nanometer spatial resolution is desired. Polymers have large

coefficients of thermal expansion (eg, several hundred ppm/°C). Image acquisition with the afm is

accomplished by raster scanning the probe across the sample. The scanner displaces the probe

relative to the sample, one line at a time. Each line is typically scanned at a rate of many hundreds

of nanometers per second in the “fast-scan” direction. After the scanner repositions the probe to

the beginning of the first line, it steps down the “slowscan” direction. This process is linear and

slow. Therefore, any thermal drift will be convoluted mainly with the slow-scan direction. The

effect will be to distort the image. This artifact is corrected for by subtracting out thermal drift

(assuming it is constant) or by rotating the scan direction by 90°. Another common

misinterpretation of afm images occurs when periodic structure is being investigated. In general,

any periodic structure that is either parallel to or perpendicular to the scanning direction should

never be believed. Sinusoidal noise in any number of the system’s components will look like

55

periodic structure perpendicular to the scan direction. Since the noise will most likely be out of

phase with the scanning frequency, the image will appear to have a “crystalline” structure.

Rotating the scan direction will not change the image. An afm is very powerful and can be made to

generate any possible image, regardless of its validity. There are several operating modes where

the afm can generate microscopic contrast10.

4.3.3 Contact Mode

These various modes can help to deconvolute topographic relief from chemical and mechanical

surface inhomogeneity. Topographic information is most precisely obtained in “contact” mode

where the afm probe is very close to the sample and interacts with the repulsive potential energy

of the surface. This is arguably the point at which the probe “touches” the sample. As long as the

cantilever force transducer is significantly softer than the material being scanned over, such that

excessive (force)/(contact area), that is pressure, does not perturb the sample, the afm will

accurately trace the surface. This is an idealized situation since there will always be some

deformation of the polymer surface by the probe even at static, zero applied force F conditions.

While in contact mode, the user can collect both sample height and probe force information as the

probe scans across the surface. In feedback mode (or height mode), the afm tries to maintain a

constant applied force by driving together or retracting apart the tip from the sample as the probe

is scanned. Ideally this would follow the topography of the sample, as the force would remain

constant. This “height mode” has two advantages over the “force mode” with the feedback

control turned off. Since the system is trying to keep a constant force, it is less likely to damage

soft polymer surfaces having significant topographic relief. Moreover, the height mode provides a

direct measure of the sample’s topography. When the afm is operated in force mode (still

addressed as contact afm), the probe-to-sample distance is not changed rapidly with varying

topography. Therefore, as the tip travels over higher areas of the sample, the interatomic forces

increase, resulting in the upward bending of the probe’s cantilever. This mode has the advantages

of allowing high scanning speeds and providing images with less noise and scanning artifacts.

These come at the expense of not being able to acquire accurate and direct height measurements

of the sample, and there is the possibility of more damage by the tip onto the polymer surface

than when running in height mode. If only qualitative microscopy is required, force mode is the

more useful technique. An afm cantilever is flexible in several directions it can be bent up and

down to measure vertical displacement of a sample and it can be torqued about its long axis

because of the moment applied to the end of the scanning tip. Because of the phenomena of

56

friction and Newton’s third law, there will be a measurable force applied to the tip, which is

perpendicular to the load force applied normal to the sample. While the probe is scanning, this

results in the twisting of the cantilever. Modern afms detect the reflection of a diode-laser from

the end of the cantilever onto a position sensitive four-quadrant photodetector. The laser spot

deflection is illustrated in figure 4.15.

Figure 4.15 – Laser spot deflection

The total laser power on the detector is the sum of the signal from each quadrant, T = A + B + C + D.

Changes in sample height will cause the cantilever to deflect up or down, and the signal is

recorded as δH = *(A + B) − (C + D)+/T. Simultaneously the cantilever may twist because of the

frictional drag on the tip as it is pulled over the sample. The torque will deflect the laser beam

horizontally and the “friction” signal is recorded as δf = *(A + C) − (B + D)+/T. Since friction is related

to both surface chemistry and surface roughness, the two signals may not be independent.

Variations in surface composition are easily detected in force mode, whereas the topography

signal may not detect any change in the surface character. Although differences in surface

composition are sought with friction mode afm, friction forces can be dominated by the near-

surface yield strength of the polymer and by any adsorbed water vapor forming a meniscus at the

tip–sample contact11.

57

4.3.4 Tapping Mode

Tapping is a potent technique that allows high resolution topographic imaging of sample surfaces

that are easily damaged, loosely hold to their substrate, or difficult to image by other AFM

techniques. Tapping mode overcomes problems associated with friction, adhesion, electrostatic

forces, and other difficulties that an plague conventional AFM scanning methods by alternately

placing the tip in contact with the surface to provide high resolution and then lifting the tip off the

surface to avoid dragging the tip across the surface. Tapping mode imaging is implemented in

ambient air by oscillating the cantilever assembly at or near the cantilever's resonant frequency

using a piezoelectric crystal. This motion causes the cantilever to vibrate with a high amplitude(

typically greater than 20nm) when the tip is not in contact with the surface. The oscillating tip is

then moved toward the surface until it begins to lightly touch, or tap the surface. During scanning,

the vertically oscillating tip alternately contacts the surface and lifts off, generally at a frequency of

50,000 to 500,000 cycles per second. As the oscillating cantilever begins to intermittently contact

the surface, the cantilever oscillation is necessarily reduced due to energy loss caused by the tip

contacting the surface. The reduction in oscillation amplitude is used to identify and measure

surface features.

During tapping mode operation, the cantilever oscillation amplitude is maintained constant by a

feedback loop. Selection of the optimal oscillation frequency is software-assisted and the force on

the sample is automatically set and maintained at the lowest possible level. When the tip passes

over a bump in the surface, the cantilever has less room to oscillate and the amplitude of

oscillation decreases. Conversely, when the tip passes over a depression, the cantilever has more

room to oscillate and the amplitude increases (approaching the maximum free air amplitude). The

oscillation amplitude of the tip is measured by the detector. The digital feedback loop then adjusts

the tip-sample separation to maintain a constant amplitude and force on the sample. When the tip

contacts the surface, the high frequency (50k - 500k Hz) makes the surfaces stiff (viscoelastic), and

the tip-sample adhesion forces is greatly reduced. Tapping mode inherently prevents the tip from

sticking to the surface and causing damage during scanning. Unlike contact mode, when the tip

contacts the surface, it has sufficient oscillation amplitude to overcome the tip-sample adhesion

forces. Also, the surface material is not pulled sideways by shear forces since the applied force is

always vertical. Another advantage of this technique is its large, linear operating range. This makes

the vertical feedback system highly stable, allowing routine reproducible sample measurements 11.

58

4.4 Gas Chromatography (GC)

Chromatography is the separation of a mixture of compounds (solutes) into separate components.

By separating the sample into individual components, it is easier to identify (quality) and measure

the amount (quantity) of the various sample components. There are numerous chromatographic

techniques and corresponding instruments. Gas chromatography (GC) is one of these techniques.

It is estimated that 10-20% of the known compounds can be analyzed by GC. To be suitable for GC

analysis, a compound must have sufficient volatility and thermal stability. If all or some of a

compound or molecules are in the gas or vapor phase at 400-450°C or below, and they do not

decompose at these temperatures, the compound can probably be analyzed by GC.

One or more high purity gases are supplied to the GC. One of the gases (called the carrier gas)

flows into the injector, through the column and then into the detector. A sample is introduced into

the injector usually with a syringe or an exterior sampling device. The injector is usually heated to

150-250°C which causes the volatile sample solutes to vaporize. The vaporized solutes are

transported into the column by the carrier gas. The column is maintained in a temperature

controlled oven. The solutes travel through the column at a rate primarily determined by their

physical properties, and the temperature and composition of the column.

Figure 4.16 – Schematic representation of a gas chromatography apparatus

The various solutes travel through the column at different rates. The fastest moving solute exits

(elutes) the column first then is followed by the remaining solutes in corresponding order. As each

solute elutes from the column, it enters the heated detector. An electronic signal is generated

upon interaction of the solute with the detector. The size of the signal is recorded by a data

system and is plotted against elapsed time to produce a chromatogram. The ideal chromatogram

59

has closely spaced peaks with no overlap of the peaks. Any peaks that overlap are called coeluting.

In the following image figure 4.17 an example of chromatogram is represented.

Figure 4.17 – Example of gas chromatogram

The time and size of a peak are important in that they are used to identify and measure the

amount of the compound in the sample. The size of the resulting peak corresponds to the amount

of the compound in the sample. A larger peak is obtained as the concentration of the

corresponding compound increases. If the column and all of operating conditions are kept the

same, a given compound always travels through the column at the same rate. Thus, a compound

can be identified by the time required for it to travel through the column (called the retention

time). The identity of a compound cannot be determined solely by its retention time. A known

amount of an authentic, pure sample of the compound has to be analyzed and its retention time

and peak size determined. This value can be compared to the results from an unknown sample to

determine whether the target compound is present (by comparing retention times) and its

amount (by comparing peak sizes). If any of the peaks overlap, accurate measurement of these

peaks is not possible. If two peaks have the same retention time, accurate identification is not

possible. Thus, it is desirable to have no peak overlap or co-elution12.

60

References

1) Keeler J. , Understanding NMR Spectroscopy, 2nd Edition, Wiley-Interscience Publications

2009.

2) Demco D.E. ; Blumich B. , Enc. of Polym. Science and Tech. Vol.10, Nuclear Magnetic

Resonance 1999.

3) Silverstein R.M.; Webster F.X.; Kiemie D.J. , Spectrometric Identification of Organic

Compounds, 7th Edition, 2006.

