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

It was also copolymerised with styrene in different ratios to form a precursor copolymer, which was then subjected to an oxidative polymerisation step to form a graft copolymer with poly

Chapter 3

Graft copolymers of polythiophene and polystyrene based

on 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene

1 Introduction:

For over two decades, conducting polymers have been hailed as futuristic materials However, the insolubility and infusibility of classical conducting polymers (polyacetylene, polyaniline, polythiophene, and polypyrrole) make them difficult to be processed, thus limiting their applications Combining the classical conducting polymers with processable insulating compounds has been demonstrated to be an effective way to improve the processability of polymers [1a-d] Graft co-polymerisation of conducting polymers with commodity polymers like poly(methyl methacrylate) (PMMA) and polystyrene (PS) as an alternative route to improve the physical properties of conducting polymers is attracting much attention lately The advantage of this method over the physical blending of conducting polymers and commodity polymers is that long-term stability problems are avoided due to the strong chemical bonds formed between the two different types of polymers [2]

There has been a report on electrochemically grafting polypyrrole on a polystyrene backbone [3], as well as grafting polyaniline on poly(p-aminostyrene) [4] Polypyrrole copolymer films can also be prepared electrochemically [5] Block or graft copolymers of polystyrene and polythiophene or its derivatives have also been described [6-8] These materials were formed by direct linkages between styrene and thiophene moieties

As discussed in the previous chapter, introducing p-vinylphenyl functional groups

to replace -bromo moiety on poly(3- -bromoalkyl)thiophene through Grignard reaction was difficult In a different approach, novel 3-{ -[1-(p-

vinylphenyl)]hexyl}thiophene was synthesised This monomer was polymerised through a two-step process to yield a graft copolymer It was also copolymerised with styrene in different ratios to form a precursor copolymer, which was then subjected to an oxidative polymerisation step to form a graft copolymer with polythiophene backbone Similar precursor copolymers were either further copolymerised with thiophene to form new graft copolymers, or copolymerised with a mixture of 3-alkylthiophene and 3-( -bromoalkyl)thiophene to give

different thiophene co styrene graft copolymers Their structures and properties

were then studied Some of the properties were compared to the blended octylthiophene) and polystyrene

poly(3-2 Experiment:

2.1 Monomer Synthesis:

The key monomer for this series of graft copolymer, 3-{

-[1-(p-vinylphenyl)]alkyl}thiophene (3), can be synthesised by either Grignard coupling

of 4-bromostyrene with 3-( -bromohexyl)thiophene or Grignard coupling of chloromethyl-4-vinyl-benzene with 3-( -bromohexyl)thiophene These two reaction routes were attempted It was found, however, that the yield of the product obtained from the Grignard coupling using 1-chloromethyl-4-vinyl-benzene was very low There are two possible reasons Firstly brominated compounds are generally more suitable for Grignard reactions, and secondly the catalyst used for these reactions affects the yield Ni(dppp)2Cl2 is known to be more effective when the coupling agents contained brominated aromatic ring structure Therefore, the monomers used in this part of the project were all afforded through 4-bromostyrene coupling reaction

1-The novel monomer 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene (3) was formed

through a Grignard reaction (see scheme 3.1) between 4-bromostyrene (2) and ( -bromohexyl)thiophene (1) Sufficient amounts of 3-( -bromohexyl)thiophene

3-(1) was prepared following the procedure reported by Bäuerle et al [9] as

described in the previous chapter

C6H12Br+

2.2 Copolymers syntheses:

Through a straightforward two-step polymerisation reaction, monomer 3 can be

converted into a graft copolymer of polystyrene and poly(3-hexylthiophene) In

the first step, AIBN was used as initiator to yield the precursor polymer 3a This

polymer was further polymerised by FeCl3 following the method reported by Casa

et al [10] to afford a graft co-polymer with two backbones Since this copolymer

was made of 100% monomer 3, it was named Graft 100 (see scheme 3.2)

