UNIVERSITE DE MONTPELLIER IIUNIVERSITY OF KENT AT CANTERBURY

CO-TUTELLE DE THESE

pour obtenir le grade de / submitted for the grade of

DOCTEUR DE L’UNIVERSITE DE MONTPELLIER II et DOCTOR OF PHILOSOPHY (PhD) OF THE UNIVERSITY OF KENT AT CANTERBURY

Discipline: Chimie Macromoléculaire / Polymer Chemistry

présentée et soutenue / presented and defendedpar / by

ROGER CLIVE HIORNSle / on the 16th of March 1999

Synthèse et Caracterisation de Nouveaux Copolymèresà Bloc à Base de Poly(méthylphénylsilylene)

Synthesis and Characterisation of Novel Block Copolymersof Poly(methylphenylsilylene)

Directeurs de Thèse / Directors of Research:Professor R. G. Jones

Professor F. Schué

JURY

Professor R. Corriu Président du JuryProfessor A. F. Johnson RapporteurDr E. Franta RapporteurDr M. J. Went ExaminateurProfessor F. Schué Directeur de ThèseProfessor R. G. Jones Directeur de Thèse

N˚ attribué par la bibliothèque

i

Abstract

Polysilylenes are an interesting class of materials. However, they often display

poor mechanical properties. The aim of this work was to synthesise and characterise block

copolymers of a representative polysilylene, poly(methylphenylsilylene) (PMPS), to

improve and diversify its properties. Reactions of oligo(α-methylstyryl) disodium and

dichloromethylphenylsilane were used to model the copolymer forming reactions.

Reactions of α,ω-dichloro-poly(methylphenylsilylene) with di-anionic and hydroxy

terminated polymers were performed. The structures of copolymers were characterised by

standard techniques. Aqueous solution aggregates of the novel polymer,

poly[poly(methylphenylsilylene)-block-poly(oxyethylene)] ((PMPS-PEO)n), were

characterised.

The Wurtz synthesis of poly(phenethylmethylsilylene) was studied, this being an

alternative to poly(methylphenylsilylene) as a polysilylene with a phenyl substituent.

The syntheses of multi-block copolymers of PMPS and polyisoprene or

poly(oxyethylene) or poly(α-methylstyrene) were optimised. Schlenk techniques were

used to prepare polyisoprenyl disodium and to manipulate polymer-polymer reactions.

Competing reactions in syntheses of block copolymers of PMPS and of ‘living’ polymers

are discussed. Poly[poly(methylphenylsilylene)-block-polyisoprene] was formed as

flexible, optically clear thin films (indicating a formation of well defined domains). In

good agreement with the Flory ‘normal’ distribution, (PMPS-PEO)n was shown to have a

number average degree of polymerisation of 2.5. Aqueous solution aggregates of this

polymer were shown to be vesicles.

It is believed that for the first time multi-block and aqueous soluble copolymers of a

poly(silylene) have been synthesised, all of which show novel properties.

ii

Acknowledgements

I would like to thank Professors R. G. Jones and F. Schué for their support

throughout the preparation of this thesis. I would also like to thank Dr S. J. Holder, Dr N.

A. J. M. Sommerdijk, especially for their characterisation of the aqueous phase behaviour

of poly[poly(methylphenylsilylene)-block-poly(oxyethylene)], Dr M. Went, Dr A. Mas

and Dr A. Collet for suggestions and help. I express my most sincere gratitude to

Professor Hoang-The-Giam for his patience and support. To the members of the groups I

have worked with, in particular, Gerard Cordina, Tony Locke, Julian Murphy, William

Wong, Graham Gray, James Parker, Andy Wiseman, Scott Hunter-Saphir, Stephane

Carlotti, Haussain Quariouh, Abdul Rahal, and Laurance Lutsen I express my thanks. I

express my gratitude to Dr. D. Smith of the NMR service of the Univeristy of Kent, and

members of the NMR Service at the Université de Montpellier II.

I am very grateful to Sophie, Mum and Tony for their support and encouragment.

Space does not allow mentioning the many other people to thank who have welcomed me

into France, and made my return visits to England enjoyable.

iii

To

Sophie, Mum and Tony.

iv

In memory of

Peter Hiorns

v

ContentsSection Title Page

Abstract iAcknowledgements iiDedications iiiContents vAbreviations xi

1 Introduction and Review 1Definitions of Polymerisations 1

1.1 Polysilylenes 21.1.1 A History of Polysilylenes 31.1.2 Synthesis of Polysilylenes 51.1.2.1 The Wurtz Polycondensation Reaction 51.1.2.1.1 Polymer Precursors 71.1.2.1.2 Reducing Agents 71.1.2.1.3 Effect of Solvents 71.1.2.1.4 Effect of Temperature 81.1.2.1.5 Crown Ethers, Cryptands and other Additives 91.1.2.1.6 Homogeneous Reagents 91.1.2.1.7 The Graphite Intercalation Compound C8K 101.1.2.1.8 Use of Ultrasound 111.1.2.1.9 Terminating Agents 121.1.2.1.10 The Mechanism of a Wurtz-type Synthesis of Polysilylenes 131.1.2.2 Catalytised Dehydropolymerisation of Silanes 231.1.2.3 Ring-Opening Polymerisation 241.1.2.4 Anionic Polymerisation of the ‘Masked Disilene’ 261.1.3 The Chemistry of Poly(methylphenylsilylene) 271.1.3.1 Functionalisation of Poly(methylphenylsilylene) 271.1.3.2 Chloromethylation of Polysilylenes and the Subsequent Chemistry 281.1.3.3 A Substitution of Phenyl Moieties with Chlorine 301.1.3.4 Block and Graft Copolymers of Polysilylenes 301.1.3.4.1 Block Copolymers by Photoinitiation 311.1.3.4.2 Ultrasound and Block Copolymers 321.1.3.4.3 Block Copolymers of Poly(methylphenylsilylene) and Amines 321.1.3.4.4 The Nucleophilic Chemistry of the Silicon-Chloride Bond 321.1.3.4.5 Preparation of α,ω-dichloropoly(methylphenylsilylene) 33

1.1.3.4.6 Block Copolymers from Dichloropoly(methylphenylsilylene) 33and Polystyryl Lithium

1.1.3.4.7 Block Copolymers from Dichloromethylphenylsilylene 35Poly(methylmethacrylyl) Lithium

vi

1.1.3.4.8 Block Copolymers from Dibromopoly(methylphenylsilylene) 35and Polystyryl lithium or Polyisoprenyl Lithium

1.1.3.4.9 Block Copolymers from Polystyryl lithium or Polyisoprenyl 36Lithium by Anionic Polymerisation of Cyclotetrasilane

1.1.3.4.10 A Reaction of Poly(methylphenylsilylene) and 37Polystyrene by Atom Radical Transfer Polymerisation

1.1.3.4.11 Graft Copolymers of Polysilylenes 381.1.4 Properties of Polysilylenes 391.1.4.1 Photophysical Properties of Polysilylenes 401.1.4.2 Ionochromism of Polysilylenes in Solution 411.1.4.3 Langmuir-Blodgett films and Polysilylenes 411.1.4.4 Electrical and Photoconductivity of Polyslylenes 411.1.4.5 The Properties of Blends of Polysilylenes and other Polymers 421.1.4.6 AB and ABA Block Copolymers of Polysilylenes and 42

their Morphology and Properties1.1.5 The Applications of Polysilylenes 431.2 Anionic Polymerisation 441.2.1 Anionic Polymerisation - a Short History 451.2.2 Initiators of Anionic Polymerisations 461.2.3 Propagation of Anionic Polymerisations 481.2.3.1 Polymers of Diene Monomers and their Geometry 491.2.3.2 Additives and 'Living' Polyisoprenyl Lithium 511.2.3.3 Anionic Polymerisations and Molecular Weight Distributions 511.2.3.4 A Propagation Reaction and Chain Termination 521.3 Block Copolymers 521.3.1 Block Copolymers through Condensation Reactions 531.3.1.1 Block Copolymers from Transformed Reaction Centres 541.3.2 Block Copolymer Morphology 551.3.3 Block Copolymers in Solution 581.3.4 Structure and Property Relationships of Block Copolymers 591.3.4.1 ‘Thermoplastic Elastomers’ 601.3.4.2 Thermal Properties of Block Copolymers 611.3.5 Applications of Block Copolymers 611.3.6 Characterising Block Copolymers 62

References 632 Aims and Objectives of the Present Work 693 Preparation of Dichloro-β-phenethylmethylsilane and its Polymerisation 70

Abstract 703.1 Introduction 713.2 Preparation and Characterisation of Dichloro-β-phenethylmethylsilane 72

3.3 The Wurtz Polycondensation of Dichloro-β-phenethylmethylsilane 76

vii

3.3.1 Effect of Solvent 763.3.2 Effect of Reaction Time 783.3.3 Effect of Rate of Addition of Reagents 783.3.4 Effect of Addition of 15-crown-5 793.3.5 The Effect of Washing Poly(β-phenethylmethylsilylene) with Hexane 80

3.4 Discussion 813.4.1 Solvent Effects 813.4.2 Rates of Polymerisation 813.4.3 Effects of Additives 823.4.4 Washing and Fractionation of Poly(β-phenethylmethylsilylene) 82

3.5 Conclusion 823.6 Suggestions for Further Work 82

References 834 Anionic Polymeristions of Isoprene Using High 84

Vacuum or Schlenk TechniquesAbstract 84

4.1 Anionic Polymerisations of Isoprene Using High Vacuum Techniques 854.1.1 Introduction 854.1.2 Methodology and Results 884.1.3 Discussion and Conclusion 904.2 Preparation of Oligo(α-methylstyryl) Disodium 90

Using High Vacuum or Schlenk Techniques4.2.1 Introduction 904.2.2 Methodology and Results 914.2.2.1 Synthesising Oligo(α-methylstyryl) Disodium Using 91

High Vacuum Techniques4.2.2.2 Synthesising Oligo(α-methylstyryl) Disodium Using 92

Schlenk Techniques4.2.3 Discussion and Conclusion 934.3 Anionic Polymerisations of Isoprene Using High Vacuum 94

or Schlenk Techniques4.3.1 Introduction 944.3.2 Anionic Polymerisation of Isoprene Using High Vacuum Techniques 944.3.3 Anionic Polymerisations of Isoprene Using Schlenk Techniques 954.3.4 Discussion and Conclusion 98

References 995 The Synthesis and Characterisation of Block and Multi-block 100

Copolymers of Poly(methylphenylsilylene)Abstract 100

5.1 Introduction 101

viii

5.2 The Reaction of Dichloromethylphenylsilane and 101Oligo(α-methylstyryl) Disodium

5.2.1 Introduction 1015.2.2 Methodology and Results 1025.2.3 Discussion 1055.2.4 Conclusion 1055.3 Reactions of Dihalo-poly(methylphenylsilyene) 106

with Di-anionic Polymers5.3.1 Introduction 1065.3.2 Methodology and Results 1065.3.2.1 Synthesis and Charaterisation of 106

α,ω-dichloropoly(methylphenylsilylene)

5.3.2.2 Synthesis of 107Poly[oligo(α-methylstyrene)-block-poly(methylphenylsilylene)]

5.3.2.3 Degradation of Methoxy Terminated 111Poly(methylphenylsilylene) by Oligo(α-methylstyryl) Disodium

5.3.2.4 Reactions of α,ω-dihalopoly(methylphenylsilylene) with 112

Polyisoprenyl Disodium5.3.2.5 Reaction of α,ω-dichloropoly(methylphenylsilylene) 116

and Poly(α-methylstyryl) Disodium

5.3.2.6 Synthesis and Characterisation of 119α,ω-dihalopoly(methylphenylsilylene)

5.3.2.7 Synthesis and Characterisation of 120Poly[poly(methylphenylsilylene)-block-polyisoprene]

5.3.2.8 Thin Self Standing Films of 125Poly[poly(methylphenylsilylene)-block-polyisoprene]

5.3.3 Discussion 1255.3.3.1 Synthesis and Charaterisation of 125

α,ω-dichloropoly(methylphenylsilylene)

5.3.3.2 Synthesis of 127Poly[oligo(α-methylstyrene)-block-poly(methylphenylsilylene)]

5.3.3.3 Degradation of Methoxy Terminated 127Poly(methylphenylsilylene) by Oligo(α-methylstyryl) Disodium

5.3.3.4 Timed Reactions of Dihalopoly(methylphenylsilylene) and 128Polyisoprenyl Disodium

5.3.3.5 Reaction of Dichloropoly(methylphenylsilylene) 128and Poly(α-methylstyryl) Disodium

5.3.3.6 Synthesis and Characterisation of 129α,ω-dihalopoly(methylphenylsilylene)

ix

5.3.3.7 Synthesis and Characterisation of 129Poly[poly(methylphenylsilylene)-block-polyisoprene]

5.3.3.8 Formation of Thin Self Standing 130Films of Poly[poly(methylphenylsilylene)-block-polyisoprene]

5.4 Synthesis and Characterisation of Block Copolymers 130of α,ω-dihalopoly(methylphenylsilylene) and poly(oxyethylene) glycol

5.4.1 Properties, Microstructure and Applications of Poly(oxyethylene) 1305.4.2 Methodology and Results 1315.4.2.1 Reaction of α,ω-dihalopoly(methylphenylsilylene) 132

and Poly(oxyethylene) Glycol5.4.2.2 UV Absorption Characterisation of 137

Poly[poly(oxyethylene)-block-poly(methylphenylsilylene)]5.4.2.3 Synthesis and Characterisation of 138

Poly poly(methylphenylsilylene)-block-poly(oxyethylene)5.4.3 Discussion 1395.4.3.1 Synthesis of 139

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)5.4.3.2 A Comparison of the Syntheses of (PMPS-PI)n and (PMPS-PEO)n 1405.4.3.3 The UV Absorption Spectrum of 140

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene) in Solution5.4.3.4 Synthesis and Characterisation of 141

Poly(methylphenylsilylene)-block-poly(oxyethylene)5.4.4 Conclusion 1415.5 Suggestions For Further Work 142

References 1436 Self Assembly of 144

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)]in Aqueous DispersionAbstract 144

6.1 Introduction 1456.1.1 Block Copolymers and Vesicle Formation in Water 1456.1.2 Encapsulaton of Fluorescent Dye in Vesicles 1466.1.3 Measuring the Vesicle Wall Size 1476.1.4 Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)] 148

used for Characterisation6.2 Methodology and Results 1486.2.1 Dispersion of 148

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)] in Water6.2.2 Vesicle Observation by Electron Microscopy 1486.2.3 Vesicle Encapsulaton of a Fluorescent Dye 1506.2.4 Investigation of the Orientation of (PMPS-PEO)n in Vesicle Walls 150

x

6.3 Discussion 1526.3.1 Orientation of (PMPS-PEO)n in the Vesicle walls 1526.4 Conclusion 1536.5 Suggestions For Futher Work 153References 154Résumé 155Appendix 1 Chemicals Used 158Appendix 2 Experimental Methods 160Appendix 3 29Si NMR Spectroscopy and Polysilylenes 172Appendix 4 Apparatus Used 174

xi

Abreviations

AIBN α,α´-azobisisobutyronitrile

ATRP atom transfer radical polymerisationBuLi butyl lithiumcmc critical micelle concentrationCr(acac)3 chromium (III) acetylacetonateDCPMPS α,ω-dichloropoly(methylphenylsilylene)

DCMPS dichloromethylphenylsilaneDCPEMS dichloro-β-phenethylmethylsilane

DXPMPS α,ω-dihalopoly(methylphenylsilylene)

DP number average degree of polymerisationDSC differential scanning calorimetryESR electron spectroscopy resonanceGC gas chromatographyGPC gel permeation chromatography (also known as size exclusion

chromatography or SEC)IR infra redMMA methyl methacrylateMn number average molecular weightMp peak molecular weightMw weight average molecular weightn-BuLi n-butyl lithiumNMR nuclear magnetic resonanceNOE nuclear overhauser effectOαMS oligo(α-methylstyryl) disodium

OαMS-PMPS Poly[oligo(α-methylstyrene)-block-

poly(methylphenylsilylene)]P.D polydispersity (Mw/Mn)PαMS poly(α-methylstyrene)

PEG poly(ethyleneglycol)PEO poly(oxyethylene)PI polyisoprenePαMSN poly(α-methylstyryl) disodium

PIN polyisoprenyl disodiumPIL polyisoprenyl lithiumPPEMS poly(β-phenethylmethylsilylene)

PMMA poly(methylmethacrylate)PMPS-PMMA poly(methylmethacrylate)-block-poly(methylphenylsilylene)

xii

PMMA-PMPS-PMMA poly(methylmethacrylate)-block-poly(methylphenylsilylene)-block-poly(methylmethacrylate)

PMMAL poly(methylmethacrylyl) lithiumPMPS poly(methylphenylsilylene)(PMPS-PEO)n poly[poly(methylphenylsilylene)-block-poly(oxyethylene)](PMPS-PI)n poly[poly(methylphenylsilylene)-block-polyisoprene]PMPS-PS poly(methylphenylsilylene)-block-polystyrenePP polypropyleneppm parts per millionPSS polysilastyrenePS polystyrenePSL polystyryl lithiumPTFE poly(tetrafluoroethylene)RI refractive indexROP ring opening polymerisationRT room temperatures-BuLi sec-butyl lithiumSET single electron transferSEM scanning electron microscopyt-BuLi tert-butyl lithiumTEM transmission electron microscopyTMS tetramethylsilaneT 1 spin-lattice relaxation timeT 2 spin-spin relaxation timeT g glass transition temperatureT m melting pointTHF tetrahydrofuranUV ultra violet

1

Not a whit, Touchstone: those that are good manners at the court are as ridiculous in the countryas the behaviour of the country is most mockable at the court. You told me you salute not at thecourt, but you kiss your hands: that courtesy would be uncleanly, if courtiers were shepherds.Pas un brin, Pierre : ces manières-là qui sont bonnes à la Cour sont aussi ridicules à la campagneque les façons de la campagne sont risibles à la Cour. Vous m’avez dit qu’on ne salue pas à laCour sans se baiser la main ; cette courtoisie serait malprop, si les courtisans étaient des bergers.Corin, the shepherd, in ‘As You Like It’, translated from the English of W. Shakespeare by J. J. Mayoux.

1 Introduction and Review

Definitions of Polymerisations

Classically, polymerisations are divided into two types. Condensation polymerisationis in which the structural unit lacks some atoms found in the monomer. Additionpolymerisation is in which the structural unit of the polymer is equivalent to the monomer.More commonly, the two groups are redefined as step growth and chain growthpolymerisation. The first includes polycondensation and polyaddition reactions (scheme A).The polymerisation of small molecules with difunctional groups are dependent on the reactionstoichiometry.

ARA + BRB → ARRB + AB i

ARA + BRB → ARABRB i i

Scheme A Representations of step growth polymerisations where A and B are functionalgroups and R is the main chain of a polymer precursor; (i) condensation polymerisation, (ii)addition polymerisation.

Chain addition polymerisations include radical, anionic and cationic polymerisations.All of these polymerisations start with an initiation step and propagate until availablemonomer is depleted or the reaction is in some way terminated (scheme B).

I* + M → M1* i

Mn* + M → Mn+1* i i

Mn* → Pn i i i

Scheme B Representations of chain addition polymerisation where I is an initiator and M is apolymer precursor; (i) initiation, (ii) propagation (iii) termination.

2

1.1 Polysilylenes

Polysilylenes, or polysilanes, consist of linear chains of silicon atoms each usually

with two organic substituents. They are, structurally, the silicon analogues of polyolefins1.

However, because of the relatively large Si-Si σ-bonding orbitals, constructed from the

silicon 3sp3 orbitals, there is geminal as well as vicinal overlap so that the electrons of the

polymer back bone are delocalised. The ionisation potentials of the electrons of the σ-

bonding orbitals of a polysilylene can be lower than those of the electrons of the π-orbitals

of many carbon polymers1. The electronic and physical properties of polysilylenes are

very different from those of the carbon based polymers2.

The affect of the delocalisation is depicted in figure 1.1. With an increasing number

of silicon atoms there is an increase in the number of σ-bonding and σ*-antibonding orbitals

and a decrease in the energy separating the σ HOMO and the σ* LUMO orbitals. With

enough silicon atoms the energy levels are so close that they collapase to a valence band (σ-

manifold) and a conduction band (σ*-manifold)3,4.

Figure 1.1 A schematic representation of the decreasing band gap, as the number of siliconatoms in the chain increases, between the filled and unfilled energy levels of a polysilylene1.

Unlike the carbon atoms of the vinyl polymers, each silicon atom in a non-

symmetrical polymer is pseudoasymmetric5. The polysilylene backbone consists of semi-

3

rigid sequences in all-trans conformations separated by occasional gauche turns. Electron

delocalisation occurs along the trans sequences, and at each gauche turn, σ-antibonding

orbitals are available to the electron1,2,6,7,8,9. However, stereo-regularity of polysilylenes

contrasts with that of carbon chain polymers. Since every silicon atom is a stereogenic

centre, as shown in figure 1.2, the notions of isotacticity and syndiotacticity appear to be

the opposite of those applied to vinyl polymers. The stereoisomers of polysilylenes,

especially those of the triads and pentads of silicon atoms, can be observed by their 29Si

NMR absorptions10.

S,S or R,R

S,R or R,S

AbsoluteConfigurations

Silicon Atoms Polysilylenes

Relative Configurations

Heterotacticr,m (m,r)

Syndiotacticr,r

Isotacticm,m

racemicr

mesom

R'

R

Si

R'

R

Si

R'

R

Si

R'

R

Si

R'

R

Si

R'

R

Si

R'

R

SiR

R'

Si

R'

R'R

R

SiSi

R'

R

Si

R

R'

Si

R'

R

Si

Figure 1.2. The relative atomic configurations of m,m, r,r and r,m (m,r) define the polymerstereoisomers that are isotactic, syndiotatic and heterotactic respectively1,2. R and R’ are thesubstituents to silicon. The pattern can be extended to pentads and so on.

1.1.1 A History of Polysilylenes

Rochow dismissed polysilylenes in 1951 as being too unstable for use as polymeric

materials11, however, the requirements of modern technology has provided a place for

them. The development of polysilylenes has taken place over three quarters of a century.

In the 1920s, Kipping discovered that a highly crystalline, intractable and uncharacterisable

4

polymer could be formed from the Wurtz polycondensation of diphenyldichlorosilane with

sodium12,13 (scheme 1.1). In 1949 Burkhard described a Wurtz polycondensation in a steel

autoclave leading to poly(permethylsilylene), and again found the polymer crystalline and

intractable14.

+ (2n - 2)Na + (2n - 2)NaCln ClSi

Ph

ClCl

Ph

Cln

Si

Ph

Ph

Scheme 1.1 The synthesis of a polysilylene by the Wurtz reductive-coupling of dihalosilanes.

Discoveries of the unusually strong UV absorptions of oligosilanes15,16, a red-shift

in these absorptions as the number of silicon atoms increased17, and a preparation of

anionic silyl radicals1 (scheme 1.2), prompted further reseach into the area. Once a

practical use for polysilylenes was found by Yajima in the synthesis of ß-SiC by a

pyrolysis of poly(permethylsilylene)18 (scheme 1.3), continuous research into

polysilylenes was inevitable.

(Me2Si)5e-, THF

-60°C(Me2Si)5

Colourless Blue

Scheme 1.2. The formation of a strongly coloured silyl radical anion1.

n

H

Me

Si CH2450°C

ArSi

Me

Men

Δ, 350°C Air(1)Δ, 1300°C N2(2)

ß - SiC

Scheme 1.3 Preparation of ß-SiCl in a three step process19 .

Wesson and Williams synthesised a pure and slightly soluble

poly(permethylsilylene)20. It could therefore be characterised to some extent. They then

synthesised the soluble copolymer of dimethylsilylene with n-propylsilane through the

coupling of lithium and chlorine ended oligosilanes21 (scheme 1.4). These materials were

found to form poor and good films respectively. Polysilastyrene, disovered by West by

5

accident at the end of the 1970s, was found both to be soluble in common organic solvents

and to be a thermoplastic material22. The synthesis gave polymers of molecular weights up

to 10000 (scheme 1.5).

2x LiCl+THF

x Cl Cl +Si

R2

R1

nLiLix

Ph

Ph

Si5

Clx5

Ph

Ph

Si

R1

Si

R2n

Scheme 1.4 The synthesis of a polysilylene by the coupling of an α,ω-dichlorodialkylsilane

oligomer with 1,5-dilithio-decaphenylpentasilane21 .

Si

Ph

Me

ClCl+Na, Toluene

110ºCSi

Ph

Me

Me

Si

Me

Me

Si

Me

ClClmn

Scheme 1.5 The synthesis of polysilastyrene22 .

When Trujillo synthesised poly(methylphenylsilylene)23, PMPS, a relatively cheap

material soluble in common organic solvents, the chemistry of polysilylenes was truly

opened. Since then, the number of publications from the area has increased exponentially

with time1.

1.1.2 Synthesis of Polysilylenes

The methods used for synthesising polysilylenes include the Wurtz

polycondensation, catalysed dehydropolymerisation of silanes, ring-opening

polymerisation and an anionic polymerisation of a ‘masked’ disilene. Each of these

methods are described in the following section.

1.1.2.1 The Wurtz Polycondensation Reaction

The heterogeneous Wurtz polycondensation of dichlorosilanes with sodium is the

most practicable method for preparing polysilylenes (scheme 1.1). The materials required

6

are readily available, and the chemistry requires no difficult procedures. Usually, the

dichlorosilane is added to finely dispersed sodium in a well-stirred solvent. After an

induction phase of between 5 and 30 minutes, the reaction goes exothermic. The mixture

warms and changes colour to a deep blue-purple24. The colour arises from the formation of

colloidal sodium and polymer suspended in a gel of the heterogeneous mixture24. It is

stable in air, but can be quenched with alcohols or cautiously with water under an inert

atmosphere.

The Wurtz reaction can be used to prepare a wide range of homo and

copolysilylenes21,25,26,27, and some are shown in figure 1.3. With a variation of

substituents, their physical properties vary from being waxy and soft through to hard and

brittle.

m nSi

CH3

CH3

Si

X

Y

(1) X = methyl, Y = n-hexyl

(2) X = methyl, Y = cy-hexyl

(3) X, Y = cy-pentamethylene

(4) X = methyl, Y = phenyl

(5) X = methyl, Y = β-phenethyl

(6) X = Y = phenyl

Figure 1.3 Some examples of copolymers that have been synthesised by the Wurtz reaction28 .

The Wurtz polycondensation is not without its problems. Yields of high molecular

weight material are low and the molecular weight distribution of polymers is often

polymodal. The yield of desirable polymer, of molecular weight greater than 5000, is often

around 20% and control over the reaction is limited. Commonly, 80% of the products are

cyclics based on Si4, Si5 and Si66,29,30. The vigourous nature of the reaction, its high

exothermicity, the often molten alkali metal, and the reflux conditions which are commonly

used, limit the range of polysilylenes that can be prepared. Any group that is susceptible

to a reaction with sodium is usually lost. Dichlorosilanes with stable alkyl and aryl groups

can be polymerised, and p-dimethylaminophenyl, alkoxyphenyl and trimethylsiloxyphenyl

moities have survived the reaction intact31,32,33. The reaction allows little stereochemical

control. The Wurtz reaction is considered a ‘last resort’ for scaling up to an industrial

process, as it would be dangerous and expensive.

7

1.1.2.1.1 Polymer Precursors

Dialkylated dichlorosilanes do not polymerise at room temperature with sodium, as

the reduction potential of these monomers is greater than the oxidation potential of the

metal34. An alloy of sodium and potassium, with a higher oxidation potential, has been used

to carry out polymerisations34. Additives such as 15-crown-5 are normally required to

promote the polymerisation35. Polymer precursors with one aryl group polymerise with

sodium at temperatures below 80 °C. The aryl group reduces the negative reduction

potential of the monomer. Copolymers often allow the polymerisation of monomers that

would otherwise be excluded from the reaction, either because of steric hindrance, or

because a product homopolymer cannot be worked-up. The polymerisation of dichloro α-

napthylmethylsilane does not normally occur2, but the steric relief allowed by the addition

of dichlorodimethylsilane makes the reaction possible36.

1.1.2.1.2 Reducing Agents

Sodium is normally used as the reducing agent in the Wurtz synthesis of

polysilanes2. Lithium tends to give rise to cyclo-oligomers37. Potassium degrades high

molecular weight polymer1,35, especially in polar solvents such as THF38. Other metals

have been used with limited success, for example gallium has been used to synthesise

polymers of a medium molecular weight39. The ionisation potential of the metal is more

important than the first electrode potential in determining reactivity. The order of

reactivity is K>Na>Ba~Ho>>Sr~Gd>Sm~La>Ca>>Mg.

1.1.2.1.3 Effect of Solvents

Wurtz syntheses of polysilylenes are most commonly conducted in toluene, THF,

diethylether, xylene and diphenylether8,40. Polymers from reactions in the less polar

solvents usually have a trimodal molecular weight distribution (less than 1500, 4000 to

30000 and 100000 to several million g mol-1 as shown in figure 1.4)2. Reactions mediated

in polar solvents synthesise polymers with bimodal molecular weight distributions (figure

1.5). High molecular weight polysilylene does not degrade when refluxed with sodium in

8

toluene, however, degradation does occur when it is refluxed with sodium in THF38,40.

Figure 1.4 A representative GPC analysis of PMPS synthesised in refluxing toluene.

Figure 1.5 A representative GPC analysis of PMPS, synthesised in refluxing THF.

1.1.2.1.4 Effect of Temperature

Molten sodium in refluxing toluene gives a continuously replenishing surface on

which polycondensations can take place. However, when a solvent refluxes at a

temperature below the melting point of the alkali metal, yields of polymers decreases

greatly2,8.

9

1.1.2.1.5 Crown Ethers, Cryptands and other Additives

Phase transfer catalysts can increase the yields of polymers and the reaction rates

when used in the anionic polymerisations of vinylic and olefinic monomers41,42. 15-crown-

5 and [2.2.2] cryptand co-ordinate strongly with the sodium positive ion43,44 and stabilise

anionic intermediates in polymerisation reactions45. When 15-crown-5 is added to the

toluene mediated Wurtz polycondensation of dichloromethylphenylsilane (DCMPS) or

dichloro(n-hexylmethylsilylene), polymers with more monomodal molecular weight

distributions and reduced peak molecular weights are synthesised at lower temperatures40.

Catalytic amounts of 15-crown-5 added to the reaction in refluxing diethylether increases

both the yield and the molecular weight of a polymer1. In THF, 15-crown-5 has the

opposite effect reducing yields and molecular weights8. Reactions in diphenylether

containing 15-crown-5 yield no product8. For reactions in toluene, diethylether and THF,

the addition of 15-crown-5 reduces the rate of disappearance of monomer during the

induction period, but propagation and rate determining steps are not affected1,8,40. The

degradation of polysilylenes with sodium in refluxing toluene is promoted by 15-crown-5,

however, with [2.2.2] cryptand only cyclic oligomers remain after 5 minutes40. The

addition of [2.2.2] cryptand to a polycondensation with sodium in toluene reduces the

induction period, does not change the overall reaction rate and gives rise to polymer of a

monomodal molecular weight40. The yields of a polycondensation with sodium in toluene

are increased on the addition of dimethoxyethane46 or even heptane47.

1.1.2.1.6 Homogeneous Reagents

The sodium complexes of napthalene, biphenyl and tetraphenylethene have been

used to promote the polycondensation in THF at low temperatures48 (scheme 1.6).

10

Na + A Na+ + A i

+ 2Cl- + 2A+ 2ACl Cl

R1

R2

Si 1/n

R1

R2

Si

i iScheme 1.6 Synthesis of polysilylenes (ii) via a preparation of an anionic radical (i) where Ais the electron acceptor molecule48 .

The strong colour of the acceptor radical anion immediately disappears upon

addition of a dichlorosilane. The reaction is rapid and exothermic and the mixture sets as a

thick pasty agglomerate of polymer and NaCl. Polymer synthesised by this method has a

mono-modal molecular weight distribution around 4000 g mol-1 but the yields are not

reproducible. The polymer is rapidly degraded if there is an excess of the acceptor

complex48,49,50.

