Synthesis and characterisation of electrical conducting polymers co polymers based on omega functionalised 3 alkylthiophenes 4

In this chapter, the different properties of the graft copolymers with polymethyl methacrylate PMMA and polythiophene backbones will be explored.. These reports described that the reacti

Chapter 4

Graft copolymers of polythiophene and PMMA

1 Introduction:

As was discussed in the previous chapter, the properties of the graft copolymers

of PS and polythiophene is affected by their backbone structure The properties of the graft copolymers can be changed by altering or replacing the polythiophene and/or PS backbone structure In this chapter, the different properties of the graft copolymers with poly(methyl methacrylate) PMMA and polythiophene backbones will be explored These graft copolymers essentially have structures similar to PS and polythiophene graft copolymers The main difference would be the presence of the PMMA in place of the PS backbone Hence any property differences of the copolymers should be caused mainly by this change of structure

The idea of integrating PMMA and polythiophene into a copolymer system has interested many A direct approach was to form linkages between thiophene or its derivatives and MMA to give block copolymers This can be achieved by both chemical [1] and electrochemical methods [2] The copolymers were reported to

be soluble and conducting although these properties depend on the amount of thiophene moiety that could be integrated into the system Poly(3-alkylthiophene) containing terminal acrylate or methacrylate functional group on the alkyl side chains have been reported Such material can be copolymerised by radiation, either UV or electron beam, to produce highly cross-linked insoluble graft copolymers, which make these materials useful for photolithographic patterning [3-6] Grafting of poly(methyl methacrylate) (PMMA) to polypyrrole and

polythiophene via an ester linkage has also been shown to give soluble graft copolymers with moderate conductivity [7a-d] More recently the syntheses of PMMA and polythiophene or polypyrrole copolymers linked through ester linkage were reported [8a-b]

The PMMA and polythiophene graft copolymers prepared in this project will have PMMA and polythiophene backbones linked through alkyl chain linkages These graft copolymers was made in a similar approach as described in the previous chapter for PS and polythiophene copolymers A novel monomer of functionalised 3-alkylthiophene, which contained a terminal , -unsaturated ketone group, was synthesised This monomer could be polymerised directly through a two-step reaction It may also be copolymerised with MMA and thiophene in turn to give another graft copolymer Similarly, this monomer can be copolymerised with MMA and 3-alkylthiophenes of different alkyl chain lengths

to afford another two graft copolymers The details of the experiments are described below

S

CH28

OH KMnO4

Charcoal S

CH28 O

4 5

Scheme 4.1 Synthesis of monomer 5

Starting material 1-hydroxylundec-10-ene (1) was brominated in excess bromine

and triphenylphosphine with high yield The brominated product was then coupled with 3-bromothiophene through Grignard reaction As reported in literature [9], this straightforward reaction gave high yield although a small amount of 2, 3 vinyl isomer (11-bromo-undec-2-ene) was detected together with

compound 2 (see scheme above)

The oxidation step using selenium dioxide was less straightforward SeO2 has been known as an effective alkene oxidising agent that forms , -unsaturated alcohol and/or , -unsaturated carbonyl products [10] The reagent is particularly interesting to many organic chemists due to its regio- and stereo-

specific nature [11, 12] These reports described that the reaction gave higher yield when the double bond groups were more substituted, as well as the use of stoichiometric amounts of SeO2 with excess t-butylhydroperoxide (tBHP) to maintain the selenium in the reactive Se (IV) oxidation state Moderate yields of allylic alcohol could still be obtained from the less substituted alkenes Another report described a method of selective oxidation of primary allylic alcohols to , -unsaturated aldehydes [13] In that experiment, an oxidsing reagent of SeO2/tBHP/SiO2 was used instead of the traditional MnO2 oxidising agent to oxidise alcohol into aldehyde It was hoped that applying the reagent mentioned

in Kalsi et al.’s paper [13] would directly oxidise 3-(unde-10-enyl)thiophene (3)

to 11-thiophen-3-yl-undec-2-enal, as shown below

‘traditional’ two-step oxidation method

In the first step, the ratio of SeO2: tBHP: alkene used was 0.5:2:1 A mixture of oxidised products was formed as expected The reaction mechanism suggests that the terminal unsaturated allylic alcohol should be the major product, as illustrated below:

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

The complex formed by SeO2 and tBHP kept the selenium centre at a higher oxidation state of IV [11, 12], which made it prone to nucleophilic attack from the alkene groups There are two possible sites of attack, namely positions 1 and 2