Scheme 3.2 Direct two-step polymerisation to afford copolymer Graft 100

It was found that the graft copolymer formed in the way described in Scheme 3.2

was not soluble at all This is despite its precursor copolymer 3a being soluble It

is therefore worthy of investigation to find out if lowering the content of polythiophene in the system will help produce a more processible copolymer since polythiophene is known to have lower solubility One way to achieve this is

to introduce styrene into the copolymer system (Scheme 3.3) When monomer 3 was polymerised together with styrene using AIBN as initiator, monomer 3 and

styrene will fuse via a radical reaction This reaction produces a copolymer that contained more phenyl rings than thiophene rings Further oxidative

polymerisation of this copolymer resulted in a copolymer with a longer polystyrene backbone compared to that of the polythiophene backbone The experiments are illustrated in Scheme 3.3

Scheme 3.3 Polymerisation of styrene and monomer in the ratio of 1:1 and 4:1

afforded 3b and 3c respectively, which can be further polymerised

to give copolymers Graft 21 and Graft 51

A series of precursor copolymers, 3b, 3c and 3d (not shown in scheme), were prepared first by co-polymerising monomer 3 with styrene in different ratios using AIBN as initiator In this step, co-polymer 3b was obtained when the feed ratio of monomer 3 to styrene is 1:1 While the feed ratio was 1:4 and 1:10 respectively,

3c and 3d was produced In the next step, when oxidative polymerisation was

carried out using copolymer 3b, a graft copolymer with a structure that is similar

to that of 3a was obtained Based on the monomer 3 and styrene feed ratio for 3b,

Graft 51(m:n=1:4)

S

3c

AIBN 4xStyrene

be 2:1 This copolymer was therefore named Graft 21 The same rationale is applied to 3c Since 3c had a monomer to styrene feed ratio of 1:4, the resultant

copolymer should theoretically contain 5 times more phenyl rings than thiophene

rings This copolymer was therefore named Graft 51

The actual ratios of phenyl rings to thiophene rings in the copolymers Graft 21 and Graft 51 will be slightly different from that of the expected theoretical

values The difference can be attributed to the first polymerisation process where not all styrene can be incorporated into the copolymer system These ‘impurities’

will be in the form of styrene oligomers or polymers that monomer 3 was not

incorporated in They are carried over to the next step These compounds should not affect the second oxidative polymerisation process and will be discarded when

the crude grafted copolymers, Graft 21 or Graft 51, are subjected to soxhlet

extraction using methanol and acetone in turn

Despite the lowered percentage molecular weight of the thiophene unit in the two

graft copolymers Graft 21 and Graft 51, these two polymers were still found to

be insoluble One possible reason for this could be the possible presence of rigid

cross-linked structures of the three grafted copolymers: Graft 100, Graft 21 and

Graft 51 This will be discussed in further detail in the later parts of this chapter

In order to obtain a more processible polymer, 3-alkylthiophene or its derivatives will be introduced into the copolymer system to impart a less rigid structure and

to promote solvent-polymer interaction The graft copolymer formed in such a

way will have two polymer backbones: the polystyrene backbone and the

polythiophene backbone Unlike Graft 100, Graft 21 or Graft 51, not all

thiophene rings are linked to the polystyrene backbones in the newly formed graft copolymers This will give the polythiophene backbones more freedom of movement and hence introduce different physical & chemical properties to these copolymers As such, experiments were carried out to form graft copolymers consisting two backbones but linked only at certain points The idea is illustrated

below:

These copolymers should have about equal amounts of thiophene and styrene in

order for a comparison to be carried out with Graft 100 In the first step of a step synthesis (see Scheme 3.4), the monomer 3 to styrene ratio was fixed at 1:10

two-to form copolymer 3d In the second step, copolymer 3d will be mixed with

3-alkylthiophene or its derivatives in a 1:10 mole ratio The resultant graft copolymer would therefore, have about equal amounts of styrene and percentage molecular weight of the thiophene unit The ratio of 1:10 was used in the hope of giving the polymer backbone enough space for movement (especially for polythiophene backbones) Ideally, on the polythiophene backbone in the graft

copolymer, there should be about ten thiophene rings that are not linked with the

polystyrene backbone alternating between two thiophene rings that are linked

with the polystyrene backbone via alkyl chain linkages to facilitate movement

In the experiment, copolymer 3d was mixed in a 1:5 mole ratio with 3(

-bromohexyl)thiophene and in a 1:5 mole ratio with 3-octylthiophene before oxidative copolymerisation was carried out for the mixture The resultant graft

copolymer 4, therefore, should theoretically have about equal numbers of phenyl

rings and thiophene rings 3-( -bromohexyl)thiophene was introduced to show if radical polymerisation step using AIBN as initiator affected the thiophene group