The K+/K- 18-crown-6 complex has been used with THF at low temperatures (-79

°C) to mediate the synthesis of PMPS of a medium molecular weight (Mw ≈ 4000 g mol-1)

and a monomodal distribution51 (scheme 1.7).

Si

Me

PhnClClSi

Me

Ph

Cl ClK+/K-/18-crown-6

THF, -78°C

Scheme 1.7 The synthesis of PMPS by use of the K+/K- 18-crown-6 complex51 .

1.1.2.1.7 The Graphite Intercalation Compound C8K

The reaction of dichlorosilanes with potassium in a graphite matrix provides a

means to altering the stereoregularity of polysilylenes10,52,53,54 (scheme 1.8). The effect is

only marginal. PMPS prepared using C8K is 50% isotactic compared to 40% isotactic

when synthesised using, for example, sodium in boiling toluene. However, it requires

considerable dexterity to extract the polymer from the graphite. The polymer from

solution and polymer from washings of the graphite phase reveal different molecular

weights and tacticities10,52.

11

+n Si

Me

Ph

Cl Cl + (2n-2)KCl + graphiteClCln

Si

Me

Ph

THFT < 0°C

(2n-2)C8K

Scheme 1.8 A synthesis of PMPS with C8K10,52,53.

The use of C8K does allow more gentle reaction conditions. The substituted

polymer poly(4,7,10,13-tetraoxatetradecylmethylsilylene), which would normally be

prepared in low yields by a sodium mediated Wurtz reaction, can be synthesised more

easily with C8K54 (scheme 1.9). However, in the temperature range -20 °C to 25 °C

excess C8K degrades polysilylenes to oligomers52.

+ Graphite + 2nKCl2 equivalents of C8K

THF/ 0°CSi

Me

O

3MeO

nn Si ClCl

Me

MeO

O3

Scheme 1.9 A synthesis of the water solublepoly(4,7,10,13-tetraoxatetradecylmethylsilylene) with C8K54.

1.1.2.1.8 Use of Ultrasound

Sonofication generates minute cavitation bubbles in a liquid phase which collapse

giving rise to considerable local forces and temperature gradients. The sodium surface is

continuously exposed to reagents34,55,56,57. A polysilylene with a monomodal molecular

weight distribution ( Mw

Mn

< 1.5) and an Mw of around 100,000 g mol-1 was synthesised albeit

with a high proportion of cyclic oligomers in 25 minutes. However, on continuing the

application of ultrasound34, the molecular weight of polymer decreased to about 50,000 g

mol-1. This limited degradation, also decreasing the polydispersity of the polymer to

around 1.2, has been observed with polystyrenes (PS), and is due to the cleavage of

polymer molecules in mid-chain by large shear forces created by ultrasound58. The

homolytic cleavage of the Si-Si bond can give rise to radicals and these react with toluene, in

hydrogen atom abstraction reactions34.

12

1.1.2.1.9 Terminating Reagents

Terminating agents have three uses. They end-cap active polymer molecules

(scheme 1.10), precipitate polymers and destroy any remaining sodium. The lower

alcohols act in all three ways. Other quenching agents have varying effects on the

molecular weight distribution. Water tends to increase the amount of high molecular weight

polymer. Trichloromethylsilane and t-butyl lithium both enhance the amount of low

molecular weight material at the expense of intermediate molecular weight polymer,

although the latter tends to narrow molecular weight distributions8.

13

Si

CH3

Cl + CH3OH + HCl

CH3

Si OCH3

i

+ HClClSi

CH3

+ H2O

CH3

Si OH

i i

OH

CH3

Si Cl

CH3

Si+ + HCl

CH3

SiO

CH3

Si

i i i

Na+

CH3

Si- + ClSi(CH3)3 Si(CH3)3

CH3

Si + NaCl

iv

+ t-BuLiSi

CH3

Cl

CH3

Si t-Bu + LiCl

vScheme 1.10 Examples of the reactions of poly(methylphenylsilylene) with (i) methanol,(ii and iii) water (including the subsequent formation of siloxy linkages),(iv) trichloromethylsilane, and (v) t-butyl lithium.

1.1.2.10 The Mechanism of a Wurtz-type Synthesis of Polysilylenes

Mechanistic theories on the Wurtz polycondensation of dichlorosilanes are

described here as they have relevance to the chemistry of polysilylenes.

In many respects the Wurtz polymerisation resembles a chain growth, rather than a

step growth reaction. The stoichiometry of condensating small molecules eliminating

14

sodium chloride would indicate the latter1. However, even from low concentrations of

starting materials, the rapid formation of high molecular weight polymer would indicate a

chain mechanism. It should be noted that knowledge of the rates of heterogeneous Wurtz

reactions of dichlorosilanes is scant38,40.

The polymodal molecular weight distributions of polysilanes were originally

explained by considering competing non-interactive mechanisms of growth. Silyl anions,

radicals, radical anions and disilenes have all been proposed as possible intermediates2,34.

The dependency of the reaction on solvent and additives was explained in the

diffusion model by Ziegler27 where the rate of diffusion of monomers to the metal surface

and the monomer concentration at the sodium/solvent interface were considered. In an

‘open’ system, growing polymer molecules extend into the solution phase and the diffusion

of monomers to the metal suface is not restricted. In a ‘closed’ system, growing polymer

molecules collapse onto the metal surface and the concentration of monomer at that surface

is low. In the former case, a high number of initation sites are formed giving rise to many

dimers and oligomers. In the latter senario, there are fewer initiation sites and a greater

chance of monomers adding to reactive chain ends. To sum, the concentration of monomer

at the interface is dependent upon the state of polymer at the metal surface. Thus, the rate

of polymerisation is dependent on whether the solvent is good or poor with respect to the

polymer (figure 1.6).

Figure 1.6 Solvent dependency of the heterogeneous Wurtz synthesis of polysilylenes27 .

The polymer extends outwards from the sodium into a good solvent. The difference

between the solubility parameters (Δδ) of the solvent (δs) and of polymer (δp) approaches

15

zero. The polymer precursor may easily access the sodium surface. In a poor solvent,

polymer is packed densely about the sodium, Δδ approaches 1, and monomers have a

restricted access to the metal surface. The polymer collapses and precipitates in an

extremely poor solvent. There is an optimal value of Δδ for each polymer precursor and

for PMPS synthesised by the method of ‘inverse addition’ this was found to be 0.62.

While the diffusion model explained the solvent dependency of heterogeneous reactions, the

polymodal molecular weight distribution of polymers indicated that there were several

other and as yet unexplained, competing mechanisms.

A chain mechanism involving single electron transfers was proposed to explain (i)

the dependence of the induction period on the size of the surface area of sodium, (ii) the

increase with time of concentrations of high molecular weight polymer and oligomers when

the former is present in concentrations even in the earliest stages of a reaction, (iii) the

likely presence of disilenes, silyl anions and silyl radical anions. Intermediate and unstable

silyl radicals have been shown to exist through the effects of solvents susceptable to

hydrogen abstraction reactions (figure 1.7)29.

SiH + RSi + RH

Figure 1.7 A representation of a hydrogen abstraction reaction at a silyl radical

In 1989, Gauthier and Worsfold proposed a mechanism to explain both the chain

growth character of the reaction40 and the polymodal molecular weight distribution of the

resultant polymers (scheme 1.11) It accounted for the formation of cyclic material38 and

was consistent with the diffusion model. However, the chain carriers were anions, so

effects of radical scavengers remained unexplained.

The copolymer of dichorohexenylmethylsilane and DCMPS has a relatively low

εmax value for a polysilylene. This is considered to be due to the incorporation of Si-C

bonds in the polymer backbone though the attack of Si. radicals on the ethylene bond of the

substituent1. In addition, 1H NMR results indicate that a large proportion of the alkenyl

groups are pendent to the backbone. The alkenyl group, which is slow to react to anions,

therefore acts as a radical trap and is incorporated into the main chain. To explain this

single electron steps were proposed as an extension to the mechanism of Gauthier and

16

Worsfold1,24,59 (scheme 1.12). Initially a silyl anion radical is formed, then through a rapid

elimination of NaCl, a silyl radical is formed. It is possible that hydrogenation may occur1.

The silyl radical is then reduced to give a silyl anion in what is probably a rate determining

step. A termination step could be the combination of the radicals, however this is thought

unlikely as they are in a low concentration and would more probably react with the solvent

or the sodium.

Initiation

+ NaCl+ 2Na -

R1

R2

Cl Si +Na

R1

R2

Cl Si Cl

i

Propagation

+ + NaClNa+Si

R2

R1

-

R1

R2

Cl Si Cl Si

R1

R2

ClSi

R1

R2i i

+ NaCl+ 2NaSi

R1

R2

ClSi

R1

R2

Na+Si

R2

R1

-

R1

R2

Si

i i i

Cyclisation

+ NaClNa+Si

R2

R1

-Si

R2

R1

3

R1

R2

SiClSi Si

SiSi

Si

iv

Back-Biting

Na+Si

R2

R1

-Si

R2

R1

3

R1

R2

Si +

R1

R2

Si -

R1

R2

Si +NaSi Si

SiSi

Si

v

Scheme 1.11 The mechanism of Gauthier and Worsfold. Step i is slow, while step ii is fastbut rate-determining, and step iii is very fast. The products of the cyclisation and back bitingreactions have been identified by 29Si NMR40 and other studies58 .

17

The ubiquitous blue colour of Wurtz reactions was shown to result from colloidal

alkali-metal particles in an agglomerate of polymer and alkali metal halide polymer matrix,

and not from F centres in the sodium metal as had been thought60. The dried colloidal

precipitate was shown to have a porous nature by SEM61. Reactions carried out in

refluxing diethyl ether in the presence of 15-crown-5 have been shown to have induction

periods of 5 minutes and, thereafter, the reactions are first order processes with half lives

of approximately 15 minutes. This is consistent with the process being diffusion

controlled. The molecular weight distributions of polymer of a solution phase and of a

colloidal phase were found to be different. The colloidal phase material was bimodal with

maxima centred at 24000 g mol-1 and at several millions g mol-1. The solution phase

material was bimodal with one peak at around 500 g mol-1 indicating oligomers and the

other at around 4000 g mol-1. This variation can be explained by considering that while the

sodium metal surface decreases, the displaced polymer molecules enter an agitated solution

to the limit of their solubility for the given solvent and temperature. Early on in the

reaction, only low molecular weight material will saturate the solution. The less soluble

higher molecular weight polymer formed later on in the reaction should not displace the

lower molecular weight material from solution even though dissolution is an equilibrium

process. The higher molecular weight material remains in the precipitate associated with

the alkali metal surface with its chain ends extending, until in turn it is displaced from the

shrinking metal surface as a precipitate. It should also be noted that the limit of saturation

of the solvent would not be reached by the ‘free’ polymer alone: the polymer attached to

the metal surface will also contribute to its saturation24. Therefore a modification of the

chain reaction involving propagation reactions which sequentially generate silyl radical

anions, silyl radicals, silyl anions, and -Si-Cl was proposed (scheme 1.12, p.19). The

growing chains constantly switch from nonpolar to polar structures, the lifetimes of which

determined by the polarity of the medium in the vicinity of the solvent-metal interface.

The polar chain ends must prefer to remain associated with the alkali metal/salt

agglomerate, and the polymers effectively remain insoluble. Only when the chain ends are

in a nonpolar structure can polymers favour entering bulk solution.

A polymer chain end that is remote from the active centre must bear a halogen atom.

Accepting the mechanism of scheme 1.12, p.19 and the above reasoning, molecules of a

low molecular weight fraction entering solution in early stages of the reaction are

dihalogenated.

18

They remain potential sites for reactions, but the probability of this occuring is low as their

rate of diffusion through a polymer/NaCl barrier to a sodium/polymer interface is low with

respect to monomers. However, in explaining the bimodiality of molecular weight

distributions it was proposed that these low molecular weight polymers may penetrate the

swollen and expanded porous structure of polymer/salt aggregates to go on to complete

coupling reactions at the alkali metal surface (scheme 1.13, p. 19)24. These reactions lead

to termination of active growth centers, but also very much higher molecular weight

polymer.

Increasing the time of a reaction does not necessarily give rise to polymers with

higher molecular weights24. An electron acceptor can solubilise the alkali metal, both

mediating a polymerisation and transporting it to polymer chains to initiate their

degradation. This can be exemplified by citing the case of a preformed polysilylene which

is completely degraded at ambient temperatures when dissolved in a solution carrying

sodium napthalide. The same system, at lower temperatures, may aid the synthesis of a

polysilylene. This can be observed similarly with the 15-crown-5/sodium system. At

higher temperatures, where the metal is in a molten state, the reaction is aided by thermal

activation. At lower temperatures, a phase transfer catalyst, such as 15-crown-5, may

activate an alkali metal so as to increase the yield of a polysilylene8 (figure 1.8). At higher

temperatures, especially in refluxing toluene, sodium transported as Na+/Na- by the 15-

crown-5, can initiate chain scission on a polymer chain (scheme 1.14)32,34. However,

where there is an excess of dichloronated monomers, any silyl anions in solution would

quickly go through an SN2 reaction with them to give a silyl chloride end-capped polymer

and back biting is arrested10,34. Where there is an excess of an alkali metal with some means

of attacking a polymer chain, degradation of the polysilylene rapidly ensues.

19

Initiation

+ Na

R1

R2

Cl Si Cl , Na+

R1

R2

Cl Si Cl

+ NaCl, Na+

R1

R2

Cl Si Cl

R1

R2

Cl Si

Propagation

+ Na

R1

R2

Si Cl , Na+

R1

R2

Si Cl

+ NaCl, Na+

R1

R2

Si Cl

R1

R2

Si

R1

R2

Si + Na

R1

R2

Si - , Na+

+ NaClSi Cl

R1

R2

Cl+

R1

R2

Si - , Na+ Si Cl

R1

R2

R1

R2

Si

Termination R1

R2

Si +

R1

R2

Si

R1

R2

Si

R1

R2

Si

Scheme 1.12 Mechanism showing evidenced intermediates (excluding silylene insertions)24,34.

+ + NaClNa+Si

R2

R1

-

R1

R2

SiSi

R1

R2

ClSi

R1

R2

Si

R2

R1 R1

R2

Si Si

R1

R2

Si

R1

R2

Scheme 1.13 The reaction of oligo(silylenes) with silyl anions at the sodium surface24 .

20

The more polar solvents, such as THF, will act to support a phase transfer reagent

carrying a metal and, to some extent, the silyl anion. This is indicated when a small amount

of 15-crown-5 is added to the THF system, where even at ambient temperatures the yield

and molecular weight distribution of polymers are reduced24.

Two different models of the structures of polysilylenes have been put forward.

The random disorder model of a polymer chain considers extended σ-orbitals within which

there are no breaks, but a structural randomness. The model does not agree well with

results of studies of polysilylenes62, for example, their observed structure in solution63 and

other theoretical considerations64. The segment model considers the polymer to be

constructed of regular segments, stabilised by extended σ-orbitals separated by units over

which the chain conformation does not permit further hybridisation. From figure 1.14, the

maximum extension of the σ-orbital is indicated to be about 50 atoms. In structural terms

this is interpreted as segments being constructed from series of atoms in low energy trans

conformation separated by gauche turns of higher energy62,65.

and so onO OO

OO = 15-crown-5

Cl O OO

OOR1 R2

Si

R1 R2

Si - Na+

Na

R1 R2

SiCl

R1 R2

ClSi Na- O OO

OONa+

Na

-NaCl

two SET

-NaCltwoSET

R1 R2

ClSiCl

Na

R1 R2

ClSiCl Na- Na+O O

OO

O

NaSETO O

OO

O+

R1 R2

SiCl

R1 R2

ClSi O OO

OO

Na

Na+R1 R2

SiCl - O OO

OO

Na-NaCl

R1 R2

ClSiCl

SET

Figure 1.8 Schematic of the Wurtz reductive-coupling reaction, leading to the preparation ofthe polysilylene, activated by the phase transfer catalyst 15-crown-58,34.

21

For PMPS, the segments have been estimated to be around 35-50 atoms in

length34,48. This corresponds well with the degree of polymerisation of medium molecular

weight fractions commonly synthesised from Wurtz polycondensations. A polymer with a

higher molecular weight is therefore constructed of sequences of trans conformers joined by

gauche turns24. The gauche links are chain defects, and as such constitute weak points in

the polymer chain. For reactions where an alkali metal is able to attack the polymer (either

through sequestation or thermal excitation), transferred electrons may move from the point

of contact, along the chain, to the nearest gauche link. This selective chain scission can

explain the disappearance of high molecular weight polymer with a consequent narrowing

of the molecular weight distribution of the intermediate fraction8,66.

+

SiSi

SiSi

SiSi

M+, M-

2e-

SiSi

SiSi Si

M+

Si SiSi

SiSi

+M+

-SiSi

SiSi

SiSi

Si-2

SiSi

SiSi-M+ Si

R1R2SiCl2

ClSi

SiSi

Si SiSi

ClSiSi

SiSi Si+

Scheme 1.14 Degradation of a polysilylene in which metal ions are sequestered 8.

Subsequently, these ideas were modified to rationalise the on-going synthesis of the

medium molecular weight fraction (c. 5000 g mol-1 for PMPS)9. If all of this fraction

evolved from a degradation of higher molecular weight polymer, then it was argued that

there was no reason why the polymer distribution should not be monomodal. It was

necessary to return to the idea that growing polysilylene chains, at a DP of around 40,

either depart to solution or stick to the sodium surface and grow by reaction with

monomer, or with oligomeric or polymeric molecules with Si-Cl end groups.

22

PMPS is modelled as a pseudo-rigid-rod (worm-like) polymer, and has a low energy

difference of HOMO-LUMO when compared to equivalent carbon based polymers64,66,67.

The maximum conjugation length of PMPS in toluene was indicated by UV absorption

analysis to be around 35 units. It is reasonable to think that a maximum gain in resonance

stabilisation energy occurs at this DP 9. A chain will therefore grow, by additions in trans

conformation, gaining stabilisation energy, to about 35 units. Beyond this point the trans

conformers added to a chain offer no energetic gain, and the addition of a gauche conformer

may occur. Once formed, the defect is held in its position on the chain. If it were to move

by conformational reorganisation, there would be a reduction in the stability of the chain.

As the defect is held close to a metal surface, the probabiliy of backbiting occuring is at a

maximum (figure 1.9). Where backbiting does occur, the formation of a cyclic oligomer

removes the reminant chain end of the polymer from the alkali metal surface. If this does

not happen the polymer continues to grow, and as it does so, the position of the defect

changes continuously as the polymer seeks to obtain the maximum overall stability for two

all-trans wormlike sequences. Thus, the defect is dislocated from the metal surface and the

probability of a backbiting reaction occuring is continuously reduced as the polymer

increases in length and increases the number of available positions for the gauche turn

(defect) to take up. Thus, a medium molecular weight region indicates a point at which the

probability of termination occuring is at its greatest.

Termination: 1 - Pi

Growth: Pi

1

Si SiSi

SiSi

Na

2

i - 1i - 1

Na

1

2 i

12

Incipient ith gauche turn

Nai - 1

Figure 1.9 Schematic of competition between growth and termination of a polysilylene9.

23

If polymerisations are carried out at a low temperature, then the probability of

backbiting occuring is reduced with a reduction in thermal activity68. This should result in

lower yields but with a more dominant high molecular weight fraction. Therefore, with a

low temperature and a solvent capable of stabilising the anionic intermediates it should be

possible to conduct syntheses which are close to living polymerisations. Among the

criteria for identifying living polymerisations are linear variations of Mn and yield with

time. This occurred when a Wurtz polycondensation of dichlorosilane was conducted in

THF at room temperature9.

1.1.2.2 Catalysed Dehydropolymerisation of Silanes

The dehydrogenative coupling of silanes provides the most promising development

towards a facile and safe synthetic route to polysilylenes. The technique was discovered

by Harrod and coworkers69 and involves, typically, a catalyst of type; Cp2MR2, where

Cp is a cyclopentadienyl ligand and M is a metal from groups 3 or 470 (scheme 1.15).

(n-1) H2catalyst

n

R'

SiH

H

H +n

R'

SiH

H

H

Scheme 1.15 Dehydrogenative synthesis of polysilylenes70 .

The mechanism is not fully understood. The one electron addition-reduction

elimination mechanism (figure 1.10) is indicated by the behaviour of an unstable catalyst

dibutyl zirconocene which is seen to undergo single electron reduction spontaneously70.

This catalyst promotes the dehydrogenative coupling reactions of silanes at room

temperature, but other catalysts which are stable at room temperature require heating to be

used in dehydropolymerisation reactions70. Some stereochemical control is possible and

the Si-H bond provides a centre for facile functionalisation of the polymer. However, the

route provides linear polymers with a molecular weight of around 4000 g mol-1 and a high

percentage of oligomers70,71,72,73. That the product molecular weight is around 3500 g mol-1

could indicate that the persistence length of σ-catenated sequences may be determining a

reaction mechanism.

24

H2

R1R2SiH2

(R1R2SiH)2

R1R2SiH2

ZrH

+ Zr III

R1R2SiH

R1R2SiH

+

R1

R2Zr H

Si

[Cp2Zr(µ-H)(SiR1R2)]2+[B(C6F5)4-nBun]2

-

Figure 1.10 The proposed one electron addition-reduction elimination mechanism of thedeydrogenative coupling of silanes (R1 = Ar or Alkyl, R2 = H or (SiHR1)nH)70 .

1.1.2.3 Ring-Opening Polymerisation

Ring-opening polymerisation (scheme 1.16) is a route to preparing polysilylenes

with a regular micro-structure74,75,76. The principal reason for using this method, in

preference to others, is the steric control offered. However, preparation, separation and

purification of cyclic monomers can be time consuming and costly75. As the reaction is an

anionic polymerisation, a high degree of control is attainable over the molecular weight of

the polymers75. The method has been used in the synthesis of well-defined copolymers

and block copolymers77.

25

Si

Ph

Me

4nMe Ph

Ph MeMe

PhMe

Ph + n-BuLiTHF

-78°Cn

Scheme 1.16 The synthesis of a PMPS by ring opening polymerisation of 1,2,3,4-tetraphenyl-1,2,3,4-tetramethylcyclotetrasilane74 .

(PhSiMe)4 is a preferred monomer due to its high solubility and ease of

polymerisation. However, its preparation and the separation of isomers (scheme 1.17) is

much more complex than that for (Ph2Si)4. The polymer microstructure depends on

configuration within the monomer, the mechanism of polymerisation and the type of

catalyst and conditions used. For monomer (a) of scheme 1.17, 3 different structures of

polymer are possible depending on whether the configuration is inversed or retained during

nucleophilic attack. Assuming this occurs with equal probability on either side of a silicon

atom, the probability of the resulting polymer triad sequences is determined by either

inversion or retention of structure at both the substitution centre and the newly formed

reactive centre (figures 1.2 and 1.11).

(b) (c)

Ph Me Ph Me

(a)

Me Ph

Ph MeMe

PhMe

Ph Me PhMe

PhMe

Ph Me Ph

Me

Ph

Me

Ph

(PhSiOTf)4MeMgBr/ether

C6H6/C6H5CH3

4 TfOHCH2Cl2

(Ph2Si)4

+ +

Scheme 1.17 The synthesis of three isomomers of (PhSiMe)4, (TfOH is triflic acid) where(a) is 55% of the full yield and the others have not been separated74 .

26

[(rrr)r/m]n [(mrm)m/r]n [(mrr)m/r]n

Twiceretention

Twiceinversion

Inversion &retention

Figure 1.11 Possible microstructures resulting from retention and inversion of configurationafter nucleophilc attack at a silicon atom during the ring opening polymerisations of structure(a) of scheme 1.1774 .

The ring opening polymerisation of phenylnonamethylcyclopentasilane has been

accomplished with silyl potassium and a sequestering agent 18-crown-676. The reaction

displays the silyl anion as a pale yellow colouration. This method gives rise to polymers

of a polydispersity of 1.8 and an Mw of 58000 g mol-1.

1.1.2.4 Anionic Polymerisation of ‘Masked Disilene’

The synthesis of polysilylenes by the anionic polymeriation of a so-called ‘masked

disilene’ is elegant (scheme 1.18), especially when compared with the Wurtz reductive-

coupling of dichlorosilanes78. Disilenes only exist as transitory species. A double bond is

imitated by the aromatic group adjacent to the silicon-silicon bond79.

Ph

Si Si

R1

R2

R3

R4(i) RLi

(ii) EtOH+

R2

R1

Si

R3

R4

Si Hn-Bu PhPh

Scheme 1.18 The anionic polymerisation of a 1-phenyl-7,8-dislabicyclo[2.2.2]octa-2,5-diene, in which RLi is n-BuLi, s-BuLi or PhLi, and R1, R2, R3 and R4: are all CH3, are all CH3

except for R1 or R4 which is n-C4H9, or both R1 and R3 are CH3 and R2 and R4 are n-C3H7 orC6H13

78,80.

Disilacyclooctadiene is prepared from the coupling of a dichlorodisilane with an

anionic radical of lithium napthalide78,79 (scheme 1.19). The isomers can only be prepared

by a route that is long and tiresome, and they have not yet been separated80.

27

THF, -78°C

Li+

SiSiCl Cl

R1

R2

R3

R4Ph

Si Si

R1

R2

R3

R4 R4

R3

R2

R1

Ph

Si Si+

Scheme 1.19 The preparation of the polymer precursors of the reaction shown in scheme1.18.

The ‘living’ polymerisation of ‘masked disilene’ in toluene or THF is indicated by

an orange-red colour. The polymerisation proceeds in a strictly head to tail fashion, giving

rise to the synthesis of regular polymers. The product polymer, with a low polydispersity

and an Mn of up to 1.1 x 105 g mol-1, is prepared in 60% yield. The polymerisation has not

been proved to be ‘living’, even though it can initiate the polymerisation of MMA to give

block copolymers of [SiMe(n-Bu)SiMe2]n-block-[CH3CMe(CO2Me)]m. A high percentage

of the initiating centres are lost to secondary reactions, and initiation of polymerisation of

styrene by the ‘living’ polymer does not occur78. The regular use of the route is generally

confounded by the difficult synthesis and near impossible separation of polymer

precursors.

1.1.3 The Chemistry of Poly(methylphenylsilylene)

In this part, the known chemistry of PMPS is described.

1.1.3.1 Functionalisation of Poly(methylphenylsilylene)

Trifluoromethanesulphonic acid, commonly called triflic acid, has been used to

replace the phenyl substituents of PMPS81 (scheme 1.20). A low degree of chain scission

occurs. The reaction provides sites for further reactions with nucleophiles. These centres

act as initiations sites for the cationic polymerisation of THF, and the polymerisation of

MMA through group transfer1.

28

mSi

CH3

n-mSi

CH3

CF3SO3HCH2Cl2n

Si

CH3

OSO2CF3

Scheme 1.20 The reaction of PMPS with triflic acid81 .

1.1.3.2 Chloromethylation of Phenyl Substituted Polysilylenes

The chloro- and bromomethylations of the phenyl moiety of PMPS82 (scheme

1.21) and of poly(phenethylmethylsilylene) (PPEMS)83 provide routes to a wide range

functionalised polysilylenes. Halomethylation, catalysed by a Lewis acid such as tin (IV)

chloride, is performed with carcinogenic halomethyl ether82. The reaction often leads to

degradation of polysilylene, and while changing the catalyst from SnCl4 to AlCl3 can reduce

this, more effective is a decrease in temperature to -18 °C82. At first, degradation was

thought to arise at weak ‘siloxy’ linkages82, but as it has since been shown that the weak

gauche links in the main chain are more likely to provide centres for attack9. The

degradation is reduced as the number of in-chain gauche defects are reduced with

temperature68. During the first 15 minutes of the reaction available gauche links are reacted

and the degradation of the polymer is most noticable. Then, a slight increase in molecular

weight is observable as a consequence of increasing halomethyl groups82. An investigation

into a model compound of the polysilylene revealed that a high proportion of the

halomethylation occured at the para position on the phenyl ring82.

CH2Cl

nn

(i) SOCl2, CH3OCH2OCH3, CHCl3(ii) SnCl4

CH3

Si

CH3

Si

Scheme 1.21 The representative in-situ chloromethylation of PMPS82.

Halomethylated sites may be reacted with a range of nucleophiles82 (scheme 1.22).

29

CH3

RLi

CpLiCp*Li

n

CH2Cpn

CH3

n

CH3

NaMn(CO)5

n

NaCH(CO.CH3)2

X= Cl,Br

n

CH3

n

CH3

CH2R CH2Cp*

n

CH3

CH2X CH2NR2

R2NH

THF FeCl3

CH2Mn(CO)5

CCH3

OO

CCH3

CCH3

OO

CCH3

FeL2

CH3

Si

CH2

Si Si

Si Si

CH3

Si

CH

Si

Si

CH2

CH

Scheme 1.22 Some functionalised polysilylenes from halomethylated PMPS84.

Chlormethylated PPEMS has been used in the syntheses of amphiphilic and

cationic polysilylenes (scheme 1.23)85,86. Monolayer films of these materials have been

formed85. The cationic aqueous soluble poly((β-phenethyltrimethylamine

chloride)methylsilylene) when complexed with arachidic acid, has formed multilayer

films86.

30

C8H17

CH3

CH3 Cl-

+NCH2

n

C6H17N(CH3)2

THF, reflux, 20 hours

(CH2)2

nCH3

Si

(CH2)2

CH3

Si

CH2Cl

Scheme 1.23 The representative synthesis of an amphiphilic polysilylene86 .

1.1.3.3 A Substitution of Phenyl Moieties with Chlorine

Poly(arylsilylenes) can be functionalised under acidic conditions replacing the

phenyl groups with chlorine1 (scheme 1.24).

pn-p mCH3

Si

CH3

CH3 CH3

SiSi

Cl

mn C6H6CH3

Si

CH3HCl/AlCl3

CH3

Si

Scheme 1.24 The reaction of HCl with ‘polysilastyrene’1.

1.1.3.4 Block and Graft Copolymers of Polysilylenes

In general, polysilylenes have poor mechanical properties; they are not easily

handled, nor do they form stable films. While their electronic and optical properties are

apparent, they are hard to use87. PMPS is widely used for research as it is the cheapest

and easiest to synthesise of a still expensive range of polysilylenes. However it is a hard,

brittle solid, and its films crack easily88. In attempts to improve the mechanical qualities of

polysilylenes while retaining their more interesting properties, blends have been made with

commodity polymers, but this has not met with much success due to the incompatibility

of PMPS89 (section 1.1.4.5). The syntheses of block and graft copolymers often provide

more satisfying solutions to incompatibilities: properties of the component polymers are

more often combined positively, rather than destructively as in the case of blends90. With

31

respect to polysilylenes, this has not been greatly explored yet, but it is an area for

development.

1.1.3.4.1 Block Copolymers by Photoinitiation

In 1986, West et al reported that a polysilylene left in a solution of styrene in sun

light initiated a polymerisation reaction91. The polysilylenes used had a low initiator

efficiency, most likely because of the ease of radical recombination and the inhibiting effect

of solvated O2. In 1994, Yagci et al described a formation of block copolymers by both

radical and cationic polymerisations initiated through degradation of polysilylenes under

UV radiation92 (scheme 1.25).

+

CH3

Si CH2

C

OCH3

O

C

CH3

CH2

C

OCH3

O

C

CH3CH3

Si

Scheme 1.25 An example of the synthesis of a block copolymer from a polymerisation ofMMA by a polysilyene radical photo-initiator92 .

In the presence of N-ethoxy-2-methylpyridinium hexafluorophosphate, silyl

biradicals and radicals formed by photolysis are oxidised by methyl pyridium cations to

yield cationic centres on a polysilylene. These can be used to initiate cationic

polymerisations92. The polymerisation continues in the dark to give block copolymers,

with a molecular weight controlled to some extent by limiting the period of illumination. In

similar experiments, diphenyliodiumhexafluorophosphate was used with PMPS to form

block copolymers of the polysilylene with polymers of THF, cyclohexene oxide, n-

butylvinylether and N-vinylcarbazol93.