4) Cheremisinoff N.P. , Polymer Characterization : Laboratory Techniques and Analysis ,

1st Edition, Noyes Publications 1996.

5) Bower I. D. , An Introduction to Polymer Physics, 2nd Edition, Cambridge University Press

2002.

6) Sperling L.H. , Introduction to Physical Polymer Science, 4th Edition, Wiley-Interscience

Publications 2006.

7) Sun S.F. , Physical Chemistry of Macromolecules : Basic Principles and Issues, 2nd Edition,

Wiley-Interscience Publications 2004.

8) Ram A. , Fundamentals of Polymers Engineering, 2nd Edition, Plenum Press New York 1997.

9) Schiraldi D.A. ; Poler J.C. , Enc. of Polym. Science and Tech. Vol.1, Atomic Force Microscopy,

John Wiley and Sons 2001.

10) Burnside S.D.; Giannelis E.P., J. Polym. Science, Vol. 36, 2000.

11) Sarid D. , Atomic Force Microscopy with Applications to Electric, Magnetic and Atomic

Forces, revised ed., Oxford University Press, New York, 1994.

12) Brun Y. , Enc. of Polym. Science and Tech. Vol.1, Chromatography, John Wiley and Sons

1999.

61

Chapter 5

5. POLYMERS SYNTHESIS : EXPERIMENTAL PROCEDURES AND

RESULTS

As already discussed, this work aims at exploring and investigating the domain of core-shell water-

soluble polymers that could be used, in the future, for applications such as, the aqueous biphasic

catalysis. The core-shell polymer consists of a crosslinked internal core linked to several radial

linear chains of amphiphilic character, which are constituted by a first hydrophobic segment of

poly(styrene-co-4-diphenylphosphinostyrene) that will provide the coordination of catalyst (for

instance rhodium complexe), and a second hydrophilic block at the exterior, that will allow the

whole polymer to dissolve in water, as shown in figure 5.1.

[Ext Block]

N

m

CH3

OO

CH3CH3CH3

m

OO

CH3CH3CH3

m

P4VP

PtBMA PtBA

m

Poly(styrene-co-4-diphenylphosphinostyrene)

P

n

Figure 5.1 – Star polymer scheme with the functional monomer

The work carried out during this thesis aims more specifically at investigating the feasibility of such

polymers, that is why the hydrophobic internal chains, described in the following chapters, will be

constructed as a polystyrene homopolymer instead of the copolymer with 4-diphenyl-phoshino-

styrene, because of the affinity in chemical properties and reactivity between styrene and the

62

functional monomer1. In the following picture (figure 5.2) the main structure of the polymer

discussed in this chapter is shown.

CH3

OO

CH3CH3CH3

m

OO

CH3CH3CH3

m

PtBMA PtBA

[Ext Block]

Polystyrene

N

m

P4VP

n

Figure 5.2- Star polymer scheme with polystyrene as hydrophobic segment

In this chapter, reactions, experimental procedures and results obtained will be presented and

discussed. We will, first, investigate the poly(vinyl pyridine) case, used as external block, then

present results obtained with the two acrylic external block, poly(tert-butyl methacrylate) (PtBMA)

and poly(tert-butyl acrylate) (PtBA), which will be hydrolyzed to respective acrylic acids, to allow

solubilization in water (figure 5.3).

mOO

CH3 CH3

CH3

Br

CH3

n

Poly(t-butyl methacrylate)-b-polystyrene-polydivinylbenzene Star Polymer

mOO

H

Br

CH3

n

Poly(methacrylic acid)-b-polystyrene-polydivinylbenzene Star Polymer

DVB DVBCF

3COOH

CH2Cl

2

mOO

CH3 CH3

CH3

Br

n

Poly(t-butyl acrylate)-b-polystyrene-polydivinylbenzene Star Polymer

mOO

H

Br

n

Poly(acrylic acid)-b-polystyrene-polydivinylbenzene Star Polymer

DVB DVBCF

3COOH

CH2Cl

2

Figure 5.3 – Star polymer hydrolyzation sequences

63

5.1 Vinyl-pyridine (4VP) and poly(4-vinyl-pyridine)

4-vinyl-pyridine (4VP) is an example of a basic/nucleophilic and coordinating monomer. The

corresponding pH-responsive polymer, poly(4-vinyl-pyridine), is a weak polybase that dissolves in

water only when sufficiently protonated. Many metal ions bind strongly to this polymeric

polydentate ligand. An important application of P4VP is in ion-exchange resins, for instance in

water purification. The mild basicity of P4VP is useful in the synthesis of alkyl triflate esters. The

polymer can serve as a precursor of N-alkyl quaternized polyelectrolytes, polybetaines (both with

carboxylate and sulfonate groups), and N-oxide.

The water soluble poly(4-vinyl-N-methylpyridinium) is indeed the target of our investigations. The

similar quaternization reactions of the isomer, P2VP, are significantly less efficient and slower due

to steric hindrance. Besides living anionic polymerization, various controlled radical polymerization

(CRP) methods have been applied to prepare well-defined polymers derived from 4VP: nitroxide-

mediated polymerization, ATRP, and RAFT2. The first technique generally requires high

temperatures (120 °C) with TEMPO or its 4-substituted derivatives or 110 °C with an α-

phopshonylated nitroxide). RAFT yields well-defined polymers derived from 4VP in bulk or in DMF,

The preparation of well-defined polymers with 4VP units by copper-mediated ATRP has proven

successful, provided a ligand that binds strongly to the copper center, such as the hexamethylated

tris(2-aminoethyl)amine (Me6TREN), which is also used in this project. The use of the copper

chloride-based rather than bromide-based initiating/catalytic system is necessary to obtain

polymers with narrow molecular weight distribution. However, detailed studies on the influence

of the ligand and halide on the polymerization rate and control as well as a quantitative

description of the side reactions that take place during the ATRP of 4VP are lacking in the

literature.

N

m

Poly(4-vinyl-pyridine)

N

4-vinyl-pyridine

ATRP

Figure 5.4 – 4VP ATRP scheme

64

5.1.1 Poly(4-vinyl-pyridine) synthesis

Here below in table 5.1 the conditions and the stochiometry for a typical ATRP of 4VP are

resumed. The ATRP is usually carried out in a thermostated oil bath at 40 °C under Argon. The

solvent used is isopropanol. Hexamethylated tris(2-aminoethyl)amine (Me6TREN) was used as

ligand and 1-phenylethylchloride (PhEtCl) as initiator. ATRP for 4VP is represented in figure 5.5.

Cl

N

m


N

4-vinyl-pyridine

ATRP

Cl

1-phenylethyl chloride

CuCl/Me6TREN

+

Figure 5.5 – 4-vinyl-pyridine initiated with PhEtCl and catalyzed by CuCl/Me6TREN system

Substance Volume(mL) Mass(mg) m moles M (mol/L) Mw(mg/mmol) Molar Ratio

CuCl (I) - 172,70 1,71 0,08 98,98 1

4VP 9,48 8989,50 85,5 4,19 105,14 50

Isopropanol 9,00 - - - - -

Me6TREN 0,67 584,7 5,13 0,24 230,39 3

Dodecane 1,00 750,00 - - 170,33 -

PhEtCl 0,23 245,4 1,71 0,08 140,61 1

Table 5.1 – 4PV polymerization parameters

Experimental Procedure

4VP (Sigma-Aldrich 99% grade) was dried over CaH2 and distilled under reduced pressure

(55°C/18mmHg), 1-phenylethylchloride (PhEtCl) and isopropanol were used as received, dodecane

was distilled under reduced pressure then used. A dry flask was charged with CuCl (172,7 mg, 1,71

mmol) under Ar atmosphere (in a glove box), then with, isopropanol (9,0 mL), and ligand Me6TREN

(0,67 mL, 2,53 mmol). The flask was kept under vigorous stirring condition, in order to let

Me6TREN coordinate CuCl and obtain a homogeneous green solution. Subsequently 1 mL of

dodecane was added as reference for the gas chromatography analyses. Monomers 4VP (9,48 mL,

85,5 mmol) and initiator PhEtCl (0,23 mL, 1,71 mmol) were then added, then the flask was

degassed and filled with argon, (3 freeze-pump-thaw cycles). The flask was then immersed in an oil

bath at 40°C of temperature, kept constant by a thermostat controller. The reaction was carried

out for a prescribed time, small amounts of sample were taken out at regular intervals with argon-

purged syringes, and diluted in THF, the conversion was calculated by gas chromatography GC,

determination of the residual monomer against the dodecane reference (see table 5.2) . The

residual resultant mixture (after 77% conversion) was then dissolved in acetone, and the solution

65

was passed through a short alumina column for removal of the copper complexes. Several

filtration cycles on alumina were necessary, because of the strong coordination attitude of the

P4VP on the catalyst, which did not allow to obtain metal-free white powders. A rotary evaporator

was employed to concentrate the eluent solution, and then a precipitate was obtained by pouring

the residual solution into a large amount of methanol. Finally the product was filtered several

times and dried at room temperature under vacuum. (Yield ca. 6 g).

Table 5.2 – 4VP ATRP conversion and Ln(M0/Mi)

In figure 5.6, the conversion for a 4VP ATRP experience, after 180 min of reaction time is shown.

Figure 5.6 – Conversion against time for 4VP synthesis

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

Co

nve

rsio

n %

Time (min)

Sample N° Time (min) Conversion(%) Ln(M0/Mi)

0 0 0 0 1 30 41,60 0,53 2 60 50,85 0,71 3 90 53,57 0,76 4 120 70,96 1,23 5 150 76,05 1,42 6 180 78,76 1,54

66

The ATRP synthesis of 4VP was repeated and operational parameters such as stochiometric ratios

were changed, as shown in table 5.3 here below, in an attempt to optimize the procedure.

