Synthesis and characterisation of electrical conducting polymers co polymers based on omega functionalised 3 alkylthiophenes 1

III 3.1 Experiments on different protecting groups 40 3.2 Structural characterisation of synthesised monomers 41 3.5 Electrical conductivity of the doped polymers 49 3.6 UV-Vis and fluor

Synthesis and Characterisation of Electrical Conducting

Polymers/co-polymers Based on -Functionalised

3-Alkylthiophenes

Ma YiFei

National University of Singapore

2004

Synthesis and Characterisation of Electrical Conducting

Polymers/co-polymers Based on -Functionalised

3-Alkylthiophenes

Ma YiFei

(B Sc (Hons.) NUS)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF

SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE

2004

I

I would also like to thank the members of Functional Polymer Group for their excellent support and stimulating advice, especially to Mr Wang Yeang Chyn for his help and friendship Many thanks to staff and technicians in Surface Science Laboratory, Honours Laboratory and Central Laboratory for their assistance

II

Table of Contents Summary

List of schemes List of figures List of tables Chapter 1 Introduction

2 Brief introduction on the conducting mechanism 5

III

3.1 Experiments on different protecting groups 40 3.2 Structural characterisation of synthesised monomers 41

3.5 Electrical conductivity of the doped polymers 49 3.6 UV-Vis and fluorescence spectroscopy 50 3.7 Thermal stability of the neutral polymers 53

3.9 Attempts to further functionalise the -bromo moiety on the

Chapter 5 Conclusion and suggestions for future work

2 Comparison between the two series of copolymers 185

Appendix List of main compounds reported in this thesis

191

1 Poly[3-( -bromoalkyl)thiophene]

n = 4 (pTHC4Br) = 6 (pTHC6Br) = 8 (pTHC8Br) = 10 (pTHC10Br) = 12 (pTHC12Br) Initially, a series of monomers, 3-( -bromoalkyl)thiophenes, were synthesised and characterised based on reported methods Subsequent oxidative polymerisation yielded a series of corresponding polymers, which were fully characterised These polymers were soluble in polar solvent such as chloroform and had conductivities comparable to those of poly(3-alkylthiophenes) However, attempts to carry out Grignard reactions using these polymers in order to further functionalise them were unsuccessful Hence 3-( -bromoalkyl)thiophenes were further functionalised in order to obtain graft copolymers of polythiophene and commodity polymers

2 Graft copolymers of polythiophene and polystyrene (PS) based on 3-{

-[1-(p-vinylphenyl)]hexyl}thiophene

S (CH2)nBr m

VI

In this approach, further functionalisation of 3-( -bromoalkyl)thiophenes resulted

in a novel monomer, 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene

This compound was polymerised in two steps: first, methylpropionitrile) (AIBN) was used as an initiator to polymerise the vinyl phenyl group Subsequently an oxidative polymerisation step produced a polythiophene structure However, the copolymer formed was not soluble and had poor conductivity On the other hand, the monomer was copolymerised with styrene in different ratios When the resultant copolymers were subjected to oxidative copolymerisation with thiophene or 3-alkylthiophene derivatives, a series of grafted copolymers of polystyrene and polythiophene with alkyl chain linkages were obtained These copolymers have improved conductivity and processibility A close look at the characterisation results revealed that the structure of the copolymers played an important role in determining their properties

2,2’-azobis(2-3 Polymethyl methacrylate (PMMA) and Poly(3-alkyl)thiophene co-polymer

A novel monomer, 11-thiophen-3-yl-undec-1-en-3-one, was synthesised and characterised

S

C6H12Ph

VII

Once again, a direct two-step co-polymerisation reaction of this monomer resulted

in a polymer that was not soluble and with poor conductivity Introducing methyl methacrylate MMA and thiophene or 3-alkylthiophene into the copolymer matrix using the process described above produced a series of grafted copolymers that have improved processibility and conductivity

The results from this work demonstrated the viable route of forming graft copolymers of polythiophene and PS or PMMA via functionalised 3-alkylthiophenes Their properties largely depend on the backbone structure of the polymers Soluble and conductive copolymers have been obtained by varying the polymer structure These materials are potentially useful as anti-static plastic products