32

1.1.3.4.2 Ultrasound and Block Copolymers

PMPS radicals formed by ultrasonification in styrene can initiate a polymerisation,

but with little control over the molecular weight of block copolymers59. Cleavage by ultra-

sound of mixed PMPS and PS can provide block copolymers through an ensuing radical

recombination reaction. This can lead to compatibilisation of polymers89.

1.1.3.4.3 Block Copolymers of Poly(methylphenylsilylene) via Reactions with

Amines

In looking to the possible chemistry at the reactive Si-Cl end-group of polysilylenes

formed from the Wurtz reaction, A.-F de Mahieu studied the reaction of propylamine with

DCMPS94 (figure 1.12). Electron delocalisation along the chains increased (indicated by

UV spectroscopy). Block copolymers containing 2 or 3 segments of PMPS were

synthesised by this simple but effective condensation reaction.

N

R

Cl Si

CH3

ClSi

CH3

m n

Figure 1.12 The product of a reaction of DCPMPS with an amine where R is a propylgroup94.

1.1.3.4.4 The Nucleophilic Chemistry of the Silicon-Chloride Bond

Depending on the sustituent, nucleophilic replacement at silicon occurs easily. It is

not thought to occur via a R3Si+ intermediate, but through an SN2 reaction with a

pentacoordinated intermediate95 (figure 1.13). The reaction is very (>80%)

stereoselective. Depending on the substituents to the silicon, the reaction undergoes either

retention or inversion of configuration95. With Si-Cl, whatever the nucleophile, inversion of

configuration is the normal outcome as Cl- is a good leaving group.

33

-

Cl

Nu

Si

Figure 1.13 The pentacoordinated silicon intermediate95 .

1.1.3.4.5 Preparation of α,ω-dichloropoly(methylphenylsilylene)

The preparation of α,ω-dichloropoly(methylphenylsilylene) (DCPMPS) relies on

the exclusion of air and water throughout the Wurtz synthesis of PMPS. Polymer in the

solid phase of the product mixture is impossible to retrieve as it would require the sodium

to be destroyed by an alcohol or water, and this would destroy the Si-Cl groups60,96.

Hence, yields of DCPMPS from the solvent, and from washings of the solid phase, are

often low (~10 - 20%). Once recovered, the polymer is fractionated to remove oligomers

and low molecular weight polymer. Although this reduces the yield still further, the

resulting DCPMPS has a narrow molecular weight distribution. Under analysis by 29Si

NMR, the Si-Cl shows a clear split resonance absorption at +15 ppm96. This splitting is

attributed to the asymmetric centre at the chain end97. Elemental analysis has confirmed

the expected ratio of chlorine to the monomodal high molecular weight PMPS96.

1.1.3.4.6 Block Copolymers from Dichloropoly(methylphenylsilylene) and

Polystyryl Lithium

Polystyryl lithium (PSL) (Mn = 3500 g mol-1 and of low polydispersity) was added

to DCPMPS (Mn = 3500, Mw 20000 - 30000 g mol-1) in toluene and stirred for 18 hours at

room temperature under N2 (scheme 1.26)98. From simple statistics, a mixture of the

polysilylene homopolymer (25%), a diblock copolymer PS-PMPS (50%) and a triblock

copolymer PS-PMPS-PS (25%) was expected, but as the molecular weight of the

component polymers were so close, it was hard to differentiate between products. On

increasing the molecular weight of PS, a good agreement was found between the expected

and GPC indicated values.

34

The limit of reaction of DCPMPS and PSL is affected by the molecular weight of

PS, the concentrations of reagents and the solvent used98. In toluene, the coupling ratio

changes with time and the molecular weight of the PS (table 1.1). The effect of the

molecular weight of PMPS below 20000 - 30000 g mol-1 has not been considered. The

coupling ratio at 93% is only just under the 94% of Si-Cl end groups of the DCPMPS.

Other end groups are most likely hydrolysed.

The morphology of solid block copolymers is complicated by PMPS forming

ordered structures in a PS matrix. In the solid state, the block copolymer assumes a

complex morphology (section 1.3) based on phase separation of PMPS and PS. In the

above example, PMPS formed lamellae, around 50Å thick - corresponding to the extended

trans conformational segments of PMPS - in a PS matrix. This tendency of PMPS to form

ordered phases was thought to be a factor in the reaction in solution, and that perhaps a

liquid-liquid phase separation, which was amplified by increasing molecular weights of

polymers, was limiting the coupling ratio of the polycondensation98.

It was concluded that as the molecular weight of PS increased, the ion pairs had a

reduced accesssibility to reaction sites. In toluene, a doubling of the concentration of

reagents was seen to reduce the coupling ratio to only 28%. Ion pairs were excluded from

reaction by polymers, but more importantly, they have a greater tendency to form Li2+PS2

-

complexes at higher concentrations (section 1.2.3). On changing the solvent to THF,

condensation of the same DCPMPS and PSL was seen to increase and to be nearly

complete after 1.8 hours as the ion pairs were no longer held in aggregates98.

x

CH3

SiCl CH2s-Buy+Cl CH CH- Li+CH2n-1m

+ LiCls-Bu CHCH2n

CH3

Si Clm

+2LiCl s-BuCH2CHn

CH3

Sim

s-Bu CHCH2n

and

Scheme 1.26 A representation of the synthesis of block copolymers of DCPMPS with amono-anionic polymer PSL98.

35

Time to reach coupling ratio maximum/ hour PS / g mol-1 Coupling ratio

1 4400 93%

1 25800 65%

3 51000 45%

Table 1.1 Coupling ratios and molecular weights of PSL in reactions with DCPMPS in THF98.

1.1.3.4.7 Block Copolymers from α,ω-Dichloropoly(phenylmethylsilylene) and

Poly(methylmethacrylyl) Lithium

Using a similar methodology, a polycondensation of DCPMPS with

poly(methylmethacrylyl) lithium (PMMAL), has been shown to give di- and tri-block

copolymers, ie. PMPS-PMMA and PMMA-PMPS-PMMA99,100.

As before, the DCPMPS fractionated from products of a toluene mediated Wurtz

reaction, had an Mw of 20000 - 30000 g mol-1, while PMMAL had a much higher molecular

weight of between 150000 and 200000 g mol-1. By using an excess of DCPMPS, an

enhancement of the block copolymer product was achieved using a cold Soxhlet extraction

with CCl4 to remove unreacted PMPS.

1.1.3.4.8 Block Copolymers from Dibromopoly(methylphenylsilylene)

and Polystyryl lithium or Polyisoprenyl Lithium

An alternative method to polycondensation to arrive at a block copolymer, is a

reaction of 'living' PS or PI with α,ω-dibromopoly(methylphenylsilylene)101 (scheme

1.27). PMPS was prepared with methoxy end groups and was then cleaved in solution by

an addition of a stoichiometric amount of bromine. This reaction took 24 hours and chain

scissions were random. The block copolymers were purified by precipitation in a selective

non-solvent.

36

BrBrn

CH3

Si OCH3H3COBr2

Benzene / 24 hours

CH3

Sim

ps-Bu CHCH2 Li2

+ 2LiBrs-Bup

CH CH2ps-Bu CHCH2

n

CH3

Si

Scheme 1.27 A representative condensation reaction of brominated PMPS with a mono-anionic polymer: PSL101.

1.1.3.4.9 Block Copolymers from Polystyryl lithium or Polyisoprenyl

Lithium by Anionic Polymerisation of Cyclotetrasilane

In attempting to control molecular weights of block copolymers and eliminate the

tedious process of their purification, Matyjazewski et al used 'living' anionic polymers of

PSL and PIL to initiate the ring opening polymerisation (ROP) of a cyclotetrasilane102.

Polyisoprene (PI) was used to confer elastomeric properties on the block copolymer. The

ROP in benzene would not proceed unless 12–crown–4 was present (scheme 1.28). GPC

and 1H NMR characterisation of product AB copolymers did not agree on the block

proportions, but considering the latter technique to be the more accurate, it was concluded

that the reaction was 70% efficient. In both cases, some homopolymers of PS and PI were

found in the product. This was most likely due to the inefficiency of the cross over

reaction and the 'killing' of ion pairs by impurities in the cyclotetrasilane solution. The PS

homopolymer was removed by precipitating the mother liquor into acetone. The PI

homopolymer was removed in a similar way using hexane. Polydispersities of the products

were low (~1.3), and the molecular weights, though not completely controllable, were an

improvement over those of copolymers prepared by polycondensation.

37

ms-Bu CHCH2

- + Li12-crown-4

n/4 (CH3)4Ph4Si4Li

n

- +s-Bu CHCH2

CH3

Sim

Scheme 1.28 The synthesis of PMPS-PS102.

1.1.3.4.10 Block Copolymers of Poly(methylphenylsilylene) with

Polystyrene by Atom Transfer Radical Polymerisation

While there was still an excess of sodium present, PMPS from a Wurtz synthesis in

toluene was end-capped with a labile organic halide to act as an initiation centre for atom

transfer radical polymerisation (ATRP) of styrene (scheme 1.29)103,104. Once DCPMPS is

end-capped by the organic halide, it is stable to air and water. In the ARTP reaction Cu (I)

halide, typically complexed with 2,2'-bipyridine, is used as a mediating agent in a 'living'

reaction which gives vinyl polymers of low polydispersities (1.1 < Mw

Mn

< 1.5) and

predetermined molecular weights.

(ii) Methanol

CH2BrBrH2Cn

(i)

(CH2)2 (CH2)2

ClCl

CH2Br

Me Me Me

n

Me

ClCl

MeNa/Toluene

(CH2)2

Me

Si

Me

Cl

Ph

Si

Ph

Si 110°C, 2 hours

Si

Me

Si

Me Ph

Si

Scheme 1.29 Preparation of organic labile sites on poly(methylphenylsilylene)104.

38

Some homopolystyrene forms during the polymerisation (scheme 1.30), most

likely due to autopolymerisation of styrene at the temperatures employed105. A shift in

the 1H NMR of CH2Br protons from 0.85 and 2.5 ppm to 2.0 and 3.7 ppm respectively

indicated the presence of CH2-CH(Ph)Br terminal groups due to ATRP. However, the

most compelling evidence a formation of block copolymer came from GPC

characterisations.

mn(CH2)2

Me Me

Si

MePh

Si CH2 Br

Ph

CH2 CH

CuBr, Styrene2,2'-bipyridyl

n(CH2)2 (CH2)2Si

Me

Si

Me Ph

Si

Me Me Me

CH2BrBrH2C

Scheme 1.30 The preparation of polystyrene-block-poly(methylphenylsilylene)-block-polystyrene104.

1.1.3.4.11 Graft Copolymers of Polysilylenes

The action of triflic acid on PMPS provides a route grafting polymers along

polysilylene chains (scheme 1.20)106. Cationic polymerisations of THF, 2-methyl-2-

oxazoline and isobutylvinylether have been initiated using these sites106. However, control

over the polydispersity and molecular weights of the graft copolymers were limited.

The ATRP reaction of styrene to halomethylated phenyl moieties of PMPS

simplified the route to graft copolymers107,108. This resulted in graft copolymers with

predictable molecular weights. There was a slight increase in the value of λmax of PMPS,

most likely due to the graft copolymer ordering and lengthening its trans segments. DSC

characterisation of a graft copolymer indicated that the polystyrene had a reduced Tg at 80

°C, presumably due to an increased free volume about the graft chains109. 1H NMR

characterisation of a product showed a peak at 4.35 ppm, indicating that some of the Si-Ph-

CH2Br groups remained unreacted. Thus, the degree of polymerisation of styrene was

greater that originally calculated.

39

1.1.4 Properties of Polysilylenes

The properties of polysilylenes depend heavily on the lengths of silicon chains and

the type of their substituent groups, and they range from soft and rubbery to hard and

brittle materials2. Their Tg varies from greater than 120 °C to -80 °C. Normally, the Tg

rises with the amount of aryl substitution2. Most non-symmetrical substituted

polysilylenes are soluble in common organic solvents. The degree of crystallinity increases

with a decreasing length of alkylated substituents1. There is a strong sub-group interaction

in the solid phase. However, polymer backbones retain a low torsional energy barrier

between trans and gauche conformers2.

Poly(di-n-hexylsilylene) has a trans zig-zag conformation at below 42 °C, and

above that, a hexagonal mesomorphic phase110. Poly(di-n-butylsilylene) exists in a 7/3

helical arrangement below 86 °C, above which it is completely helical110. An

unsymmetrically substituted polymer, poly(n-hexyln-butylsilylene), is a rubbery solid at

room temperature. Below 220 °C the polymer is mainly amorphous, but X-ray

measurements indicate it to have a hexagonal lattice of partial crystallisation110,111.

Temperature changes merely alter the lattice distances and, as is common for polysilylenes,

it can be considered a conformationally disordered crystal lattice.

PMPS, an amorphous material, exhibits a Tg at around 120 °C, although it may be

concluded that most instruments are not sensitive enough to observe this112,113. PMPS is

about 10% crystalline. Dynamic mechanical thermal characterisation of PMPS showed

peaks at 97 °C and 185 °C113. Thus, the Tg of PMPS of a low molecular weight is around

95 °C, and that of high molecular weight is around 120 °C. At 190 °C, the structure

changes from attactic chains packed in a monoclinic crystalline lattice of near hexagonal

symmetry to a liquid crystalline-like columnar hexagonal packing. Between 150 °C and

250 °C, a phase disordering transition and softening of the polymer occurs113. PMPS in N2

is stable up to 300 °C113.

In solution, PMPS retains a semi-rigid rod-like character, each segment having a

persistance length of around 35Å - 40Å. The molecular weights of PMPS measured by

light scattering and GPC indicate that there is a near equivalence between the GPC value,

drawn from PS standards, and the actual value66. This result contradicts previous

indications that a conversion of GPC results was required67.

40

1.1.4.1 Photophysical Properties of Polysilylenes

Polysilylenes exhibit strong UV absorptions from 280 nm to 400 nm depending on

the number of monomer units and the substituent groups17,114. Alkyl substituted polymers

have a λmax between 303 nm and 309 nm, where as aryl substituted polymers normally

absorb between 335 nm and 347 nm17. A red shift of UV absorption with the increasing

number of catenated silicon atoms for per-alkyl polysilylenes has been depicted (figure

1.14)17. A red shift of this absorption band on substitution with aryl groups may be due to

LUMO stabilisation through an interaction of Si-Si σ*-antibonding orbital with a π*-

antibonding orbital of pendent groups17,76. Not only are σ→σ* transitions, possible but

also σ→π* transtions may occur62,63,114.

Figure 1.14. A plot of UV absorption maxima against chain length for the polyalkylsilylenes() Me(Me2Si)nMe, and () [(n-dodecyl)(Me)Si]n

17 .

Thermochromism results from a change in length of the all-trans segments of

polysilylenes. Normally, a bathochromic shift in absorption occurs with a reduction in

temperature, due to the loss of interrupting gauche turns. However, due to an admixing of

adjacent orbitals of the main chain and of subgroups, substituents affect the absorption

spectrum. With aryl groups, a σ→π* transition is available and this causes a red-shift62.

41

However, this is reliant on the conformation of aryl groups to the main chain. A spacer

group, such as -(CH2)2-, between the main chain and an aryl group reduces possibility

transitions between the aryl ring and polymer backbone31. Solvents may induce dipoles at

a polymer, polarising the trans segments and altering its UV absorption spectrum115.

1.1.4.2 Ionochromism of Polysilylenes in Solution

Polysilylenes, such as poly[(4-ethoxyethoxybutyl)octylsilylene], which are easily

polarisable, are susceptible to ionic reagents like lithium trifluoromethane sulphonic acid.

In solution the polymer exhibits iononchromism, however, the addition of a salt prevents

the normal bathochromic shift with increasing temperature116. The lithium ion can

effectively lock the chain conformation by interacting with side groups.

1.1.4.3 Langmuir-Blodgett films and Polysilylenes

To more fully exploit the UV absorption and conductivity qualities of

polysilylenes, Langmuir-Blodgett films have been prepared of amphiphilic polysilylenes

containing phenolic or alkoxy side groups117. The alkoxyphenyl substituted polymers

were found to be exceptionally good in forming films117,118. With an ammonium moiety on

the phenyl ring, a polyarylsilylene forms films dependent on the presence of a hydrophilic

counter ion119.

1.1.4.4 Electrical and Photoconductivity of Polysilylenes

Electrical conduction in polysilylenes is often limited by their poor ablity to form

films. In an ordered poly(cyclohexylmethylsilylene) film charge transfers by the

movement of holes through σ-conjugated chains120. PMPS may be induced by microwaves

to transfer charge by holes, which can be partly created by dopants121. Doping

polysilylenes with AsF5, SbF5 or H2SO4 improves their conductivities up to levels

equivalent to those of semiconductors1. Indeed, the holes can be moved by UV when under

an electric field2.

42

1.1.4.5 The Properties of Blends of Polysilylenes and Other Polymers

Blends of polymers often combine the qualities of component materials. However,

they often display poor interfacial adhesion between the droplets of one polymer

dispersed in the matrix of the other. Their properties may add destructively resulting in a

poor material90,122. Blends of polysilastyrene (PSS) with polystyrene or polypropylene

(PP) exemplify this point well123. PSS, at low concentrations, disperses in polystyrene.

At concentrations of PSS above 5%, the polymers separate. The dispersed mixture

displays hardness, and polystyrene protects the PSS from light stimulated degradation. PP

blends poorly with PSS, even at concentrations of 1% PSS. PMPS disperses in a matrix of

PS giving rise to a material with low interfacial adhesion and poor mechanical properties87.

Commonly, blends may be improved by adding block copolymers of the components,

which act to 'glue' the interfaces122. On incorporating PMPS-PS (10%) into the respective

blend, dispersion of polymers was improved along with mechanical properties87.

1.1.4.6 AB and ABA Block Copolymers of Polysilylenes and their Morphology

and Properties

The amount of literature on block copolymers of polysilylenes is relatively small.

To my knowledge, only AB and ABA type polymer have been considered to the exclusion

of multi-block copolymers.

It is expected that block copolymers of polysilylenes should have improved

mechanical properties. It is also possible that copolymers of particular molecular weights

and low polydispersities may give materials with defined morphologies101 (section 1.3). A

mixture of block copolymers and homopolymers (35% PS-PMPS-PS, 26% PMPS-PS,

37% PS and 2% PMPS) displayed a two phase system with the immiscible PMPS forming

lamellae98.

A study has been performed on the diblock copolymer PMPS-PS (PS Mn = 18700,

PMPS Mn = 9000)124. A film was cast from 1,4-dioxane, which favours the solvation of

PS, and therefore could aid the formation of miscelles with PMPS cores. Surface electron

microscopic characterisation showed a relatively smooth surface as the PMPS had

degraded, but surface force microscopy revealed cylinders of PMPS in the PS matrix. At

43

low concentrations, the film formed worm-like patterns - a cylindrical morphology of

PMPS in PS - of a length of about 7 nm, equivalent to 40 Si units. This approximates to

the length of σ-conjugated orbital persistence of PMPS.

1.1.5 The Applications of Polysilylenes

Polysilylenes, at present, are still ‘materials of the future’. There are many possible

uses for polysilylenes, however they are used in only a few applications. The use of

polysilylenes in the synthesis and manufacture of β-SiC, an important hard material, has

been discussed (section 1.1.1), as has the use of polysilylenes as photoinitiators of radical

and cationic polymerisations (section 1.1.3.4.1) and their properties of conductivity and

photoconductivity (section 1.1.4.4) which may be of use in the xerographic industry2.

The possible use of polysilylenes in the microlithographic industry is being

explored1,2,125,126. They show a broad wavelength lability, can be prepared as optical

quality films, and in having a high silicon content may be used in the formation of

multilayer chips, as by dry etching with O2 plasma, they form a tough SiOx surface. This

may be used in a + ve process, to give the well controlled topography required (figure

1.15). The preparation of -ve process resists of polysilylenes has been tried although there

is less scope for progress126.

In light emitting diodes there is a high possibility for the use of polysilylenes;

PMPS shows a sharp photoluminescence, a high quantum efficiency, and a high charge

mobility by holes of up to 10-3 cm2/Vs. PMPS has emission bands of a frequency and

intensity dependent on the temperature, and could give rise to high frequency LEDs,

something that is hard to achieve with organic materials87,127. As polysilylenes are

compatible with silicon technology, and are transparent in the visible region, they can be

easily combined with semiconductor circuitry and with other organic materials in layers87.

44

A polysilylene resist top layer,

degraded by the application of a UV, X-

ray or electron beam, when developed,

may leave a pattern on the surface

which protects the underlying polymer

from O2 reactive ion etching1,2.

Figure 1.15 The construction of a bilayer resist with a polysilylene1.

Polysilylenes exhibit large non-linear susceptibilities with rapid response times2,

and as 3rd order devices, are not restricted by a requirement to be symmetrically substituted

as is the case for 2nd order non-linear materials. The use of PMPS as a non-linear optical

device may encounter problems as it shows a high non-linear susceptibility but

photodecomposes.

1.2 Anionic Polymerisation

Ideally, ‘living’ polymers propagate while their termination or chain transfer is

rigorously prevented. More realistically, their definition is of "polymers that retain their

ability to propagate for a long time and grow to a desired maximum size while their degree

of termination or chain transfer is still negligible"128. The C- M+ ion pair (where C- is the

propagating carbanion and M+ is the metal gegen ion) is a reactive centre (scheme 1.31).

Negative charge is stabilised by a local conjugation of orbitals across conjugated groups129.

Thus, vinyl and diene polymer precursors are preferred and ethylene, for example, will not

easily polymerise130. The carbanion is extremely sensitive (hence the redefinition above),

and contact with water or air or impurities will arrest the polymerisation. Polymerisations

are normally conducted in a dry solvent under a high vacuum or a dry inert gas. The 'living'

polymer is coloured, varying from a deep blood red for those of α-methylstyrene to a light

orange for those of dienes.

45

CH2 C- M+

R''

R'

CH2 C

R''

R'

+ M+CH2

R''

R'

CH2C C-

R''

R'

Scheme 1.31 A typical propagation of the anionic polymerisation.

Some particular features of anionic polymerisations are notable. The molecular

weight of synthesised polymers can be predicted from simple stoichiometry. For

monofunctional initiators where the rate of initiation is faster than that of propagation,

equation 1.1 holds true. A sequential addition of monomers should give rise to block

copolymers. The synthesis of functionalised polymers is facile due to the reactive end

groups130.

molecular weightpolymer = weightmonomer (grams) / molesinitiator 1.1

1.2.1 Anionic Polymerisation - a Short History

In 1910 Matthew and Strange polymerised isoprene using an alkali metal in a

heterogeneous reaction, however, the mechanism of the reaction was not understood131. In

1934 Ziegler coined the term 'living' polymers, outlined a now accepted mechanism, and

used alkyllithium to initiate a polymerisation132. After World War II, the reaction was

thought of as a nucleophilic attack of a growing carbanion on the double bond of a

monomer133. The mechanism was further elucidated by the polymerisation of styrene in

ammonia with sodamide as the initiator134. However, the anionic polymerisation

mechanism was hard to comprehend, not because it is implicitly complex - as it is not, but

because of its sensitivity to impurities and a tendency to undergo side reactions.

Flory advanced the idea of the probability of growth of polymer chains, and

stipulated that without termination, the molecular weight of a polymer should approach

that of the 'Poisson ' distribution135 (section 1.2.3.3).

Two main advents developed what would now modernly be called anionic

polymerisation; (i) the initiation of polymerisation of isoprene by alkyllithium reagents

and the discovery by IR that the synthesised polymer, like natural rubber, contained over

90% of the cis 1,4 isomer136 (figure 1.16), and (i) the discovery by Szwarc et al that a

46

solution of sodium napthalide could initiate a 'living' polymerisation of styrene137.

1.2.2 Initiators of Anionic Polymerisations

Initiators of anionic polymerisations may be classed into three groups; alkali

metals, aromatic complexes of alkali metals and organometallic compounds.

Polymers arising from initiation by alkali metals have relatively high

polydispersities: the system is heterogeneous, and there is the possibility of a second

electron transfer step occuring at the metal surface138 (scheme 1.32, step iii).

Na + CH2= CH-CH = CH2 Na+CH2 -CH = CH -CH2 i

Na+ -CH2-CH = CH-CH2-CH2-CH = CH-CH2- Na+2 Na+CH2 -CH= CH -CH2

i i

Na+ -CH2-CH = CH-CH2- Na++ NaNa+CH2 -CH= CH -CH2 i i i

Scheme 1.32 A representative formation of carbanion and gegen ion pairs: butadiene withsodium130.

Aromatic complexes of alkali metals are actually radical anions supported by a

solvent (usually THF) (scheme 1.33). ESR analysis indicates that the extra electron on

napthalene moves to a π LUMO130.

Na++ Na

O

O

Scheme 1.33 The representative formation of a radical anion of napthalene with sodium andTHF130.

The initiator may transfer charge to a monomer (scheme 1.34).

Thermodynamically, this step (i) should act in reverse, but the dianion is considerably

more stable and pulls the reaction forward. In the example, the dianion formation is faster

47

than that of propagation of the polymerisation. Thus, the molecular weight distribution of

the polymer approaches the Poisson distribution137 (section 1.2.3.3).

Na+++ CH CH2Na+ CH CH2

i

2 Na+ -CH-CH2-CH2-CH- Na+ Na+CH CH2

i iScheme 1.34 A representation of a formation of a styrene dianion130.

α-methylstyrene may be used to form dianionic oligomers. This occurs at

concentrations of alkali metals which thermodynamically exclude the possibility of a

polymerisation. Depending on the concentration of the reagents, dimers, trimers and

pentamers, all based on the same head-to-tail structure (scheme 1.34) may arise139. It may

be used to initiate polymerisations leading to polymers of a low polydispersity, for

example, poly(α-methylstyrene) with Mw

Mn

= 1.06140.

Initiators synthesised from aromatic alkali metal complexes are dianionic, and any

growth in a polymer chain occurs at both chain ends. Any prediction of the molecular

weight of a product polymer must therefore take this into account, as in equation 1.2.

1/2 molecular weightpolymer = weightmonomer (grams) / molesinitiator 1.2

Alkylated alkali metal initiators are monofunctional and are commonly based on

lithium. The advantages of their use are that they are soluble in polar and non-polar

solvents, and they allow a measurement of rates of initiation and propagation because of

their rapid formation of coloured ion pairs130. However, the order of reaction is not simple.

Normally, the reaction is of first order with respect to the monomer concentration, but it is

also of a fractional order with respect to initiator concentration. Initially this was thought

48

to be due to a complexing of up to 6 ion pairs, thus reducing the initiation capability of the

alkyl lithium (scheme 1.35)129.

Ion pairAggregate

ContactPair

SeperatedPairs

FreeIons

n(R-) n(M+)(R- M+) n(R-M+)n

n(R- M+)

Scheme 1.35 A representation of the formation and dissociation of ion pair aggregates ofalkyl lithium. The separated pairs are occur more readily in polar solvents129.

This however has since been shown to be a superficial hypothesis as (i) there is no

apparent connection between the order of kinetics and the states of ion pair aggregates, (ii)

the aggregates require a high concentration of alkyl lithium to exist - higher than that

normally found in anionic polymerisations. It is more likely that a cross association of

initiator and organolithium chains occurs130. It is also possible that there is a reaction

between the monomer and the associated forms of initiator141. For dienes, the reactivity of

alkyl lithium initiator is generally in the order s-butyl lithium > t-butyl lithium > n-butyl

lithium130. Aging of the living end groups of polystyryl lithium initiated by butyl lithium

has been observed, and is most likely due to the formation of lithium hydride142.

1.2.3 Propagation of Anionic Polymerisations

The rate of propagation of an anionic polymerisation is independent of the type of

initiator used, but dependent on the concentrations of reagents and type of solvent. In

principle, an anionic polymerisation is a bimolecular reaction, and should therefore be first

order with respect to monomer and initiator. This can occur in polar solvents, such as

THF, where a single chain polymerisation is thought to exist143. In polar solvents, the end

group alternates rapidly between free ions, contact ion pairs and solvent separated ion

pairs, and the polydispersity of synthesised polymer is therefore low. However, in non-

polar solvents this is rarely the case as the active polymer chains associate as dimers130,144.

Solvent effects are readily seen when polymerising styrene in benzene. This is only a rapid

reaction when a few drops of THF are added130.

49

Analysis by viscosimetry has indicated that the molecular weights of ‘living’

polymers in non-polar solvents are twice that of the terminated polymers144,145.

Furthermore, analysis of polyisoprenyl lithium in toluene by light scattering indicates that

the ‘living’ polymers are associated in pairs146. In ether solvents, this association between

'living' polymers is not indicated, as metal ions are solvated, separated from carbanions and

the reactive ion pairs are stabilised.

1.2.3.1 Polymers of Diene monomers and their Geometry

Polyisoprene, as the representative example, can be formed of four isomers (figure

1.16). The isomer formed depends on the solvent, the type of gegen ion (normally of Li+

or Na+ or K+) and even temperature130.

CH CH2CH2

CH3

C1 2 3 4

5

CCH2

CH3C H

H2CCC

HH2C

H3C CH2

CH2

CH

CH3

CH2 C CH2

CH2

CH

CH3C

cis 1,4 trans 1,4 1,2 3,4

Figure 1.16 The various isomers of polyisoprene129.

IR spectroscopy has aided the determination of isomers136, but more effectively

NMR can be used to indicate the relative concentrations of isomers147,148. Polar solvents,

even in low concentrations, tend to enhance the concentration of 1,2 and 3,4 isomers (or

'vinylic' groups). In a polar solvent, the reactive ion pair is separated, negative charge is

delocalised, and the dimerisation of reactive polymers is reduced. This has the overall

effect of reducing the steric constriction on reacting monomers149. Non-polar solvents

favour the formation of 1,4 units. Aliphatic solvents increase the concentration of cis

isomers. Lithium also promotes the presence of 1,4 units, but sodium is more ambivalent

in effect. In addition, another possible coordination is that of 4,1 (figure 1.17).

50

M+CH2-CCHCH2

CH3

M+CH2-C CHCH2

CH3

Figure 1.17 A representation of 4,1 and 1,4 isomers as reactive end groups130.

As to whether a 1,4 or a 4,1 (collectively called 1,4) addition occurs, the approach

of the monomer to the reactive centre decides. In non-polar solvents, the reaction centre

tends not to be widely delocalised, and exists mostly in either cis or trans 1,4 forms

(figure 1.18). Most reactions occur at the α position with respect to the ion pair150.

M+CH2-CHCH

CH2M+CH2-

CHCH

CH2

cis trans

αβ δ γγδ β α

Figure 1.18 A representation of charge delocalisation at a polymer end group in a non-polarsolvent130,151.

To give rise to a 1,2 or 3,4 isomer in a polymer chain, the approaching monomer

must react at a γ position with respect to the ion pair. In polar solvents this is more

possible due to the delocalisation of negative charge and a relatively free placing ions

(figure 1.19).

αβγδ

M+CH2

-CHCHCH2

Figure 1.19 A representation of charge delocalisation at the polymer end group, which isnormally in the cis form, in polar solvents130.

Especially in THF, the carbanion of isoprene with Li+ or Na+ is relatively unstable,

and may undergo transfer or secondary reactions152. The sodium ion pair is much more

reactive than that of lithium, and polyisoprenyl sodium in THF is particularly susceptible

to side reactions at even -40 °C.

51

1.2.3.2 Additives and 'Living' Polyisoprenyl Lithium

Triethylamine, tetrahydrofuran, 2-methoxyethylether and 1,2-dipiperidinoethane

have been used as additives with 'living' polymerisations of isoprene in toluene or

cyclohexane with n-butyl lithium as initiators153. Their addition increases the proportion

of 3,4 isomers. In the abscence of these Lewis bases, a high proportion of the structural

units are in the cis 1,4 form.

1.2.3.3 Anionic Polymerisations and Molecular Weight Distributions

The relationship between the molecular weight of a polymer and its polydispersity

was determined by Flory135. In an ideal system, the polydispersity of a polymer is

inversely proportional to its molecular weight. A number fraction (Pj) and weight fraction

(Wj) of a polymer of a degree of polymerisation (x) of monomers (j), can be related as in

equations 1.3 and 1.4.