Sample Mw th Mw (g/mol) PDI CuCl 4VP Me6TREN

P4VP(1) 4630 82100 1,08 1 50 1

P4VP(2) 4630 29750 1,01 1 50 3

Table 5.3 – P4VP samples properties

Working in excess of ligand does not seem to influence the polymerization, the solutions obtained

after complexing appear to be all homogeneous, are not encountered major problems compared

to the reagent solubility. Isopropanol not only solubilizes P4VP but also allows the reduction of

green coloring due to catalyst in the whole process of synthesis, possibly through hydrogen

bonding to 4VP/P4VP, decreasing the contamination of the catalyst. 4VP polymerizations can also

be carried out in DMF which solubilizes both monomer and polymer, but it presents several

problems related to the precipitation of the solid polymeric phase and possible coordination of the

solvent to copper reducing the amount of complexed catalyst and consequently its efficiency in

the reaction. Polymerization takes about 4 hours and conversions achieved are all around 80%,

calculated by gas chromatography (GC). It is important to notice, molecular weights obtained are

much higher than theoretical ones, this effect could be attributed to an excessive loss of initiator

functionalities, that cause an uncontrolled chain growth, even if polydispersities remain low. In

figure 5.7 the chromatogram for P4PV(1) is shown.

Figure 5.7 – P4VP(1) size exclusion chromatogram carried out in DMF

35 36 37 38 39 40

Elution Time (min)

67

In figure 5.8 the 1HNMR spectra of sample P4VP(1) performed in CDCl3 is rapresented.

Figure 5.8 – P4VP(1) 1HNMR spectrum in CDCl3

The peaks at δ=6,3 – 6,6 ppm and δ=8,3 ppm are ascribed respectively to ortho and metha

protons of pyridine ring, protons of CH2 and CH on the main chain are respectively contained in

the broad peaks at δ=1,45 ppm and δ=2,67 ppm. Peaks at δ=3,20 and δ=7,20 can be associated

to toluene solvent used for the polymer filtration.

5.1.2 Copper Catalyst Removal in Poly(4-vinyl-pyridine) ATRP

After filtration on alumina or silica columns, the solutions were precipitated and a green polymer

powder was obtained. The green color is obviously caused by the presence of the copper based

catalyst, probably coordinated by the pyridine nitrogen atoms of the macromolecular chains. The

catalyst removal from the polymer after the polymerization is a general problem in ATRP, which is

obviously accentuated when the polymer contains coordinating functions such as the N atoms in

P4VP. The usual procedure for laboratory scale reactions involve, precipitation of the polymer with

nanosolvents, filtration of the polymer solution through aluminum oxide or silica, which adsorb

the catalyst or precipitation of copper as hydroxide or sulfide by shaking the organic polymer

solutions with aqueous NaOH or Na2S. These techniques have disadvantages, such as cost,

68

difficulties in scaling up, important loss of polymer, and difficulties in separating the catalyst from

functional polymers that interact with the copper complexes. Better methods are required for

production scale reactions3. A possible and suitable solution is the immobilization of the catalytic

system on a solid support, providing a more efficient way of separating, and potentially recycling

the catalyst. The immobilization of metal complexes on silica gel can be easily done by attaching

ligands to the silica surface, which has been shown to work quite well for many catalytic systems

and in particularly for copper. The modification of silica surfaces by attachment of silicon alkoxides

is a very particular technique to chemically bind different substrates to surfaces 4. The reaction

takes place between silanol groups present on the surface of the particle and the silicon alkoxides

of the substrate, leading to elimination of an alcohol and condensation of the coupling agent onto

the surface as illustrated in the scheme below (figure 5.9).

OH

OH

OH

OH

OH

OH

OH OH

+ (MeO)3Si NH

NHNH2

O

O

OH

OH

OH

OH

OH

O

Si NHNH

NH2 + 3 MeOH

Toluene

Reflux 48h

N1-[(3-trimethoxysisyl)-propyl] diethylenetriamine

Silica-supported diethylenetriamine

Silica

Silica

Figure 5.9 – Silica functionalization with diethiletriamine

The amount of coupling agent attached on the surface depends on the number of surface OH

groups available, the activity of the alkoxide, and the reaction time. Ideally, a full y covered surface

is targeted, but only a fraction of the OH groups may react with the alkoxides. Functionalization of

69

silica surface with linear triamines was conducted by the procedure according to literature 5, 8, and

reported here below:


The silica was heated at 150°C under vacuum for 24 hours before modification. A 10 g sample of

silica particles was suspended in 80 mL of dry toluene. A 2.00 g (0.034 mol) sample of N1-[3-

(trimethoxysisyl)-propyl] diethylenetriamine was added to the suspension and heated under reflux

for 48 hours. Toluene was removed by evaporation, and the particles were dried at 50°C under

vacuum for 24 hours. To further remove residues of the coupling agent, the particles were

extracted in a Soxhlet apparatus with methanol for 24 hours and finally dried again at 50°C under

vacuum for 24 hours.

Figure 5.10 shows two sample of P4VP powder, before (green one on the left) and after (white on

the right) filtration on diethylenetriamine-modified silica column.

Figure 5.10 – P4VP sample before (left) and after (right) filtration on diethylenetriamine functionalized silica

70

5.1.3 P4VP-b-PS and PS-b-P4VP blockcopolymers

In order to verify the possibility of obtaining well-defined diblock P4VP-b-PS and PS-b-P4VP

copolymers , several syntheses were performed. The goal is to utilize a first linear block

polymerized via ATRP, as macroinitiator (MI) and to switch to a second block still by ATRP. For this

purpose, different samples of chlorine terminated polystyrenes (differing by their Mn) were

synthesized according to literature9, as shown in figure 5.11.

Cl

m

Polystyrene Styrene

ATRP

Cl

PhEtCl

CuCl/Me6TREN

+

Figure 5.11 – Styrene ATRP reaction scheme

An example of polystyrene homopolymerization made use of the quantities summarized in table

5.4. The solvent used for this experiment was anisole and PhEtCl was used as initiator. The

reaction was carried out at 100°C.


CuCl (I) - 85,40 0,84 0,08 98,98 1

Styrene 10,00 8989,50 84,6 4,13 104,10 100

Anisole 9,00 - - - - -

Me6TREN 0,36 292,40 1,26 0,06 230,39 1,5

Dodecane 1,00 750,00 - - 170,33 -

PhEtCl 0,11 121,40 1,71 0,04 140,61 1

Table 5.4 – Chemicals used for styrene homopolymerization


Styrene (Sigma-Aldrich 99% grade) was distilled at 40°C/18mmHg and stored at 4°C Other

chemicals were used as received. Into a dried glass flask with a magnetic stirrer bar, CuCl (85,4 mg,

0,84 mmol), Me6TREN (0,36 mL, 1,26 mmol) and anisole (9 mL) were added, the reaction mixture

was stirred for 20 minutes, then styrene (9,99 mL, 84,6 mmol) and the 1-chloro-1-ethylbenzene

initiator (0,11 mL, 1,71 mmol) were added. The mixture was degassed with 3 freeze-pump-thaw

cycles then the flask was sealed under argon and immersed in an oil bath thermostated at 100°C.

At regular time intervals a sample was withdrawn for conversion monitoring by GC analysis .

71

An example is summarized in table 5.5. After the reaction was carried out for a prescribed time

the flask was rapidly cooled down.

Sample N° Time (min) Conversion (%)

1 0 0

2 30 22

3 60 37

4 90 41

5 120 49

6 150 55

Final conversion 75% after 5 hour of reaction.

Table 5.5 – Conversion datas for styrene ATRP

The whole range of PS-Cl MIs synthesized is shown in table 5.6. The size exclusion chromatogra m

of sample PS-Cl(1) is shown in figure 5.12.

Figure 5.12 – SEC chromatogram for sample PS-Cl(1), THF solvent (1ml min-1 flow rate)

Mn=3380 g/mol

PDI=1,03 PS-Cl(1)

72

Main properties of PS-Cl MIs synthesized are reported table 5.6.

PS-Cl (1) PS-Cl (2) PS-Cl (3) PS-Cl (4)

[M]/[I] 30 50 100 100

Mn (g/mol) 3380 6520 30000 4800

PDI (Mw/Mn) 1.07 1.05 1.1 1.09

Table 5.6 – Polystyrene samples properties

With chlorine-terminated P4VP-Cl and PS-Cl macroinitiators obtained, several copolymerizations

were performed in order to investigate the reactivity of the chlorine terminated homopolymers

and to verify the feasibility of the switching from one type of polymer chain to the other (P4VP to

PS and PS to P4VP). An example of copolymerization starting from a P4VP-Cl macroinitiator is

shown in figure 5.15 and table 5.7. The solvent used in this case was DMF (dimethylformamide),

because it is a good solvent for both the P4VP-Cl macroinitiator and for polystyrene.