S

CH28 O

VIII

List of Schemes:

Scheme 2.1 Monomer synthesis Reagents and conditions: (i) KOH/MeOH, acetone,

reflux; (ii) Mg, I 2 , anhydrous ether, reflux; (iii) Ni(dppp)Cl 2 , anhydrous Ether; (iv) HBr/Ac 2 O, 100 C

Scheme 3.1 Synthesis of the monomer 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene Scheme 3.2 Direct two-step polymerisation to afford copolymer Graft 100

Scheme 3.3 Polymerisation of styrene and monomer in the ratio of 1:1 and 5:1

afforded 3b and 3c respectively, which can be further polymerised to give copolymers Graft 21 and Graft 51

Scheme 3.4 Polymerisation of styrene and monomer 3 in the ratio of 10:1 produced

3d, which can be further co-polymerised with 3-octylthiophene and

thiophene Scheme 4.1 Synthesis of monomer 5

Scheme 4.2 Possible mechanisms for the formation of 4 as the major product

Scheme 4.3 Synthesis of graft copolymer 7 from monomer 5

Scheme 4.4 Synthesis of graft copolymer 9

Scheme 4.5 Syntheses of graft copolymers 10 and 11

IX

List of Figures:

Fig 1.1 Proposed conducting unit of polyacetylene Soliton may be neutral (radical),

positive (carbocation), or negative (carbanion) Fig 1.2 The formation of polaron and bipolaron in polythiophene Fig 1.3 Interchain hopping of bipolaron according to Bredas et al

Fig 2.1 Overlaid FT-IR spectra of all monomers From top to bottom: THC4Br,

THC6Br, THC8Br, THC10Br, and THC12Br Fig 2.2 FT-IR spectra of polymers before and after soxhlet extraction (doped and

undoped) From top to bottom: (a) undoped pTHC4Br; (b) pTHC6Br; (c) pTHC10Br; (d) doped pTHC6Br and (e) 3-( -bromohexyl)thiophene

Fig 2.3 NMR spectrum of pTHC6Br Fig 2.4 TGA curve of pTHC6Br in air Fig 2.5 Deconvolution of C1s environment in XPS spectrum of pTHC4Br Fig 3.1 1 H NMR spectrum of monomer 3

Fig 3.2 13 C NMR spectrum of monomer 3

Fig 3.3 NMR of precursor polymer 3a

Fig 3.4 NMR spectrum of precursor copolymer 3c

Fig 3.5 NMR spectrum of the resulted copolymer from 3-( -bromohexyl)thiophene,

monomer 3 and styrene in 1:1:10 mole ratio

Fig 3.6 1 H NMR spectrum of the polymer blend of PS and poly(3-octylthiophene) Fig 3.7 1 H NMR spectrum of grafted copolymer 4

Fig 3.8 1 H NMR spectrum of grafted copolymer of PS and polythiophene (5)

Fig 3.9 IR spectra of copolymers 4, 5 and the copolymer blend

X

Fig 3.10 IR spectrum of copolymer Graft 100 Fig 3.11 XPS spectrum of copolymer 4 Fig 3.12 C 1s core level of XPS spectrum of copolymer 5 Fig 3.13 C 1s core level of XPS spectrum of copolymer Graft 100 Fig 3.14 S 2p core level XPS spectrum of copolymer 4

Fig 3.15 XRD spectrum of copolymer Graft 100 Fig 3.16 XRD spectrum of copolymer 4

Fig 3.17 XRD spectrum of copolymer 5 Fig 3.18 Thermal degradation pattern for copolymers (from top to bottom) Graft 21,

Graft 51 and Graft 100

Fig 3.19 TGA plot for copolymer 4 in air Fig 3.20 Overlaid DSC plots for copolymers (from top to bottom) 4(1), Graft 100(2)

and 5(3)

Fig 3.21 Illustration of possible structures of copolymers Graft 100 and 4

Fig 4.1 1 H NMR of 3-(unde-10-enyl)thiophene (3)