Pj =

e-x.x j-1

( j −1)!1.3

Wj =

xx + 1

. je-x .xj-2

( j −1)!1.4

Where the polydispersity H is;

H =

xw

xn

1.5

where xw and xn are the weight average and the number average chain lengths, equations

1.3, 1.4 and 1.5

=> H = 1 +

(xn -1)xn

2 1.6

and ∴ H = 1 +

1xn

where x is a high value. 1.7

This would imply that for a polymer of 100 units, its polydispersity is 1.01 or

very low. However there are two assumptions; that the initiation is rapid, and that there

is perfect mixing of reagents and solvent.

52

1.2.6.5 Propagation Reaction and Chain Termination

Chain termination may occur by impurities, destroying the 'living' chain ends, or by

transfer reactions from the chain ends to solvents or other reactive groups130. The

molecular weight indicated for a polymer will always be higher than expected, as it is

inevitable that some distruptions occur during the reaction. In the case of dianionic 'living'

polymers, for example those initiated by oligo(α-methylstyryl) disodium, the polymer

may have a bimodal molecular weight distribution. This is because the probability of a

destruction of one reactive group on a polymer (p) is much higher than that occuring on the

same polymer twice (p2). For all types of 'living' polymers, the indeterminate termination

of reactions will broaden the molecular weight distribution, eventually leading to the most

probable type, that is xw / xn = 2.

1.3 Block Copolymers

Block copolymers may be prepared by several synthetic routes. There are two

principal methods; (i) to functionalise the end groups of a polymer and then conduct a

condensation reaction, or (ii) to use the reactive end group of one polymer to initiate the

polymerisation of another90. The first has two distinctive disadvantages. Firstly, mutually

reactive polymers may be thermodynamically incompatible, or rather immiscible in

solution, thus limiting synthetic routes. Secondly, when linking two hetero-reactive groups

to give a block copolymer of a high molecular weight, reagent stoichiometry must be as

close as possible to equivalence. The latter method can over come these disadvantages,

although termination of the reactive centre may result upon addition of impurities with a

second monomer. In two stage processes, both AB and ABA type block copolymers are

possible, by using monofunctional or bifunctional initiators respectively (scheme 1.36).

Multi-block copolymers are possible by this method, but they are limited by the

impurities with each addition of new monomer. In this respect, anionic polymerisation is

an elegant but delicate route to block copolymers of a controlled molecular weight (section

1.2).

53

AR B R+ AB+RR

i +A2 R B B 2 AB+RR

i i ++n n B BAA A B

nn AB

i i i n M+*R R * iv

2n M+* ***v

Scheme 1.36 A representation of two principal routes to block copolymers; condensationreactions to give (i) a di-block copolymer, (ii) a tri-block copolymer (iii) a multi-blockcopolymer, and an initiated polymerisation to give (iv) a di-block copolymer and (v) a tri-block copolymer. R, A and B, and * are unreactive, reactive chain ends, and ion pairsrespectively. M is a monomer.

1.3.1 Block Copolymers through Condensation Reactions

The degree of polymerisation of a copolymer can be estimated with regards to the

stoichiometry of well mixed reagents. The degree of polymerisation, xn (the number

average of repeat units in the chain), is related to the extent of the reaction (p) by;

xn =

1(1− p)

1.8

The molecular weight of the repeat unit (Mo) is related to the number average molecular

weight Mn by;

Mn =

Mo

(1− p)1.9

This holds true for a system that is stoichiometrically balanced, with no side

reactions nor monofunctional polymers present. A good example of this is the reaction of

polystyryl lithium, in THF at –78 °C, with α,ω-dichloroalkanes154,155. The molecular

weight may be controlled and predicted, to a degree, by adding an excess of one reagent. In

reality, the reaction equilibrates, and in industrial or bulk systems, the condensates such as

HCl are removed by high temperatures and vacuum. There is a slow arrival to higher

conversions as the concentration of the reagents decreases156. The analysis of the kinetics

describe the time dependency of the reaction. Where k is the rate constant and co the initial

reactional group concentration;

54

p =

kcot(1+ kcot)

1.10

xn = kcot +1 1.11

Statistical treatment of the reaction leads to the conclusion that at equilibriation, the

distribution of chain lengths is a function of p156;

wx = x(1-p)2 px−1 1.12

Here wx is the weight fraction of block copolymers consisting of chains of a degree of

polymerisation x. This in turn can be used to determine the weight average of the degree of

polymerisation xw ;

xw =

(1+ p)(1-p)

1.13

As;

Mw = Moxw 1.14

Where the polydispersity is a simple ratio of xw xn , the ‘breadth’ of the molecular weight

distribution can be determined as;

xw

xn

=Mw

Mn

= 1+ p 1.15

As p approaches unity, the polydispersity arrives at 2, the most probable distribution.

1.3.1.1 Block Copolymers from Transformed Reaction Centres

In deactivating ‘living’ poly-anions with a suitable reagent, the stable polymers may

be used for further reactions. Thus, polymers end-capped in this way may be immune to

impurities and still be used for condensation reactions to form block copolymers. This

route may also be used in bringing together reactive end groups of polymers.

Commonly, the reagent polymers have different solubilty parameters, and so in

reaction they have a certain immiscibility. If the polymers are immiscible, then the reaction

forming the block copolymers cannot ensue. Separation can occur more readily with

polymers of a high molecular weight, and there can be a drive towards a liquid-liquid phase

separation between polymers. It is possible to choose polymers and end-groups so that

the reaction is promoted at a reaction interface (figure 1.20). In the example shown, a

reaction is performed from a hydrophilic polymer with a hydrophobic end-group, and a

hydrophobic polymer with a hydrophilic end-group. The reactive end-groups (which are

55

added to the polymers after they are prepared from anionic polymerisation) tend to move

towards a organic solvent-water interface157. This circumscribes the problem of polymer

immiscibility and, in addition, localises the concentration of the reactive end-groups.

AAABBB

organic solvent

water

Figure 1.20 A representation of the synthesis of a block copolymer at a solvent inteface,where A and B are hydrophilic and hydrophobic end-groups respectively157.

1.3.2 Block Copolymer Morphology

The morphology of a solid block copolymer is dependent on the purity,

polydispersity, molecular weight, crystallinity and compatibility of the component

polymers. Importantly, block copolymer type determines the available morphology

(figure 1.21). Morphology may even be altered by the method of casting90. The

microstructure of a block copolymer may also change with temperature and complex

calculations can be used to make phase diagrams of possible morphologies158,159.

56

a

bFigure 1.21 Representations of block copolymer architectures; (a) an A-B type polymer,and (b) an ABA polymer. An (AB)n polymer is much like a ABA polymer, but the domains aremore interlinked90.

Morphologies of block copolymers can vary from the simple to the extremely

complex. The latter is more prevalent for incompatible component polymers, where for

reasons of thermodynamics, the boundaries are sharpened and interfacial areas are

minimised90.

Altering ratios of molecular weights of the component polymers can, in effect,

control their mixing ratio. Thus, some control is gained over the domains of the block

copolymer. At a ratio of 1:3 the lesser component forms spheres in a matrix of the other

polymer. Usually the main component forms the continuous phase (figure 1.22). If

component polymers of a block copolymer are mixed more equally than this, more

morphologies become available, including cylinders, rods, or continuous phases. The

blending of a block copolymer with homopolymers can also be used to control

morpholgies. Transitions between strict morphologies may be observed by DSC159.

The melting point and/or Tg of the component polymers may be changed by their

forming a block copolymer. An example is that of PEO in poly(oxyethylene)-block-

polystyrene-block-poly(oxyethylene) (PEO-PS-PEO) which exhibits a change in melting

point due to the polystyrene disrupting the structure of PEO90.

57

a b c d e

Figure 1.22 Representations of changes in morphology of a block copolymer with increasingA domain content going from left to right; (polymer A is white and polymer B is black);(a) A-Spheres occupying a body centered cube lattice in a B-matrix,(b) A-cylinders occupying a hexagonal lattive in a B-matrix,(c) A-B alternating lamellae,(d) B-cylinders in an A-matrix, and(e) B-Spheres in an A-matrix159.

Multi-block copolymers with low polydispersities may form numerous types of

phase separated morphologies90. The morphologies arise from a reduction of unfavourable

short range monomer-monomer interactions, and the long range entropic effect by chemical

links of the A and B blocks. In theory, an (AB)n model, in which n = ∞, may describe the

possible effects of the block copolymer on the morphology (figure 1.22). The chemical

links are at the polymer domain boundaries, and the polymers align themselves across the

domains perpendicular to the boundaries160. The lamellae spacing increases with the

molecular weights of polymers. In a system that is strongly segregated, domain size D is

related to the size of molecule M by;

D α Ma 1.16

where a is a constant approximately equal to 2/3.

For AB polymers, the repeat structure is A-BB-A, but ABA polymers tend to

form loop like structures (figure 1.23). These changes in the structures can have adverse

effects on the properties of the copolymers.

58

Figure 1.23 Representations of the chain conformations in block copolymer domains;(a) an AB type, (b) a (AB)n=∞ type, and (c) a (AB)2 type160.

1.3.3 Block Copolymers in Solution

In aqueous solution, micelles of an amphiphilic block copolymer exist above the

critical micelle concentration (cmc). These aggregates of polymer have hydrophobic chains

at the centre, and hydrophilic chains at the exterior of the colloid161. The cmc occurs when

it is possible, on a formation of aggregates, that there is a reduction in the interfacial energy

of polymer with solution. In water, ‘string’ hydrogen bonding reduces the cmc of many

block copolymers containing polar polymer chains163. Adding electrolyte to a solution can

reduce the cmc by decreasing the repulsion between the micelle surface groups and solvent.

Spherical, cylidrical and lamellar structures are possible (figure 1.24). Bilayer spherical

structures (vesicles) normally exist only when there are high concentrations of the

polymers. The lengths of polymer chains determine the micelle radius, and can provoke

distortions of regular structures. As in the solid structures, block copolymers of a low

polydispersity are required to form regular aggregates162.

Star micelles are those with small cores, made of the more lyophobic polymers of

low molecular weight. ‘Crew cut’ micelles are the inverse: they have large cores with short

coronal ‘hairs’163. Incompatibility between the blocks of the polymers decreases the cmc.

Various types of aggregates of poly(oxyethylene)-block-polystyrene (PEO-PS)

occur in water90. They can form ‘pearl necklaces’ which are chains of aggregates with a

structure in-between that of spheres and rods. These ‘crew cut’ aggregates form in dilute

59

solutions with an addition of ions, and their structure may be controlled by altering the

solvent164. However, very low polydispersities are required to give these sorts of

structures163. There is a negligible exchange of the polymers between the micelles when the

core material is immiscible with the solvent90. As the PEO content of the ‘crew cut’

micelles decreases, the morpholgy changes sequentially from spheres to rods to lamellae to

vesicles, and often the structures coexist165. The miscelles may be trapped in polymer

matrixes, or in frozen water for characterisation, or prepared as a dry film162.

a b

Figure 1.24 Two representative micelles; (a) spherical micelle, and (b) spherical vesiclebilayer structure161.

1.3.4 Structure and Property Relationships of Block Copolymers

Block copolymers exhibit various properties which are dependent on the

component materials, the type of morpholgy present (if it occurs) and the material casting.

Block copolymers are expensive to make, but industrially they are prepared in large

amounts.

Single phase copolymers have a combination of the properties of components.

Two phase materials have more interesting properties, including ‘thermoplastic elasticity’

(section 1.3.4.1)90. Unlike blends, block copolymers are nomally optically clear, due to

domain sizes being smaller than the wavelength of visible light90. Block copolymers have,

in general, superior physical properties to blends, as the domain sizes are smaller and the

interfaces are chemically linked. The qualities of blends may be enhanced by the addition

of small quantities of block copolymers, improving their impact properties. However, the

60

best results are found when the molecular weights of block copolymers exceed those of the

homopolymers. Examples include block copolymers of poly(α-methylstyrene) which are

mostly tough and optically clear90. Another example is that of the block copolymer PS-

PEO, which forms cylindrical, lamellae and spherical type domains and has a shear modulus

of 2 orders of magnitude higher than the equivalent blend90.

ABA and (AB)n type block copolymers show drastically different properties to

those of the AB type90. Only the first two can exist as ‘thermoplastic elastomers’, as the

AB type cannot form a networked structure. This is most clearly illustrated by stress and

strain curves of each system (figure 1.25).

Figure 1.25 A representative diagram of stress and strain curves of polymers of differenttype structures90 .

AB polymers are easier to melt process but of the three types (AB)n polymers

show the highest recovery properties, the finest most networked morphology and high

optical clarity90.

1.3.4.1 ‘Thermoplastic Elastomers’

ABA and (AB)n block copolymers combine the behaviour of crosslinked rubber and

the processability of thermoplastics, and thus they are called ‘thermoplastic elastics’. The

properties arise from the network structure of the interlaced chains90. The two phase

structures required for this behaviour derives from an incompatibility of components, and

the microphase separation of small (100Å - 300Å) domains dispersed in a rubber matrix.

61

These domains act to link the elastomer and they can be melted or softened90. If the

molecular weight of a polymer is too low, domains cannot form, if it is too high,

thermoplasticity is obliviated.

1.3.4.2 Thermal Properties of Block Copolymers

A random copolymer normally exhibits a Tg which is midway between those of the

components, and single phase materials exhibit a similar Tg. Like blends, two phase

materials show Tgs, but a constant modulus plateau exists between the two phases (figure

1.26). A flat plateau indicates a high degree of phase separation. As the composition

changes of a block copolymer changes, the vertical position of the plateau alters, but the

Tgs do not change.

Figure 1.26 A graphical representation of glass transition temperatures of a block copolymerAB, and with a high proportion of the A polymer (Tg

2), or a high proportion of the Bpolymer (Tg

1)90 .

1.3.5 Applications of Block Copolymers

Block copolymers have been around for 30 or so years. The market place is used to

having these materials made available90. The ‘thermoplastic elastomers’ have gained wide

acceptance. For example ‘Lycra’ of Dupont plc is used in clothing, and the car industry

uses various block copolymers for tubes, car interiors and paints. The surfactants of PEO

and PPO are used in drug delivery systems, as the hydrophobic core can carry

62

hydrophobic drugs165. Vesicles have also been used to model cell structures164.

1.3.6 Characterising Block Copolymers

GPC is the single most important method for indicating the synthesis of block

copolymers from precursor polymers90. Depending on the type of polymers used

(whether they absorb UV or alter a refractive index of a solution), the relative amount of

block copolymer formed can be indicated. Light scattering may be used to determine the

size of block copolymer aggregates in solution. In the solid phase TEM is most useful for

determining domain sizes. DSC may be used to determine the Tg and/or the Tm of the

component polymers. These values may indicate whether or not there is a dissruption of

one polymer structure by the other, and qualitively determine the degree of phase

separation. Other methods of characterisation include x-ray scattering, for determining

domain sizes, IR and NMR for studying the component polymers. By simply observing

the optical clarity of a material, a block copolymer can be indicated and observed90.

63

References

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69

2 Aims and Objectives of the Present Work

The aim of this work was to improve upon and diversify the mechanical and other

properties of poly(methylphenylsilane) through its modification to graft and block

copolymer structures with carbon-based polymers.

Since the synthesis of graft copolymers of PMPS from its chloromethylated and

bromomethylated derivatives was already well understood, the synthesis of an alternative

polysilylene with an aromatic substituent, poly(β-phenethylmethylsilylene) (PPEMS),

was considered worthy of assessment under the conditions of the Friedel-Crafts chemistry

which was required to prepare such precursors of graft copolymers. The insertion of the

spacer group between the electron rich silicon backbone and the phenyl ring was considered

as a means of limiting the degradation that accompanies the halomethylation reactions.

Thus, in the first instance it was decided to synthesise the polymer precursor, dichloro-β-

phenethylmethylsilane, and to investigate the optimisation of its polymerisation by the

Wurtz polycondensation reaction.

The block copolymers of PMPS that were envisaged were multi-block structures with

polyisoprene or poly(oxyethylene), and ABA and AB type block copolymers of PMPS

and poly(oxyethylene) or poly(α-methylstyryl). The syntheses of these structures were

found to be very successful. Accordingly, the optimisation of these reactions and the

characterisation of the products took precedence over those leading to the graft

copolymers.

The amphiphilic block copolymer, poly[poly(methylphenylsilylene)-block-

poly(oxyethylene)], was found to display unusual aqueous solution properties so a final

objective was the determination this polymer’s aggregation in aqueous dispersion.

70

3 Preparation of Dichloro-β-phenethylmethylsilane

and its Polymerisation

Abstract

The preparation and purification of dichloro-β-phenethylmethylsilane was

undertaken with the view to studying its Wurtz polycondensation. Two synthetic routes to

dichloro-β-phenethylmethylsilane were attempted and the most effective found was that via a

Grignard reagent.

The molecular weight distributions of the polymer were monomodal from those

reactions conducted in THF, bimodal from those in diethylether and multimodal from those

in toluene. The variations of the molecular weight distributions of the polymer formed in

toluene and in diethylether were followed with time. The effect of the rate of addition of the

monomer on the molecular weight of the polymer was also determined. The effect of the

addition of 15-crown-5 to the reaction system was studied. The possible reasons for the

differences found in molecular weights of formed polysilylenes are discussed and compared.

It is proposed that the differences partly arise from the separation of the phenyl π-orbitals

from the σ-orbitals in poly(β-phenethylmethylsilylene), a feature which effects its solubility.

Grateful thanks are extended to NEDO, the Japanese Ministry of International

Trade and Industry (MITI) and the Japan Chemical Innovation Institute for the financial

support of this work which has in part been presented at the 2nd International Symposium

on Silicon-Based Polymers, Tokyo, Japan, June 1997.

71

3.1 Introduction

The polymerisation of dichloro-β-phenethylmethylsilane (DCPEMS) was studied

so as to optimise its synthesis. The reaction was followed in THF, toluene and

diethylether. The use of 15-crown-5 was studied as phase transfer catalyst. The effects of

temperature and the rate of addition of monomer to the reaction were also followed.

Pure DCPEMS is not commercially available. A sample of DCPEMS from

Fluorochem™ was indicated by 1H NMR to contain a high proportion (~ 50%) of a

structural isomer dichloro(1-phenethyl)methylsilane1.

To obtain DCPEMS there were two apparent routes; (i) to prepare it by the

hydrosilylation of dichloromethylsilane with styrene with a platinum catalyst2,3, or (ii) to

prepare it by the reaction of the Grignard reagent phenylethyl magnesium bromide with

trichloromethylsilane4. Both routes were attempted.

The Wurtz polycondensation of DCMPS in various solvents, with and without

additives, at different temperatures and reducing agents has been well studied5,6,7,8,9,10,11

(section 1.1.2.1). Studies of the polymerisation of DCPEMS are less numerous, but it is

known that it reacts more slowly than DCMPS7,12.

At the time of this work, it had not yet been realised that the extended σ-bonding of

the catenated polysilylene chain determines to a large degree the molecular weights of the

products of the Wurtz polycondensation of dichlorosilanes5,10,11. Now that the mechanism

is more fully understood, it is considered possible that the following results may be used to

indicate; (i) what effect the phenyl ring to the PMPS σ-conjugated backbone may have on

its synthesis, and (ii) to define, approximately, what the all-trans segment length is in

poly(β-phenethylmethylsilylene) (PPEMS)10.

Where appropriate, the samples were characterised by standard methods (GPC,

NMR, DSC, UV and IR). The experimental conditions and the preparation of samples are

detailed in appendix 2.

72

3.2 Preparation and Characterisation of

Dichloro-β-phenethylmethylsilane

The first route detailed in scheme 3.1 proved unreliable as the catalyst was suppled

as a thick gel which was not easy to weigh or transfer to a reaction vessel. Thus, the

second route mentioned above was chosen to be followed to completion.

+Cl

H

Cl

CH3

CHH2CSiPlatinum catalyst

Toluene, RT, 60 hoursClCl

CH3

Si

CH2

CH2

Scheme 3.1 The preparation of DCPEMS with a platinum catalyst2,3.

The two step process of the preparation of phenethyl magnesium bromide and its

subsequent reaction with trichloromethylsilane is shown in scheme 3.2. The second step

also gave (PhCH2CH2)2SiMeCl and (PhCH2CH2)3SiMe in low amounts. As the former

would inhibit subsequent polymerisation reactions, the product mixture was redistilled five

times. The details of this process are shown in table 3.1. The polymerisation of

DCPEMS was performed directly after the fifth distillation. The product DCPEMS was

colourless, viscous at room temperature, corrosive and fuming in air.

Distillation temperature ± 0.5 °C, pressure ± 0.05 mmHg DCMPS (gas

chromatography)

1 80 °C @ 1.3 mmHg to 90 °C @ 2.4 mmHg -

2 62.5 °C @ 1.0 mmHg to 71.5 °C @ 0.4 mmHg -

3 82 °C @ 1.0 mmHg 91.8 %

4 75 to 78 °C @ 0.6 mmHg 96.8 %

5 91 °C @ 1.1 mmHg -

Table 3.1 The 5 step purification of DCMEPS by distillation.

73

CH2CH2Br + MgDiethylether

Reflux, 12 hoursCH2CH2MgBr

+

+ (CH2)2Cl

CH3

Si(CH2)2

DiethyletherReflux, 24 hours

(CH2)2

CH3

Si

(CH2)2

(CH2)2

CH2

ClCl

CH3

Si

CH2

SiCl3CH3+

CH2CH2MgBr

Scheme 3.2 A representation of the preparation of phenethyl magnesium bromide and itssubsequent reaction with trichloromethylsilane4.

Purified DCPEMS was characterised by 1H, 13C and 29Si NMR (figures 3.1-5, and

tables 3.2-4).

Figure 3.1 The 1H NMR of the 5 times distilled product dichloro-β-phenethylmethylsilane

(at 60 MHz.)

74

Peaks / ppm Number of peaks Relative number of hydrogens by

integration

Resonance

indicated1,2

7.2 multiplet 5 a

2.8 triplet 2 b

1.4 triplet 2 c

0.6 1 + minor peaks 3 d

3.35 multiplet - Impurity

3.85 multiplet - Impurity

Table 3.2 Results of 1H NMR analysis of DCPEMS.

Notes to the table;

1) shown in figure 3.2

CH2 CH2 CH3Si

Cl

Cl

H

a b c d

Figure 3.2 The structure of DCPEMS for the 1H NMR.

2) Minor peaks were thought to be due to di- and tri-substituted molecules. The

peaks at 3.35 ppm and 3.85 ppm were thought to be due to methylene hydrogen

absorptions on the di- and tri- substituted molecules respectively.

Figure 3.3 The 13C NMR of the 5 times distilled product dichloro-β-phenethylmethylsilane

(at 67.8 MHz).

75

Peak / ppm Indicated1,2

142.3, 128.5, 127.8, 126.2 a

28.5 b

23.4 c

5.19 d

Table 3.3 The results of the 13C analysis of DCPEMS.

Notes to the table;

1) shown in figure 3.4

CH2 CH2 CH3Si

Cl

Cl

ab c d

Figure 3.4 The structure of DCPEMS for its 13C NMR.

2) several smaller peaks are seen, most likely due to impurities.

Figure 3.5 The 29Si NMR of the 5 times distilled product dichloro-β-phenethylmethylsilane

(at 67.8 MHz).

Peak / ppm Indicated

56.4 Impurities1

24.1 a2

14.5 Impurities3

Table 3.4 Results of the 29Si NMR analysis of DCPEMS.

76

Notes to the table;

1) the peak at ~56 ppm was seen to reduce in size from sample 3 to sample 5,

indicating that it is due to an impurity. It is possibly an oxidised DCPEMS as

detailed in scheme 3.3, or a substituted derivative. It is rare for alkyl substituted

silanes to show absorptions as high as 50 ppm, thus the signal is most likely due to

a siloxy impurity.

Si

CH3

Cl

OH(CH2)2 + Si

CH3

Cl

(CH2)2 Cl

CH3

ClCH3

Cl

(CH2)2 Si (CH2)2Si O

a

Scheme 3.3 The structure of DCPEMS for the 29Si NMR and the reaction of DCPEMS withwater.

2) the absorption frequence concurs with that previously observed13 (scheme 3.3).

3) an absorption, at a higher field, of an impurity - most likely derived from the

multiple substitution of the trichlorosilane by the Grignard reagent.

3.3 The Wurtz Polycondensation of Dichloro-β-phenethylmethylsilane

To study the effects of using DCPEMS in place of DCMPS in a Wurtz reaction, the

effects of varying solvent, time of reaction, rate of addition of monomer, use of a phase

transfer catalyst and, the type of the terminating reagent were investigated.

3.3.1 Effect of Solvent

Three solvents were chosen for the study: toluene, THF and diethylether. All

three are commonly used in the Wurtz polycondensation of dichlorosilanes8,9. Polar and

non-polar solvents are represented. The reaction methodology used in each case was the

same; the ‘normal’ mode of addition, 3.5 hours at reflux temperature and with the same

77

amounts of reagents. Each sample was analysed by GPC using a UV detector set at 254

nm (table 3.5).

Solvent M p / g mol-1 M n / g mol-1 M w / g mol-1 Polydispersity Yield / %

toluene 2960 3420 103700 30.3 20.2

diethylether 5060 4030 19720 4.9 19.2

THF 1940 1640 34100 2.1 13.8

Table 3.5 Molecular weight parameters and yields of PPEMS prepared in different solvents.

Diethylether and toluene yielded most polymer. However, with all the solvents

used yields were low with respect to those of polymerisations of DCMPS5. Molecular

weight distributions were found to be trimodal in toluene and in diethylether, and bimodal

in THF (figure 3.6). In all cases the lower molecular weight fraction can be considered to

be cyclosilanes. All the polymer samples were sticky and greyish white in colour. On

exposure to daylight over several days, the colour of the samples changed to a light brown.

Figure 3.6 Representative GPC curves of PPEMS prepared in; (a) toluene, (b) diethyletherand (c) THF.

78

3.3.2 Effect of Reaction Time

The effects of duration of reaction were studied for reactions in toluene (table 3.6)

and in diethylether (table 3.7). Again, the other conditions of reaction were maintained.

Time / minutes M p / g mol-1 M n / g mol-1 M w / g mol-1 Polydispersity Yield / %

15 3520 4740 635400 134 10.8

30 2610 3530 228600 65 8.2

120 2400 2630 206500 78 15.3

210 2960 3420 103700 30.3 20.2

390 2140 2400 240800 100 32.2

Table 3.6 Representative polymerisations of DCPEMS in toluene.

Time / minutes M p / g mol-1 M n / g mol-1 M w / g mol-1 Polydispersity

30 580 1100 7000 6.4

60 580 1330 11120 8.4

210 5060 4030 19720 4.9

8700 6250 6260 24900 4.0

Table 3.7 Representative polymerisations of DCPEMS in diethyether.

In toluene, PPEMS with a significant high molecular weight fraction was formed

quickly. As the reaction proceeded, there was a noticable formation of oligomeric material.

The yield of polymer increased to around 30%. In diethylether, the molecular weight

distribution of polymer was less variable than that found for the reaction in toluene, and

the proportion of medium molecular weight fraction (~ 3000 to 30000 g mol-1) increased

with time.

3.3.3 Effect of Rate of Addition of Reagents

Two reactions, each of 3.5 hours overall, were performed in diethylether using the

‘normal’ mode of addition (table 3.8), i.e. DCPEMS was added to the sodium dispersion9.

79

Rate of addition M p / g mol-1 M n / g mol-1 M w / g mol-1 Polydispersity

Instantaneous 5060 4030 19720 4.9

Over 60 minutes 4230 4120 16600 4.0

Table 3.8 Effects of instantaneous and slow additions of DCPEMS to a reaction.

Compared to the method of instantaneous addition, the slow addition of the

monomer to the reaction gave rise to a polymer of a lower polydispersity, but the

differences were not significant.

3.3.4 Effect of Addition of 15-crown-5

The phase transfer catalyst 15-crown-5 co-ordinates with and solvates the sodium

cation4,14. On addition to reactions in THF and in diethylether, the formation of high

molecular weight material was no longer evident. With diethylether, the values of the

medium molecular weight material decreased. With THF, only the oligomeric material was

formed as shown in tables 3.9 and 3.10. In both reactions, the addition of 15-crown-5

reduced yields.

Additive Reaction time / minutes M p M n M w Polydispersity Yield / %

No 210 5060 4030 19720 4.9 19.2

Yes 30 2230 2100 3570 1.7 18.4

Yes 210 2390 2190 3500 1.6 10.4

Yes 8700 3410 2650 4060 1.5 negligible

Table 3.9 Results of polymerisations of DCPEMS with and without 15-crown-5 indiethylether (molecular weights are in g mol-1).

Additive M p / g mol-1 M n / g mol-1 M w / g mol-1 Polydispersity Yield / %

No 2960 3420 103700 30.3 20.2

Yes 730 1000 1700 1.7 negligible

Table 3.10 Results of polymerisations of DCPEMS with and without 15-crown-5 in THF.

80

3.3.5 The Effect of Washing Poly(β-phenethylmethylsilylene) with Hexane

PPEMS was washed by hexane in a cold Soxhlet extractor over a period of 24 hours.

Samples were removed periodically and characterised by GPC, the results of which are

shown in figure 3.7 and table 3.11. There was a steady reduction in the relative amount

of low and medium molecular weight material. The higher molecular weight material that

remained was hard and glassy.

Figure 3.7 Representative GPC results following the washing of PPEMS with hexane; (a)unwashed PPEMS, (b) after washing for 180 minutes and (c) washing for 1440 minutes.

Washing /

minutes

M p / g mol-1 M n / g mol-1 M w / g mol-1 Polydispersity

0 2960 3420 103700 30.3

180 160000 10200 271000 26.5

780 152800 55800 387000 6.9

1440 152500 54600 402000 7.3

Table 3.11 Molecular weights of PPEMS washed with hexane.

81

3.4 Discussion

3.4.1 Solvent Effects

The effects of various solvents on the polymerisation of DCPEMS are similar to

those observed for polymerisations of DCPMPS (section 1.1.2.1). The main difference is

that for DCPEMS, a use of diethylether does not result in a lower yield of polymer,

although the use of THF does. It could be that diethylether is a better solvent of PPEMS

than it is of PMPS, and it stabilises the polymer’s formation. The molecular weight

distributions of PPEMS follow a similar pattern to those shown by PMPS (section

1.1.2.1). However, the molecular weights for a similar reaction time are lower, and even

when the reaction is prolonged, the medium molecular weight fraction does not approach

that attained by PMPS. This could be due to the steric effects of the -CH2-CH2-Ph

substituent.

From figure 3.6, it is evident that the intermediate molecular weight fraction

appears at between 940 and 5060 g mol-1. However, for syntheses in diethyl ether it is

clear that there are overlapping peaks in this region and that one of the lower molecular

weight region appears as a shoulder at about the same position as the peak for polymers

synthesised in toluene. From the totality of the results it therefore appears that an Mp of

about 2500 for this fraction would be a reasonable value to use in an estimation of the

average length of all-trans sequences in PPEMS. Assuming that the molecular weights

determined relative to polystyrene standards are accurate, this corresponds to 17 chain

units.

3.4.2 Rates of Polymerisation

The polymerisation of DCPEMS is slower than that of DCMPS. This may be due

to steric effects of the larger CH2CH2Ph group7, but perhaps more important is the effect

of the extended σ-orbital of the all-trans segments. In PMPS, the σ-orbital interacts with

phenyl π-orbitals15. Therefore, it is more stable than the σ-orbital of PPEMS.

82

3.4.3 Effects of Additives

15-crown-5 transports sodium as Na+/Na- to PPEMS in toluene. The transfer of an

electron from the sodium to PPEMS causes a polymer chain scission. An electron is

transferred along the polymer chain to a cis linkage by mobile σ*-orbitals. At high

temperatures there are many cis links, thus the break down of the chain may occur more

rapidly resulting in a formation of oligomers and cyclic oligomers10.

3.4.4 Washing and Fractionation of Poly(β-phenethylmethylsilylene)

The washing of PPEMS exposes a large difference in the solubility of this polymer

and PMPS. With PMPS, only very low molecular weight material can be extracted by

hexane5. A large proportion of the medium molecular weight of PPEMS was removed due

to its higher solubililty in hexane, perhaps because of a higher aliphatic content of PPEMS

substituents.