Cl

N

m

Styrene

ATRP

CuCl/Me6TREN

+N

Cl

nm

Poly(4-vinyl-pyridine)-b-polystyrene-Cl

Figure 5.13 – P4VP-Cl styrene copolymerization


CuCl (I) - 68,70 0,68 0,03 99,00 1

Styrene 4,00 8989,50 34 1,52 104,10 50

DMF 16,00 - - - - -

Me6TREN 0,20 292,40 0,68 0,03 230,39 1

Dodecane 1,00 750,00 - - 170,33 -

P4VP(2) - 3150,00 0,68 0,03 4630 1

Table 5.7 – Stochiometry for a P4VP-styrene copolymerization

73


As a representative example, the following quantities of substances were taken for a certain

amount of P4VP-Cl macroinitiator. Into a dry round-bottom flask with a magnetic stirrer bar, the

copper catalyst CuCl (68,7 mg, 0,68 mmol) , the ligand Me6TREN (0,197 mL, 0,68 mmol) and 8 mL

of DMF were added and the reaction mixture was vigorous stirred until the catalyst was solubilized

in the organic phase (20 min). In a separate flask, 1,705 g of P4VP-Cl (0,68 mmol) were dissolved in

8 mL of DMF, then the two solutions were combined. The mixture was degassed by three freeze-

pump-thaw cycles, then the flask was sealed under argon and immersed in an oil bath

thermostated at 100°C. During the reaction, samples were withdrawn at regular intervals, filtered

on modified silica columns and diluted in THF for the gas chromatographic analysis. After a

prescribed time, the flask was rapidly cooled down to room temperature, then the mixture was

diluted in THF and filtered several times through a modified silica column in order to remove the

copper catalyst present in the resulting polymer. A large amount of MeOH was added to the

solution to precipitate the polymer, which was then filtered and dried overnight under vacuum at

room temperature.

Concerning the synthesis of the diblock copolymers, the procedures were not successful, whether

they started from the P4VP-Cl or the PS-Cl macroinitiators. All the experiments were carried out in

a common solvent for monomers and polymers of the two species, such as dimethylformamide

(DMF). The monomer/macroinitiator ratios and the overall concentration (from bulk solution to

very dilute solution) were varied, but very weak conversions were always achieved as indicated by

the results of the GC analyses.

Here below (table 5.8), resuming monomer/initiator ratios and relative conversions for ATRP

copolymerizations.

P4VP(1) -PS P4VP(2)-PS P4VP(2)-PS PS(1)-P4VP PS(2)-P4VP

[M]/[MI] 100 50 400 30 100

Conversion (%) 2.74 1.60 3.45 2.31 3.15

Table 5.8 - monomer/initiator ratios and relative conversions for ATRP copolymerizations.

74

In figure 5.14 the 1HNMR spectrum of PS-Cl(1) is shown.

Figure 5.14 – PS-Cl (1) 1HNMR spectrum performed in CDCl3 (300MHz)

1H NMR spectrum (300 MHz) of PS-Cl (1) in figure 5.14. The broad signals in the region 6.2 – 6.4

ppm are characteristic of the aromatic protons of the styrene unit, whereas those in the region

1,2 – 2,2 ppm belong to the alkyl protons of the hydrocarbon backbone. The sharp signal at 7,30

ppm is the CDCl3 solvent resonance, whereas those at 3,85 and 1,55 ppm assigned to THF.

75

5.1.4 Competitive Side Reactions in ATRP Synthesis Involving 4VP

Prior to polymerization and copolymerization experiments, it could be important to take in

account the side reactions that can take place in ATRP involving 4VP. As already said nitrogen

atom on 4VP ring, could be seen as a strong binding ligand that can interact with catalyst (CuCl)

and reduce copper efficiency during the synthesis. Moreover, if the coordination reaction cannot

compete against the complexation of 4VP and P4VP to copper ions, slow polymerization will occur.

There is another category of competitive reaction that can affect the polymerization of such

monomers which is the nucleophilic attack. When styrene-like polymers (including 4VP) forming a

secondary alkyl halide dormant species (halogen terminations), are polymerized under ATRP

conditions, the chain ends are susceptible to nucleophilic substitution or elimination reaction

(figure 5.15), which are particularly pronounced when the solvent or the monomer itself (like 4VP)

or the polymer (like P4VP) have nucleophilic properties . These reactions effectively “kill” the

polymer chain ends, resulting in a relatively broad molecular weight distributions and a low degree

of halide end-functionalization.

Cl

N

m m

ClN

+

N

n

Cl

m m

ClN

+n

Nucleophilic substitution on Poly(4-vinyl-pyridine) halogen

terminated chain

Nucleophilic substitution on polystyrene halogen

terminated chain

ATRP

ATRP

Figure 5.15 – 4VP and P4VP side reactions

Competitive reactions shown above, can occur in most cases, between the growing linear chains

(P4VP or PS) and nitrogen atoms from another or even the same P4VP chain 6,7.

76

5.2 Poly(tert-butyl methacrylate) and Poly(tert-butyl acrylate)

Tert-butylmethacrylate (tBMA) is another monomer which could be considered interesting to

obtain amphiphilic block copolymers and of course water-soluble star polymers, instead of 4-

vinylpyridine. The tBMA monomer is largely used in the assembly of double hydrophilic block

copolymers (DHBCs), which represent a new class of macromolecular systems exhibiting stimuli-

responsive properties in water. These copolymers may associate one polyelectrolyte block and a

neutral one, but they may also contain two basic or two acidic blocks. Similar to amphiphilic block

copolymers constituted of hydrophobic and hydrophilic moieties, these kind of copolymers can

self-assemble and form reversible micelle-like structures in the submicron or the micron size

range, for instance, by adjusting the pH or temperature of the solution. This can be achieved in

aqueous media by manipulating the hydrophilycity of the responsive block through stimulation,

whereas the role of the water-dispersible and non-interacting block is to prevent the aggregates

from precipitating. However, the range of monomers that can undergo such polymerizations is

somewhat limited. In this respect, controlled/living radical polymerization techniques such as

ATRP, owing to their much less demanding experimental conditions and their tolerance to polar

functions as compared to ionic procedures, seem very promising for the synthesis of new DHBCs.

Interest in new types of DHBCs that exhibit a star like architecture is driven by the expectation that

a well-controlled number of branching points can induce different properties as compared to their

linear counterparts. Attempts to derive such star-like copolymers are part of continuing efforts to

tailor well-defined branched amphiphilic systems such as star-block copolymers (as in the present

work), mikto-arm stars, and dendrimer-like copolymers10.

CH2

CH3

OO

CH3 CH3

CH3

CH3

OO

CH3 CH3CH3

nATRP

Tert-butyl methacrylate Poly(tert-butyl methacrylate)

Figure 5.16 – tBMA ATRP reaction scheme

77

Poly(tert-butyl acrylate) (PtBA), obtained from the polymerization of the corresponding monomer

tert-butyl acrylate (tBA), is another polymer of great importance for the synthesis of amphiphilic

diblock copolymers, because of its similarities to PtBMA, differing by just a methyl group in the

linear chain, which could make the polymer, once hydrolyzed to the corresponding poly acid

(PAA), probably even more suitable for the dissolution in water.

CH2

OO

CH3 CH3

CH3

H

OO

CH3 CH3CH3

nATRP

Tert-butyl acrylate Poly(tert-butyl acrylate)

Figure 5.17 - tBA ATRP reaction scheme

In the following section, ATRP conditions for the synthesis of linear amphiphilic diblock copolymers

were employed (as in the 4-vinylpyridine case), to derive star like polymers that include peripheral

PS-b-PMAA or PS-b-PAA diblocks (after hydrolysis of PS-b-PtBMA or PS-b-PtBA diblocks). We will

therefore describe here the synthesis of such linear copolymers. This was achieved by ATRP of

polystyrene using tBMA-X and tBA-X as macroinitiators 11.

5.2.1 Poly(tert-butyl methacrylate) synthesis

Differently from 4-vinylpyridine, PtBMA polymerizations were carried out in toluene. The ligand

used was the PMDETA (penthamethyl diethyl triamine) and the initiator phenyl-tosyl chloride,

figure 5.18. Because this protocol seems to give best results.

p-Tosyl chloride

CH2

CH3

OO

CH3 CH3

CH3

CH3

OO

CH3 CH3CH3

Cl

nATRP

Tert-butyl methacrylate Poly(tert-butyl methacrylate)

CH3

S OO

Cl

+CuCl/PMDETA

Figure 5.18 – p-tosyl chloride initiated ATRP of tBMA

78

The stochiometry used for a typical PtBMA ATRP polymerization catalyzed by CuCl is reported in

table 5.9.

Substance Volume (mL) Mass (mg) m moles M (mol/l) Mw (mg/mmol) Molar Ratio

CuCl (I) - 471,30 4,60 0,10 99,00 1

tBMA 23,22 19908,00 140 3,05 142,20 30

Toluene 20,00 - - - - -

PMDETA 0,96 809,20 4,60 0,10 173,39 1

Dodecane 1,00 750,00 - - 170,33 -

p-TsCl 0,69 907,90 4,60 0,10 190,65 1

Table 5.9 - The stochiometry used for a typical PtBMA ATRP


CuCl (471 mg, 4,60 mmol) and PMDTEA (0,96 mL, 4,60 mmol) were introduced in a Shlenck flask.

Then 10 mL of toluene and 1mL of dodecane were added and the mixture was stirred to complex

and solubilize the catalyst. Then, 908 mg of p-Tosyl-Cl (4,60 mmol) were dissolved in 10 mL of

toluene and added to the reaction mixture. Finally, 23,2 mL of tBMA (140 mmol) were introduced.

After three freeze-pump-thaw cycles, the flask was placed in a preheated and thermally regulated

oil bath at 80°C. Samples for GC analysis were withdrawn at regular intervals. The conversions

values calculated from the GC analyses are shown in table 5.10. After a prescribed time (ca.75% of

conversion), the reaction mixture was rapidly cooled down, diluted in toluene and filtered on

alumina columns several times. The resulting solution was evaporated and concentrated, then the

polymer was precipitated in MeOH, recovered by filtration and dried overnight under vacuum at

room temperature.

Sample N° Time Conversion (%) ln(M0/Mi)

0 0 0 0

1 30 49,89 0,691

2 60 62,79 0,988

3 90 70,93 1,235

4 150 88,74 2,184

Table 5.10 – Conversion and Ln(M0/Mi)

79

Plots of the conversion and of Ln([tBMA]0/[tBMA]) vs time are reported in figures 5.19 and 5.20,

respectively.