The results will be discussed later in this chapter Graft co-polymer 5 was produced based on precursor co-polymer 3d Polymerisation of a mixture of 3d and thiophene in 1:10 mole ratio using Casa’s method [10] produced 5 (see

scheme 3.4)

As comparison, a polymer blend of polystyrene and poly(3-octylthiophene) was also prepared Polystyrene was formed by polymerising styrene in a 1:1 styrene and 3-octylthiophene mixture using AIBN as initiator Subsequently, CH3NO2

solution of FeCl3 was added in situ to polymerise 3-octylthiophene

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

3 Results and Discussion:

3.1 Monomer synthesis and characterisation of

The novel monomer 3 was characterised by 1H, 13C NMR and elemental analysis The NMR spectra confirmed the structure of the monomer

3.1.1 NMR The 1H NMR is shown in Fig 3.1

The multiplets between 1.30-1.65 ppm are a result of hydrogen resonance of methylene protons on the alkyl chain that is not adjacent to the aromatic rings The quartet at 2.65 ppm is due to the overlapping of two methylene groups that are immediately neighbouring the thiophene and benzene rings The chemical environment of the two methylene groups is similar which resulted in the peaks appearing at the same chemical shift The peaks appearing at 5.22 and 5.73 ppm should be viewed as a doublet of doublets Together with another doublet of doublets at 6.73 ppm, they account for the coupling of the alkene protons The cis and trans hydrogen on the terminal alkene carbon split the signal caused by proton atom that is adjacent to the benzene ring Apart from coupling with the other allylic proton, the cis and trans proton atoms also have strong geminal coupling, giving rise to doublets at 5.22 and 5.73 ppm

Fig 3.2 shows the 13C NMR spectrum of the monomer

Fig 3.1 1 H NMR spectrum of monomer 3

Fig 3.2 13 C NMR spectrum of monomer 3

The peaks at 136.7 ppm and 136.6 ppm in 13C NMR were caused by C at phenyl 1 position and thiophene 3 position respectively The double bond C that was closer

to the phenyl ring produced a peak at 135.0 ppm The peak at 131.3 ppm was

caused by the C at p position on the phenyl ring, whereas the deshielded C at

position 4 of the thiophene ring gave rise to the peak at 130.2 ppm The peaks at

around 128 ppm were attributed to the o position carbon on the phenyl ring and

peaks at 126 ppm can be ascribed to C on position 5 of the thiophene ring Carbon

on m position of the phenyl ring should account for the peak at 125.0 ppm, while

C on position 2 of the thiophene ring resulted in a peak at 119.7 ppm The double bonded C that was further from the phenyl ring produced peaks around 113 ppm The peaks in the region of 29.0 ppm to 37.3 ppm were due to the carbons on the alkyl long chain, with Cs closer to the aromatic ring having higher chemical shifts

The NMR spectra also indicate that the monomer was not quite pure In 1H NMR, the singlet at 2.9 ppm and some small peaks in the aromatic region could not be accounted for In the meantime, chemical shifts seen at ~141 ppm in 13C NMR were not likely to be caused by the carbon on the monomer One possible source

of impurity could be the stabiliser, 4-tert-butylcatechol, used in the starting material, i.e 4-bromostyrene Attempts were made to wash off the stabiliser with NaOH However, this results in the Grignard reaction producing monomer with compromised yield On the other hand, when washing was omitted, substantial amounts of monomers were obtained from the same reaction The only drawback

of the latter method is the production of an impurity that cannot be removed despite repeated column chromatography Distillation under reduced pressure was not carried out to avoid loss of monomer as a result of premature polymerisation Another likely source of impurities could be oligomers with very few repeating units In spite of the presence of impurities, the integration ratio of double bonded

Hs (the doublet of doublet at ~6.72 ppm and ~5.47 ppm) and methylene Hs next

to the aromatic rings (multiplets at ~2.64 ppm) on 1H NMR was 3:4 This is in agreement with the theoretical value, indicating that the impurities did not contain any vinyl phenyl group and hence should not interfere with the polymerisation of the monomer Therefore the polymerisation step was carried out using this product