3.5 Conclusion

The results indicate that due to the high number of weak gauche links in PPEMS

relative to that found in PMPS, PPEMS is unsuitable as a polymer for further Friedel-

Crafts chemistry at its phenyl substitutents.

3.6 Suggestions for Further Work

The following experiments would advance this study of the Wurtz

polycondensation of DCPEMS:

to follow the polymerisation of DCPEMS in THF, at room temperature and at

reflux temperature over a longer period of time.

to follow the variations of the polymer molecular weight fractions with time.

to characterise the proportions of polymer remaining in colloid and solution.

83

References

1 Conversation with A. Wiseman2 Albinati, Caseri, Pregosin, Organomet., 788, 6, (1987)3 S. J. Webb, PhD Thesis (1995)4 Advanced Organic Chemistry, 3rd Edition, John Wiley and Sons; New York,

Chichester, Brisbane, Toronto, Singapore, 77-79 (1985) Ed. Jerry March5 R. G. Jones, U. Budnick, S. J. Holder, W. K. C. Wong, Macromolecules, 8036, 29

(1996)6 R. G. Jones, R. E. Benfield, R. H. Cragg, A. C. Swain, S. J. Webb, Macromolecules,

4878, 26 (1993)7 P. Trefonas, R. West, R. D. Miller, D. Hofer, J. Polym. Sci., Polym. Lett. Ed., 823, 21

(1983)8 Inorganic Polymers, Ed. J. E. Mark, H. R. Allcock, R. West, Prentice-Hall, New Jersey,

Chapt. 5 (1992)9 R. D. Miller, J. Michl, Chem. Rev., 1359-1410, 89 (1989)10 R. G. Jones, W. K. C. Wong, S. J. Holder, Organomet., 59, 17(1) (1998)11 R. G. Jones, R. E. Benfield, P. J. Evans, S. J. Holder, J. A. M. Locke, J. Organomet.

Chem., 171, 521 (1996)12 Y.-L. Hsiao, R. M. Waymouth, J. Am. Chem. Soc., 9779, 116 (1994)13 Schroml, Ngugen-Dui-Chuy, Chvalousky, J. Organomet. Chem., 5, 51 (1973)14 Pedersen, J. Am. Chem. Soc. 89, 2495, 7017 (1967)]15 J. Michl, Synth. Met., 367, 49-50 (1992)

84

4 Anionic Polymeristions of Isoprene Using High

Vacuum or Schlenk Techniques

Abstract

Anionic polymerisations of isoprene were conducted both under high vacuum

conditions, and using Schlenk techniques. The two methods are compared. The

preparation of the di-anionic initiator, oligo(α-methylstyryl) disodium, and its subsequent

use in the polymerisations of isoprene was studied. Polymers of low polydisperisties were

formed by both methods. It is concluded that the latter method is more convenient, both in

terms of general applicability, and for further syntheses.

This work has been presented, in part, at Conferences in Barcelona (9/1995),

Canterbury (9/1997), Genoa (3/1996), Madrid (3/1997), and Valencia (9/1996) under the

auspices of the ARPEGE project of the EU.

Thanks are extended for the financial support of the ARPEGE project of the EU

Human Capital and Mobility Programme (Contract No. ERBCHRXCT 940517).

85

4.1 Anionic Polymerisations of Isoprene Using High Vacuum Techniques

4.1.1 Introduction

Anionic polymerisations are traditionally carried out using high vacuum techniques

to maintain the purity of the reagents and the solutions1. While this rigorous method

ensures the synthesis of polymers of a low polydispersity, it can be time consuming,

requires a skilled hand to complete and it does not facilitate the handling of samples for

further chemistry. In using Schlenk techniques and inert gases, it was seen that this method

presented a simpler and faster route to performing anionic polymerisations. However, it

could not be assured that this route would give rise to polymers and oligomers of an

equivalent purity and low molecular weight polydispersity to those found when using the

high vacuum line. The reactive anionic ‘living’ ion pair is extremely sensitive to impurities

such as water or air1, and only a few polymerisations have been carried out by this

method2,3,4. It is for these reasons that the use of a high vacuum line has been preferred,

and the method has been used with a variety of reagents to tailor the synthesis of a wide

variety of products, typically in the preparation of ‘star’ polymers from a ‘buckminster

fullerene’ complex5,6. The high vacuum, maintained in a hand built system, allows the near

eradication of impurities, both in the volume of the manifolds, and at the glass walls of the

reaction vessel. This enables the narrowest of molecular weight distributions of the

synthesised polymers1 (figure 4.1). The perfection of the evacuated and purified systems,

represents for many a reliable method for anionic polymerisations.

The experimental conditions and the preparation of samples mentioned in this

chapter are detailed in appendix 2. Where appropriate, the samples were characterised by

standard methods (GPC, NMR, DSC, UV and IR).

86

Figure 4.1 A representational figure of a typical high vacuum line.Notes to Figure:

1 The internal diameter of the manifold glass tubes is no less than 3 cm, to aid

evacution.

2 The oil pump, reducing the pressure to 10-1 to 10-2 mmHg.

3 Oil (or more dangerously, mercury) diffusion pump reducing the pressure to

approximately 10-6 mmHg.

4 McLeod gauge.

5 Flask for storing degassed solvents on the line.

6 Flask for storing solvents on butyl lithium.

7 Attachment point for a reaction vessel.

The glassware is cleaned and then flame dried while under vacuum1. All reagents are

distilled, dried, and stored either on the vacuum line, or in ampoules with glass break-seals,

or in fragile glass bulbs. The last two may be joined to the reaction vessel and then broken

by a magnetic breaker (figure 4.2).

87

Figure 4.2 An example of a reaction system with reagents stored in break-seals and fragilebulbs.

The sealed systems for the preparation of ampoules, bulbs, and for performing the

polymerisations are evacuated and washed with a weak solution of butyl lithium. Up to

one third of the initiator may be lost to the solvent1. Comparing the results of anionic

polymerisations can be nigh impossible, primarily because the reagents are extremely

sensitive to both impurities and reaction conditions. The same polymerisation performed

by different groups can give different results in terms of the polydispersities of the

products and the rates of reactions observed5. It is also for this reason that the high

vacuum system is widely used6,7,8,9.

88

4.1.2 Methodology and Results

The polymerisations were performed with isoprene for three reasons. Firstly,

isoprene is easy to handle: it is a liquid with a low boiling point and may be cold distilled

about the vacuum line and purified quickly. Secondly, its rate of initiation is greater than

its rate of propagation, thus allowing the simple addition of the monomer to solutions of

the initiator. Thirdly, polyisoprene (PI) may be expected to impart strength, flexibility and

impact resistance to block copolymers with PMPS. The monomer was distilled from over

n-butyl lithium (n-BuLi). Three solvents were chosen in which to perform the anionic

polymerisations; cyclohexane, toluene, and toluene with THF. These gave both a range of

results and allowed the author to try different techniques. The GPCs of the products

indicated a synthesis of polyisoprene of a low polydispersity, thus verifying the purity of

the system (table 4.1, figure 4.3).

Prior to the addition of isoprene, a high proportion of initiator is lost to impurities

in the solvent1. This is amplified by using a low concentration of initiator. The low rate of

polymerisation of isoprene in toluene reduces the polydispersity of the polymer, as its rate

of initiation is relatively fast, however the reaction requires 20 hours or so to reach

completion.

Figure 4.3 A representative GPC of a polyisoprene formed using high vacuum techniques.

89

Solvent /

volume

s-BuLi / mol

(conc. / M)

Isoprene / mol

(conc. / M)

Expected

MW

M p M n M w P.D

Cyclohexane/

3 ml

2.7 x 10-4

(9.0 x 10-3)

0.020

(0.67)

5000 5600 5240 5670 1.08

Toluene /

30 ml

3.6 x 10-5

(1.2 x 10-3)

0.016

(0.52)

30000 58000 51700 54000 1.04

Toluene /30 ml

THF /3 ml

4.5 x 10-5

(1.5 x 10-3)

0.020

(0.67)

30000 54900 46960 50360 1.07

Table 4.1 GPC results of three representative polymerisations of isoprene with s-butyllithium. (All at -78 °C, yields consistently near 100%. Molecular weights are in g mol-1).

The 1H NMR characterisation of the samples can be used to indicate the effects of

the solvents on the configuration of the polyisoprene (table 4.2, figure 4.4). Toluene

promotes the formation of 1,4 trans conformers, and THF, 1,4 cis conformers5.

Figure 4.4 A representative example of the 1H NMR analysis of a polyisoprene in C6D6,showing the principal peaks used for defining the microstructure (at 270 MHz).

90

Solvent 1,2 configurations/% 3,4 configurations/% 1,4 configurations

(including cis and trans)/ %

Cyclohexane 10.5 67.7 21.8

Toluene 0.0 42.9 57.1

Toluene/THF 23.0 66.0 11.0

Table 4.2 Results of the 1H NMR analyses of three exampled polyisoprenes.

4.1.3 Discussion and Conclusion

The polyisoprene samples were prepared with a low polydispersity. However, the

setting up and manipulation of apparatus required time and great effort. Due to the limit in

the number of break seals that could be set on each reaction vessel, the number of samples

that could be drawn from a single reaction was limited. The use of this apparatus would

restrict a determination of the optimum conditions for the reaction of dichloro-

poly(methylphenylsilylene) with ‘living’ anionic polymers. In addition, the manipulation

of the preformed dichloro-poly(methylphenylsilylene) would be restricted due to the

techniques of glass blowing required to form the reaction vessel.

4.2 Preparation of Oligo(α-methylstyryl) Disodium Using

High Vacuum Line or Schlenk Techniques

4.2.1 Introduction

The initiator, oligo(α-methylstyryl) disodium (OαMS), is usually prepared using

high vacuum techniques from a solution of α-methylstyrene in contact with a sodium

mirror1,10,11,12. An equilibrium exists between the OαMS, α-methylstyrene and the metal

in THF11. Originally it was thought that the structure of the oligomer arose from sequential

head to tail additions of the monomer11 but it was later shown, by mass spectrometry,

NMR and infra red spectroscopy to have that shown in scheme 4.112. Typically, the

reaction can give rise to dimers, trimers, tetramers and pentamers, all configured in the same

fashion as the exampled tetramer.

91

OαMS was chosen for several reasons: it is simple to prepare, it is stable over long

periods of time in a sealed vessel and it is characterised12. When used in polymerisations it

does not leave impurities in the product mixture and it can be used as a di-anionic initiator

of polymerisations which can facilitate the formation of multi-block copolymers.

Na+2 CH CH(CH3)

Na+ -C(CH3)-CH2-CH2-C(CH3)- Na+ + 2CH C(CH3)

Na+ -C(CH3)-CH2-CH2-C(CH3)- Na+

Na+ -C(CH3)-CH2-C(CH3)-CH2-CH2-C(CH3)-CH2-C(CH3)- Na+

Scheme 4.1 Representative oligomers: di(α-methylstyryl) disodium and tetra(α−

methylstyryl) disodium.

4.2.2 Methodology and Results

4.2.2.1 Synthesising Oligo(α-methylstyryl) Disodium Using High Vacuum

Techniques

The techniques used have been documented10,11,12. The requisite sodium mirror was

hard to reproduce through melting and vapourising the metal onto a glass wall. Once the

reagents had been mixed under high vacuum conditions, the solution was left stirring for 24

hours so that an equilibrium was reached11. The GPC of a methanol quenched sample

indicated a high polydispersity (Mn = 550 g mol-1, Mw = 4500 g mol-1). This indicated that

the contact between the sodium film and the solution of the monomer was not good.

92

4.2.2.2 Synthesising Oligo(α-methylstyryl) Disodium Using

Schlenk Techniques

In synthesising OαMS under inert gases in Schlenk tubes, a good contact was

achieved between dispersed sodium and solution, and samples were easily manipulated.

The reaction was followed over time (table 4.3).

The oligomers were precipitated in methanol and the calculated molecular weights of

products are; dimer, 298.4 g mol-1; trimer, 416.6 g mol-1; tetramer, 534.8 g mol-1; and

pentamer, 652.96 g mol-1. A GPC of a sample withdrawn after 7 days indicated the

principal oligomers to be dimers, trimers, tetramers and pentamers (figure 4.5).

Time M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

3 hours 520 700 1230 1.8

18 hours 380 320 420 1.3

7 days 430 330 390 1.2

13 days 340 300 460 1.4

24 days 340 360 510 1.4

Table 4.3 Representative preparations of OαMS sustained over different times.

Figure 4.5 A representative GPC of a oligo(α-methylstyryl) disodium after 7 days.

93

Over 7 days, the equilibrium between the oligomers and sodium and α-

methylstyrene favours the oligomers. After 13 days, and more visibly so on the 21st day, a

low amount of polymers was indicated by GPC to have formed. This would suggest that

the system is well protected from impurities and can be kept for use over a period of one

week as its equilibrium is negligibly disturbed.

In testing the initiator (at a concentration of 0.13 M) by polymerising isoprene in

THF it was found that it remained stable over 10 days, with a negligible loss of strength,

although after 14 days, the strength had dropped by 20 %, and after 17 days, the

concentration of the disodium oligomer had dropped to 0.06 M. This would suggest that

Schlenk tubes may be used to store this sensitive reagent with a negligible loss of reactivity

for at least one week.

4.2.3 Discussion and Conclusion

The methods are easy to compare. The high vacuum line technique requires a high

degree of expertise, whereas the Schlenk techniques require care but no great effort. The

high vacuum technique requires reagents to be fastidiously purified and separated into

break-seal ampoules, whilst the Schlenk techniques require only simple preparations that

can be completed in less than a few hours which are less dangerous. The products from the

latter system are easier to use and characterise, and the results indicate that there is no great

loss of the initiator to impurities. While the stability of the initiator may be enhanced by

the use of high vacuum techniques, the method probably requires one week to set up.

For reactions of ‘living’ polymers with dihalo(polysilylenes), initiators should be

prepared using Schlenk techniques. Initiator samples may be used over a period of one

week.

94

4.3 Anionic Polymerisations of Isoprene Using High Vacuum

or Schlenk Techniques

4.3.1 Introduction

Investigations of the polymerisation of isoprene in THF with OαMS have shown

that in polar solvents with a sodium counter-ion, isoprene carbanions are unstable10,13,14.

Side and reversible reactions and cis-trans isomerisation occur during the polymerisations,

although these are reduced with temperature10,12,13. Why then study a polymerisation of

isoprene under such conditions? Firstly, because any reactions of ‘living’ polymers with

dihalpoly(silylenes) were expected to proceed much more smoothly with a sodium gegen-

ion since it is more reactive than the lithium equivalent10. Secondly, in THF the possibly

disruptive dimeric association of active ion pairs does not occur5 (section 1.2.3). Knowing

that ‘living’ polymers are extremely sensitive to impurities, it was worthwhile investigating

the possible effects of the two available synthetic methods.

4.3.2 Anionic Polymerisation of Isoprene Using High Vacuum Techniques

Polyisoprene samples were prepared using high vacuum techniques. The

polymerisations were in THF, using OαMS as an initiator. The products were

characterised by GPC the results of which are shown in table 4.4. The high

polydispersity of product polymers is due to the destabilising effect of THF on the

reacting isoprenyl sodium ion-pair.

Sample M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

1 2400 1990 2500 1.24

2 5040 2820 4470 1.58

Table 4.4 Representative results from GPC analyses of polyisoprene.

95

4.3.3 Anionic Polymerisations of Isoprene Using Schlenk Techniques

GPCs of polyisoprenes synthesised in THF using OαMS as an initiator indicated

them to have a polydispersities similar to those of polymers formed under high vacuum

conditions (tables 4.4 and 4.5 and figure 4.6).

The polydispersities of the polyisoprenes resulted from a destabilsation of reactive

ion-pairs in THF, and this masked possible effects of a use of Schlenk techniques. To

confirm that Schlenk techniques could be used to conduct reactions leading to polymers of

low polydispersities, a polymerisation of isoprene was performed in a toluene/ THF (9:1)

solvent. Polyisoprene of a polydisperisty of 1.07 was attained as shown in figure 4.7,

thus indicating Schlenk techniques to be effective in preparing polymers of low

polydispersities.

Figure 4.6 A representative GPC analysis of polyisoprene synthesised using OαMS in THF.

Sample M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

1 6330 5040 6510 1.29

2 10290 7860 10520 1.34

Table 4.5 Representative molecular weights of polyisoprenes synthesised from OαMS in

THF.

96

Figure 4.7 A representative GPC curve of polyisoprene, prepared in toluene/THF (9:1) usingOαMS (Mp = 15180 g mol-1, Mn = 13380 g mol-1, Mw = 14350 g mol-1, P.D = 1.07).

GPCs of product polyisoprenes formed using Schlenk techniques indicated that

considerable care was required when performing polymerisations. This was especially so

when working at low concentrations of reagents. An example is shown in figure 4.8,

where a polyisoprene with a bimodal molecular weight distribution resulted from the use of

an insufficiently dry inert gas.

Figure 4.8 An example of a bimodal molecular weight distribution of polyisoprene.

97

A polyisoprene synthesied in THF using OαMS under Schlenk conditions was

further characterised by 1H NMR and DSC. 1H NMR indicated that the polymer had a

high content of 1,2 units (δ = 5.1 and 5.9 ppm), 3,4 units (δ = 4.8 ppm) but a low content

of 1,4 (inseperable cis and trans) units as evidenced by resonances at δ = 5.3 ppm (table

4.6, figure 4.4 and 4.9). The Tg value of 18.7 °C is consistent with a structure of low 1:4

content (figure 4.10) 15.

1,2 units 3,4 units 1,4 units

22.2 % 73.9 % 3.9 %

Table 4.6 Representative 1H NMR calculated integrals of resonances of polyisoprene in C6D6.

Figure 4.9 A representative 1H NMR of polyisoprene synthesised in THF with OαMS (at

270 MHz).

98

Figure 4.10 A representative DSC curve of polyisoprene (second run, passage rate of 10 °Cminute-1).

4.3.4 Discussion and Conclusion

The results indicate that the anionic polymerisations of isoprene may be carried out

using Schlenk techniques with an exactitude similar to that obtained using high vacuum

techniques. The former method facilitates further chemistry, allows samples to be removed

easily, and is less dangerous than the latter.

Polymerisations of isoprene in THF with sodium tend to give polymers of a

relatively high polydispersity. This is due to the active isoprenyl sodium ion-pair in THF

being extremely sensitive to impurities5. However, this is outweighed by their

applicability to polycondensation reactions.

Polyisoprene synthesised in THF with sodium as a gegen-ion shows a high Tg due

to the low proportion of 1,4 cis and trans units in chain.

99

References

1 M. Morton, R. Milkovich, J. Polym. Sci., Pt. A, 443, 1 (1963)2 S. K. Varshney, J. P. Hautekeer, R. Fayt, R. Jerome, Ph. Teyssie, Macromolecules,

2618, 23 (1990)3 R. G. Jones, L. Lutsen, Polym. Int., 46(1) (1998)4 S. Demoustier-Champagne, A.-F. de Mahieu, J. Devaux, R. Fayt, Ph. Teyssie,

J. Polym. Sci., Polym. Chem., 2009, 31 (1993)5 M. Morton, ‘Anionic Polymerisation: Principles and Practise’, Academic Press Inc.,

(1983)6 Y. Ederlé, C. Mathis, Macromolecules, 4262, 30(15) 19977 M. Morton, L. J. Fetters, J. Polym. Sci., Pt A., 3311, 2 (1964)8 D. Rahlwes, J. E. L. Roovers, S. Bywater, Marcomolecules, 604, 10 (1977)9 T. Fujimoto, M. Nagasawa, Polym. J., 397, 7(3) (1975)10 A. Garton, R. P. Chaplin, S. Bywater, Eur. Polym. J., 697, 12 (1976)11 A. Vrancken, J. Smid, M. Szwarc, 2036, Trans. Faraday. Soc., 2036, 58 (1962)12 D. H. Richards, R. L. Williams, J. Polym. Sci., Polym. Chem. Ed., 89, 11 (1973)13 S. Bywater, A. F. Johnson, Can. J. Chem., 1255, 42 (1964)14 A. Garton, S. Bywater, Macromolecules, 694, 8 (1975)15 C. Kow, M. Morton, L. J. Fetters, Rubber Chem. Technol., 245, 55(1) (1982)

100

5 The Synthesis and Characterisation of Block and

Multi-block Copolymers of Poly(methylphenylsilylene)

Abstract

Multi-block copolymers of poly(methylphenylsilylene) and polyisoprene, and of

poly(methylphenylsilylene) and poly(oxyethylene) were synthesised via condensation reactions

of α,ω-dihalopoly(methylphenylsilylene) with di-anionic ‘living’ polyisoprenyl disodium and

with poly(oxyethylene) glycol respectively. ABA and AB type block copolymers of

poly(methylphenylsilylene) with poly(oxyethylene) or poly(α-methylstyrene) were also

synthesised. The optimum conditions required for reactions involving di-anionic polymers

were sought using time varient studies and through a model copolymerisation of

dichlorormethylphenylsilane with oligo(α-methylstyryl) disodium. The multi-block

copolymers have normal size distributions, which are related to the most probable

distributions expected. The limits of the reactions are discussed in terms of the reactivity of

the polymer end groups, the character and the immiscibility of the polymers in solution. This

is the first synthesis of multiblock copolymers of poly(methylphenylsilylene), and, in the case of

the poly(methylphenylsilylene) and poly(oxyethylene) copolymers, the first synthesis of block

copolymers of poly(methylphenylsilylene) which are water dispersible.

This work, has in part been published or presented as follows:

(i) in S. J. Holder, R. C. Hiorns, N. A. J. M. Sommerdijk, S. J. Williams, R. G. Jones

and R. J. M. Nolte, Chemical Communications, 1445, 14 (1998)

(ii) in R. G. Jones, R. C. Hiorns, S. J. Holder, R. M. Nolte, N. Sommerdijk, S. J. Williams,

Abstracts of the 31st Organosilicon Symposium, New Orleans, USA B2 May 1998.

(iii) at the 2nd International Symposium, Silicon-Based Polymers, Tokyo, Japan, June 1997

(iv) at conferences in Madrid (3/1997) and Canterbury (9/1997) for ARPEGE

(v) at the 11th International Symposium on Organosilicon Chemistry, France (9/1996) and

at the Macrogroup Conference for Young Scientists, Leeds, England (4/1997).

Thanks are extended for the financial support of this research by the ARPEGE project

of the EC Human Capital and Mobility Programme (Contract No. ERBCHRXCT 940517).

101

5.1 Introduction

This chapter details the preparation of block copolymers of

poly(methylphenylsilylene) (PMPS) with polyisoprene (PI), or poly(α-methylstyrene)

(PαMS), or poly(oxyethylene) (PEO). PMPS is a brittle and unmaleable material

(section 1.1.4). Block copolymers with the aforementioned materials were thought not

only to ameliorate its properties, but also provide materials with qualities specific to other

components of the block copolymers. A model condensation reaction of

dichloromethylphenylsilane and oligo(α-methylstyryl) disodium was explored and all the

reactions were followed by GPC analysis. These experiments facilitated the optimisation

of the block copolymer syntheses.

Thus, this chapter is divided into three sections; (i) the model condensation

reaction, (ii) the optimisation of reactions of α,ω-dichloropoly(methylphenylsilylene)

(DCPMPS) and α,ω-dihalopoly(methylphenylsilylene) (DXPMPS) with ‘living’ dianionic

polymers, and (iii) the reactions of DXPMPS with the hydrophilic polymers. Finally, the

different reactions leading to the aforementioned lyophilic and amphiphilic block

copolymers are compared.

The experimental conditions and the preparation of samples are detailed in

appendix 2. Where appropriate, the samples were characterised by standard methods

(GPC, NMR, DSC, UV and IR).

5.2 The Reaction of Dichloromethylphenylsilane and

Oligo(α-methylstyryl) Disodium

5.2.1 Introduction

The reaction between OαMS and DCMPS was studied as a model of the planned

syntheses of block copolymers. It may be argued that the latter reaction may depend

greatly on the behaviour of polymers in solution and the susceptibility of PMPS to

degradation (section 1.1.3), and the model does not address such issues. However, the

rates of the reaction and susceptibilities of the reactions to impurities were either unknown

102

or at best predictable and the model would at least address these issues. Reactions leading

to this alternating copolymer have been studied before, however, the only publication has

been as a foot note to an article1. The reaction is a simple condensation (scheme 5.1).

Si

CH3

ClCl Na+CC

CH3

C C

CH3

--Na+n

p p+

H

H H

H

pp-1 NaCl+Cl

H

H H

Hn

Na+-C

CH3

C C

CH3

CSi

CH3

THF, R.T5 minutes

Scheme 5.1 A representation of a synthesis of poly[olig(α-methylstyrene)-co-

methylphenylsilane], in which n = 1, 1.5, 2 and 2.5 and p = 3 to 30.

5.2.2 Methodology and Results

A solution of DCMPS in THF was titrated against the strongly blood-red solution

of ‘living’ OαMS in THF. Once the colour had disappeared, the solution was left to stir.

Almost immediately the solution gelled. After this point, no variation in the molecular

weight of the product was indicated by analysis by GPC (figure 5.1, table 5.1). The

copolymers were isolated by precipitation in methanol.

Characterisation by GPC indicated that poly[oligo(α-methylstyrene)-co-

methylphenylsilane] (OαMS/MPS) was synthesised with a degree of polymerisation

ranging from approximately 3 to 30.

103

Figure 5.1 A representative GPC curve of the alternating copolymer OαMS-MPS.

Compound M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

OαMS 340 340 440 1.30

OαMS-MPS 2010 2030 3069 1.51

Table 5.1 Representative GPC results of OαMS and the product of its reaction with DCMPS.

GPC results indicate that the copolymer has a multi-modal molecular weight

distribution. However, considering that OαMS was also multi-modal, this is not

surprising. The reaction is fast and yields 28% product precipitated from methanol.

The 1H NMR of the product polymer (figure 5.2) indicates the formation of an

alternating copolymer. Resonances due to phenyl protons pendent to carbon and silicon

are centred about 7.3 ppm. Peaks at around 3.5 ppm arising from methoxy protons show

those adjacent to silicon resonating slightly downfield from those bonded to carbon atoms.

Assuming that both chain ends in the copolymer are capped with methoxy groups, then

from the ratio of phenyl to methoxy protons (23 to 2 respectively) determined from the

integrals of the 1H NMR spectrum indicate an average molecular weight for the copolymer

of 2700.

Knowing the molecular weight of each oligomer (dimer, trimer and so on) of OαMS

and of the methylphenylsilane group, an attempt to calculate the average structure of

OαMS/MPS was performed. OαMS is known to exist as a head-to-tail structure2. The

104

ratio of the OαMS oligomers was estimated to be around 5 dimers to each 2 trimers from a

simple inspection of GPC curves. The above calculated average molecular weight of

OαMS/MPS thus indicated that the average copolymer molecule was formed of 5 dimers

and 2 trimers of OαMS with 7 linking methylphenylsilanes. This could not be further

confirmed due to the overlapping, broad resonances displayed by the in-chain methylene

protons adjacent to silicon (around 2.6 ppm), β to carbon (around 2.0 ppm), and the

methyl protons at carbon and silicon (two peaks at around 1.2 ppm and 0.9 ppm

respectively). The molecular weight so calculated is in reasonable agreement with that

determined by GPC, however the latter is with reference to polystyrene standards, and

may not be exact.

Figure 5.2 A representative 1H NMR of the alternating copolymer OαMS-MPS in CDCl3 (at

270 MHz).

To confirm again that the polymer was an alternating copolymer, an infra red

analysis was performed. No absorbance of an Si-Si bond stretch normally at c. 470 cm-1

was observed. DSC characterisation of the sample showed a single Tg at 79 °C, indicating

the formation of a regularly structured alternating copolymer3 (figure 5.3).

105

Figure 5.3 A representative DSC curve of the alternating copolymer OαMS-MPS (at a

heating rate of 10 °C per minute).

5.2.3 Discussion

That this reaction occurs rapidly should not surprising; Si-Cl and carbanions are

extremely reactive. The reaction most likely occurs via an SN2 subsitution at the silicon

atom4 (section 1.1.3). The rapid formation of a gel during the reaction is the most likely

reason for both the yields and molecular weights being low. This has important

implications for the planned reactions of α,ω−dichloropoly(methylphenylsilylene) in that

the precipitation of the product could determine the degree of polymerisation attainable.

5.2.4 Conclusion

On the equimolar mixing the reagents OαMS and DCMPS, the initial concentration

of reactive ends groups decreases rapidly as the average degree of polymerisation of

OαMS/MPS increases. As the rate of the reaction is proportional to the multiple of the

concentration of each reagent, it can be seen that the rate of the reaction decreases rapidly

as the formation of OαMS/MPS occurs. However, the formation of a gel arrests the

reaction.

The reactions of α,ω−dichloropoly(methylphenylsilylene) with other polymers are

expected to be much slower than those of the model system, as the concentration of end

groups will be considerably lower. Thus, the formation of block copolymers in these

reactions would be aided by a high concentration of reagents, even though this may risk the

formation of a gel.

106

5.3 Reactions of Dihalo-poly(methylphenylsilyene) with Di-anionic

Polymers

5.3.1 Introduction

Rapid ‘normal’ addition of DCMPS to a fine dispersion of sodium in THF at room

temperature affords α,ω-dichloropoly(methylphenylsilylene) (DCPMPS) of a monomodal

distribution and high molecular weight with some oligomeric material5. The oligomers are

easily removed by fractionation6.

The mono-modal distribution of the DCPMPS should aid the formation of regular

block copolymers with ‘living’ anionic polymers. However, in order to obtain a higher

concentration of more reactive chain ends and to reduce the polydispersity of PMPS to

obtain more regular copolymers, it was decided to react DCPMPS with bromine to yield

DXPMPS. The Si-Br end groups were protected during work-up using well established

procedures7. Reactions leading to block copolymers could then be attempted using this

polymer.

5.3.2 Methodology and Results

5.3.2.1 Synthesis and Characterisation of

α,ω-dichloropoly(methylphenylsilylene)

Instead of quenching PMPS synthesised by a Wurtz reaction (scheme 1.1) with

methanol (scheme 1.10), the reactive Si-Cl chain ends can be maintained by excluding

nucleophiles from the reaction medium. Accordingly, DCPMPS prepared in THF was

fractionated by the gradual addition of hexane or petroleum ether to remove the oligomers

normally formed with the polymer. The polymeric product has a monomodal molecular

weight distribution (figure 5.4). 1H NMR shows peaks similar to those of a methoxy

terminated PMPS, with resonances at around 7 ppm and 0 ppm corresponding to phenyl

and methyl protons respectively8. However, 29Si NMR reveals a split resonance at +15

ppm due to the Si-Cl chain ends6 (figure 5.5). For the reactions described below, the

107

DCPMPS was used within one week of its preparation so as to minimise adventitious

hydrolysis of chain ends.

Figure 5.4 A representative GPC (UV at 254 nm) curve of fractionated DCPMPS preparedfrom a THF mediated Wurtz reaction (Mp = 19090 g mol-1, Mn = 13110 g mol-1, P.D = 3.3).

Figure 5.5 A representative 29Si NMR of DCPMPS showing the typical silicon main chainresonances at around -40 ppm and of the Si-Cl end group at around 15 ppm (at 67.8 MHz).

5.3.2.2 Synthesis of

Poly[oligo(α-methylstyrene)-block-poly(methylphenylsilylene)]

Before attempting the coupling of DCPMPS with dianionic polymers, a simple

coupling of OαMS with DCPMPS was carried out (scheme 5.2). This was followed with

respect to time by GPC analysis (figures 5.6 and 5.7).

108

Figure 5.6 Representative GPC curves (UV at 254 nm) of; (a) OαMS, (b) DCPMPS, and (c)

the reaction products.

Figure 5.7 Results of GPC analysis (UV at 254 nm) of a timed reaction of OαMS with

DCPMPS, in which Mp is ·······, Mn is ——, and Mw is -------.

109

Si

CH3

ClCl Na+CC

CH3

C C

CH3

--Na+n

p p+

H

H H

Hm

pmp-1 NaCl+Cl

H

H H

Hn

Na+-C

CH3

C C

CH3

CSi

CH3

THF, -78°C30 minutes

Scheme 5.2 A representation of the reaction between DCPMPS and OαMS in which n is

defined in scheme 5.1.