H

H

H

C (CH2)7-THH

H Se

O O HO

Se OH O

HO

OtBu

H H

H

C (CH2)7-THH

Se O

O

HO

OtBu

H H

H

C (CH2)7-TH H

Se O

OH

1

4

(see scheme above) Attack at position 1 would form a six-membered ring transition state, which would result in a stabilised allylseleninic ester transition state with a di-substituted double bond Subsequent C-Se bond rotation and sigmatropic rearrangement would give an ester that was readily hydrolysed by tBHP to form the alcohol At the same time, the oxidising agent would be regenerated This is a thermodynamically favoured reaction route

The alternative attack is at position 2 At the intermediate stage, the adjacent alkyl group may help to stabilise the electron poor carbon 2 compared to the attack at position 1, as shown below:

Se O

Se HO

O

OH

OtBu

1 2 3

This might lower the activation energy for the reaction to take place However, such an attack would also lead to a carbocation transition state that is highly unstable

Continued rearrangement of this intermediate would lead to an allylic alcohol with a terminal alcohol that could be one of the minor products Further oxidation

of the alcohol would result in an aldehyde As mentioned earlier, some ,

-unsaturated aldehydic product was detected when 3 was oxidised with

SeO2/tBHP/SiO2 The abovementioned reaction intermediate might have contributed to the formation of that product

The oxidation reaction of 3 using SeO2 and tBHP in 0.5 to 2 ratio was usually allowed to react overnight About 50% of the starting material was recovered in most cases Prolonged reaction time did not improve the yield much Moreover, more side products were produced thus creating complications during purification

In the second oxidation step, a more conventional oxidising agent was prepared

by adsorbing KMnO4 on charcoal, as described in literature [14] The reaction

gave the desired product in moderate yield (~35%)

CH2

8 On

S

CH2

8 On

m

Scheme 4.3 Synthesis of graft copolymer 7 from monomer 5

The precursor polymer 6 formed after the first step was a sticky solid that could

be dissolved in solvents such as CHCl3 and CCl4 After the second polymerisation step, the resultant polymer was found to be insoluble It seemed to ‘swell’ in the

solvents, a sign of cross-linkage in its structure

In order to obtain a more processable copolymer, the monomer was first copolymerised with MMA in 1:9 ratio using AIBN as initiator The precursor

copolymer (8) obtained was dissolved in CCl4 and further copolymerised with

thiophene in a 1:9 ratio to give graft copolymer 9 It was also copolymerised with 3-butylthiophene in a 1:9 ratio to yield graft copolymer 10 Similar to the case for

PS graft polythiophene as discussed previously, the conductivity and solubility of the graft copolymer containing 3-butylthiophene improved drastically The

precursor copolymer (8) was also copolymerised with 3-dodecylthiophene in 1:9 ratio using the same method to produce graft copolymer 11 The effects of chain

length on the properties of the graft copolymers can be investigated by comparing

the properties of copolymers 10 and 11

S

CH28 O

m

FeCl3S

Scheme 4.4 Synthesis of graft copolymer 9

S

CH28 O

m (H2C)

FeCl3

thiophene

Scheme 4.5 Syntheses of graft copolymers 10 and 11

3 Result and Discussion:

3.1 Monomer synthesis and characterisation

1-bromoundec-10-ene were characterised by 1H NMR (NMR not shown here) The appearance of a quartet at 3.39 ppm evidenced the bromination of the starting material The Hs on the double bond was intact too However, an impurity was detected at 5.48 ppm, which could not be removed despite repeated column chromatography and distillation This impurity could most likely be due to isomerisation of the double bond from terminal to 2, 3 positions on the alkyl chain

The Grignard reaction product, 3-(unde-10-enyl)thiophene, was characterised using FT-IR, 1H NMR and elemental analysis On the NMR spectrum (see Fig 4.1), there were no peaks observed at 3.30 ppm, which was an indication that Br from the starting material was not present in this compound Instead, a triplet appeared at 2.63 ppm owing to the –CH2 group next to a thiophene ring The multiplets at around 6.93 and 7.24 ppm confirmed the presence of thiophene groups All are evidence that the substitution of Br by thiophene have taken place The isomerisation of the double bond is verified by the weak band at 5.48 ppm

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

The 1H NMR of 11-thiophen-3-yl-undec-1-en-3-ol is shown below (Fig 4.2) The peak at 4.1 ppm, which accounted for one hydrogen atom on the molecule, accounts for the –CH next to a hydroxyl group The high chemical shift was a result of the deshielding effect inserted by the double bond neighbouring it Similarly, the presence of allylic alcohol also affected the chemical shift of alkene