3.2 Copolymers characterisation

3.2.1 Physical Properties

All the intermediate co-polymers, 3a, 3b, 3c and 3d, were white powders that

were soluble in common polar solvents such as chloroform and THF, similar to polystyrene However, upon the second polymerisation process, the morphology

of the grafted co-polymer changed drastically Copolymer Graft 51 was a yellow powder Copolymers Graft 21 and Graft 100 were orange-coloured powders These three co-polymers were found to be insoluble Copolymer 4 was a dark brown powder Copolymer 5 was a red powder About 10% of copolymer 5 can

be soxhlet-extracted by CHCl3 whilst about 20-30% of 4 can be extracted The

soluble parts of these two polymers were subjected to further interpretation by NMR, GPC, UV-Vis etc Finally the co-polymer blend of polystyrene and poly(3-octylthiophene) was in the form of black powder and was soluble in common polar solvents

3.2.2 NMR

The NMR spectrum of 3a is shown below The broad band at ~6.9 ppm and ~6.5

ppm suggests that polymerisation of styrene had occurred Sharp peaks at ~7.25 ppm and ~6.9 ppm were caused by aromatic hydrogen resonance on thiophene rings Traces of the monomer are evidenced by the small peaks at ~5.7 ppm and

~5.2 ppm Polymerisation of p- substituted vinyl phenyl groups induced two

broad peaks in the lower field One weaker broad peak at ~2.8 ppm represents methine (-CH) hydrogen on the alkyl backbone of the polystyrene chain, the other stronger broad band at ~2.5 ppm is attributed to methylene groups neighbouring the phenyl rings The peaks caused by –CH2 next to the thiophene rings at ~2.6 ppm overlapped these two peaks slightly

Fig 3.3 NMR of precursor polymer 3a

S

C6H12Ph

n

3a

NMR spectra of 3b, 3c and 3d showed similar features except that the peaks

caused by the polystyrene backbone are stronger due to co-polymerisation with

styrene An example spectrum of 3c is shown in Fig 3.4

Fig 3.4 NMR spectrum of precursor copolymer 3c

While studying the NMR spectra of these precursor copolymers, one of the complications encountered is that if some of the thiophene moiety in the monomer was polymerised at this stage, they would have produced peaks at ~2.8 ppm and

~2.5 ppm too, depending on the way the thiophene rings were connected It is therefore difficult to exclude the possibility of the thiophene polymerising based

on the NMR spectrum alone In order to study if the thiophene moiety had

C6H12Ph Ph

n

S

3c

polymerised, 3-( -bromohexyl)thiophene was added together with monomer 3

and styrene in 5:1:10 mole ratio After radical polymerisation, the reaction mixture was extracted and washed Its NMR spectrum is shown below:

Fig 3.5 NMR spectrum of the resulted copolymer from 3(

-bromohexyl)thiophene, monomer 3 and styrene in 1:1:10 mole ratio

The sharp and obvious triplet that was caused by –CH2Br hydrogen resonance clearly indicated that polymerisation of 3-( -bromohexyl)thiophene did not occur Hence, we can conclude that thiophene rings did not polymerise under these experimental conditions This is further supported by the presence of the peaks at ~7.25 ppm and 6.9 ppm Consequently, a graft copolymer system containing two different polymer backbones that are connected through an alkyl

chain linkage was formed through the second oxidative polymerisation of the copolymers

The abovementioned precursor copolymer mixture was further oxidised with

3-octylthiophene to give graft copolymer 4 The NMR spectra of the polymer blend and 4 are depicted in Fig 3.6 and Fig 3.7 respectively

Fig 3.6 1 H NMR spectrum of the polymer blend of PS and

poly(3-octylthiophene)

The two spectra have some similar features:

i) the multiplets between 6.9 to 7.2 ppm were caused by aromatic Hs at

positions o and p on the phenyl rings, as well as protons at positions 4 on

the thiophene rings of the polymer

ii) The multiplets between 6.5 to 6.6 ppm account for the protons at position

m on the phenyl ring

Fig 3.7 1 H NMR spectrum of grafted copolymer 4

The two broad bands at ~2.5 ppm and ~2.8 ppm on the spectra of the blended

copolymer and grafted copolymer 4 do not represent the same functional group

In the polymer blend these two peaks are ascribable to –CH2 protons on the alkyl

x

alkylthiophenes are joined in head to head (HH) fashion this peak appears at 2.55 ppm If they were joined in head to tail (HT) fashion, the peak would appear at