There is an initial growth in the polymer chain length followed by a slight decrease,

after which there is a second growth period followed by a gradual decrease in the molecular

weight. This sort of reaction has been observed in the Friedel-Crafts chemistry of PMPS9.

The initial fast growth can be attributed to the rapid SN2 chemistry at silicon chloride chain

ends, and the first drop in molecular weight to degradation of the polysilylene by excess

carbanion after which the SN2 chemistry continues to build on the copolymer chain9

(section 1.1.2.1). Eventually, of the two competing reactions, the degradation wins. This

coincided with the slow appearance of a golden-yellow colour due to a formation of silyl

anions10. Presumably, the reaction sites for chain extension have become depleted or

entangled in the thickening solution.

For this reaction, it would seem that the best time to isolate the product would be

after 360 minutes. At this point, the number average molecular weight of the product

indicates that the average block copolymer structure was OαMS–PMPS–OαMS–PMPS–

OαMS. The product was hard and brittle, consistent with it being mostly of PMPS. 1H

NMR characterisation of poly[oligo(α-methylstyrene)-block-poly(methylphenylsilylene)]

(OαMS-PMPS) showed the expected resonances at around 7 ppm, and from -1 to 0 ppm

for the protons of the phenyl rings and of the methyl groups respectively. The

110

absorptions between 1 and 2 ppm are assigned to methylene protons of the OαMS (figure

5.8).

Of more interest was the DSC characterisation of OαMS-PMPS (figure 5.9).

Second order transitions are evident at 78 °C, 120 °C and 190 °C. However, these

disappeared between the first and second cycles through an annealing effect. These

transitions could be just part of the thermal history of the sample, but the results were

repeatable from sample to sample. They correspond more or less to previously found Tgs

and phase-disordering transitions of high and low molecular weight PMPS at 120 °C and 95

°C, and 220 °C and 195 °C respectively11. However, such transitions are notoriously

difficult to observe in a sample of high molecular weight PMPS so it would seem that the

linking reaction imposes a regular long-range macro-structure on the PMPS.

Figure 5.8 An 1H NMR spectrum of OαMS-PMPS (at 270 MHz).

111

Figure 5.9 A typical DSC of OαMS-PMPS.

5.3.2.3 Degradation of Methoxy Terminated Poly(methylphenylsilylene) by

Oligo(α-methylstyryl) Disodium

The forgoing results indicated that ‘living’ OαMS could induce the degradation of

PMPS. In order to check this possibility the degradation of methoxy terminated PMPS by

‘living’ OαMS was followed using GPC. Figure 5.10 shows an initial sharp reduction in

molecular weight, followed by a slower attack on the PMPS backbone. After 30 minutes, a

feint golden colour was observed in solution.

Figure 5.10 Results of the GPC analysis following the degradation of methoxy terminatedPMPS by OαMS, in which; Mp is ·······, Mn is ——, and Mw is -------.

112

5.3.2.4 Reactions of α,ω-dichloropoly(methylphenylsilylene) with

Polyisoprenyl Disodium

The reaction of DCPMPS with polyisoprenyl disodium (PIN) was again followed

with respect to time, to establish the rate of reaction (scheme 5.3) and any possible

degradation of DCPMPS by polyisoprenyl carbanions. Nevertheless, based on the

corresponding reaction of PMPS with OαMS, the reactions were carried out at -78 °C in

order to minimise the rate of the PMPS degradation reaction.

+n

Na+ _Cl

y

CH3

Si

CH3

x/23x/2CHCH2

CH2

CH3 C

C CH2

CH3 C

CH2

CH2 CH ClNa(n-1)

n

THF9 minutes

-78°C

+

CH3

_Na+

x/23Na+ _

x/2CHCH2

CH2

CH3 C

C CH2

CH3 C

CH2

CH2 CH Si ClCl

CH3

yn

Scheme 5.3 A representative synthesis of a multi-block copolymer of PMPS and PI.

A range of molecular weights were reacted with DCPMPS (figures 5.11, 5.12 and

5.13). The concentrations of reactive end-groups in each reaction were equivalent. The

following points concerning the GPC characteristics should be noted: (i) PMPS has UV

chromophores at 254 nm (due to the phenyl π→π* transition) and at around 339 nm (due

to the σ→σ* transition of the backbone) and either can be used for detection by GPC. (ii)

PI has no accessible UV chromophores, so the refractive index detector was used in the

determination of the molecular weight of PI. Block copolymers were determined using a

113

UV detector as the experiment was simply designed to follow the reaction.

For a reaction of a PIN of low molecular weight with DCPMPS in THF at -78 °C,

the medium appeared to have a maximum viscosity 9 minutes after mixing. GPC results

indicated a considerable amount of poly[poly(methylphenylsilylene)-block-polyisoprene]

((PMPS-PI)n) to be present (figure 5.11 and table 5.2).

GPC results indicated that for low and medium molecular weight polyisoprenes, the

reaction can occur in the first hour. For higher molecular weight polyisoprenes, a greater

reaction time (c. 18 hours) is required, and this agrees with previous work12. However,

with higher molecular weight polymers, the PMPS is more stable and there is a slower fall

off in molecular weights. When PMPS, of any molecular weight, was left with PIN for

longer than 15 minutes, the reaction solution displayed a distinctive golden brown colour.

Figure 5.11 A representation of a formation of (PMPS-PI)n indicated by GPC in which; (a)is PI (RI), (b) is DCPMPS (UV, 254 nm), (c) is (PMPS-PI)n (RI), and (d) is (PMPS-PI)n (UV,254 nm).

Polymer Detector M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

DCPMPS UV (254 nm) 19570 12720 49980 3.93

PIN RI 8400 7430 11020 1.48

(PMPS-PI)n UV (254 nm) 88630 25970 557650 21.5

(PMPS-PI)n RI 45570 20770 11100 5.34

Table 5.2 Representative molecular weights of DCPMPS, PIN and (PMPS-PI)n indicated byGPC.

114

Figure 5.12 Representative GPC results of the timed reactions of DCPMPS (Mp = 8840 gmol-1, Mn = 4530 g mol-1, Mw = 19060 g mol-1) with PIN (Mp = 5830, Mn = 5570, Mw = 7035)in which; Mp is ·······, Mn is ——, and Mw is ------. The UV detector was set at 254 nm.

Figure 5.13 A representative set of results of a timed reaction of DCPMPS (Mp = 8840 gmol-1, Mn = 4530 g mol-1, Mw = 19060 g mol-1) with PIN (Mp = 16300 g mol-1, Mn = 20620 gmol-1, Mw = 29100 g mol-1), in which; Mp is ·······, Mn is ——, and Mw is -------. The UVdetector was set at 254 nm.

Peak resolution of a GPC curve (RI) of (PMPS-PI)n indicated that a significant

proportion of the starting materials remained in the mixture (figure 5.14).

115

Figure 5.14 A representative peak resolution of a GPC (RI) analysis of (PMPS-PI)n,describing some of the most probable types of block copolymers.

Polymer fractionation was accomplished by the progressive addition of isopropanol

to a solution in THF (figure 5.15 and table 5.3)13. Centrifugation was used to separate

out the very fine suspension of polymer from the various fractions.

Figure 5.15 A typical GPC of a fractionated sample of (PMPS-PI)n using a UV detector set at254 nm.

116

Isopropanol

added / ml

Ratio of

THF / Isopropanol

M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

380 0.79 / 0.21 146400 67000 452700 6.7

460 0.66 / 0.34 134400 58200 366520 6.3

520 0.58 / 0.42 46250 26200 44400 1.7

610 0.49 / 0.51 32450 19100 29300 1.5

1020 0.29 / 0.71 14070 14490 20100 1.4

Table 5.3 A fractionation of a mixture of PMPS, PI and (PMPS-PI)n (3.5 g) where theamount of THF used as a solvent was 300 ml.

Results indicate that this method allowed the separation of the multi-block

copolymer from the reagent polymers in 50% yield.

5.3.2.5 Reaction of α,ω-dichloro-poly(methylphenylsilylene) and

Poly(α-methylstyryl) Disodium

The reaction detailed in scheme 5.4 of of low molecular weight poly(α-

methylstyryl) disodium (PαMS) and DCPMPS was attempted.

117

+ 2n NaClNa+_

xxNa+ _

C CH2

CH3

CH2 C

CH3

ySi

CH3

n

CH3

2n Na+-x

Na+ _C CH2 + Cl

yCl Si

CH3

n

THF

30 minutes-78°C

Scheme 5.4 A representation of a synthesis of PαMS-PMPS-PαMS.

A multi-block copolymer was expected from the equimolar mixing of the reagents

but the GPC (UV at 254 nm) indicates that PαMS-PMPS-PαMS was the dominant

product (figure 5.16). However, the GPC of the PαMS is clearly bimodal indicating that a

proportion of the PαMS had only one ‘living’ end-group. This can be explained by the

effect of trace impurities on the ‘living’ dianionic poly(α-methylsyryl) disodium, perhaps

from wet N2. Figure 5.17 shows the 1H NMR of the purified material with the proposed

resonance assignments. The polydispersity of the DCPMPS is high, and some multi-block

copolymers may have been formed readily from the lower molecular weight DCPMPS.

118

Figure 5.16 Representative GPC curves of (a) PαMSN, (b) DCPMPS, and

(c) PαMS-PMPS-PαMS (UV at 254 nm).

Figure 5.17 1H NMR of PαMS-PMPS-PαMS (in CD2Cl2) with proposed absorption

assignments (at 270 MHz).

The block copolymer was in the form of hard and white granules which could be

bent and shaped without rupturing.

119

5.3.2.6 Synthesis and Characterisation of

α,ω-dihalopoly(methylphenylsilylene)

In order to achieve a greater extent of reaction and with a more effective linking of

polymers, it was decided to reduce the molecular weight of DCPMPS. This was

accomplished by bromination of high molecular weight DCPMPS (scheme 5.5). This has

previously been investigated with the reaction of a near stoichiometric amount of bromine

and DCPMPS7. However, the method was time consuming and risked the exposure of the

polymer to adventitious impurities which could hydrolise the Si-X chain ends. It was

thought better to use an excess of bromine on an unfractionated DCPMPS for a lesser

period of time.

The reaction was followed by GPC (table 5.4) and 1H and 29Si NMR, the results of

which concurred with those already published7. It was found that the duration of the

reaction had to be strictly controlled otherwise the resulting polymer would either be of too

high or too low a molecular weight. The product DXPMPS was precipitated into n-hexane

to remove oligomeric material. One iodination of DCPMPS was also attempted but while

the molecular weights of the α,ω-dihalopolysilylene were found to be controllable, it was

found that Si-I groups did not promote the linking reaction as effectively as Si-Br groups.

mSi

CH3

X X where X is Br or Cln

(n/m)-1 Br220 minutesTHF, R.T

+ClCl Si

CH3

Scheme 5.5 Bromination of DCPMPS.

Polymer M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

DCPMPS 45190 3910 55360 14.2

DXPMPS 9810 6110 10050 1.6

Table 5.4 Exampled GPC results: a bromination of DCPMPS, (UV at 254 nm).

120

A solution of DXPMPS in THF was slowly dropped into methanol, and after

drying under vacuum, the precipitated polymer was characterised by 29Si NMR (figure

5.18). The characterisation reveals a number of halogenated end-groups remaining on the

polymer. These must be protected from the methanol by being buried within the

precipitated mass of the polymer. Assigments were made from those already determined,

as described in appendix 3. The unknown assigment for the resonance of silicon atoms

bonded to bromine was determined on the basis of the electronegativity of bromine with

respect to chlorine.

Figure 5.18 Results from an 29Si NMR of the DXPMPS precipitated from THF intomethanol (at 67.8 MHz).

5.3.2.7 Synthesis and Characterisation of

Poly[poly(methylphenylsilylene)-block-polyisoprene]

A reaction of fractionated DXPMPS with PIN, over 9 minutes in THF at -78 °C

was performed (figure 5.19, scheme 5.3, table 5.5). The reagents were mixed at high

concentrations to reduce the time of the reaction.

121

Figure 5.19 Representative GPC curves of (a) PMPS (UV, 254 nm), (b) PI (RI), and (c)product (RI) and (d) product mixture (UV, 254 nm).

Polymer (detector) M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

PIN (RI) 10290 7860 10520 1.34

DXPMPS (UV 254 nm) 10290 6110 10050 1.64

Products (RI) 10860 15250 52970 3.47

Products (UV 254 nm) 44900 20380 76850 3.77

Table 5.5 Representative molecular weight parameters of reagents and products of a reactionof PIN and DXPMPS.

At higher molecular weights the RI and UV chromophores of the product are close

and it is clear that linking has occured. In order to purify the (PMPS-PI)n, results from the

previous experiment (table 5.3) were used to indicate the required ratio of solvent and non-

solvent required (table 5.6, figure 5.20). Thus, 870 ml of isopropanol was added to 400

ml of a THF solution of the polymers. The precipitate was recovered by filtration. The

multi-block copolymer was white and slightly rubbery (yield = 35%).

122

Figure 5.20 GPC curves of a mixture prior to and after purification; in which (a) mixture(RI), (b) mixture (UV, 254 nm), (c) is (PMPS-PI)n (RI), and (d) (PMPS-PI)n (UV, 254 nm).

Polymer (detector) M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

Mixture (RI) 10860 15250 52970 3.47

Mixture (UV 254 nm) 44900 20380 76850 3.77

(PMPS-PI)n (RI) 51770 39300 68750 1.75

(PMPS-PI)n (UV 254 nm) 56150 32350 77530 2.40

Table 5.6 Molecular weights of a purified (PMPS-PI)n indicated by GPC.

In contrast to the unpurified product, the GPCs using RI and UV detectors of the

purified material are almost identical. Barely any of the homopolymers are detectable.

The UV (254 nm) GPC curve of copolymer was resolved in order to gain an

understanding of its structure. The results of the peak resolution are shown in figure 5.21.

The magnitude of the absorption is directly proportional to the incidence of phenyl

substituents of PMPS passing through the detector. Accordingly, the weight of each peak

was adjusted to take account of the perceived phenyl content of the appropriate

copolymer structure in arriving at the last column of table 5.7. This calculation was

performed by a simple division of the perceived incidence of phenyl groups by the relative

proportion of phenyl groups for each copolymer structure. The resolved chromatograms

indicated a wide distribution of block copolymer structures. The number average weight of

the polymer indicates an average structure close to (PMPS-PI)3.

123

The polymers DXPMPS and PI were mixed in equimolar proportions. The average

number of phenyl units to each DXPMPS polymer was indicated by GPC to be around 51

(table 5.5). The polyisoprene was indicated by GPC to consist of an average number of

115 repeat units. Thus, the ratio of the number of repeat units in DXPMPS to PIN was

calculated to be 1:2.6 respectively. The GPCs of the purified product copolymer indicate

the equivalent ratio in the polymer to be of 1 methylphenylsilane repeat unit to 2.3 of each

isoprene repeat unit. This would indicate that, overall, the purified copolymer contained

more PMPS than was in the reagent mixture.

The resolution is not a determinate characterisation: the results can only be

indicative of average structures contained within the mixture. The polydispersities of

PMPS and of PI are great enough to distribute what otherwise would be regular structures.

The peak resolution software that was used is more accurate for the purpose of dealing

with curves that have several identifiable peaks, i.e. the program has guides as to where

specific components may lie. For a smooth monomodal chromatogram such as this, the

software has no indicators, other than the overall dimensions of the plot.

Figure 5.21 Resolved GPC (UV, 254 nm) of (PMPS-PI)n.

124

Peak M n / g mol-1 M w/Mn Polymer molecular

structure

Relative

Absorption %

Number average

proportion %

1 7800 1.09 PMPS 0.7 3.7

2 11800 1.06 PMPS-PI 1.1 5.8

3 16400 1.04 PMPS-PI-PMPS 1.8 4.7

4 21700 1.04 (PMPS-PI)2 2.3 6

5 29500 1.04 (PMPS-PI)2-PMPS 5.6 9.8

6 40900 1.04 (PMPS-PI)3 9 15.7

7 53500 1.03 (PMPS-PI)4 9.6 12.6

8 68200 1.03 (PMPS-PI)5 11.5 12

9 86300 1.02 PI-(PMPS-PI)6 11.8 10.3

10 108600 1.02 (PMPS-PI)8-PMPS 11.2 6.5

11 134500 1.02 (PMPS-PI)10-PMPS 9.1 4.3

12 163100 1.02 PI-(PMPS-PI)12 7.1 3.1

13 194500 1.01 (PMPS-PI)14 5.8 2.2

14 231800 1.01 PI-(PMPS-PI)17 4.9 1.5

15 276600 1.01 (PMPS-PI)21 3.3 0.8

16 327900 1.01 (PMPS-PI)25 2.3 0.5

17 387300 1.01 PI-(PMPS-PI)29 1.4 0.25

18 459800 1.01 (PMPS-PI)35 1 0.15

19 559600 1.01 PI-(PMPS-PI)42 0.45 0.06

20 669100 1.00 (PMPS-PI)51 0.05 0.005

Table 5.7 A representative peak resolution of a GPC curve (UV, 254 nm) of (PMPS-PI)n.

Figure 5.22 shows the 1H NMR characterisation of a sample of (PMPS-PI)n. The

integrals of the 1H spectrum indicate a ratio of the number of repeat units in PMPS to PI to

be 1: 2.5. This is in close agreement with the mixing ratio of repeat units of the reagents (at

1:2.6), and not far from 1:2.3 indicated by the peak resolution of GPC results.

125

Figure 5.22 1H NMR analysis of (PMPS-PI)n, in C6D6, with proposed assignments (at 270MHz).

DSC analyses of the block copolymer and PMPS (figure 5.23) are both featureless.

Thus, the Tg at about 19 °C for PI that is evident in figure 4.10 is not evident in the DSC

of the copolymer. This is unusual and indicates a different microstructure for PI within the

copolymer.

Figure 5.23 Representative DSC results from analyses of (a) PMPS, and (b) (PMPS-PI)n.

126

5.3.2.8 Thin Self Standing Films of

Poly[poly(methylphenylsilylene)-block-polyisoprene]

PMPS forms unstable films from solvents which evaporate slowly. In contrast,

thin films of (PMPS-PI)n were easily prepared by the evaporation of a solution in

chloroform. The films were optically clear, self standing, resistant to handling and light

brown in colour.

5.3.3 Discussion

5.3.3.1 Synthesis and Characterisation of

α,ω-dichloropoly(methylphenylsilylene)

The PMPS used in the preparation of block copolymers was prepared by a THF

mediated Wurtz polycondensation. As such, its molecular weight and its resistance to

degradation allowed the synthesis of the block copolymers presented here to be carried out

rapidly. In the previous work of Demoustier-Champagne et al12, the PMPS was prepared

from a toluene mediated reaction and had a molecular weight of between 20000 and 30000 g

mol-1, and the reaction leading to block copolymers took up to 18 hours to complete and

had to be performed at low concentrations. There are three probable reasons why the

reactions presented here were relatively fast; (i) there is a higher relative concentration of

chain ends, (ii) the chain ends were less likely to be crowded by an entangled polysilylene,

and (iii) because of a more ‘open’ polysilylene structure, the phase separations of the

reagents was less likely to occur12. This has been shown to allow the syntheses of block

copolymers of PMPS with PI, PαMS and OαMS of high molecular weights and it has

allowed the formation of multi-block copolymers of PMPS for the first time.

The synthesis of DCPMPS from a Wurtz reaction in THF is simple to perform;

there is no need for fractionation to remove high molecular weight material. However,

DCPMPS requires strict handling under anaerobic conditions, as the reactive Si-Cl groups

may easily hydrolyse.

127

5.3.3.2 Synthesis of

Poly[oligo(α-methylstyrene)-block-poly(methylphenylsilylene)]

The reaction of PMPS and OαMS is fast. This indicates that a polysilylene of a

medium molecular weight in solution has an ‘open’ structure, and it is only when the

copolymer is formed that phase separation can occur14. Hence, this is an important limit

on the formation of (OαMS-PMPS)n. The DSC results indicate that (OαMS-PMPS)2-

OαMS precipitated directly from solution is partially crystalline, at least more so than

PMPS itself. While it may be argued that this sort of crystallinity results from a method of

precipitation, this was not observed for other PMPS samples nor other block copolymers.

As in all other reactions described in this chapter, the addition of the polymeric

anions to DCPMPS occurs through an SN2 reaction via a penta-coordinated silicon with an

inversion of configuration4. PMPS degradation also occurs, by an anionic attack but at the

silicon chain. It would be expected that this reaction occurs more slowly than that at a

chain end, because of steric effects.

5.3.3.3 Degradation of Methoxy Terminated Poly(methylphenylsilylene) by

Oligo(α-methylstyryl) Disodium

The tentatively proposed route to a degradation of PMPS by oligo(α-methylstyryl)

disodium shown in scheme 5.6 is indicated by the slow appearance of a golden colour in

the reaction. This is due to the presence of silyl anions10. The degradation of PMPS by

OαMS occurs more slowly than that found with PIN. This could be due to the stabilising

effect of phenyl rings on OαMS. The negative charge is delocalised to some extent and

therefore a nucleophilc attack at a silicon on PMPS occurs less readily.

128

Na+_

Si

Si

Na+_

Si

Si

+

Me

Ph MePhMe

Ph Me Ph

Scheme 5.6 A proposed route to the degradation of PMPS by ‘living’ anionicoligo(α-methylstyryl) disodium.

5.3.3.4 Timed Reactions of Dihalopoly(methylphenylsilylene)

and Polyisoprenyl Disodium

The reaction of DCPMPS and PIN are faster than those previously observed of

DCPMPS with polystyryl sodium in toluene12. There are several possible reasons for this;

(i) THF dissociates the ‘living’ ion pair of anionic polymers and may lead to a higher

reactivity, (ii) greater concentrations of end-groups, (iii) end-groups that are more open to

react, (iv) a lesser tendency to phase separation of reagents, and finally (v) PIN is a more

reactive species than polystyryl sodium. It has been seen, both here and in previous

work12, that the molecular weight of the reagents plays an important part in determining

the outcome of a reaction. It seems, from simple inspection of the results, that the phase

separation of the reagent polymers becomes an important factor when their molecular

weights are above around 10000 g mol-1.

5.3.3.5 Reaction of Dichloropoly(methylphenylsilylene) and

Poly(α-methylstyryl) Disodium

The reaction of DCPMPS and PαMSN clearly demonstrates the importance of

eliminating impurities from reaction vessels, reagents and solvents. This system probably

is the most sensitive to reactive impurities due to the low rate of reaction arising from the

delocalisation of charge at the ‘living’ polymer chain ends.

129

5.3.3.6 Synthesis and Characterisation of

α,ω-dihalopoly(methylphenylsilylene)

The method described for the preparation of DXPMPS was easier to complete and

more rigorous in giving rise to a polysilylene of a low polydispersity than a method

previously used7. DXPMPS should be more reactive than DCPMPS for three reasons; (i)

a relatively high proportion of end groups, (ii) the polymer is not so entangled in solution

as it has a semi-rigid rod conformation, rather than the semi-coiled struture of high

molecular weight PMPS5,14 and (iii), Si-Br groups are likely to be more reactive than Si-Cl.

5.3.3.7 Synthesis and Characterisation of

Poly[poly(methylphenylsilylene)-block-polyisoprene]

In the formation of the multi-block copolymer (PMPS-PI)n, there are two

competing reactions, that of the addition and that of the degradation of PMPS by the

‘living’ carbanion of PIN in a reaction similar to that shown in scheme 5.6 Thus an

optimum time was chosen for the reaction. The reaction was fast (9 minutes). Any longer,

and degradation of the PMPS would have ensued.

Peak resolution of the GPC (UV, 254 nm) curve indicated that there is more PMPS

in (PMPS-PI)n than PI. While the argument is tenuous, as the molecular weights of the

polyisoprene are not absolute, the results do indicate a conclusion of sorts. The PIN is

either lost to terminating impurities in the solution of DXPMPS, or there is a phase

separation in solution between PMPS and PI, and (PMPS-PI)n and PI. The first point is

supported by the observation that the PIN ‘living’ ion-pairs are considerably more

sensitive to terminating agents than DXPMPS. Concealment of the Si-Cl/Br end groups by

DXPMPS in solution is possible. This may be occurring in the reaction mixture, where PI

in THF is a non-solvent. Coagulation has been observed during the reactions, which would

indicate that some phase separation of (PMPS-PI)n and PI and/or PMPS is occurring.

Whatever occurs during the reaction, the overall limit is that of the degradation of the

polysilylene by the carbanions.

130

DSCs of homopolymers and (PMPS-PI)n indicate; (i) the Tg of the PI to be 19 °C,

as would be expected for a polymer containing a low quantity of 1,4 isomers15, and (ii)

that the near amorphous PMPS ‘masks’ glass transition temperature of PI in (PMPS-PI)n.

Thus, there is a high order of integration of the PMPS and the PI in the precipitated block

copolymer.

Purification of the block copolymer by fractionation was easy to perform.

5.3.3.8 Formation of Thin Self-Standing Films of

Poly[poly(methylphenylsilylene)-block-polyisoprene]

The thin films of (PMPS-PI)n, which were optically clear, strong, flexible, and rigid

enough to be self-standing would indicate that (PMPS-PI)n has separate macrophase

domains3 (section 1.3.5). It was disappointing that time did not allow the characterisation

of the films by TEM, as it would be expected that this technique would have revealed the

size of any domains that might be present, which can in turn be related to the size of the

component polymers. Previous work has shown that a low polydispersity (c. 1.1) of

similar rod-coil block copolymers is required to form domains of this sort16. The

polydispersity of (PMPS-PI)n is considerably greater than this but the domain formation

may be aided by the semi-rigidity of the PMPS units.

5.4 Syntheses and Characterisations of Block Copolymers of

α,ω-dihalopoly(methylphenylsilylene) and Poly(oxyethylene) Glycol

In this sub-section the synthesis and characterisation of the block and multi-block

copolymers of DXPMPS and poly(oxyethylene) glycol (PEG) are studied and the results

are presented. These were chosen as they are hydrophilic and would give rise to

amphiphilic copolymers possibly displaying unusual properties in solution, as is the case

for the block copolymers of poly(oxyethylene) and polystyrene17.

131

5.4.1 Properties, Microstructure and Applications of Poly(oxyethylene)

Poly(oxyethylene) was first discovered in 1859 by Lourenço18, and since then the

polymer, which has some unusual properties, has been marketed for and used in a wide

range of applications.

Unusually for a polymer, especially for those of high a molecular weight, PEO is

water soluble in all proportions19. However, in water between 20 °C and 70 °C it does

exhibit a tendency to form fibrillar colloidal particles, which behave like crystals20. It is

soluble in common organic solvents and may even be extracted from water by chloroform.

Toluene, at room temperature, is a poor solvent, although at higher temperatures (c. 60 °C)

it is a good solvent for PEO21. In toluene, crystallisation of the polymer occurs, even in

dilute concentrations22. High concentrations of NaOH decrease PEO’s solubility in water,

whereas high acidic concentrations increase its solubility. Salts, of sodium and potassium

in particular, may be used to 'salt out' and increase the solubility of the polymer in water

through ionic co-ordination23. The complexes formed of metal ions and PEO, which acts

like a crown ether, form polyelectrolites and exhibit ionic electrical conductivity19. In pure

water, at lower temperatures, a sheath of water molecules are attached to and surround the

polymer chain, although at higher temperatures the polymer shakes off this layer and tends

to form colloids19. In most solvents, the chain assumes a random coiled conformation24.

PEO, when freezing in the pure state, tends first to form dominant lamellae, then

secondary lamellae are formed between the extended branches and finally a crystallisation

occurs forming 'flower petal' macrostructural spherulites25. The melting point of PEO is

around 69 °C25, and its Tg is at approximately -52 °C19.

Poly(oxyethylene) is widely used as an emulsifier. PEO, because of its high

lubricity, has been used as a synthetic lubricant for the internal combustion engine, for

metal working and as a solution of water and isopropanol, to aid the mounting of tyres19.

Because of its low toxicity, it is used in the manufacture of supositories, lotions and

pharmacetical salves19. It is resistant to oxygen, ozone and heat. Through its ability to

form ionic complexes with salts it is used to manipulate cells as the PEO aids their

penetration without damaging them. Block copolymers of PEO have received much

attention. For example the block copolymers of PS and PEO form micelles in aqueous

solutions consisting of PS cores19.

132

5.4.2 Methodology and Results

5.4.2.1 Reaction of α,ω-dihalopoly(methylphenylsilylene)

and Poly(oxyethylene) Glycol

DXPMPS and PEG were reacted to form poly[poly(methylphenylsilylene)-block-

poly(oxyethylene)] and corresponding AB and ABA type copolymers. DXPMPS was

used for these reactions for the reasons outlined in section 5.3.2.6. All the reactions were

performed in rigorously dried Schlenk tubes under inert gases. While PEG is not so

sensitive to impurites as the ‘living’ anionic polymers (section 1.2), it is extremely

hydrophilic, and for this reason it was thoroughly dried before use, and then stored in

toluene. It was considered that if it were stored in THF, there could be a higher risk of

contamination by water. PEG crystallised in the toluene, so solutions were made up just

before use, and the toluene was warmed to increase the solubility and reduce its

crystallisation. The concentrations of the reagents were high so as to favour the reactions.

Pyridine was used as a sequestering agent to react with the HCl co-product of the reaction

shown in scheme 5.7. It was found that when a low amount of pyridine was used, the

reaction would not proceed, and it was only with an extreme excess of pyridine, added after

mixing the reagent polymers, that a reaction would occur.

y

THF, toluene 15 minutespyridine

+p ClCl Si

CH3

xHp OHO CH2CH2

pxCl Si

CH3

yOHO CH2CH2 + (p-1) HCl

Scheme 5.7 A synthesis of (PMPS-PEO)n.

133

GPC analysis of the reagent polymers and the products after 10 minutes indicated

that the reaction was fast (figure 5.24, table 5.8). PEG can only be determined using the

RI detector, as unlike PMPS, it does not carry a UV chromophore.

Figure 5.24 Representative GPCs of; (a) DXPMPS, (b) PEG, (c) unpurified block copolymer(RI), and (d) unpurified block copolymer (UV, 254 nm).

Polymer (detector) M p M n M w P.D

DXPMPS (UV 254nm) 8900 5280 9070 1.72

PEG (RI) 7440 7060 7230 1.02

(PMPS-PEO)n unpurified (RI) 102900 29200 100900 3.45

(PMPS-PEO)n unpurified (UV 254 nm) 100300 34150 134700 3.94

(PMPS-PEO)n purified (RI) 34210 26700 41900 1.57

(PMPS-PEO)n purified (UV 254 nm) 35200 24160 43900 1.82

Table 5.8 GPC results from reagents, and purified and unpurified (PMPS-PEO)n. Molecularweights are in g mol-1.

A purification process was devised to separate the block copolymer from the

reagent polymers. This was done as a two stage precipitation and fractionation of the

copolymer using methanol and pentane as detailed in Appendix 2. The overall yield of a

yellowish-white powder was 60%.

134

Figure 5.24 GPC curves of; (a) DXPMPS, (b) PEG, (c) purified block copolymer (RI), and(d) purified block copolymer (UV, 254 nm).

The GPC shown in figure 5.24 demonstrates that PMPS and PEO have

substantially been removed from the product. The GPC (obtained using the UV detector

set at 254 nm) of the purified block copolymer was resolved as shown in figure 5.25. The

low molecular weight polymers had to be excluded from the calculations to realise a normal

distribution of the fitting curves. This excluded approximately 5% of homopolymeric

material.

135

Figure 5.25 A representative peak resolution of a GPC spectra (UV 254 nm) of (PMPS-PEO)n where the peak numbers relate to those in table 5.9.

The measured absorption is directly proportional to the amount of phenyl

chromophores passing through the UV detector. The number average proportion of each

block copolymer could be calculated by a simple division of its absorption by the relative

amount of PMPS in each copolymer chain. The results of the peak resolutions and the

calculations performed upon them are summarised in table 5.9.