Figure 5.19 - Conversion against time Figure 5.20 - Ln([tBMA]0/[tBMA]) against time

The PtBMA synthesis was carried out using both CuCl and CuBr as catalyst. The bromine-based

catalysts seemed to allow in general a better control on methacrylates ATRP, although in the

specific case of this work, the polydispersity values of the obtained polymers does not differ too

much, as shown in the following SEC curves (figure 5.21 and 5.22).

Figure 5.21 - Size exclusion chromatogram of sample PtBMA-Cl (1) obtained from the CuCl/PMDETA catalyzed ATRP of tBMA with p-TsCl initiator

0

20

40

60

80

100

0 30 60 90 120 150 180

Co

nve

rsio

n (

%)

Time (min)

0

0.5

1

1.5

2

2.5

0 100 200Ln

(M0/M

i)Time (min)

y = 0.0137x + 0.1165 R² = 0.9781

Mn=6730 g/mol

PDI=1,07 PtBMA-Cl (1)

80

Figure 5.22 - Size exclusion chromatogram of sample PtBMA-Br (2), obtained from the CuBr/PMDETA catalyzed ATRP of tBMA

with p-TsCl initiator

The [M]/[I] ratios and catalysts used for the various experiments are summarized in table 5.11.

PtBMA-Cl PtBMA-Br (1) PtBMA-Br (2)

[M]/[I] 30 50 70

Mn th (g/mol) 4280 4570 8580

Mn SEC (g/mol) 6730 4380 7730

PDI 1.07 1.05 1.03

Catalyst Cl Br Br

Table 5.11 - [M]/[I] ratios and catalysts used for the various PtBMA experiments

Differences between theoretical and experimental molecular weights may be due by an

incomplete efficiency of the initiators.

Mn=4380 g/mol

PDI=1,05

PtBMA-Br (1)

81

The isolated solid powder of PtBMA-Br is shown in figure 5.23. Contrary to the crude P4VP

polymers, the obtained PtBMA solid powders are clear and white, with no traces of immobilized

copper in the final polymer after filtration. This confirms the much smaller tendency of PtBMA

relative to P4VP to act as a ligand for the copper ion and simple filtration over an alumina column

is sufficient to trap the copper catalyst in this case.

Figure 5.23 – PtBMA-Br (1) metal-free white powder

82

5.2.2 PtBMA-b-PS blockcopolymers

Similarly to the poly-4-vinylpyridine case, a number of ATRPs of styrene were carried out using

PtBMA-X as macroinitiators. These polymerizations were much more successful than when

starting from the P4VP-X macroinitiator, with clean and well controlled switch to the polystyrene

block.

CH3

OO

CH3 CH3CH3

Cln

StyrenePoly(tert-butyl methacrylate)

+ATRP

CuCl/PMDETA

CH3

OO

CH3 CH3CH3

Cln m

Poly(tert-butyl methacrylate)-b-polystyrene

Figure 5.24 – PtBMA and Styrene Copolymerization sequence


In a typical procedure, to a Shlenck flask, the macroinitiator CuBr(71,5 mg, 0,7 mmol), PMDETA

(0,15 mL, 0,7m mol) and 6 mL of dry toluene were added. The mixture was stirred to dissolve the

catalyst, yielding a green homogeneous solution. About 4,5 g of PtBMA-Cl were dissolved in dry

toluene using a round-bottom flask, then the macroinitiator solution and styrene (5 mL, 42,5

mmol) were purged and added to the reaction mixture. After three freeze-pump-thaw cycles, the

flask was sealed under argon and immersed in a preheated oil bath thermostated at 100°C. During

the reaction, several samples (1 mL of solution) were withdrawn, dissolved in 3 mL of toluene and

filtered on small alumina columns in order to remove the catalyst. The conversion values were

estimated by GC analysis (see table 5.13). After a prescribed time (60% conversion) the reaction

mixture was removed from the thermostated bath, rapidly cooled down to room temperature,

dissolved in 30 mL of toluene and passed several times through an alumina column to remove the

residual catalyst in solution. Finally, it was concentrated with a rotavapour apparatus and

precipitated by adding cold methanol. After filtration, the solid powder was dried overnight under

vacuum at room temperature11.

For all the experiments, the conversions reached 60 % after 5 hours of reaction. The

polymerizations were stopped on purpose before 70% of conversion in order to avoid excessive

losses of halogen terminations by disproportional coupling between growing polymer chains .

83

An example of PtBMA-b-PS-Cl copolymerization using a low molecular mass (Mn=6730 g/mol)

chlorine-terminated PtBMA homopolymer as macroinitiator is detailed in table 5.12.

The synthesis was catalyzed by CuCl and carried out at 100°C in toluene to allow a better

solubilization for PtBMA macroinitiators, PMDTEA was used as ligand.

Table 5.12 – Stochiometry for PtBMA and styrene copolymerization

Sample N° Time Conversion (%) Ln (M0/Mi)

0 0 0 0

1 40 5,88 0,06

2 120 20,39 0,22

3 180 39,21 0,49

4 300 61,76 0,96


The conversion and ln(M0/Mi) as a function of time are shown respectively in figures 5.25 and

5.26.

Figure 5.25 – Conversion vs. time Figure 5.26 - ln(M0/Mi) vs. time

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350

Co

nve

rsio

n (

%)

Time (min)

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400

Ln (M

0/M

i)

Time (min)

Substance Volume(mL) Mass(mg) m moles M(mol/L) Mw(mg/mmol) Molar Ratio

CuCl (I) - 71,50 0,70 0,03 98,98 1

Styrene 5,00 4421,20 42,45 1,88 104,15 60

Toluene 12,00 - - - - -

PMDTEA 0,15 122,70 0,70 0,03 173,39 1

Dodecane 1,00 750,00 - - 170,33 -

PtBMA-Cl 4,42 4532,30 0,70 0,03 Mn=6730 1

y = 0.0033x - 0.0676

R² = 0.9728

84

The SEC analysis of the isolated polymer, shown in figure 5.27, revealed a very narrow

polydispersity (PDI = 1.10), although slightly higher than that of the macroinitiator (PDI = 1.07).

The diblock copolymer molecular weight (Mn = 20780 g/mol), is quite close to the designed

molecular weight.

Figure 5.27 - Size exclusion chromatogram of sample DB1 diblock copolymer, starting from PtBMA-Cl macroinitiator

In the following page, 1HNMR spectra of PtBMA-Cl and DB1 (PtBMA-b-PS-Cl) are shown and

discussed. In this work a large use of NMR spectroscopy was made, in order to investigate the

chemical nature of synthesized polymers, and in the particular case of diblock copolymers, 1HNMR

analyses were very useful to proof that the second block copolymerizations occurred.

Mn=20780 g/mol

PDI=1,10

DB1

Diblock

85

The 1H NMR spectrum in figure 5.28 shows a sample of the PtBMA-Cl macroinitiator. The CH2 and

CH3 groups are detected respectively at 1.8 and 1.1 ppm, and the t-butyl protons are observed at

1.45 ppm

Figure 5.28 – 1H NMR spectrum of PtBMA-Cl macroinitiator carried out in CDCl3

The 1H NMR spectrum in figure 5.29 shows a PtBMA-PS sample (DB1). Its shape in the high field

region is essentially identical to that of the PtBMA-Cl macroinitiator, since the CH and CH2 protons

of the PS block resonate at similar chemical shift as the CH2 and tBu protons of the PtBMA block

(cf. with the PS-Cl NMR spectrum shown in figure 5.14). The low field region shows the distinctive

aromatic C-H resonances of the phenyl groups as broad bands around 7 ppm.

Figure 5.29 - 1H NMR spectrum of PtBMA-b-PS-Cl copolymer carried out in CDCl3

86

5.2.3 Poly(tert-butyl acrylate) synthesis

For poly(tert-butyl acrylate), a single attempt of synthesis was made to test whether the

conditions for greater solubility can be achieved through the use of poly acrylic acid (PAA) instead

of poly methacrylic acid (PMAA), as a hydrophilic outer block. It is reasonable to assume that the

absence of the methyl group in the chain of PtBA can significantly increase the favorable

conditions for the entire final star-like polymer solubility in aqueous solution. Note that the

initiator used in this polymerization is methyl-2-bromopropionate (Me2BrPro), which is more

effective and soluble in the monomer, unlike the p-Tosyl chloride, which in the early attempts of

ATRP synthesis of PtBA proved to have poor solubility and induced the precipitation of CuBr. The

ATRP of tert-butyl acrylate is shown in figure 5.30.

Tert-butyl acrylate Poly(tert-butyl acrylate)

CH2

OO

CH3 CH3

CH3

OO

CH3 CH3

CH3

Br

nATRP+

CuBr/PMDETA

CH3OCH3

Br

O

Methyl-2-bromopropionate

Figure 5.30 – tBA ATRP sequence

The operating conditions of are summarized in table 5.14. The reaction was carried out in toluene

at 80°C and PMDETA was used as ligand as in the previous synthesis.