Fig 4.2 1 H NMR of 11-thiophen-3-yl-undec-1-en-3-ol (4)

Fig 4.3 13 C NMR spectrum of 11-thiophen-3-yl-undec-1-en-3-ol (4)

Fig 4.4 1 H NMR spectrum of 11-thiophen-3-yl-undec-1-en-3-one (5)

Fig 4.5 13 C NMR spectrum of 11-thiophen-3-yl-undec-1-en-3-one (5)

Fig 4.6 1 H NMR spectrum of 11-thiophen-3-yl-undec-3-en-2-one Fig 4.7 1 H NMR spectrum of copolymer 10

Fig 4.8 Overlaid IR spectra of copolymers 7, 9, 10 and 11

Fig 4.9 XPS spectrum of copolymer 9

Fig 4.18 TGA plot of copolymer 10 in N 2

Fig 4.19 TGA plot of copolymer 11 in N 2

Fig 4.20 Stacked TGA plots for copolymers 7, 9, 10 and 11 (the % weight change is

only indicative of the scale)

Fig 4.21 SEM image of (from top to bottom, left to right) copolymers 7, 9, 10 and 11

Fig 5.1 Illustration of possible structures of copolymers formed from 100% monomers

A: commodity polymer backbone TH: thiophene/3-alkylthiophene

melting points Table2.7 Percentage yields of 3-( -bromoalkyl)thiophenes and their boiling points Table 2.8 Percentage yields of poly-3-( -bromoalkyl)thiophenes

Table3.1 Elemental analyses results of grafted copolymers

Table 3.2 Elemental content of copolymers 4, 5 and Graft 100 determined by XPS Table 3.3 C 1s of XPS spectrum data of copolymer 5

Table3.4 Summarised 2 peaks of copolymers Graft 100, 4 and 5

Table 3.5 Summarised thermalgravimetric analyses results Table3.6 GPC analyses, conductivity measurements and UV-Vis results for copolymers

formed Table4.1 Elemental analyses results of the graft copolymers Table4.2 Summary of XPS elemental content of the copolymers

Table 4.3 C1s core level data of copolymer 11 Table 4.4 C1s core level data of copolymer 7 Table 4.5 C1s core level data of copolymer 9

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Table 4.6 C1s core level data of copolymer 10 Table 4.7 Summary of the 2 peaks of the copolymers 7, 9, 10, 11 Table 4.8 Thermogravimetric studies of the graft copolymers 7, 9, 10, 11 Table 4.9 Thermal analyses results for copolymers 7, 9, 10 and 11 Table 4.10 Atomic ratio of copolymers 7, 9, 10, 11 calculated based on EDX spectra Table 4.11 Maximum conductivity achieved by the copolymers 7, 9, 10, 11

Chapter 1 Introduction

2

1 Overview of common conducting polymers

Most commodity polymers such as polyethylene, polystyrene and poly(vinyl chloride) are insulators Some conduct electricity to a certain extent, e.g., poly(N-vinyl carbazole) is used in the photocopying industry due to its photo conducting properties [1]

In the 1970s it was found that certain polymers especially poly(sulfur nitride) [2] and polyacetylene [3] are highly conducting when doped with certain additives (dopants)

As a result of this important finding, major research efforts have been initiated over the next three decades to explore these new materials’ conducting mechanisms and application potential

Factors such as method of synthesis, type and concentration of dopant and polymer morphology affect the conductivity of the polymers Although the more stable polymers such as those heterocyclic conducting polymers may not be as conductive as polyacetylene, nonetheless, the amount of conductivity is considered acceptable for commercial development

poly(N-vinyl carbazole)