Hs The deshielded cis and trans Hs gave rise to a doublet, which was further split

by the hydrogen on carbon 2 of the terminal double bond to give a doublet of doublets at 5.2 ppm The H at the C2 position had a more complicated chemical environment that produced multiplets at 5.83 ppm The presence of thiophene was confirmed by the triplet at ~2.6 ppm and the multiplets at 6.9 and 7.26 ppm in the

S

93

aromatic region The spectrum below proved that the oxidation reaction did produce 11-thiophen-3-yl-undec-1-en-3-ol as a major product

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

However, the weak bands between 5.2 ppm and 5.83 ppm indicated the presence

of an impurity that was most likely the oxidised product of the isomer of 10-enyl)thiophene Repeated column chromatography could not remove this impurity Distillation was not carried out for fear of losing the major product to heat-induced polymerisation The amount of impurity was considered negligible (less than 4 mol %) in all the experiments carried out Hence, the following step was allowed to proceed with the impurity present

3-(unde-The 13C NMR spectrum (Fig 4.3) also confirmed the presence of the allylic alcohol The ethylene carbons caused peaks at 143 and 141 ppm, whereas the deshielded hydroxyl group connected to C gave rise to the peak at 73.2 ppm The peak at 37 ppm is ascribable to the alkyl –CH2 group next to the hydroxyl group

S

8OH

4

12

The 3-substituted thiophene ring was evidenced by peaks at 128.2, 124.9, 119.7 and 114.5 ppm, together with a peak at 30.5 ppm that was a result of the –CH2 group beside the ring

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

The spectroscopic evidences conclusively proved the formation of the allylic alcohol through oxidation by SeO2/tBHP reagent Despite the moderate yield, which can be attributed to the difficulty of oxidising a mono-substituted alkene, the reaction was considered feasible and provided enough products for subsequent oxidation

S

8OH

4

The 1H spectrum of 11-thiophen-3-yl-undec-1-en-3-one is depicted below

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

The ethylene hydrogen atoms that were deshielded by the carbonyl group gave rise to complicated multiplets at ~5.8 ppm and ~6.3 ppm Compared to the starting material, the shift of these Hs downfield had to be due to the presence of the carbonyl group Since the carbonyl carbon has no H attached to it, the H atoms on the double bonded carbons self coupled One can expect the H that is cis

to the carbonyl double bond to have a high chemical shift The strong geminal coupling with the trans H may even split the doublet so strongly that it resulted in the peak at ~5.8 ppm The integration of the six peaks in this region accounts for the three alkene Hs Coincidentally, the –CH2 group next to the carbonyl group produced peaks at the same chemical shift as the methylene group adjacent to the thiophene group The integration of these muliplets at ~2.6 ppm indicated the

S

8O

5

presence of two methylene groups The thiophene ring was not affected and showed coupling at ~6.9 and 7.2 ppm

On the 13C NMR spectrum, the peaks at 200.1, 143.1, 136.6 ppm proved that the major product of the oxidation reaction did contain primary , unsaturated ketone groups Other peaks were similar to those observed on the spectrum of the starting material, except the peak at 39.6 ppm, which in fact represents two carbon atoms, one next to the carbonyl and the other adjacent to the thiophene ring

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

The major side product of the reaction was identified by NMR (Fig 4.6) to be thiophen-3-yl-undec-3-en-2-one The alkene Hs on this compound were even more deshielded compared to the major product and caused multiplets at ~6.8 ppm and 6.1 ppm Another feature that distinguished this compound from the major compound was the triplets at ~2.61 ppm and ~2.51 ppm, attributable to –

11-CH2 groups next to the thiophene ring and the carbonyl group respectively As

S

8O

5

was mentioned earlier, compound 4 was used with traces of an isomer The

oxidation of this isomer most likely produced the side product,

11-thiophen-3-yl-undec-3-en-2-one Although the impurity level in the starting material 4 was low,

substantial amounts of this side product was able to be accumulated throughout the many oxidation reactions carried out Nevertheless, since this by-product can

be separated in pure form from the major product it should not interfere with the polymerisation process

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

S

CH2

7

O

thiophene/3-MMA, and 3-butylthiophene (10) is shown below:

Fig 4.7 1 H NMR spectrum of copolymer 10

The broad peak at 3.6 ppm was caused by the methyl ester group is typical for PMMA The peaks at 2.85 ppm and 2.58 ppm were due to –CH2 groups next to the thiophene backbone connected in HT and HH fashion, as discussed in the

previous chapter On the spectrum of the precursor copolymer 6, this same group

produced a triplet at ~2.6 ppm The weak peak at ~2.2 ppm was a result of

hydrogen resonance of the –CH next to the carbonyl group, which originated from the monomer This peak shifted more up field since the deshielding effect of the double bond is absent The existence of this peak proved that the monomer units were integrated into the polymer backbone The methylene group found on the pendant chain next to the carbonyl group should have hydrogen resonance at ~2.5 ppm This peak could have overlapped with the peak caused by the –CH2 group next to the thiophene ring The broad band at 7.0 ppm was the result of the only aromatic H left on the thiophene backbone