Table 5.14 – Chemicals used for tBA ATRP


Tert-butyl acrylate (tBA) (Sigma-Aldrich 99% grade) was stored over CaH2 and vacuum distilled

before polymerization. Methyl-2-bromopropionate (Me2BrPro) (Fluka 99%) was stored at room

temperature and used as received. A 50 mL Shlenck flask equipped with a magnetic stirring bar

was charged with 146 mg (1,00 mmol) of CuBr and 22,4 mg (0,1 mmol) of CuBr2 under inert

conditions (glove box), then distilled toluene (15 mL), dodecane (0,5 mL) and PMDETA (0,205 mL,

Substance Volume (mL) Mass (mg) m moles M(mol/L) Mw (mg/mmol) Molar Ratio

CuBr (I) - 146,40 1 0,040 143,45 1

CuBr2 - 22,40 0,1 0,004 223,35 0,1

tBA 10,50 8974,00 70 2,650 128,20 70

Toluene 15,00 - - - - -

PMDETA 0,205 173,40 1 0,040 173,39 1

Dodecane 0,500 375,00 - - 170,33 -

Me2BrPro 0,250 169,30 1 0,040 167,61 1

87

1,00 mmol) were added followed by stirring for 20 minutes in order to coordinate and solubilize

the catalytic species. Subsequently, the Me2BrPro initiator (0,251 mL, 1,00 mmol) and tBA (10,5

mL, 70,0 m mol) were added and the reaction mixture was evacuated with three freeze-pump-

thaw cycles. The flask was sealed under argon and immersed in a preheated oil bath thermostated

at 80°C. Several samples were withdrawn at regular time intervals and the conversion was

estimated by GC (table 5.15). After 5 h, the mixture was removed from the oil bath and rapidly

cooled down to room temperature, diluted with 20 mL of toluene, and filtered through an alumina

column. The resulting solution was concentrated and the polymer was precipitated in MeOH, then

filtered and dried for 24 h under vacuum at room temperature12. Yield: 7 g (60%), Mn = 5310 g/mol

(PDI = 1,15).


In figure 5.31 and 5.32 conversion and Ln(M0/Mi) versus time are plotted.

Figure 5.31 – Conversion vs. time Figure 5.32 - Ln(M0/Mi) vs time

0

10

20

30

40

50

60

70

0 100 200 300 400

Co

nve

rio

n (

%)

Time (min)

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400

Ln(M

0/M

i)

Time (min)

Sample Time (min) Conversion (min) Ln (M0/Mi)

0 0 0 0

1 40 7,43 0,07

2 60 18,41 0,22

3 120 28,26 0,34

4 180 39,70 0,50

5 300 61,00 0,92

y = 0.0032x - 0.0278

R² = 0.9841

88

By a similar method to that used previously for PtBMA-b-PS-Cl, an amphiphilic block copolymer

PtBA-b-PS-Br was synthesized by ATRP (figure 5.33).

+

Poly(tert-butyl acrylate)-b-polystyrene

OO

CH3 CH3

CH3

Br

nATRP

CuBr/PMDETA

OO

CH3 CH3

CH3

Br

n

Poly(tert-butyl acrylate) Styrene

m

Figure 5.33- PtBA styrene copolymerization reaction scheme

The size exclusion chromatogram of the PtBA-Br macroinitiator and the corresponding PtBA-b-PS-

Br diblock copolymer are shown in figure 5.34. The figure shows quite clearly that the overall

molecular weight distribution shifts to higher molecular weights while remaining narrow and

monomodal, indicating a good switch on going from the first to the second block and a controlled

chain extension of the PtBA macroinitiator by the styrene monomer.

Figure 5.34 - Molecular weight distribution shift to higher molecular weights after the copolymerization of styrene block

8 10 12 14 16 18Elution time (min)

PtBA-Br

DB7

Mn=5310 g/mol

PDI=1,15

Mn=8620 g/mol

PDI=1,30

89

PtBA spectrum performed in CDCl3 250 MHz.

Figure 5.35 - 1H NMR spectrum of the PtBA-Br sample (solvent CDCl3)

The 1H NMR spectrum of the PtBA-Br sample is shown in figure 5.35. The CH and CH2 protons are

observed at 1.8 ppm, whereas the tert-butyl protons give rise to the strong peak at 1.45 ppm.

The starting chain end does not give rise to distinctive peaks because it has the same chemical

nature as the monomer after incorporation in the polymer chain.

90

In summary, the entire range of linear polymers synthesized is listed below: the homopolymer

macroinitiators in Table 5.16 and the diblock copolymers in Table 5.17.

Homopolymer macroinitiators

Sample Mn (g/mol) N° (Metha)Acrylic

Units PDI

Halogenated Termination

PtBMA-Cl

6730 46 1,10 Chlorine

PtBMA-Br (1)

4380 30 1,05 Bromine

PtBMA-Br (2)

7730 53 1,03 Bromine

PtBA-Br

5310 40 1,15 Bromine

Table 5.16 – Homopolymer macroinitiators

Linear diblocks

Sample Mn (g/mol) N° tB(M)A-sty units tB(M)A-Sty (% wt) PDI

DB1

20780 46-135 33/67 1,16

DB2

26140 46-186 25-75 1,10

DB3

17850 53-97 43-57 1,25

DB4

6360 30-19 68-32 1,22

DB5

9120 53-13 84-16 1,45

DB6

9340 53-16 82-18 1,42

DB7

8620 40-31 61-38 1,30

Table 5.17 – Linear diblocks

91

5.3 Star Polymers Synthesis: Crosslinking

Crosslinked star polymers have a unique three dimensional architecture that consists of a

crosslinked core surrounded by a number of radiating linear arms. Synthesis of this class of

polymer is usually carried out in a two-step process known as “arm-first” approach, where living

linear arms capable of further chain extension are initially synthesized. These terminally reactive

linear polymer chains are subsequently used to initiate the polymerization of a cross-linkable

monomer (generally bifunctional), such that the active arm ends are coupled together to form star

shaped polymers with a crosslinked core. Controlled radical polymerization (CRP) such as atom

transfer radical polymerization (ATRP), are typically employed to synthesize crosslinked star

polymers, resulting in a high degree of structural control and narrow molecular weight

distribution12. In the following scheme, an example of “arm-first” crosslinking procedure, for a

PtBMA-b-PS-Br amphiphilic diblock copolymer is shown. It is noticeable that in the present work,

the cross linking agent that will be used is divinyl benzene (DVB), figure 5.36.

Br

OO

CH3 CH3

CH3

n n

Br

OO

CH3 CH3

CH3

m

CuBr/PMDETA

Toluene 100°C

PtBA-BrStyrene

PtBA-b-PS-Br

CuBr/PMDETA

Toluene 90°C

DVB

n m

Br

OO

CH3 CH3

CH3

m

OO

CH3 CH3

CH3

Br

n


Figure 5.36 – Cross linking sequence to form the final star polymer

92

In table 5.18 here below, an example of crosslinking polymerization using PtBMA-b-PS as

macroinitiator. The reaction is carried out at 90°C in toluene or anisole, PMDETA was used as

ligand, a small amount of CuBr2 was used to obtain a more stable ATRP equilibrium.

Substance Volume (mL) Mass (mg) m moles M(mol/L) Mw (mg/mmol) Molar Ratio

CuBr (I) - 89,90 0,61 0,035 143,45 0,9

CuBr2 - 15,30 0,07 0,004 223,35 0,1

DVB 2,00 1776,40 13,64 0,781 130,19 20

Toluene 20,00 - - - - -

PMDETA 0,15 118,30 0,68 0,039 173,39 1

PtBMA-PS-Br 4,31 4424,90 0,68 0,039 6730 1

Table 5.18 – Stochiometry for a crosslink experiment


Divinyl benzene (DVB) (Sigma-Aldrich 87% grade) was stored at 4°C and used as received. In a

typical polymerization, a 50 mL shlenck flask equipped with a stirring bar, was charged with 1 m

mole of CuBr and 0,1 m moles of CuBr2 then distilled toluene (10 mL) and ligand (PMDETA) were

added and stirred for 20 minutes in order to coordinate and solubi lize catalytic species, after that

4,3 g of macroinitiator (PtBMA-PS-Br) were diluted in 10 mL of toluene, then macroinitiator

solution and DVB (13,6 m moles) were added and the reaction mixture was evacuated with three

freeze-pump-thaw cycles. The shlenck flask, was sealed under argon then immersed in a

preheated thermostated oil bath at 80°C for 36h. The shlenck was then removed from the oil bath

and rapidly cooled down to room temperature, the reaction mixture was successively diluted and

filtered on alumina column13. The resulting solution was concentrated and polymer precipitated in

MeOH, then dried for 24h under vacuum at room temperature.

During the crosslinking reactions, the quantity of catalyst was increased to investigate a possible

influence on the efficiency of cross-linking due to a higher concentration of catalyst solution. In the

following page a table summarizing the entire range of star polymers obtained by crosslinking of

diblock copolymers previously synthesized (table 5.19). Crosslinking reactions for polymers STP3

and STP5 did not give satisfactory results (crosslinking did not occurred), so they are not listed in

the table and will not be taken into account for the star polymer characterization in chapter 6.