Peak M n / g mol-1 M w/Mn Polymer structure Absorption % Number average

proportion %

1 7800 1.06 PMPS 0.6 2.1

2 12000 1.07 PMPS-PEO 3.9 12.9

3 18000 1.09 PMPS-PEO-PMPS 14.6 24.2

4 29700 1.10 (PMPS-PEO)2-PMPS 27.4 30.4

5 48400 1.09 (PMPS-PEO)4 27.4 22.8

6 76500 1.08 (PMPS-PEO)6-PMPS 17.0 4.3

7 119000 1.07 PEO-(PMPS-PEO)9 8.5 3.2

8 195000 1.02 (PMPS-PEO)16 0.6 0.1

Table 5.9 Representative results of calculations on the peak resolution of a GPC UV 254 nmanalysis of (PMPS-PEO)n.

136

The polydispersity of the block copolymer is indicated to be 1.6 by the RI detector

spectrum. Using equations 1.15 and 1.8, the probability26 (p) of the reaction having

occurred is therefore 0.6. This in turn indicates that the probable degree of polymerisation

is 2.5 i.e. the average block copolymer structure is (PMPS-PEO)2-PMPS. This is in

excellent agreement with the results of the peak resolution of the GPC UV spectrum and

shows that the Flory ‘normal’ distribution is followed by the block copolymer.

Figure 5.26 shows an 1H NMR of (PMPS-PEO)n with resonances attributable to

the methyl and phenyl protons of PMPS, and a resonance about 3.6 ppm attributable to

the protons of PEO.

Figure 5.26 A representative 1H NMR spectra (in CDCl3) of (PMPS-PEO)n (at 270 MHz).

An 29Si NMR of (PMPS-PEO)n in CDCl3, and again in deuterated THF was

attempted. However, due to a crystallisation of (PMPS-PEO)n in solution over the long

period of time required for data acquisition, a clear spectrum could not be obtained.

137

5.4.2.2 UV Absorption Characterisation of

Poly[poly(oxyethylene)-block-poly(methylphenylsilylene)]

The UV absorption spectrum of (PMPS-PEO)n in pure water was distorted by light

scattering at all frequencies but a λmax at 348 nm is discernable (figure 5.27). This light

scattering was reduced rapidly on the addition of THF to the solution. With dilution of a

water solution of (PMPS-PEO)n by THF, the value of λmax decreased from 348.0 nm, and

then slightly increased. This absorption is associated with the σ - σ* transition at the

silicon back bone of PMPS (section 1.1.4.1). In the same way, the value of εmax increased

steadily, only to decrease in pure THF (table 5.10).

Figure 5.27 UV absorption spectrum of (a) (PMPS-PEO)n in water and (b) (PMPS-PEO)n inwater and THF (2:3 ratio).

Water % THF % (PMPS-PEO)n/ M λmax/ nm Absorption εmax/ M-1cm-1

100 0 2.39 x 10-6 348 0.141 59000

99 1 2.30 x 10-6 344 0.119 52000

40 60 9.55 x 10-7 336.8 0.566 5.9 x 105

8 92 7.64 x 10-9 338.4 0.817 1.1 x 109

0 100 9.55 x 10-8 339.2 0.21 2.2 x 106

Table 5.10 Representative results of UV absorption analysis of (PMPS-PEO)n in water andTHF.

138

5.4.2.3 Synthesis and Characterisation of

poly(oxyethylene)-block-poly(methylphenylsilylene)

A copolymer was made by mixing two parts of PEG with one part DXPMPS.

From GPC results, a linking was seen to have occured (figure 5.28, table 5.11). However,

a satisfactory fractionation of the product polymers could not be obtained due to the

proximity of their molecular weights.

Figure 5.28 GPC spectra of; (a) DXPMPS, (b) PEG, (c) product block copolymer and PEG(UV, 254 nm) and (d) product copolymer and PEG (RI).

Polymer (detector) M p / g mol-1 M n / g mol-1 M w / g mol-1 P.D

DXPMPS (UV 254nm) 8900 5280 9070 1.72

PEG (RI) 7440 7060 7230 1.02

Copolymer (RI) 18200 12100 18290 1.51

Copolymer (UV at 254 nm) 18470 14740 21760 1.48

Table 5.11 Molecular weights indicated by GPC of DXPMPS, PEG, and of PEO-PMPS-PEOand PEG.

The GPC results indicate that the product copolymer consisted of a major

compponent of PMPS-PEO and a minor component of PEO-PMPS-PEO.

139

DSC characterisations of PEG, PMPS-PEO and (PMPS-PEO)n are shown in figure

5.29. There is a noticable decrease in the melting point of PEO going from the first to the

latter of these polymers. This may indicate an increasing disruption of the PEO by the

PMPS in the same order.

Figure 5.29 DSC curves of (a) (PMPS-PEO)n, (b) PEO-PMPS and (c) PEO. All analyseswere taken from the second heating passage at 10 °C / minute.

5.4.3 Discussion

5.4.3.1 Synthesis of

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)]

Reaction conditions for the synthesis of (PMPS-PEO)n were severe; a high purity

of reagents was required, a high concentration of the reagents facilitated the reaction

kinetics, and pyridine had to be used in an excess. The use of two solvents, THF and

toluene was also found to optimise the reaction.

The pyridine may play some role as a co-solvent. The reaction proceeds quickly:

it is complete within 10 minutes, and this would indicate that the molecular weights of the

polymers determine the extent of phase separations. The peak resolutions of GPC spectra

are not completely accurate: the resolved curve was nearly ‘normal’ in its shape, and no

140

distinguishing features could be used to aid the peak resolution program. The results

though do agree with the calculated most probable molecular weight distribution of the

polymer as being (PMPS-PEO)2-PMPS.

5.4.3.2 A Comparison of the Syntheses of (PMPS-PI)n and (PMPS-PEO)n

During the synthesis of (PMPS-PEO)n an increasing degree of coagulation was

observed in the reaction mixture, indeed more so than that seen for the reaction

synthesising (PMPS-PI)n . The two reactions were performed with polymers at similar

concentrations and molecular weights over similar times. PEG, while hydrophilic, could

co-ordinate with water to arrest the reactions, but it was well purified and is more stable

than the di-anionic PIN. This may explain the lower molecular weights of the product

(PMPS-PEO)n.

While it has been shown that the Si-Br and Si-Cl chain-ends of PMPS are not

always readily accessible to reagents, especially hydrophilic ones, (PMPS-PEO)n contains

a higher proportion of PMPS than the (PMPS-PI)n polymer. This would indicate that

PEG is shielded from reactive Si-Br and Si-Cl sites in solution more effectively than PIN,

and that it is the liquid-liquid phase separation of the multi-block copolymer from PEG or

PIN, and not from PMPS which is a more important limit to these reactions. The greater

hydrophilic quality of PEG, when compared to PI, explains this increased liquid-liquid

phase separation. It can be hypothesised therefore that in the case of the reaction

synthesising (PMPS-PI)n, the phase separation is therefore more important than the

susceptibility of ‘living’ PIN to impurities, and this agrees with the conclusion of previous

work12.

5.4.3.3 The UV Absorption Spectrum of

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)] in Solution

The disappearance of light scattering by aqueously solubilised (PMPS-PEO)n on

the addition of THF can be explained through the hydrophilicity of the component

polymers of (PMPS-PEO)n. Molecularly assembled aggregates may be formed by the

hydrophobic action of PMPS of (PMPS-PEO)n. PEO being more hydrophilic would shield

141

the PMPS from water. THF is a non-selective solvent of the component homopolymers,

and on its addition to the solution, the thermodynamically force to form aggregates is

reduced.

It is known that the environmental perturbation of PMPS in solution can cause

inhomogeneous segmental geometries in the silicon backbone and hence variations in λmax27.

Changes in λmax were observed with a change in solution. The variation found in λmax and

εmax indicate the presence of molecularly assembled aggregates. In water, PMPS is forced

to take up a close packed conformation inside an aggregate28, thus giving rise to an extended

all-trans conformation. This leads to a bathochromic shift in its UV absorption curve7.

With the addition of THF, such structures dissipate, the PMPS is disorganised and the

increased number of gauche links in the silicon chain lead to a hypsochromic shift of λmax.

5.4.3.4 Synthesis and Characterisation of

Poly(methylphenylsilylene)-block-poly(oxyethylene)

The GPC results of the formed poly(oxyethylene)-block-

poly(methylphenylsilylene) indicate that only a poor control over the structure of product

copolymers can result from a variation of the proportions of reagent polymers.

5.4.4 Conclusion

It is believed that for the first time, both multi-block copolymers, and a water

dispersable multi-block copolymer of PMPS have been detailed. The synthetic routes are

simple, and while they require care in the handling of reagents and apparatus, they can

easily be extended to include a wide range of commodity polymers, which may improve

and diversify the mechanical properties of polysilylenes.

Reactions synthesising (PMPS-PEO)n are limited to high degree by the effects of

phase separations. It is tentatively suggested that this is principally due to the solution

effects of PMPS and the formed copolymers. The molecular weights of the reagents, in

part, determine the limit to the linking of polymers.

142

5.5 Suggestions For Further Work

If the work were to be continued, it would be desirable to follow:

❃ a study of possible methods to separate block copolymers from the reagents.

❃ an experiment to synthesise multi-block copolymers of PMPS and commodity

polymers using ATRP

❃ the involvement of pyridine in the reaction.

❃ the reactions of DXPMPS with other polymers such as polystyrene and to see if

the reactions can be performed with polymers having more than 2 reaction sites.

❃ to attempt a low temperature synthesis of DCPMPS through a Wurtz

polycondensation reaction using an electron transfer reagent such as 1,1,4,4-

tetraphenyl-1,3-butadiene or C60. Both of these compounds may solvate sodium

into the reaction medium. PMPS is susceptible to attack by electron tranfer agents

at weak cis links in its silicon backbone. It is considered that the size of the

reagents may sterically restrict this reaction. If the reaction were to proceed, then it

may be possible to attempt ‘one-pot’ reactions of DCPMPS with reagents such as

PIN and PEG, thus minimising reagent contact with impurities and forming

polymers of predetermined molecular weights.

143

References

1 D. H. Richards, R. L. Williams, J. Polym. Sci. (Polym. Chem. Edn) 11 , 89 (1973)2 A. Vrancken, J. Smid, M. Szwarc, Trans. Faraday Soc., 2036, 58 (1962)3 A. Noshay, J. E. McGrath, ‘Block Copolymers’, Academic Press, New York, (1977)4 R. J. P. Corriu, C. Guerin, J. Organomet. Chem., 231, 198 (1980)5 R. G. Jones, W. K. C. Wong, S. J. Holder, Organomet., 59, 17(1) (1998)6 S. Demoustier-Champagne, J. Marchand-Brynaert, J. Devaux, Eur. Polym. J., 1037,

32(9) (1996)7 E. Fossum, J. A. Love, K. Matyjaszewski, J. Organomet. Chem., 253, 449 (1995)8 J. Michl, R. D. Miller, Chem. Rev. 89 , 1359 (1989)9 R. Hiorns, MSc (1994)10 M. Suzuki, J. Kotani, S. Gyobu, T. Kaneko, T. Saegusa, Macromolecules 27 , 2360

(1994)11 S. Demoustier-Champagne, S. Cordier, J. Devaux, Polymer, 1003, 36(5) (1995)12 S. Demoustier-Champagne, I. Canivet, J. Devaux, R. Jerome, J. Polym. Sci., Polym.

Chem., 1939 (1997)13 J. Devaux, Bull. Soc. Chim. Belg., 803, 98(9-10) (1989)14 C. Strazielle, A.-F de Mahieu, D. Daoust, J. Devaux, Polymer, 4174, 33(19) (1992)15 C. Kow, M. Morton, L. J. Fetters, Rubber Chem. Technol. 55 , 245 (1982)16 L. H. Radzclowski, S. I. Stupp, Macromolecules, 7747, 27 (1994)17 D. H. Richards, M. Szwarc, Trans. Faraday Soc. 55 , 1644 (1959)18 A. Lourenço, Compt. Rend., 619, 49 (1859)19 F. E. Bailey, J. V. Koleske, ‘Alkyene Oxides and Their Polymers’, Marcel Dekker Inc.

New York (1991)20 V. I. Klenin, N. K. Kolibolotchuk, N. A. Solonina, Vysokomol. Soedin. Ser. A., 2076,

30(10) (1988)21 G. M. Powell, F. E. Bailey, ‘Encyclopedia of Chemical Technology’, Second

Supplement Volume, Wiley Interscience, New York (1960)22 N. Ding, E. J. Amis, M. Yang, R. Salovey, Polymer, 2121, 29 (1988)23 Seymour , Carrahar, ‘Polymer Chemistry’, 4th Ed. Marcel Dekker, Inc., New York

(1996)24 S. Bekiranov, R. Bruinsma, P. Pincus, Phys. Rev. E, 577, 55 (1997)25 N. B. Bikales, ‘Water Soluble Polymers’, Plenum, New York (1973)26 P. J. Flory, J. Am. Chem. Soc.,1561, 62 (1940)27 T. Seki, A. Tohnai, T. Tamaki, A. Kaito, Macrolmolecules 29 , 4813 (1996)28 G. H. Fredrickson, Macromolecules 26 , 2825 (1993)

144

6 Self Assembly of

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)

in Aqueous Dispersion

Abstract

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)] ((PMPS-PEO)n,) is shown

to form vesicular aggregates in aqueous solutions. The structure of the aggregates are

confirmed by electron microscopy and dye entrapment. In addition, the dimensions of the

walls of vesicles are indicated by the use of π-A isotherms. The vesicles are tentatively

ascribed as having walls formed of parallel PMPS chains, each side of which being covered

by a corona of hydrophilic PEO chains. This is believed to be the first time that such

aggregates have been observed of a multi-block copolymer and also of a polysilylene. The

results indicate that a polymer of a low polydispersity for the formation of vesicles may not

be necessary, and the reasons for this are discussed.

This work in part has been published in:

(i) S. J. Holder, R. Hiorns, N. A. J. M. Sommerdijk, S. J. Williams, R. G. Jones and

R. J. M. Nolte, Chemical Communications, 1445, 14 (1998)

(ii) R. G. Jones, R. C. Hiorns, S. J. Holder, R. M. Nolte, N. Sommerdijk, S. J. Williams,

Abstracts of the 31st Organosilicon Symposium, New Orleans, USA B2 May 1998.

Thanks are extended to NEDO, the Japanese Ministry of International Trade and

Industry (MITI) and the Japan Chemical Innovation Institute in conjunction with the

University of Nijmegen for the financial support for this work.

The characterisation of the aqueous solution behaviour of (PMPS-PEO)n was

performed by Dr S. J. Holder and Dr N. A. J. M. Sommerdijk at the University of

Nijmegen, Holland.

145

6.1 Introduction

In this chapter the behaviour of poly[poly(methylphenylsilylene)-block-

poly(oxyethylene)] ((PMPS-PEO)n) as an aqueous dispersion is described and discussed.

PMPS is hydrophobic and PEO is hydrophilic. Therefore, the copolymer was expected to

form aggregates in water. However, this is believed to be the first time such aggregates have

been observed for a polysilylene, and more over, for a multi-block copolymer1.

The experimental conditions and the preparation of samples are detailed in

appendix 2.

6.1.1 Block Copolymers and Vesicle Formation in Water

Prior to this work, only AB and ABA type block copolymers were known to form

vesicles. An example is that of PS-PEO which displays a wide range of aggregate

morphologies in aqueous solutions2. These morpholgies can depend on several parameters;

the relative molecular weights of each block, the polarities of the solvent and the polymer

blocks, the temperature and the presence of salts in solution. The types of aggregate

known include rods, cylinders, worm-like vesicles, ‘pearl necklaces’, micelles and ‘crew

cut’ miscelles3,4 (section 1.3). Vesicles are rarely seen and are sensitive to changes in the

aforementioned parameters2. They can normally be identified by transmission electron

microscopy (TEM) in which a higher transmission is recorded at the centre with respect to

the edges of the aggregates5. Vesicles have been proposed for delivering drugs and as cell

models6.

Vesicles and other aggregates can be prepared by dissolving block copolymers in a

solvent common to both constituent polymers. On slowly adding a selective solvent, for

example water, the turbidity of the solution increases as the hydrophobic polymer is forced

to aggregate. Removal of the co-solvent, by dialysis (figure 6.1) often gives a wide

distibution of vesicles sizes. If the temperature of the mixture is below the Tg of the core

polymer, then the aggregates are locked, and a negligible transfer of polymers occurs

between the micelles (section 1.3). Block copolymers that form vesicles commonly have a

polydispersity of less than 1.1. As a comparison, it is thought that to obtain domain

formation in solid samples requires a copolymer polydispersity of less than 1.37.

146

Figure 6.1 A representation of a dialysis bag. The bag is stirred to maintain a high rate ofdiffusion across the membrane6.

Vesicles are known to fuse during the dialysis procedure, and the rate of their fusion

can have an impact on their size distribution8. Any changes in a morphology of aggregates

is thought to occur from a change in the entropy of the core polymer. Thus, the formation

of aggregates is a thermodynamic process. With aggregates of PS-PEO, PS is in the cores of

the micelles. If the size of the PEO is changed and the PS is kept the same, the PS stretchs.

This may continue to a point where the entropy has decreased enough to force a change in

morphology3.

6.1.2 Encapsulation of Fluorescent Dye into Vesicles

A water soluble fluorescent dye such as carboxyfluorescein can be trapped into

solution voids inside aggregates to confirm a formation of vesicles. To do this, a copolymer

is dissolved in a common solvent. A thin film of the solution is made on the side of a

container and the solvent evaporated. After an aqueous solution of the dye is added and

‘incubation’ or dialysis vesicles forms holding the dye. Dye inside the vesicles can then be

separated from outside by passing the mixture through a column, separating the vesicles

and the free dye9. Characterisation of the vesicles can be made through a fluorescence from

the carboxyfluorescein dye at 520 nm resulting from an excitation by light at 491 nm.

147

6.1.3 Measuring the Vesicle Wall Size

The internal part of a vesicle wall usually consists of the more lyophobic polymer

of the copolymer. A deposition of a single layer of block copolymer on a water surface can

be used to indicate the dimensions of the internal domain of the vesicle wall. This is

possible as the hydrophilic polymer rests in the water. On a known surface area of water,

the expanding polymer film acts against the water’s surface tension. This force (π) is

related to the tension of a clean interface (γ0) and the tension between the interface and the

monolayer (γ) through equation 6.16. To see if the copolymers are aligned on the water

surface, salts may be added to the solution. If polymer alignment does not change, and the

measured surface area remans constant, the hydrophobic polymer controls the effective

surface area.

π = γ0 - γ equation 6.1

An example of the equipment used to measure the forces is the Langmuir-Adams

balance (figure 6.2). Here a film of copolymer is trapped between a moveable barrier and

a float fixed to a torsion wire. The barrier movement results in pressure being exerted on

the float, and a plot of the force against the resulting area can be made.

Figure 6.2 A representation of a surface balance6.

148

6.1.4 Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)]

used for Characterisation

Purified (PMPS-PEO)n used for the formation of aggregates was indicated by GPC

to have the molecular weights shown in table 5.8. It had a polydispersity of 1.6 and a

number average degree of polymerisation of 2.5, equivalent to (PMPS-PEO)2-PMPS. This

copolymer structure was indicated by peak resolution to make up around 30% of the

copolymer. The copolymer was of a purity of approximately 95% (section 5.4.2.1).

6.2 Methodology and Results

6.2.1 Dispersion of

Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)] in Water

To disperse (PMPS-PEO)n in water three different methods were used. Firstly,

simply dropping the polymer into water resulted in a poor dispersion although vesicles

were observed by TEM. Secondly, the copolymer was disolved in THF and water was

added dropwise. From this, a turbid mixture arose. This mixture was subjected to

ultrafiltration, and with continuous concentration and water dilution, a homogenous white

dispersion of copolymer in water resulted (of a concentration of 1.2 g / l). This sample

could then be characterised. A third method used was dialysis5. A solution of the

copolymer in THF and water was placed in a dialysis bag (with an exclusion limit of 20000

g mol-1), and dialysed against pure water.

6.2.2 Vesicle Observation by Electron Microscopy

The first method preparing a water solution of the copolymer, when subjected to

TEM, revealed vesicles in an aqueous portion of the mixture (figure 6.3).

149

Figure 6.3 A transmission electron micrograph of vesicles of (PMPS-PEO)n through negativestaining (the bar represent 300 nm).

A homogeneous white liquid of (PMPS-PEO)n in an aqueous solution, prepared by

ultrafiltration, revealed vesicles on characterisation (figure 6.4). This method of

preparation is shown to have excluded the formation of micelles and other types of

aggregates. The diameter of the vesicles, determined by simple inspection, are of between

100 nm and 180 nm.

Figure 6.4 A transmission electron micrograph of a freeze fractured sample of (PMPS-PEO)n (the bar represent 300 nm).

An SEM analysis of the dialysed sample showed vesicles to be the most dominate

aggregate to be formed in an NaCl (0.05 M) aqueous solution by (PMPS-PEO)n (figure

6.5).

150

Figure 6.5 A platinum shadowed electron micrograph of (PMPS-PEO)n from a NaCl (0.05M) aqueous solution.

6.2.3 Vesicles Encapsulaton of a Fluorescent Dye

To confirm the presence of vesicles, an encapsulation of aqueous soluble fluorescent

dye 5-carboxyfluorescein in vesicles was performed10. The dye was placed into an aqueous

solution during the dialysis of the vesicles in THF and water against ionised water. The

resultant mixture was passed through a Sephadex column, and the elution volume between

the vesicles (at between 30 ml and 110 ml) and the free dye in the solution (at between 160

ml and 190 ml) was great enough to separate encapsulated dye from free dye. Vesicles

containing the dye were then analysed by fluoresence spectroscopy, and emissions of the

copolymer (at 355 nm) and of the dye (at 519.5 nm) were found from the same sample.

This confirmed that closed vesicles were formed1,11 (section 1.3).

6.2.4 Investigation of the Orientation of (PMPS-PEO)n in Vesicle Walls

To investigate the orientation of the copolymers which made up the walls of the

vesicles of (PMPS-PEO)n, an experiment with a double barrier R & K trough (similar to

that shown in figure 6.2) was performed. (PMPS-PEO)n was dissolved into chloroform (a

solvent of both PMPS and PEO), and then dropped onto a known enclosed area of water.

Once the chloroform had evaporated, a thin film of the copolymer was left on the water

surface, and by measuring the pressure the film exerted on a springed float as the surface

area of the film was reduced, a plot of the pressure (π) against the area (A) could be made

151

(figure 6.6). The lift-off area at about 30 nm2 / molecule corresponds approximately to the

area of 3 PMPS chains parallel to the water surface12. There is a continuous plateau in the

plot, between 30 and 5 nm2 / molecule. This is followed by a distinct decrease in the area

of the film starting at around 4.7 nm2 / molecule which would indicate that the copolymer

was reorganising, and going through a pseudo first-order transition. The copolymer is

indicated to have changed from a parallel to a perpendicular orientation with respect to the

water surface. The point of collapse, where the copolymer can no longer retain its regular

structure on the water surface, was found to be attained at around 3.7 nm2 / molecule. The

film displayed a negligible change in behaviour when analysed on an aqueous solution of

NaCl (100 mM). To characterise the effect of aggregation of (PMPS-PEO)n on the rigid-

rod like PMPS chains, vesicles were analysed by UV spectroscopy. The σ→σ*

absorption of the PMPS back-bone was observed to increase from 339 nm to 342 nm

(section 1.1).

Figure 6.6 π-A isothermal plot for (PMPS-PEO)n at (a) the pure water interface, and (b) at

the interface of an aqueous solution of NaCl (100 mM). The indicated orientations of themost probable copolymer (PMPS-PEO)2-PMPS are shown. PMPS is represented by straightlines, and PEO by curved lines.

152

6.3 Discussion

The results indicate the formation of vesicles of (PMPS-PEO)n in aqueous

dispersions. This is the first time that a block copolymer of PMPS has shown such

properties. The formation of vesicles could be due to the relatively large size of the PEO

polymer with respect to PMPS (section 5).

The range of vesicle sizes observed in the transmission electron micrograph

indicates that they fuse during dialysis, ultrafitration or simple solvation. The dye

encapsulation in the vesicles during dialysis indicates that their fusion is susceptible to the

effects of solvent concentration8.

6.3.1 Orientation of (PMPS-PEO)n in Vesicle walls

The collapse point of the vesicles of (PMPS-PEO)n, observed where the area per

molecule is 3.7 nm2, is an approximation to a calculated end area of three parallel chains of

PMPS (3.6 nm2 / molecule)12. These figures indicate that (PMPS-PEO)n is mostly formed

of three PMPS chains which align themselves in parallel, thus making the walls of the

vesicles. Thus, PEO is at the (PMPS-PEO)n - water interface. This would indicate that

the average structure of the copolymer is (PMPS-PEO)2-PMPS, agreeing with calculations

performed on the GPCs of (PMPS-PEO)n (section 5).

The molecular weight distribution of the copolymer is broad, and the

polydispersity of the copolymer is therefore not particularly important with respect to the

formation of solubilised aggregates. What may be more important though is the

polydispersity and structure of the consitutuant homopolymers. The PEO used in the

synthesis of (PMPS-PEO)n had a polydispersity of 1.03. Hypothetically, what may be a

remarkable effect is that of the all-trans sements of PMPS on the formation of the vesicle.

The length of these segments may be determining the depth of the PMPS walls of the

observed vesicles. The indicated molecular lengths of (PMPS-PEO)n, the results of the

electron microscope and the measurements of the areas of the molecules of (PMPS-PEO)n,

can all be tentatively summed up in figure 6.7.

153

Figure 6.7 A representation of vesicle walls of (PMPS-PEO)n aqueous soluble aggregates1.

6.4 Conclusion

For the first time, multi-block copolymers of polysilylenes have been tentatively

shown to form vesicles in aqueous solutions. The results of the characterisations have

indicated that (PMPS-PEO)n is not required be of a low polydispersity to form regular

aggregates. This is believed to be the first known case of a multi-block copolymer forming

vesicles.

6.5 Suggestions For Futher Work

If this work were to be continued it would be interesting to:

❀ vary the polydispersity and molecular weights of (PMPS-PEO)n and characterise

the various expected forms of aggregates in aqueous solution.

❀ synthesise and characterise various types of block copolymers of PMPS with

hydrophilic polymers, which may have interesting aqueous solution properties.

154

References

1 S. J. Holder, R. C. Hiorns, N. A. J. M. Sommerdijk, S. J. Williams, R. G. Jones,R. J. M. Nolte, Chem. Comm., 1445, 14 (1998)

2 L. Zhang, A. Eisenberg, Science, 1728, 268 (1995)3 K. Yu, A. Eisenberg, Macromolecules, 6359, 29 (1996)4 L. Zhang, K. Yu, A. Eisenberg, Science, 1777, 272 (1996)5 L. Zhang, A. Eisenberg, J. Am. Chem. Soc., 3168, 118 (1996)6 J. D. Shaw, ‘Introduction to Colloid and Surface Chemistry’, Butterworth-Heinemann

Ltd., Oxford, 4th Ed., (1992)7 L. H. Radzilowski, S. I. Stupp, Macromolecules, 7747, 27 (1994)8 M. Ueno, H. Kashiwagi, N. Hirota, Chem. Lett., 217 (1997)9 J. N. Weinstein, S. Yoshikami, P. Henkart, R. Blumenthal, W. A. Hagins, Science,

489, 195 (1977)10 J. H. Fendler, ‘Membrane Mimetic Chemistry’, Wiley, New York 198211 Y. Liu, S. L. Regen, J. Am. Chem. Soc., 708, 115 (1993)12 W. J. Welsh, J. R. Damewood Jr., R. C. West, 2947, 22 (1989)

155

Résumé

Introduction

Les objectifs de cette thèse étaient la synthèse et la caractérisation de nouveaux

copolymères à blocs et multi-blocs à base de poly(méthylphénylsilylene) (PMPS). Aussi,

un modèle de la réaction du dichlorométhylphénylsilane et d’un oligo(α-méthylstyrene)

disodé a été étudié. Finalement, les agrégats en solution aqueuse d'un nouveau copolymère

multi-blocs, poly[poly(méthylphénylsilylene)-bloc-poly(oxyethylene)] ((PMPS-PEO)n),

ont été étudiés et caractérisés.

Les polysilylenes, ou polysilanes, se composent de chaînes linéaires d’atomes de

silicium chacun d’eux étant habituellement lié avec deux substituents organiques. Ils sont

communément préparés par la réaction de polycondensation de Wurtz. Ils ont des

propriétés intéressantes. En raison des orbitales σ délocalisées le long de la chaîne

polymère, les photons UV rompent les chaînes en excitant les électrons liés situés sur les

orbitales antiliantes. Cependant, dans le cas particulier du PMPS - lequel est sans doute le

plus facile et le moins cher des polysilylenes à synthésiser, celui-ci manifeste de mauvaises

propriétés physiques. En effet, le polymère n’est pas malléable et ne forme pas facilement

des films. Aussi, certains travaux de recherches visaient à améliorer et à diversifier les

propriétés physiques du PMPS, en préparant des copolymères blocs de PMPS avec des

polymères commerciaux .

Le polyisoprene a de très bonnes propriétés mécaniques et il peut être préparé par

polymérisation anionique avec un contrôle de la répartition de la masse moléculaire. Les

polymérisations anioniques ou ‘vivantes’, de monomères diéniques ou styrèniques, sont

habituellement effectuées dans des conditions de vide poussé. En effet, les réactions sont

extrêmement sensibles à de faibles quantités d'impuretés. Aussi, pour garantir la réactivité

continue du carbanion, plusieurs techniques sont employées pour éliminer l'air et l’eau.

Considérant que la technique réalisée sous vide ne pourrait pas être facilement adaptée à la

copolymérisation en bloc d’un poly(silylène) dihalogéné aussi, l'emploi, plus simple, de la

technique de Schlenk fut étudiée et comparée à la technique précédente.

Par opposition aux mélanges de polymères, lesquels présentent une mauvaise

résistance aux impacts, des caractéristiques physiques inintéressantes, une clarté optique

156

amoindrit et une faible adhérence interfacial entre les homopolymères, les polymères ABA

et AB ont des propriétés mécaniques plus élevées. Cependant, à notre connaissance, rien

n’a été réalisé dans le domaine de copolymère multi-blocs de PMPS. En général, les

copolymères multi-blocs donnent des propriétés physiques meilleures que celles des

copolymères de type ABA et AB, en raison de leur micro-structure régulière, et des

adhésions interfaciales plus fortes entre les domaines. Pour ces raisons, la synthèse de

copolymères de PMPS avec des polymères commerciaux a été effectuée.

Résulats et Discussions

La synthèse de poly(phenethylméthylsilylène) (PPEMS) a été étudiée pour en

savoir plus sur un possible candidat pour la synthèse des copolymères d’un polysilylène.

Les polymérisations conduisant au PPEMS ont formé des polymères avec des masses

molaires en rapport avec la longueur persistente des orbitales σ.

La polydispersité et les masses molaires des polymères formés par polymérisation

anionique « vivante » sont parfaitement contrôlables, et en utilisant le système sous vide

élevé, les polydispersités des polymères restaient inférieures à 1,1. Cependant, des

résultats similaires ont été réalisés en utilisant la technique dite de « Schlenk » avec des gaz

inertes. Considérant que cette dernière méthode facilite d’avantage les manipulations des

polymères, les copolymérisations en bloc de polymères « vivants » et de PMPS halogéné

nous semblaient être facilitées par cette méthode.