3.2.3 FT-IR Infrared spectra provided important insight into the structure of the graft

copolymers This method is especially important for characterising copolymer 7,

the only copolymer where the NMR spectrum could not be acquired due to its

poor solubility The diagram that follows illustrates the IR spectra of copolymer 7 (direct two step polymerisation of monomer 5), copolymer 9 (graft copolymer of monomer 5, MMA and thiophene), copolymer 10 (graft copolymer of monomer 5, MMA and 3-butylthiophene) and copolymer 11 (graft copolymer of monomer 5, MMA and 3-dodecylthiophene) (see Fig 4.8)

The spectra of the four copolymers were quite similar due to the similarities in their structures The bands between 3100-3000 cm-1 were due to thiophene aromatic C-H stretch present in all four polymers The bands at around 2922 cm-1 was a result of asymmetric -CH2 stretch and the band at 2849 cm-1 was ascribed to symmetric -CH2 stretch These two bands were relatively weaker on the spectrum

of copolymer 9, due to the lack of alkyl chains in this polymer

The spectrum of copolymer 7 can be distinguished from the spectra of the other

three copolymers because of the peak at 1709 cm-1 This band was caused by C=O

stretch of ketone groups, which were abundant in copolymer 7 On the other three

copolymers, the C=O stretch of the ester groups from MMA gave rise to a band at

~1728 cm-1 The large amounts of such ester groups in copolymers 9, 10 and 11

would most likely have overlapped with the weak band at 1709 cm-1 caused by

the presence of a small amount of ketone groups that originated from monomer 5

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

Thiophene aromatic ring C-C stretching bands were evident on the spectra of all copolymers at around ~1456 and ~1370 cm-1 The lack of ester groups in

copolymer 7 was once again noticeable due to the absence of the bands at

1238-1146 cm-1 attributed to esters’ C-O stretch These peaks were evident on the IR

spectra of copolymers 9, 10 and 11

The spectra of copolymers 7, 10 and 11 shared a common medium peak at around

825 cm-1, which was found as a weak peak on copolymer 9 On the other hand, the spectrum of copolymer 9 had a medium peak at 785 cm-1 that was not detected on the other spectra The band at 825 cm-1 was caused by the C-H out-of-plane bending of 2, 3, 5-trisubstituted ring [15] of the polymerised 3-alkylthiophene rings, as discussed in the previous chapter These tri-substituted polythiophene

rings were abundant in all the copolymers except copolymer 9, which consisted

mainly of di-substituted polythiophene rings Hence, in its spectrum, aromatic

C-H out of plane bend showed up as 785 cm-1 and there was only a weak peak appearing in this region that was caused by the little amounts of tri-substituted

polythiophene rings originating from its precursor copolymer 8

The IR spectra of the four graft copolymers confirmed the presence of the major functional groups in their structures One can also gain insight to the differences

in structure between these copolymers, however subtle they are

3.2.4 Elemental Analysis Based on the elemental analysis results, the empirical formulae of the graft copolymers were calculated and compared with the theoretical formula calculated based on the feed ratio of the monomer The results are shown in the table below Also listed is the percentage molecular weight of the thiophene unit in the graft copolymers estimated as follows:

Percentage molecular weight of the thiophene unit

Experimental the percentage molecular weight

of the thiophene unit (%) c

Theoretical the percentage molecular weight

of the thiophene unit (%)

(graft copolymer of monomer 5, MMA and 3-dodecylthiophene) has the closest

agreement with the expected percentage molecular weight of the thiophene unit The other two graft copolymers’ empirical formulae showed discrepancy from the formula calculated based on their feed ratio

The close agreement between the empirical and expected formula of copolymer 7

can be attributed to the fact that this graft copolymer was formed exclusively

using a single monomer 5 The differences between theoretical and experimental formulae for copolymers 9, 10 and 11 were most likely the result of a complicated polymerisation process that involved three monomers

3.2.5 XPS The XPS of the four copolymers revealed lines of C 1s, O 1s and S 2p An

example of the XPS for copolymer 9 is shown below:

Fig 4.9 XPS spectrum of copolymer 9

The elemental content on the surface of the copolymers as analysed by XPS are listed in the table below:

Table4.2 Summary of XPS elemental content of the copolymers

polymers

Co-O 1s (%) C 1s (%) S 2p (%) Element

ratio

Elemental analysis formula

m

9