93

Star-like Polymers

Sample Crosslinked

Linear Copolymer

tBMA/tBA N° Units Mn (g/mol) Catalyst

Equivalents

STP1

DB1 46 PtBMA 280400 1

STP2

DB2 46 PtBMA 191000 1

STP4

DB3 53 PtBMA 48320 1

STP6

DB4 30 PtBMA 105100 1

STP7

DB5 53 PtBMA 68940 2

STP8

DB6 53 PtBMA 102900 2

STP9

DB7 40 PtBA 663700 1

Table 5.19 – Obtained star polymers properties

94

5.4 Polymer Hydrolysis and Solubilization

This section illustrates the results obtained for the hydrolysis of some of the synthesized polymers

previously described. As mentioned above, in order to achieve the solubilization in water of these

objects, the polymers (both linear and star-polymer) were put under moderate acid conditions,

hydrolysis affects only the outer chains, such as PtBMA and PtBA blocks, with the consequent

removal, via etching, of tert-butyl groups in chain, giving as result, the corresponding poly acid,

poly (methacrylic acid) (PMAA) and (poly acrylic acid) (PAA). Considering that, in such

macromolecular objects, there is a significant amount of hydrophobic polymer (polystyrene

chains and PDVB core), it is reasonable to assume that the solubility in water, could not be not

achieved with the only contribution of external acid chains. Actually, both poly (acrylic acid) and

poly (methacrylic acid), are soluble in water when used alone, but not as a copolymer with

polystyrene14. The following image (figure 5.37) illustrates the hydrolysis reaction of the tert-butyl

groups and the synthesis of poly acid external segment. The last step for a complete sol ubilization

of diblocks and star polymers will be reported on the next page.

mOO

CH3 CH3

CH3

Br

CH3

n

Poly(t-butyl methacrylate)-b-polystyrene-polydivinylbenzene Star Polymer

mOO

H

Br

CH3

n

Poly(methacrylic acid)-b-polystyrene-polydivinylbenzene Star Polymer

DVB DVBCF

3COOH

CH2Cl

2

mOO

CH3 CH3

CH3

Br

n


mOO

H

Br

n

Poly(acrylic acid)-b-polystyrene-polydivinylbenzene Star Polymer

DVB DVBCF

3COOH

CH2Cl

2

Figure 5.37 – Hydrolysis reaction for PtBMA and PtBA external blocks

95


Into a round-bottom flask 6mL of dichloromethane (CH2Cl2) were added, 300 mg of polymer

(PtBMA-X, PtBA-X, diblock copolymer or star polymer) were dissolved in the solution at 0°C and

vigorous stirred. With a dropping funnel (0.9 mL, 11.7 mmol) of trifluoroacetic acid were slowly

added. The reaction mixture was kept at 0°C for 3h and then overnight at room temperature. The

reaction mixture was carefully evacuated and then redissolved in dioxane for purification by

freeze-drying. The 1HNMR characterization before (a) and after (b) hydrolysis is illustrated in

figure 5.38.

Figure 5.38 – PtBMA-b-Ps-Br before (a)and after (b) hydrolysis

In the figure above, are represented in the two 1H-NMR spectra of a copolymer PtBMA-b-PS-Br

before (a) and after hydrolysis (b). In the second spectrum it is noticeable the absence of the peak

(a)

(b)

96

corresponding to butyl groups (δ= 1.45 ppm), showing that complete hydrolysis occurred. Solvents

used for 1H-NMR analysis were CDCl3 (a) and dimethyl sulfoxide-d6 (b).

Copolymers and star polymers in the acidic form do not dissolve directly in water. They have to be

ionized and heated in order to obtain homogeneous solutions of the polyelectrolytes 15.


For each experiment, 30 mg of block or star copolymers were introduced into 0.4 M aqueous

solutions of K2CO3 (potassium carbonate). The pH of all solution was above 7 (complete

neutralization of methacrylic and acryl acid units), the concentration of acrylic acid units was

below the salt concentration. The samples were heated for 45 min under vigorous stirring

conditions at 70°C to complete solubilization. At this stage, the acrylic acid units are in the

potassium salt form.

In the following scheme (figure 5.39) ionization of polymers and resulting polymer in the

potassium salt form are described.

mOOH

Br

CH3

n

Poly(methacrylic acid)-b-polystyrene-polydivinylbenzene

mOO

-

Br

CH3

n

Poly(methacrylic acid)-b-polystyrene-polydivinylbenzene Potassium salt

DVB DVBK2CO3 in H2O (0.4 M)

70°C

mOOH

Br

n

Poly(acrylic acid)-b-polystyrene-polydivinylbenzene

mO

Br

O-

n

Poly(acrylic acid)-b-polystyrene-polydivinylbenzene Potassium salt

DVB DVB

K+

K+

K2CO3 in H2O (0.4 M)

70°C

Figure 5.39 – Ionization of hydrolyzed star polymers with K2CO3

In the following page, results obtained during the solubilization of part of previously synthesized

polymers are shown in tables 5.20 and 5.21. The polymers on which water solubility tests were

97

performed are listed here: PAA-Br PMAA-Cl, hydrolyzed DB1, DB3 DB4 and DB7, hydrolyzed STP1,

STP4, STP6 and STP9.

Cloudy White Solutions

Hydrolyzed Polymer Resulting Polymer N° of acid units Ionization Solution Obtained

DB1 (PMAA+K)46-(PS)-Cl 46 V White Solution

DB3 (PMAA+K)53-(PS)-Br 53 V White Solution

DB4 (PMAA+K)30-(PS)-Br 30 V White Solution

STP1 (PMAA+K)46-(PS)-PDVB 46 V White Solution



Table 5.20 – Cloudy white solutions obtained

Clear Colorless Solutions

Hydrolyzed Polymer Resulting Polymer N° of acid units Ionization Solution Obtained

PtBMA-Cl (PMAA)46-Cl 46 X Clear Solution

PtBA-Br (PAA)40-Br 40 X Clear Solution

DB7 (PAA+K)40-(PS)-Br 40 V Clear Solution

STP8 (PAA+K)40-(PS)-PDVB 40 V Clear Solution

Table 5.21 – Clear colorless solutions

Figure 5.40 – White water solutions (a) and colorless water solutions (b) obtained by solubilization experiments

(a) (b)

98

References

1) Terashima T.; Ouchi M.; Ando T.; Kamigaito M.; Sawamoto M., Macromolecules 2007,40.

2) Gao, H.; Matyjaszewski, K. Macromolecules 2007, 40.

3) Hadjichristidis, N. Journal of Polym. Sci., Part A: Polym. Chem. 1999, 37.

4) Georgiades, S. N.; Vamvakaki, M.; Patrickios, C. S. Macromolecules 2002, 35.

5) Du, J.; Chen, Y. Macromolecules 2004, 37.

6) Yang R.; Wang Y.; Pan C.; Eur. Polym. Journal 2003 ,39.

7) Zhang, X.; Xia, J.; Matyjaszewski, K. Macromolecules 2000, 32.

8) Kickelbick G. ; Paik H ; Matyjaszewski, K. Macromolecules 1999, 32.

9) Gobius G.; Voet V.;Loos K.; Macromolecules 2008,41.

10) Hou S.; Chaikof E.; Gnanou Y. ; Macromolecules 2003,36.

11) Wang G.; Yan D.; Journal of Appl. Polymer Science 2001, 82.

12) Gao H.; Matyjaszewski, K. Macromolecules 2006, 39.

13) Terashima T.; Ouchi M.; Baek K.Y. ; Kamigaito M.; Sawamoto M., Journal of American

Chem. Soc. 2003,125.

14) Davis K.A.; Matyjaszewski K.; Macromolecules 2000, 33.

15) Burguiére C.; Chassenieux C.; Charleux B. Elsevier Science Polymer 2002,44.

99

Chapter 6

6. STAR POLYMERS CHARACTERIZATION

In this chapter, some of the results obtained from characterization tests conducted on star

polymers previously synthesized are shown and discussed. Characterization tests were performed

by size exclusion chromatography (SEC), nuclear magnetic resonance (1H-NMR) and atomic force

microscopy (AFM). Size exclusion chromatography is a very useful tool that allows to investigate

the progressive growth of the molecular weight of polymers, this analysis can reveal important

information about the mass evolution of such macromolecular objects. Cross-linking and diblock

copolymerization can be verified by taking into account the shift to lower elution times (on the

left) of the chromatogram. SEC analysis were performed in THF at 1 mL/min, instrument

specification are reported in (see Appendix). The nuclear magnetic resonance 1H-NMR were

performed in order to verify the success of the ATRP polymerization for a specific polymer

(especially for diblock copolymer). An important use of this technique was made for the

recognition of the protons characteristic of particular polymers, such as benzene aromatic ring

protons of styrene, as a confirmation of the successful diblock copolymerization, or the

characterization of hydrolyzed polymers. Unfortunately, given the significant amount of solvent

present in the spectra, it was not possible to perform calculation on molecular masses, and any

recognition of the proton near the atom of bromine or chlorine on the terminal repeating unit.

The solvent used to perform 1H-NMR analysis was CDCl3 for non-hydrolyzed polymers and

deuterated DMSO for hydrolyzed ones, further information about the spectrometers used are

reported in (see Appendix). Further analysis of morphological character of star polymers were

performed by atomic force microscopy (AFM). The samples were dispersed on silicon substrates

using the spin coating techniques .It was possible to recognize the spherical structures operating in

contact mode on scanning areas ranging from 20x20µm to 600x600nm. Technical specification

about the AFM apparatus used in this work are reported (see Appendix).

100

6.1 Size Exclusion Chromatography (SEC)

Size exclusion chromatography analysis of linear and star polymers were performed in filtered THF

(flow rate 1mL/min) at 25°C, 4 mg of polymer were diluted in 1 mL of THF then filtered before

injection in the column. Multiangle light-scattering (figure 6.1a) and refractive index (figure 6.1b)

signals were detected as shown in the following example (Star Polymer 7) for PtBMA-Br (2) , DB5

and the resulting star polymer STP7.

Figure 6.1 – Refractive index detector size exclusion chromatogram for PtBMA macroinitiator DB5 copolymer and star polymer STP7

Figure 6.2 – Light scattering detector size exclusion chromatogram for PtBMA macroinitiator DB5 copolymer and star polymer

STP7

5 7 9 11 13 15 17 19

Elution Time (min)

PtBMA-Br

DB5

STP7

5 10 15 20

Elution Time (min)

PtBMA-Br

DB5

STP7

Mn=7730 g/mol

PDI=1,03

Mn=9120 g/mol

PDI=1,45

Mn=68940 g/mol

101

Chromatogram in figure 6.1 shows the increasing molecular weight from right to left, going from

the external PtBMA-Br homopolymer to the final star polymer (STP7). The refractive index signal

in, associated to the green curve, shows a bimodal distribution, with a smaller peak corresponding

to the presence of the diblock copolymer, this effect can be explained by the fact that there is still

a fraction of a linear polymer that does not participate to the crosslinking reaction. In figure 6.2

(light scattering detector) it is possible to notice a remarkable growth of molecular weight, shown

by the green curve (star polymer) as result of the cross-linking of DB7 diblock copolymer.

