Synthesis and characterisation of electrical conducting polymers co polymers based on omega functionalised 3 alkylthiophenes 5

Summary of the project As mentioned in Chapter 1, the aim of this project is to develop graft copolymers of polythiophene/poly3-alkylthiophene with commodity polymers like polystyrene an

Chapter 5

Conclusion and Suggestion for Future Work

1 Summary of the project

As mentioned in Chapter 1, the aim of this project is to develop graft copolymers

of polythiophene/poly(3-alkylthiophene) with commodity polymers like polystyrene and PMMA through an alkyl chain linkage in order to form materials with good conductivity and processability The following work was carried out to achieve this goal:

1) A series of functionalised alkylthiophene monomers, 3( -bromoalkyl)thiophenes, were synthesised based on a reported method

These monomers were polymerised using oxidative polymerisation method to give a series of novel polymers, poly[3-( -bromoalkyl)thiophenes] Both the monomers and polymers were fully characterised Attempts to further functionalise these polymers at the position by replacing the bromo moiety through Gringnard reaction have proven to be difficult Although it is possible to replace the moiety by other reactions, this approach was not pursued further

2) In the second attempt to form a graft copolymer of polythiophene and

S

(CH2)nBr [ ]

m

n = 4 pTHC4Br

6 pTHC6Br

8 pTHC8Br

10 pTHC10Br

12 pTHC12Br

Based on this monomer, a series of graft copolymers of polystyrene and polythiophene can then be realised It was found that the most conductive copolymer in this series is a system in which this monomer is copolymerised with alkylthiophene and styrene The experimental data seemed to support the idea that cross linkages in the copolymer system affect the properties of the copolymers The commodity polymer backbone also played an important role in determining the graft copolymers’ conductivity These points will be further discussed in the next section

3) The graft copolymers of polythiophene and commodity PMMA were synthesised based on a novel compound, 11-thiophen-3-yl-undec-1-en-3-one

This compound was formed through a new process starting from 3-(unde-10-enyl)thiophene All intermediate products were characterised A novel side product, 11-thiophen-3-yl-undec-3-en-2-one, was also identified Amongst the graft copolymers formed, it was found that the most conductive polymer was a

S

CH2 8 O

5

S

C6H12Ph

3

system that incorporated the monomer, alkylthiophene and MMA The conductivity and processibility of polythiophene and PMMA graft copolymers were also found to be improved compared to polystyrene and polythiophene graft copolymer The most conducting graft copolymer achieved a conductivity of 3.3 S/cm

In conclusion, graft copolymers of polythiophene and two commodity polymers, polystyrene and PMMA were formed Through trial and error the most conducting systems were identified Some of the graft copolymer systems possess good processibility as well as conductivity and has the potential to be used as anti static material etc

2 Comparison between the two series of copolymers based on PS and PMMA

The two main monomers used to synthesise the two series of copolymers were 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene and 11-thiophen-3-yl-undec-1-en-3-one These two monomers can undergo chemical polymerisation themselves using AIBN as initiator followed by oxidative polymerisation using FeCl3 However, by doing so, two graft copolymers that have poor conductivity and solubility were afforded This could be attributed to possible cross-linking in the structure of the

copolymers The idea is illustrated as follows:

In the first step, the commodity polymer backbone was formed:

Ideally, in the second oxidative polymerisation step, the following will form:

Based on the experimental data, it was more likely that the following actually

occurred:

A

TH

TH

A

TH

A

TH

TH

A

TH

A

TH

n

m

A

TH

AIBN A

TH

A

TH

A

TH

n

Fig 5.1 Illustration of possible structures of copolymer formed from 100%

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

After the first polymerisation process, the thiophene group would be ‘dangling’ on the commodity polymer backbone to form a ‘comb’ shaped structure If these thiophene groups did not join the neighbouring thiophene groups but rather, connect with the thiophene groups on other commodity polymer backbones during the following oxidation state, a three-dimensional network would result In which case, a double- layered copolymer structure with extended conjugation length would be unlikely to form This would result in low conductivity and insolubility The possible reason for the occurrence of this cross linkages could be due to the steric effect exerted from the polymer backbone or simply, random association

A

TH

A

TH

A

TH

n + A

TH

A

TH

A

TH

n

A

TH

A

TH

A

TH

n

A

TH

A

TH

A

TH

n A

TH

A

TH

A

TH

n

A

TH

A

TH

A

TH

n

A

TH

A

TH

A

TH

n

A

TH

A

TH

A

TH

n

A

TH

A

TH

A

TH

n

Reducing the concentration of the thiophene groups by copolymerising the monomer

(3-{ -[1-(p-vinylphenyl)]hexyl}thiophene) with PS initially followed by FeCl3

oxidation did not improve the properties of the copolymers either The properties of the copolymers only improved after large amounts of ‘spacer’ units, 3-alkylthiophenes, were introduced into the system The presence of these units during the second polymerisation process would have prevented the thiophene groups on the precursor copolymers from conjoining, thus reducing the chances of the formation of

a ‘closed’ network In doing so, a polythiophene backbone with extended conjugation can be formed which explains the improved conductivity