Les voies de synthèse des copolymères multi-blocs de PMPS avec le polyisoprene

(PI) ou poly(α-méthylstyrene) ou poly(oxyethylène) (PEO) ont été menées à bien et

optimisées. Les synthéses ont été optimisées en fonction des solvants utilisés, du temps de

réaction, de la température de réaction, en fonction des masses molaires des

homopolymères, et dans le cas des polymères hydrophiles, en fonction de l'utilisation des

agents complexants. Des réactions concurantes lors de la synthèses des copolymères blocs

de PMPS et de polymères « vivants » ont été mises en évidence et ont fait l’objet de

commentaires. Le copolymère de (PMPS-PI)n, où n est compris entre 3 et 16, a donné

d’excellentes propriétés physiques. Il permet l’obtention de films fins, optiquement clairs,

flexibles et résistants. L’obtention de films clairs laisse supposer la formation de

microdomaines. Le copolymère (PMPS-PEO)n, par déconvolution des résultats de GPC,

157

avait un degré de polymérisation moyen de 2,5 - équivalent au (PMPS-PEO)2-PMPS, et

ceci était en accord avec la distribution de Flory normale d'un copolymère. Le polymère se

disperse dans l'eau. Il montre en plus quelques formations d’agrégats inhabituels. Ces

agrégats ont été davantage étudiés en préparant, par dialyse, une solution de polymère dans

l'eau, qui a ensuite été analysée par microscopie électronique. Conjointement avec

l’encapsulation d’un colorant fluorescent et des études des π-A isothermes d’un film fin sur

l’eau, il a été démontrée que le copolymère formait des vésicules en solution. Les

copolymères ont été caractérisés par RMN, GPC, DSC, UV et IR.

Conclusion

Les copolymères multi-bloc d'un poly(silylène), ne montrent pas seulement une

grande amélioration des propriétés physiques de polysilylènes, mais gardent les

caractéristiques essentielles de ces derniers. La technique de copolymérisation en blocs

peut s’appliquer à une large gamme de polymères commerciaux avec les poly(silylènes).

Pour la première fois, un copolymère multi-blocs de poly(silylène), soluble en mileux

aqueux, a été synthétisé et ce copolymère forme des agrégats de vésicules dans des

solutions aqueuses.

158

Appendix 1

Chemicals Used

Chemical Quoted Purity / % Supplier

1,2-dibromoethane 99 Aldrich

15-crown-5 98 Aldrich

2-(bromoethyl)benzene 98 Aldrich

α-methylstyrene 97 British Drug Houses Ltd

bromine 99.5 Aldrich

calcium hydride 95 Aldrich

calcium sulphate 99 Aldrich

chloroform synthesis grade sds France

cyclohexane 99 Aldrich

dichlorobenzene synthesis standard Merck

dichloromethylphenylsilane 97 Lancaster

dichloromethylsilane 97 Aldrich

diethyl ether AR Fisons

heptane 95 Aldrich

hexane 98.5 Aldrich

iodine 99 Aldrich

isoprene (stabilised by 100 ppm butylcatechol) 99 Aldrich

2-propanol 99 Aldrich

lithium 99.9 Aldrich

magnesium turnings 98 Aldrich

methanol 99.8 Aldrich

napthalene 98 Aldrich

n-butyl lithium - Aldrich

(1.6M solution in hexanes)

pentane 98 Aldrich

petroleum ether (40-60) ACS Aldrich

159

poly(oxyethylene-co-propylene) - Aldrich

(average Mn = 12000)

poly(oxyethylene) glycol (average Mn = 8000) - Aldrich

(measured Mn = 7010)

potassium - Aldrich

pyridine 99 Aldrich

sec-butyl lithium - Aldrich

(1.3M solution in cyclohexane/heptane)

sodium 98 Aldrich

styrene 99 Aldrich

(inhibited with 10-15 ppm 4-t-butylcatechol)

tert-butyl lithium - Aldrich

(1.7 M solution in pentane)

tetrahydrofuran 99.9 Aldrich

tetrahydrofuran EPR (99.5) Carlo- Erba

(stabilised with 0.05% hydroquinone)

tetramethylsilane 98 Aldrich

toluene 99 Aldrich

trichloromethylsilane 97 Aldrich

triethylamine 99 Aldrich

160

Appendix 2 Experimental Methods

Preparation of β-phenethylmagnesium bromide

Into a three necked flask were placed magnesium turnings (80.8 g, 3.3 mol). To the

flask were fitted 2 double jacketed condensers (in series), an over-head stirrer and a rubber

septum. The apparatus was flushed with argon. Diethyl ether (1.90 l) was injected and

brought to reflux. 1,2 dibromoethane (2.5 ml, 0.029 mol) was added over 10 minutes to

start the Grignard reaction. Phenethyl bromide (215.4 ml, 291.9 g, 1.58 mol) was added

slowly over 75 minutes. After 12 hours the reaction was assumed to have gone to

completion.

Preparation of Dichloro-β-phenethylmethylsilane

A 5 l three necked flask was fitted with 2 double surface condensers (in series), an

over-head stirrer and a rubber septum and was flushed with argon. Diethyl ether (1.2 l)

was injected and brought to reflux. Trichloromethylsilane (185.2 ml, 1.577 mol) was

added, and with vigourous stirring, β-phenethylmagnesium bromide (1.577 mol) was added

as a solution in diethylether (0.7 l) via a metal siphon over a period of 2 hours. The

mixture was stirred for 24 hours.

Standard Wurtz Polycondensation of Dichloro-β-phenethylmethylsilane

This is the general method. Into a 50 ml two necked round bottomed flask, fitted

with a condenser and a magentic stirring bar, was placed freshly cut sodium (0.92 g, 0.04

mol) with some oil (circa 10 ml). The sodium was melted and finely ground with a

dispersion unit. Once the sodium had solidified, the oil was removed by repeated washing

with THF. The solvent (THF, toluene or diethylether) (45.5 ml) was added and once

refluxing, dichloro-β-phenethylmethylsilane (4 g, 3.55 ml, 0.018 mol) was added. After

refluxing for a predetermined time, the quenching agent was added. After two hours of

stirring, water was added to the mixture, and the polymer recovered by filtration, or left in

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a flask to settle overnight, and then by decantation, the liquid was removed. The polymer

was dried over 72 hours in a vacuum oven at 50 °C.

Preparation of Tetrahydrofuran on the High Vacuum Line

All glassware was built by hand to design, and where appropriate was flame dried

under vacuum (10-6 mmHg).

THF (2.5 l) was left over dry calcium hydride in a 5 l one necked flask over a period

of 4 days. On filtering the THF into a 5 l flask, potassium (25 g) was added, and the THF

was distilled over a column of 160 theoretical plates (approximately 1.5 m high). The

central distilate (1.5 l) was recovered on to dry calcium hydride (100 g) under nitrogen in a

3 l one necked flask. The THF was left overnight and then the flask was attached to a high

vacuum line (figure 4.1). Over a period of several hours the THF was degassed (at which

point the THF ‘bolts’ up the line on the application of a vacuum). This is a general

method and it was applied to the solvents cyclohexane and toluene.

Sodium (2 g) and potassium (4 g) were placed into a tube attached to a pair of 250

ml flasks (one a container, the other a valve (figure A2.1)). The tube was sealed, the unit

evacuated, and the Na/K mixture was flamed across into the flask. After cooling, the tube

was removed, and the THF was distilled onto the Na/K. The mixture was left to stir until

it attained a pale blue colour.

To transfer the THF to fragile bulbs, it was distilled across to a 50 ml flask of a

glass unit (figure A2.1). Each bulb, when filled with THF, was sealed off and stored in a

fridge until use. A similar system was used in the preparation of fragile bulbs of methanol.

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THF and K/Na alloy

250 ml flask

Magnetic barstirrer

MeOH/liquid nitrogen

250 ml flask‘valve’

50 ml flask

Vacuum

Tap

Tap

Sealingpoint

THF to measuringmark (15ml)

Fragile bulb with1 ml measuringunit (x 12)

Figure A2.1 A representation of the apparatus used to transfer THF to fragile glass bulbs.

Preparation of Isoprene for Polymerisation

Isoprene was distilled from over calcium hydride, and then stored and degassed on a

vacuum line from over molecular sieves (figure A2.2). In preparation for polymerisations,

the isoprene was distilled into a 50 ml flask into which BuLi (1 ml of a 1.6 M solution) had

previously been injected. As soon as the isoprene started to warm and oligomerise, it was

163

distilled into ampoules, and each of these was then individually sealed off. The ampoules

were stored in a fridge until use.

Figure A2.2 A vacuum line apparatus for the preparation of isoprene in break seals.

Preparation of sec-butyl Lithium

s-BuLi as supplied was found to have some precipitates. As a standard method,

the s-BuLi (7.2 ml, 1.3 M solution in pentane, and assumed to have a 50% strength) was

injected into a vessel under a high vacuum which was set just below a cold finger. Once the

solvent had been evaporated, the vessel was sealed, and liquid nitrogen placed in the cold

finger. Thus, the s-BuLi was separated from impurities. The cold finger was sealed off and

then attached to a distributer, whereby purified s-BuLi could be resolvated in cyclohexane

(12 ml) at a known concentration (0.3M) and separated off into individual ampoules fitted

with breakseals. These were stored in the fridge until use.

164

Preparation of Fragile Bulbs Containing Butyl Lithium

The method for cleaning reaction vessels is a general one and was used in

preparations for anionic polymerisations under a high vacuum. First, an excess of

cyclohexane was distilled into the vessel under a high vacuum (figure A2.3). The vessel

was sealed off from the high vacuum line, and n-BuLi (1.5 ml of a 2.0M solution in

cyclohexane) was injected into the lower flask. The injection point was sealed off. The

solution was distributed about the unit for 1 hour so as to clean the walls of impurities and

then collected in the lower flask. To rinse the glass walls of n-BuLi, cyclohexane was

distilled about the vessel by applying a cold rag to the higher parts of the vessel. Then,

cyclohexane (12 ml) was distilled from reservoir (a), to mixing flask (b), (the apparatus

having been turned upside-down to that shown in figure A2.3). The reservoir holding any

remaining cyclohexane and n-BuLi was then sealed off and removed. This method left a

mixing vessel with clean glass walls, a high vacuum and containing cyclohexane.

The s-BuLi breakseal was broken and initiator and cyclohexane were well mixed.

Each fragile bulb was filled with 1 ml of the solution and sealed off. The fragile bulbs were

stored in a fridge until use.

165

Figure A2.3 Equipment on a high vacuum line for the preparation of fragile bulbs containingthe initiator s-BuLi.

Polymerisation of Isoprene under High Vacuum Conditions

The apparatus used in the polymerisation of isoprene is shown in figure 4.2.

After washing this vessel, a magnetic stirrer was released. A fragile bulb containing s-BuLi

was then broken. The solution was cooled to -78 °C. Isoprene in a breakseal was cooled

to -60 °C prior to adding it to the mixture, so as to ensure that the vessel would not

explode upon opening the breakseal. The typical colour of ‘living’ polyisoprene - orange -

was observed and the reaction was left to continue for 2 hours. It was arrested by breaking

a fragile bulb of methanol. Once the vessel had been broken open, the solution was added

dropwise to methanol (400 ml) to precipitate the polymer, which was then collected by

filtration and dried, in a vacuum oven for 36 hours. A similar method was used for the

166

polymerisation of isoprene with oligo(α-methylstyryl) disodium.

Preparation of Oligo(α-methylstyryl) Disodium under High Vacuum Conditions

Freshly cut sodium metal (0.92 g, 0.04 mol) was placed in to a vessel which was

then sealed, placed under a high vacuum, and flame dried. The sodium was heated and spun

with a magnetic stirring bar, creating a fine spray of metal on the sides of the vessel. Once

cool, the breakseaks to THF (100 ml) and α-methylstyrene (distilled from over CaH2,10.4

ml, 0.08 mol) were broken and the mixture left to stir for 24 hours. The solution was

passed into individual breakseals via a glass filter, to remove the remaining sodium

particles. Each ampoule of initiator was sealed off and stored for use at -20 °C.

Preparation of Oligo(α-methylstyryl) Disodium under Inert Gases

A Schlenk tube was evacuated, well flame dryed and flushed with nitrogen. Freshly

cut sodium (0.92 g, 0.04 mol) was placed into the tube along with toluene (10 ml) and

melted. With a strong flush of dry nitrogen, a dispersion unit was used to make a fine

powder of sodium. The cooled toluene was removed, partly by syringe, partly by

evaporation. THF (100 ml) (double distilled over sodium wire) and α-methylstyrene

(distilled under vacuum and collected at 31 °C at 3.0 mmHg to 36 °C at 4.0 mmHg, 10.4 ml,

0.08 mol) were injected into the tube, and the mixture was stirred for 8 hours. After which

the blood-red solution was canulated, via a glass paper filter (Whatman type ‘B’) into a

clean Schlenk tube to remove remaining sodium particles. The solution was stored under a

positive pressure of nitogen until use.

Synthesis of Poly[oligo(α-methylstyrene)-co-methylphenylsilane]

Into a Schlenk tube fitted with a magnetic stirring bar was injected a solution of

oligo(α-methylstyrene) disodium in THF (0.13 M, 23.7 ml, 3.08 x 10-3 mol). To this

rapidly stirred solution was added dropwise dichloromethylphenylsilane (0.5 ml, 0.588 g,

3.08 x 10-3 mol). Almost instantaneously the red colour of the sodium anion had

167

disappeared. After 5 minutes the solution was added dropwise to stirred methanol and the

precipitate was collected over a Buchner funnel. The product was dried under vacuum over

48 hours to yield a white powder. The yield was 0.525 g or 28%.

Preparation of α,ω-dichloropoly(methylphenylsilylene) and a Subsequent

Synthesis of α,ω-dihalopoly(methylphenylsilylene)

Sodium (20.03 g, 8.712 x 10-1 mol, 120% relative to chlorine of

dichloromethylphenylsilane) was placed into an argon flushed two necked 1 l flask fitted

with a condenser and a large magnetic stirring bar. Toluene (30 ml) was injected in. The

sodium was warmed until molten, at which point it was powdered using a dispersion unit.

On cooling, the toluene was removed, and THF (600 ml) was injected in followed by a

rapid addition of dichloromethylphenylsilane (60 ml, 69.38 g, 0.363 mol). After 15

minutes the ubiquitous blue colour of the Wurtz reaction appeared along with a slight

warming of the mixture. After 3 hours, the heterogeneous mixture was canulated and

filtered into a second 1 l two necked flask under argon. This yeilded a clear liquid, which

was reduced by vacuum to approximately 200ml of a slightly yellow and viscous solution.

At this point, the polymer was either precipitated by the slow addition of petroleum ether

(750 ml), collected by filtration and dried under a vacuum over 48 hours, or an aliquot was

removed for characterisation and the solution was used for a synthesis of α,ω-

dihalopoly(methylphenylsilylene). To synthesise the latter, to the rapidly stirred solution

was injected an excess of bromine (0.1 ml, 1.9 x 10-3 mol). The solution changed colour

from a pale yellow to a strong orange, and over the course of 5 minutes, to a pale orange.

After 20 minutes hexane (700 ml) was injected into the stirred solution to fractionate the

polymer.

By canulation through a PTFE tube fitted with a glass paper filter, the orange liquid

was withdrawn from the mixture to leave a white powder. The solid was dried under

vacuum overnight. Analysis of the polymer by GPC of a small sample indicated the

presence of a negligible amount of oligomers. The yield of the polymer was 6.98 g (15.7%).

168

Synthesis of Poly[poly(methylphenylsilylene)-block-oligo(α-methylstyrene)]

α,ω-dichloropoly(methylphenylsilylene) (1.0 g, 7.8 x 10-5 mol) was transferred in

to a flame dried Schlenk tube and dissolved in to 30 ml of THF under argon. Slowly, an

excess of oligo(α-methylstyryl) disodium was added (0.9 ml of a 0.13M solution in THF,

1.2 x 10-4 mol). An excess was used to remove impurities from the solution. The blood-

red colour of the initiator disappeared, and the solution became cloudy. After 30 minutes,

the mixture was added drop-wise to methanol (200 ml) and the precipitate was collected by

filtration. The white polymer was dried over 24 hours in a vacuum oven.

An Attempted Synthesis of

Poly[poly(methylphenylsilylene)-block-poly(α-methylstyrene)]

This method is similar to that used for the synthesis of

poly[poly(methylphenylsilylene)-block-polyisoprene]. A solution of poly(α-

methylstyryl) disodium (Mn = 2030 g mol-1, Mw/Mn = 1.8, 3.6 x 10-4 mol) in THF (20 ml)

was used in this reaction.

Preparation of Polyisoprenyl Disodium under Inert Gases using Schlenk

Techniques

Into a flame dryed and dry nitrogen flushed Schlenk tube was injected THF (double

distilled, 30 ml) and isoprene (freshly distilled, 5 ml, 3.4 g, 0.05 mol). At -78 °C the

initiator, oligo(α-methylstyryl) disodium, was added drop-by-drop until the orange colour

of the isoprenyl anion became just stable. Immediately afterwards, oligo(α-methylstyryl)

disodium (6.8 ml of a 0.10M solution, 6.8 x 10-4 mol) was rapidly injected and the solution

was stirred quickly for two hours. An aliquot was removed for characterisation, prior to

the polyisoprenyl disodium being used for other reactions, or the polymer was

precipitated, drop-wise into methanol and then dried in a vacuum oven over 36 hours.

169

Synthesis of Poly[poly(methylphenylsilylene)-block-polyisoprene]

A solution of polyisoprenyl disodium (Mn = 7860 g mol-1, 25 ml, 5.6 x 10-4 mol) in

THF (30 ml) was prepared under dried nitrogen in a Schlenk tube at -78 °C. To this rapidly

stirred solution was added α,ω-dihalo-poly(methylphenylsilylene) (Mn = 5250 g mol-1,

2.9g, 5.6 x 10-4 mol) in THF (25 ml) via a PTFE tube. Over the period of the reaction (9

minutes), the mixture was allowed to warm slowly to room temperature. The polymers

were precipitated by adding the mixture drop-wise to methanol. The precipitate was

collected over a Buchner funnel and dried in a vacuum oven for 48 hours.

Polymer fractionation, removing the multi-block copolymers from the remaining

homopolymers, was performed by dissolving the mixture in THF (400 ml), and to this

stirring solution isopropanol (870 ml) was added slowly, before leaving the mixture to

stand overnight. The mother liquor was removed by canulation via a glass filter paper.

Remaining solid precipitate was dried in a vacuum oven over 48 hours. The yield of a

white copolymer was 2.25 g (35%).

Preparation of Thin Films of Poly[poly(methylphenylsilylene)-block-polyisoprene]

Poly[poly(methylphenylsilylene)-block-polyisoprene] (Mw = 77500 g mol-1, 1.4 g,

1.8 x 10-5 mol) was placed with chloroform (8.5 ml) in a sealed jar with a magnetic stirring

bar. The mixture was stirred for 24 hours to dissolve the polymer. After standing for 2

hours to release bubbles of trapped air, the solution was drawn out on to glass plates using

a glass rod lifted just above the surface. Films were left to dry in a dark and well ventilated

room. Three films fabricated by this method were found to have thicknesses of 10, 22 and

40 µm. The clear and flexible films were of sizes A5, A5 and 3 cm x 15 cm respectively.

Drying Poly(oxyethylene) glycol

Poly(oxyethylene) glycol (manufacturers quoted Mn = 8000 g mol-1, GPC analysis

indicated Mn = 7010 g mol-1) was dried by heating in a Schlenk tube at 80 °C under vacuum

for 2 hours and then dissolving in warm toluene to give a concentrated solution (8.8 x 10-2

M) ready for use.

170

Synthesis of Poly[poly(methylphenylsilylene)-block-poly(oxyethylene)]

Using an argon flushed Quick fit glass tube, α,ω-dihalo-poly(methylphenylsilylene)

(Mn = 5280 g mol-1, Mw/Mn = 1.7, 3.6 g, 6.8 x 10-4 mol) was transferred into a Schlenk

tube fitted with a magnetic stirring bar. The Schlenk tube was sealed, evacuated and

reflushed with argon. THF (15 ml) and toluene (20 ml) were injected into the Schlenk tube.

The mixture was stirred for 15 minutes until homogeneous. A concentrated solution of

poly(oxyethylene) glycol (Mn = 7010 g mol-1, Mw/Mn = 1.03, 4.8 g, 6.8 x 10-4 mol) in

toluene was injected into the Schlenk tube. Pyridine (3 ml, 3.7 x 10-2 mol) was injected

after 10 minutes of stirring. The stirring was continued for 30 minutes. The solution was

then added drop-wise to methanol (300 ml) and to this mixture was slowly added pentane

(400 ml). The precipitate, was collected over a Buchner funnel and dried in a vacuum oven

for 72 hours. The yield of the slightly yellow white powder was 5.05 g (60.3%). GPC

indicated that no further purification of the product was required.

Synthesis of Poly(methylphenylsilylene)-block-poly(oxyethylene)

The synthesis of poly(methylphenylsilylene)-block-poly(oxyethylene)was by the

same method as that for used to synthesise poly[poly(methylphenylsilylene)-block-

poly(oxyethylene)]. However, the ratio of the reagents was one part α,ω-dihalo-

poly(methylphenylsilylene) (Mn = 5280 g mol-1, 2.1 g, 4.0 x 10-4 mol) to 2.2 parts

poly(oxyethylene) glycol (Mn = 7010 g mol-1, 6.2 g, 8.9 x 10-4 mol) in toluene. To

fractionate the product polymers, a dried mixture of the product was dissolved in THF

(200 ml) and to this heptane (600 ml) was slowly added. The precipitate was collected

over a Buchner funnel and dried in a vacuum oven for 24 hours. In an attempt to remove

unreacted poly(oxyethylene) glycol, the mixture was dissolved in THF (200 ml) to which

methanol was added until a slight clouding appeared. The solution was warmed to remove

the cloud and then left to settle overnight. After further gentle cooling with a dry

ice/acetone bath, the precipitate was collected over a Buchner funnel and a compound dried

in a vacuum oven for 72 hours at 30 °C. The yield of a white powder was 2.22 g (26.6%).

171

Dispersion and Characterisation of Poly[poly(methylphenylsilylene)-block-

poly(oxyethylene)] in Aqueous Solutions

1 mg of (PMPS-PEO)n was shaken in 1 ml of water, however after leaving for 72

hours and heating the mixture to 70 °C, most of the solid remained undispersed. The

homogeneous aqueous solutions of (PMPS-PEO)n were prepared by two different

methods. Firstly, by dissolving the copolymer (100 mg) in THF (10 ml) and then after

adding water (3.5 ml) drop-wise the solution was subjected to ultrafiltration (using a

membrane with a 10000 g mol-1 cut-off) with continuous concentration and water dilution

(3 x 10 ml). A white opaque dispersion resulted (with a concentration of 1.2 g / l).

Secondly, a solution of (PMPS-PEO)n in THF (3 ml) and water (1 ml) was placed in a

dialysis bag (exclusion limit of 20000 g mol-1) and dialysed against pure water (500 ml) for

72 hours.

Samples of solutions of (PMPS-PEO)n for electron micrographs were dried, by

evaporation, on a carbon coated copper electron microscope grid, stained with uranyl

acetate and then platinum shadowed.

Dye Encapsulation

The procedure detailed for the preparation of aggregates by dialysis was used,

except that 5-carboxyfluorescein was added to the pure water. The dispersion solution

was eluted through a Sephedex column (G150, mesh size 40 - 120 µm).

Characterisations of Mono-films of Poly[poly(methylphenylsilylene)-block-

poly(oxyethylene)]

The analyses of the mono-layered films were completed at 20.0 ± 0.1 °C using a

double barrier R & K trough of dimensions 6 x 25 cm with a compression speed of 8.8 cm2

minute-1. The copolymer was spread from a solution of chloroform.

172

Appendix 3

29Si NMR Spectroscopy and Polysilylenes

The NMR responsive 29Si atoms make up 4.7% of the isotopes of silicon1. 29Si

atoms have long spin-lattice relaxation times (T1 ~ 60 s), a low negative magnetogyric ratio,

a spin I = 1/2, and anti-parallel magnetic moments. The effects of interacting protons are

difficult to predict. The nuclear overhauser effect (NOE) may suppress any resonances of29Si by coupling 1H atoms to heteroatoms. There are several methods to eliminate this

problem. One is to add Cr(acac)3, a paramagnetic species, which reduces T1 and

suppresses the NOE. Another is to 'gate decouple' protons by using a pulse of radio

frequency at their frequency to excite them, leaving them unresponsive to 29Si nuclei.

Another and more common method is to use, with FT NMR, 'Insensitive Nuclei Enhanced

by Polarisation Transfer' pulse techniques2 (INEPT) which can retain the 'gated decoupling'

of hydrogen atoms. This uses the faster T1 of protons coupled to 29Si, and not the T1 of

the 29Si2,3. 'Distortionless Enhancement of NMR signals by Polarisation Transfer' (DEPT),

a modern version of the INEPT process, enhances 29Si absorptions by the use of a series of

radio frequencies which 'flip' the nuclei spin and decouple interacting protons.

For PMPS in solution, some of the resonance peaks are known to indicate certain

groups of atoms (table A3.1). For randomly atactic alkylated polysilylenes the

absorptions are generally broad. Structural defects in PMPS, probably due to a formation

of organosilyne units interacting with methylphenylsilylene units, have been observed as

broad peaks at -35 ppm and -40 ppm4.

173

Absorption / ppm Indication Reference

-21.0 (PhMeSiO)3 from hydrolysis 5

-22.0 Polydimethylsiloxane (vac grease) 5

-39.2 PMPS main chain 2, 6, 7, 8

-39.9 PMPS main chain 2, 6, 7, 8

-40.2 PMPS main chain 2, 6, 7, 8

+8 Si-OMe, multiplicity observed due to associated Si triad 5

+15 Si-Cl, multiplicity observed due to associated Si pentad 5

-45.4 Si β to Si-OMe group 5

-41.2 Si β to Si-Cl group 5

-35 Si γ to end groups 5

Table A3.1. Representative absorption peaks associated with PMPS in solution.

References

1 R. D. Miller, J. Michl, Chem. Rev., 1359-1410, 89 (1989)2 A. R. Wolff, PhD Thesis (1984)3 J. M. Ziegler, L. A. Harrah, Macromolecules, 601, 20 (1987)4 M. Fujiki, Chem. Phys. Lett., 177, 198(1-2) (1992)5 A.-F. de Mahieu, J. Devaux, C. Dereppe, G. Baudoux, J. Delhalle, J. Polym. Sci., Polym.

Chem., 763, 34 (1996)6 R. G. Jones, R. E. Benfield, P. J. Evans, S. J. Holder, J. A. M. Locke, J.

Organomet.Chem., 171 - 176, 521 (1996)7 R. G. Jones, R. E. Benfield, P. J. Evans, A. C. Swain, J. Chem. Soc., Chem. Commun.,

1465, (1995)8 E. Fossum, K. Matyjaszewski, Macromolecules, 1618, 28 (1995)9 J. Michl, Synth. Met., 367, 49-50 (1992)

174

Appendix 4

Apparatus Used

GPC

The molecular weights of the oligomers, polymers and copolymers were indicated,

relative to polystyrene standards, by size exclusion chromatography (GPC) using

equipment supplied by Polymer Laboratories Ltd. All determinations were carried out at

room temperature, using a dual column bank of 5 µm particles with 500 and 104 Å pores,

with unstabilised THF as the eluent at a flow rate of 1.0 ml/min.

The system was equipped with a refractive index (RI) detector and a Knauer

variable wavelength UV-visible spectrophotometric detector set at either λ = 254 nm or

334 nm.

NMR

1H, 13C and 29Si NMRs were recorded on a Jeol PS 270 all at 67.8 MHz, and for

investigative 1H NMRs only, a Jeol PMX 60ni was used at 60 MHz. All NMRs were

recorded at room temperature. Tetramethylsilane was used as a reference point for

downfield calibrations (in ppm).

DSC

The apparatus used was a Perkin Elmer DSC-7. Scans were made at a rate of 10 °C

minute-1, and taken from the second heating passage, unless otherwise noted.

Infra Red

The machine used was a IFS 25, SCA Infra Red, connected to a PC for data

management.

175

Gas Chromatography

Samples were analysed on an Altech Econocap EC-30 silica column with an internal

diameter of 0.5 mm with Pye Unicam PV4500 equipment.

Dispersion Unit

An Ultra-Turrax T8 dispersion unit was used with a maximum speed of 25000 l/min

with the dispersing elements S 8 N-5G and S 8 N - 8G fitted. It was supplied by Janke

and Kunkel GmbH and Co. KG, IKA Labortechnik, Germany.

Ultra Violet

A scanning PU8700 UV/V spectrophotometer with an attached printer was used.

Document Scanning

Non-ditigal data was scanned on a Umax S-6E at 1200dpi at 10% and then

tranferred to the Macintosh™ format for working up.

Graphics

Graphs were calculated in Excel™ from ASCII data. Peaksolve™ was used to

deconvolute the GPC curves, and this data was worked up in Excel™. Figures and

schemes were designed in the Macintosh™ format using either Graphic Converter™,

PhotoShop™ or ChemIntosh™ and then transferred to Word™ on a Macintosh™ system.

The font used throughout the text was Times New Roman (11 and 12 point). The whole

text was exported to and printed from a PDF™ format.

Résumé

Ce travail visait à améliorer les mauvaises propriétés physiques des poly(silylène)s, en préparant descopolymères à blocs de poly(méthylphénylsilylene) (PMPS) avec des polymères commerciaux. A cet effet,un système modèle de la réaction du dichlorométhylphénylsilane et de l’oligo(α-méthylstyryl) disodé a étéétudié. Les copolymères étaient caractérisés par les méthodes classiques de même que les agrégats en solu-tion aqueuse de poly[poly(méthylphénylsilylene)-bloc-poly(oxyethylène)] ((PMPS-PEO)n).

La synthèse de Wurtz du poly(phénethylméthylsilylène) a été étudié en complément du PMPS. De nouvelles synthèses de copolymères multi-blocs du PMPS avec le polyisoprene ou le

poly(α-méthylstyrene) ou le poly(oxyethylène) ont été optimisées. La préparation du polyisoprenyl disodéa été faite selon la technique de Schlenk. Le poly[poly(méthylphénylsilylene)-bloc-polyisoprene] a permisla formation de films fins, flexibles et optiquement clairs. En accord avec la distribution normale de Floryd’un copolymère, le (PMPS-PEO)n était obtenu avec une valeur de n = 2,5. Le polymère ainsi formé donnedes aggrégats ayant les caractéristiques des vésicules.

Abstract

Polysilylenes are an interesting class of materials. However, they often display poor mechanicalproperties. The aim of this work was to synthesise and characterise block copolymers of a representativepolysilylene, poly(methylphenylsilylene) (PMPS), to improve and diversify its properties. Reactions ofoligo(α-methylstyryl) disodium and dichloromethylphenylsilane were used to model the copolymer formingreactions. Reactions of α,ω-dichloro-poly(methylphenylsilylene) with di-anionic and hydroxy terminatedpolymers were performed. The structures of copolymers were characterised by standard techniques.Aqueous solution aggregates of the novel polymer, poly[poly(methylphenylsilylene)-b l o c k-poly(oxyethylene)] ((PMPS-PEO)n), were characterised.

The Wurtz synthesis of poly(phenethylmethylsilylene) was studied, this being an alternative topoly(methylphenylsilylene) as a polysilylene with a phenyl substituent.

The syntheses of multi-block copolymers of PMPS and polyisoprene or poly(oxyethylene) orpoly(α-methylstyrene) were optimised. Schlenk techniques were used to prepare polyisoprenyl disodium andto manipulate polymer-polymer reactions. Competing reactions in syntheses of block copolymers of PMPSand of ‘living’ polymers are discussed. Poly[poly(methylphenylsilylene)-block-polyisoprene] was formedas flexible, optically clear thin films (indicating a formation of well defined domains). In good agreementwith the Flory ‘normal’ distribution, (PMPS-PEO)n was shown to have a number average degree ofpolymerisation of 2.5. Aqueous solution aggregates of this polymer were shown to be vesicles.

It is believed that for the first time multi-block and aqueous soluble copolymers of a poly(silylene)have been synthesised, all of which show novel properties.

Disipline / Discipline: Chimie Macromoleculaire / Polymer Chemistry

MOTS-CLES / KEYWORDS: Polysilanes, poly(methylphenylsilylene), anionic polymerizations, multi-block copolymers, polyisoprene, poly(oxyethylene)

Laboratoire de Chimie Macromoleculaire, Université de Montpellier II,Place E. Bataillon, 34095 Montpellier cedex 5, FRANCE

Centre for Materials Research, School of Physical Sciences, University of Kent at CanterburyKent CT2 7NR, GREAT BRITAIN