In the example below, the evolution of molecular weight as a function of elution volume for

polymers PtBMA-Br, DB3 and star polymer 4 (STP4)

Figure 6.3 – Molecular weight evolution for STP4 sample

Similar results to the examples shown in this section, were obtained for all the other star polymers

synthesized in this work.

102

6.2 Proton Nuclear Magnetic Resonance (1H NMR)

1H NMR were recorded on a Bruker ARX 250 spectrometer and a Bruker Avance 300. Analysis

were carried out in deuterated chloroform (CDCl3) for all the polymers, except for hydrolyzed

ones, which were performed in deuterated dimethyl sulfoxide-d6 (DMSO). Below some examples

of spectrum of particularly relevance (linear diblock DB3 and star polymer STP7) are presented

and discussed.

DB3

Figure 6.4 - 1H NMR for the sample DB3 carried out in CDCl3

In figure 6.4, 1H NMR for the sample DB3 is shown, signals a and b are associated respectively with

methyl protons and CH2 protons in chain at 0.9 and 1.1 ppm. Tert butyl peak appears at 1.45 ppm

and protons near the aromatic ri ng in the styrene block can be seen at 1.85 ppm (signals d).

Aromatic protons are associated at the two broad peaks between 6.5 and 7.5 ppm.

The presence of aromatic protons, corresponding to two very broad peaks (orto and para protons)

at relatively high chemical shifts, demonstrates the successful “switch” on the second polystyrene

block during the diblock copolymerization step.

H

H

COOtBu

CH3

H

H

Ph

H

Br

mna

b a

b

c c

e

e

d

d

103

STP7

Figure 6.5 - 1H NMR for the sample STP7 carried out in CDCl3

In figure 6.5, 1H NMR for the star polymer STP7 is shown, signals a and b are associated (as already

seen for DB7) respectively with methyl protons and CH2 protons in chain at 0.9 and 1.1 ppm.

Protons belonging to the tert butyl group appear at 1.45 ppm (signal c) and those near the

aromatic ring in the styrene block can be found at 1.85 ppm (signals d). Aromatic protons are

associated at the two broad peaks between 6.5 and 7.5 ppm. Note, that in this case we find

aromatic protons also in the crosslinked core (crosslinked divinyl benzene). Signal s f, are

associated to unreacted vinyl groups in the core.

H

H

COOtBu

CH3

H

H

Ph

H

mn

PDVB

a

a b

b

c c

d

d

e

e

e

f

f

104

6.3 Atomic Force Microscopy (AFM)

Atomic force microscopy analysis were performed on the whole range of star polymers

synthesized. Polymers were diluted in acetone solutions (3 mg of polymer in 2 mL of acetone),

polymer solutions were casted by spin coating on silicon substrates, with initial acceleration a = 70

rpm/s2 and speed 1500 rpm. Silicon substrates were previously cleaned in a distilled water

ultrasound bath 1,2. Here below an example of AFM analysis performed in tapping mode, of star

polymer 1 (STP1).

Figure 6.6 – Topography (a) and demodulation (b) images (scanning area 20x20 µm)

Figure 6.7 – Topography (a) and demodulation (b) images (scanning area 10x10 µm)

a

a b

b

105

In the previous page figure 6.6 and 6.7 show respectively 20x20 µm and 10x10 µm scan size for

the sample STP1 using tapping mode. Figures 6.6a 6.7a are related to the topography of star

polymer films deposited. It is possible to recognize ellipsoidal objects on the silicon substrate

corresponding to the core shell polymers. Demodulation images (figure 6.6b and 6.7b) confirm

the presence of the polymers forming a coating on the substrate.

The objects detected on the surface, appear to reach an order of magnitude of a few hundred

nanometers in width, and a few tens of nanometers in height, above the substrate. It is reasonable

to assume that the structures that are spin coated on silicon have assumed a flattener shape,

resulting an increase in width and a decrease in height.

Figure 6.8 – Three dimensional topography image for sample STP1 (scanning area 10x10 µm)

Figure 6.7 shows a three-dimensional topography image for sample STP1, performed in tapping

mode on a scanning area of 10x10 µm.

106

A further enlargement was made, on scanning area of 1x1 µm as shown in Figure 6.9a

(topography) 6.9b (demodulation).

Figure 6.9 – Topography (a) and demodulation (b) image for sample STP1 (scanning area 1,35x1,35 µm)

Figure 6.9 – 3D image of topography star polymer 1 (STP1) on a scanning area of 1.35x1.35 µm

a b

107

References

1) Njikang G.N.; Ouchi M.; Cao L.; Gauthier M.; Macromol.Chem.Phys. 2008,19.

2) Iijima M.; Yoshimura M.;Tsuchiya T. Langmuir 2008, 24.

108

Chapter 7

7. CONCLUSION

A new approach for the realization of star polymers with hydrophilic properties was studied and

developed in this work. It is clear from experimental evidence that the first approach, which uses

monomer 4-vinylpyridine as the initiator, appears not to be the best way to obtain star polymers

following the procedure used in this work. Although its suitable properties for use as

macroinitiator, the reaction with polystyrene in an attempt to add a second block does not lead to

the desired result. This is mainly due to the difficulties related to its synthesis, the strong affinity of

the catalyst to the nitrogen atoms present on the pyridine ring, and the tendency of the catalyst to

react with halogenated endings, which inhibits the addition of the block polymer. Nevertheless it

is likely that a realizable method appropriate to these polymers can be found in a more detailed

study focusing on the technical specifications regarding the choice of ligand and solvent used. It

would also be interesting to investigate the behavior of 4-vinyl pyridine in a RAFT experiments

followed by quaternization, as a means for using the monomer to form a water-soluble star block

polymer as was envisaged at the start of the project. Unfortunately, such a study was not possible

in the duration of the 10 month project.

The use of the acrylic monomers tert-butyl methacrylate and tert-butyl acrylate has proved much

more successful and the second block was synthesized without undue difficulty, allowing the

objectives of the project to be fulfilled. The cross linking step posed no further problems, though

partially cross linked product is obtained as a function of amount of double blocks MI used in the

synthesis. Troublesomely, there is difficulty separating the two phases. Maybe a more

homogeneous structure could be obtained after an accurate study on how ATRP parameters play

their role on the entire process, for instance , the addition of a larger quantity of cross linking

agent (DVB) or catalyst (CuBr, CuCl), could reasonably minimize the amount of wasted

macroinitiator. Moreover, it is likely that the synthesis of a block copolymer having the second

109

block of polystyrene, shorter than those obtained and showed, would certainly facilitate the

achievement of a greater number of linear chains on the cross linked DVB core. This would also

improve hydrolyzation of PtBMA and PtBA. Another structural obstacle is the difficulty in

controlling the number of “arms” of each star block polymer synthesized using the arm first

approach.

Concerning the solubilization of polymers obtained, it seems to be clear that poly acrylic acid

(PAA) is more suitable than poly methacrylic acid (PMAA) for obtaining clear colorless water

solutions, and assure a complete solubilization, possibly because micellae formed by polymer

containing PAA are smaller than those formed by polymers containing PMAA. White cloudy

solutions were obtained in solubilization experiments with PMAA, as the larger aggregates lead to

the dispersion of visible light. Future study could usefully be conducted into the proportions of

acrylic and polystyrene chains in the external linear block. Such work could lead to greater

understanding of parameters governing the solubility of similar structures in water, in relation to

the different contributions made by hydrophobic and hydrophilic blocks.

110

Appendix

Nuclear Magnetic Resonance Spectrometer

1H NMR spectra were recorded on a Bruker ARX 250 spectrometer and a Bruker Avance 300.

Analysis were carried out in deuterated chloroform (CDCl3) for all the polymers, except for

hydrolyzed ones, which were performed in deuterated dimethyl sulfoxide-d6 (DMSO).

Figure I - Bruker Avance 300

Size Exclusion Chromatograph

Size exclusion chromatography (SEC) of PtBMA, PtBA, blockcopolymers and star polymers

was carried out in filtered THF (flow rate: 1 mLmin-1)at 25°C on a 3007.5 mm PL gel 5 mm

mixed-D column (Polymer Laboratories),equipped with multiangle light-scattering

(miniDawn Tristar, Wyatt Technology Corporation) and refractive-index (RI2000, Sopares)

detectors. Size exclusion chromatography (SEC) analysis of poly 4 vinyl pyridine samples,

were performed in DMF (flow rate: 1 mLmin-1 ) with a Waters column pack (3007.5 mm,

Ultrastyragel 104, 103, 100 Å),equipped with multiangle light scattering (miniDawn Tristar,

Wyatt Technology Corp.) and refractive index (Waters 410) detectors.

Figure II - SEC Apparatus

111

Atomic Force Microscope

Atomic force microscopy scans were performed in tapping mode (0.5 Hz) with a NSCRIPTON

DPN Writer (Nanoink, USA), with variable scanning areas of 20x20 µm, 10x10 µm and 0.6x0.6

µm. Topographic images were digitalized by using Nano-R AFM software supplied by Pacific

Nanotechnology Advancing Nanotechnology (USA). Cantilever tips were supplied by Pacific

Nanotechnology Advancing Nanotechnology (USA).

Figure III - Nano Ink NSCRIPTON DPN Writer

Gas Chromatograph

Conversion measures were carried out on a Hewlett Packard Gas Chromatograph 5890 Series

II. Detector FID 300°C, column Beta DEX™ 225 capillary, column size 30m x 0.25mm x 0.25 µm

film thickness.

Figure IV - HP Gas Chromatograph 5890 Series II

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