Nn

S Npoly(sulfur nitride)

polyacetylene

Poly(p-phenylene) 103

Poly(p-phenylenevinylene) 103

Polyaniline 102-103 Polypyrrole 102-103 Polythiophene 102

Polyacetylene has been the most widely studied conducting polymer However, the polymer has limited commercial value due to its instability when exposed to moisture and air Consequently, research attention has diverted to other more stable conducting polymers such as polyaniline [5] in various oxidation states,

polypyrrole [6], polythiophene [7], phenylene) [8] and

poly(p-phenylenevinylene) [9] as well as their derivatives

respectively upon complexation An unusual dopant is HCl, which was employed

in the doping of polyaniline [10] Apart from chemical synthesis, conducting polymers may also be formed from their monomer solutions via electro chemical polymerisation in a reaction cell as a continuous film [11] In this method, dopant

ions are introduced directly from the monomer solution

Conducting polymers generally have densities of around 1 g/cm3, which is much lower than that of metals (e.g., 8.92 g/cm3 for copper; 19.3 g/cm3 for gold) Many areas of application have been developed for these polymers [12] Taking advantage of this and other unique properties of these polymers, advanced technological material from lightweight batteries, sensors, thin film transistors, light emitting diodes (LEDs), solar cells and antistatic material to solid electrolytes are envisaged In recent years, conducting oligomers have also drawn much attention [13, 14] There have been reports on developing ‘molecular wires’

NHpolyaniline

CH

poly(p-phenylenevinylene)

CH

NHpolypyrrole

Spolythiophene poly(p-phenylene)

5

which consist of conducting polymeric material that bridge metal surfaces for use

in molecular scale electronic devices [15, 16]

The research effort in the last three decades have solved many previously unanswered questions and at the same time, paved way for new challenges With the amount of growing interest, one can certainly hope that the full potential of conducting polymers can be realised

2 Brief introduction to the conducting mechanism

Conductivity ( ) has units of siemens per centimetre (S/cm) Usually materials are deemed as insulators when their conductivity are below 10-8 S/cm, semiconductors when their conductivities are between 10-7 and 1 S/cm and conductors when they have conductivities higher than 100 S/cm Most conducting polymers in their neutral states are insulators or weak semiconductors They only achieve high conductivity after being ‘doped’ by either an oxidising [17] or a reducing [18] dopant It is generally believed that the so-called ‘soliton’

or ‘polaron’ and ‘bipolaron’ structure is/are formed as a result of a redox reaction Polyacetylene will be used here to demonstrate the concept of solitons and polythiophene will be used to explain the concept of polaron and bipolaron

Neutral cis polyacetylene is an insulator ( = 1.7 x 10-9 S/cm), while its neutral trans isomer is a weak semiconductor ( = 4.4 x 10-5 S/cm) Upon doping, either cationic radicals or anionic radicals are formed, depending on the dopant used:

as well

Fig 1.1 Proposed conducting unit of polyacetylene Soliton may be neutral

(radical), positive (carbocation), or negative (carbanion)

However, when the soliton extends along the polymer backbone, energy accumulates due to double bond stretching and single bond constriction This build-up of energy will eventually outweigh the stabilisation energy that arises from delocalisation Therefore, solitons can only be elongated to a certain extent Conductivity of the bulk polyacetylene though, depends not only on the solitons’ length and intramolecular conductivity Electrons moving intermolecularly across solitons also contribute to conductivity The conduction thus generated is highly dependent on the polymer’s crystalline structure For example, the conductivity of polyacetylene film can be increased to 105 S/cm level by properly orienting the polymer chains [3]

Soliton

7

From the discussion on the conducting mechanisms of polyacetylene, we can conclude that there are three important factors for a conducting polymer to achieve high conductivity: doping, backbone conjugation and polymer morphology This point is reiterated below where the conducting mechanism of polythiophene is discussed

Upon doping polythiophene, polarons and bipolarons are formed [19] A polaron

is a radical cation that stabilises itself by polarising the medium around it Each polymer segment containing polaron can be further oxidised to form two polarons

or a bipolaron Fig 1.2 illustrates the formation of polaron and bipolaron

S S

S

S

S S S

2

A-Bipolaron

Fig 1.2 The formation of polaron and bipolaron in polythiophene

A bipolaron usually stretches from 1 to 6 carbon atoms and has a structural deformation associated with it For example, the rearrangement of the arylene rings form aromatic to quinoidal configuration, thus maintaining planarity along this chain and moving in tandem as a pair Movement of a polaron or a bipolaron