A comparison between the PS co alkylthiophen) and PMMA co

poly(3-alkylthiophene) shows that the PMMA copolymers possess relatively better properties The PS copolymer containing 3-octylthiophene groups is not very soluble and can only achieve a conductivity of ~0.2 S/cm Their PMMA counterparts, both copolymers containing 3-butylthiophen and 3-dodecylthiophene, are much more soluble and have conductivities one order higher Since the difference between these two groups of copolymers is the commodity polymer used, the results suggest that the phenyl rings in the PS backbone may adversely affect the planarity of the polythiophene backbone and thus disrupt its conjugation which would have resulted

in lowered conductivity

When thiophene, instead of 3-alkylthiophene, was used as ‘spacer’ units, PMMA copolymers exhibit better conductivity and solubility as compared to its PS

counterpart XRD of the two copolymers suggests that the PMMA copolymer has a higher level of crystallinity This demonstrated that the morphology of the conducting backbone still plays an important role in determining the conductivity of these types

of copolymers

In summary, comparisons between the two series of copolymers demonstrated the importance of polymer backbone structure, which exerts a direct influence to the resultant polymer properties The conductivity of such graft copolymer was found to not only depend on the conductive polymer backbone but also on the commodity polymer backbone structure as well This information could be useful when altering and improving the electrical properties of these types of graft copolymers

3 Suggestions for Future Work

1) The reactivity of the bromo moiety on poly[3-( -bromoalkylthiophene)] can

be further explored Although Gringnard reaction on the polymer did not succeed,

it is still very possible to replace the group with another functional group, e.g., a methacrylate group, to provide an easy route for further reaction

2) The yield for the selenium dioxide oxidation was not very high mainly due to the low reactivity of the mono-substituted alkene As have been discussed in Chapter 4, a di-substituted or tri-substituted alkene will give much higher yield Further, one step oxidation from alcohol to carbonyl may also be attempted by varying the ratio and amount of SeO2/tBHP

3) In this study, the commodity polymers were first (co)polymerised instead of the polythiophene backbone This is to avoid the difficulty of having to deal with the bulky functionalised poly(3-alkylthiphene), which was expected to be more difficult to process However, this would not have been a problem if the conducting backbone had been oligomers Copolymerisation with commodity polymers will be easier to control in that way, which will result in many strings of polythiophene on the surface of the commodity polymers Both monomers mentioned may be used for this purpose

4) If the polythiophene backbone wass regioregular, the properties of the copolymer will definitely improve The carbonyl group on the monomer,

11-thiophen-3-yl-undec-1-en-3-one, can be protected before carrying out polymerisation to form regioregular polymers Subsequent deprotection would

‘reactivate’ the alkene groups for further reactions

5) Instead of 3-alkylthiophene, other functionalised thiophene derivatives can also

be integrated into the system to explore their unique electronic and optical properties

6) On top of using thiophene, the same method of synthesis of the two monomers can also be applied to form similar derivatives of pyrrole, aniline, phenylene etc, for further development

Appendix List of main compounds reported in this thesis Charpter 2

Compound Code

Compound name/structure Remarks

THC 4 Br 3-( -bromobutyl) thiophene

THC 6 Br 3-( -bromohexyl) thiophene

THC 8 Br 3-( -bromooctyl) thiophene THC 10 Br 3-( -bromodecyl) thiophene

THC 12 Br 3-( -bromododecyl) thiophene n=4 pTHC4Br

6 pTHC6Br

8 pTHC8Br 10pTHC10Br 12pTHC12Br

S

(CH2)nBr

m

Poly[3( -bromoalkyl)thiophenes]

PBHT2 Poly[3-( -bromohexyl)thiophene] Formed by rapid addition

of the monomer solution

in one portion following

the method of Sugimoto et

al

PBHT3 Poly[3-( -bromohexyl)thiophene] Formed by adding the

dropwise following the

method of Sugimoto et al

of Casa et al

Chapter 3

1

S

-bromohexyl)thiophene

2

Br

4-bromostyrene

3

S

-[1-(p-vinylphenyl)]hexyl}thioph ene

3a

S

C6H12Ph

n 3a is precursor copolymer

of graft copolymer Graft

100

Graft 100

C6H12Ph

n

3b, 3c and 3d

C6H12Ph Ph

n

S

3b is precursor copolymer

of graft copolymer Graft

21; 3c is precursor copolymer of graft

copolymer Graft 51;

3d is precursor copolymer

of graft copolymers 4 and

5 Graft 21 and

Graft 51 C6H12Ph Ph

n

m S

Graft 21: m:n=1:1 Graft 51: m:n=1:4

4

C6H12Ph Ph

C8H17

S

S

S

(CH2)6 Br

n

5

C6H12Ph Ph

S

S

n

m

Chapter 4

2 Br CH

2

9

3

S

CH2

9

4

S

CH2 8

OH

5

S

CH 2

8

11-thiophen-3-yl-undec-1-en-3-one

S

CH2 7

O

11-thiophen-3-yl-undec-3-en-2-one

6

S

CH2

n

Precursor copolymer of

graft copolymer 7

7

S

CH2

n

m

8

S

CH2

CH2

CH3

CH2

n

Precursor copolymer of

graft copolymers 9, 10 and

11

9

S

CH2

CH2

CH3

CH2

n S

m

10 (x=4)

11 (x=10)

S

CH2

CH2

CH3

CH2

n S

m (H2C)