2.80 ppm [12] In copolymer 4, the peaks caused by the –CH2 and –CH groups neighbouring the phenyl rings coincide with peaks due to methylene groups next

to the thiophene rings at 2.5 and 2.8 ppm, as discussed above The major

difference between the two spectra is the broad peak at ~3.5 ppm on 4, which was

caused by –CH2Br

It has to be noted that the NMR spectra of grafted copolymers may give qualitative structural information of the polymer, e.g., type of bonding and functional groups The integration of the peaks, on the other hand, might not have provided the correct information on the chemical composition of each functional group in the bulk of the copolymer This is due to the fact that a major portion of the copolymer was not soluble for NMR analysis

In the case of 5, weak broad bands were detected between 2.5 and 2.8 ppm The

shape of the bands resembles its precursor copolymers Peaks at 2.5 and 2.8 ppm are the result of –CH2 protons on the alkyl side chain adjoining the phenyl rings

and the –CH group at p position of this methylene group, as was discussed Since

copolymer 5 was formed by polymerising 3d and thiophene in 1:10 ratio, we can

expect that most of the mono-substituted thienylene rings originating from

monomer 3 to be linked with thiophene (added during the second polymerisation

step) This would give rise to tri-substituted thienylene rings in the graft

copolymer The methylene groups on the alkyl chain that is neighbouring these tri-substituted thiophene rings have similar chemical environment and give a single broad peak at 2.6 ppm The low intensity of the band is due to the fact that the alkyl side chain content is relatively low in this copolymer

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

CH2Ph

c b a

3.2.3 FT-IR

Fig 3.9 is the IR spectra of graft copolymers 4, 5 and the blended copolymer

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

The spectra of the other six copolymers were quite similar due to their similar structures The bands between 3100-3000 cm-1 are due to aromatic C-H stretch present in all three polymers The band at around 2922 cm-1 is a result of asymmetric –CH2 stretch and the band at 2849 cm-1 is ascribable to symmetric –

CH2 stretch A series of weak bands between 2000-1650 cm-1 observed are the overtones or combination region that is characteristic of phenyl ring containing compounds Aromatic ring C-C stretching bands give rise to the bands at around

1600, 1495 and 1450 cm-1 The bands at 1070 and 1025 cm-1 are caused by in plane C-H bending The band at around 750 cm-1 is due to out of plane aromatic ring C-H bend

The most noteworthy difference between the IR spectra of copolymer Graft 100

(see Fig 3.10) and the other copolymers is the absence of the peak at 700 cm-1,

which is a result of mono substituted aromatic ring bands [13] Graft 100 is

formed exclusively by the monomer 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene,

hence there is no mono substituted phenyl ring in its structure All phenyl groups were di-substituted therefore unlike all other copolymers, the peak at 700 cm-1 is

absent on the spectrum of copolymer Graft 100

On the spectrum of copolymer 5, a very weak peak appeared at around 825 cm-1

whereas on all other spectra, this peak is quite obvious The band is caused by

C-H out of plane bending of 2,3,5-trisubstituted rings [13] of the polymerised alkylthiophenes These tri-substituted polythiophene rings are abundant in all the

3-copolymers except copolymer 5 This copolymer mainly consists of di-substituted polythiophene rings Hence in the spectrum of copolymer 5, only a weak peak

appeared in this region, which is caused by its precursor copolymer, i.e

copolymer 3d

Fig 3.10 IR spectrum of copolymer Graft 100

3.2.4 Elemental Analysis The elemental analyses results are shown in Table 3.1 The empirical formula and percentage molecular weight of the thiophene unit were calculated based on the elemental analysis data Sulphur content was used as the base unit value to calculate the ratio of other elements in the samples The results suggest that the experimental formula of the polymers deviated from the expected formula

depending on the way each polymer was synthesised Copolymer Graft 100

shows the best agreement with the expected formula, which is most likely due to

the fact that it was formed exclusively from monomer 3 The percentage molecular weight of the thiophene unit in both copolymers Graft 21 and Graft 51