Pyrrole and Thiophene Polymers

The electrochemical polymerization of monomers 41-43 resulted in the isolation of polythiophenes with ferrocene groups pendent to the backbone. It was noted that the redox potential of the polymer from the ferrocene-ftmctionaUzed monomer 41 was the same as from n-hexylferrocene. However, increases of about 

03 and 0.15 V were observed for the polymers derived from monomers 42 and 43 because of stronger interactions between the ferrocene moieties and the polymer backbone. As the spacer between the ferrocene groups and the polymer backbone increased from one to six methylene groups, there was a corresponding decrease in redox conductivity. The redox conductivity of the polymer prepared from 41 was enhanced via copolymerization with 4-hexylcyclopentadithiophene and reached 1 S/cm. The in situ conductivity of the homopolymer prepared from electrochemical oxidation of 41 was 40 S/cm. 

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M. Armour, A. G. Davies, J. Upadhyay, and A. Wassermann. 1967. Colored electrically conducting pol5miers from furan, pyrrole, and thiophene. / Polym Sci A-1 5 1527-1538. 

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

Technically important electrochemical reactions of pyrrole and thiophene involve oxidation in non-nucleophilic solvents when the radical-cation intermediates react with the neutral molecule causing polymer growth [169, 191], Under controlled conditions polymer films can be grown on the anode surface from acetonitrile. Tliese films exhibit redox properties and in the oxidised, or cation doped state, are electrically conducting. They can form the positive pole of a rechargeable battery system. Pyrroles with N-substituents are also polymerizable to form coherent films [192], Films have been constructed to support electroactive transition metal centres adjacent to the electrode surface fomiing a modified electrode,... 

Many aromatic ring systems including aniline, pyrroles and thiophenes form extensively conjugated, electroactive polymers, which can be oxidised or reduced between... 

The other important electrochromic polymers are the polypyrroles and polythiophenes, obtained by polymerisation of the parent pyrrole and thiophene or, more importantly, their 3,4-substituted derivatives. The most widely studied of these two classes of polymers in electrochromic outlets are the poly thiophenes, which are readily synthesised by the reaction of the substituted monomer with FeClj in chloroform solution. The colour change properties of a variety of poly thiophenes in the presence of a counter-ion are shown in Table 1.14. ... 

Virtually all of the real interest in electroinitiated synthesis of conducting polymers has focussed on the anodically active aromatic monomers, of which the most highly studied examples are pyrroles and thiophenes (Table 1). 

Electrochemical synthesis utilizes the ability of a monomer to be self-coupled upon irreversible oxidation (anodic polymerization) or reduction (cathodic polymerization). While this method does not always produce materials with well-defined structures (as do the three other polymerization methods to be discussed), electropolymerization, nonetheless, is a rather convenient alternative, avoiding the need for polymer isolation and purification. Of these two routes, anodic polymerization is the most widely explored as monomers such as pyrrole and thiophene are relatively electron-rich and prone to oxidation. For this reason the anodic route will be the focus of the remainder of this presentation. 

It has been mentioned already that polypyrrole (25) and polythiophene (26) play an important role as electrical conductors and polymeric anodes in battery cells [2,47,226]. Since the charging and discharging of the conjugated polymer is accompanied by the incorporation and removal of counterions it is clear that the material can also act as a carrier of chemically different anions which influence the physical, chemical and physiological properties of the material [292]. With regard to the full structural elucidation of the polymers it must be added, however, that the electropolymerization process of pyrrole and thiophene does not provide a clean coupling of the heterocycles in the 2,5-positions. Instead, the 3- and 4-position can also be involved giving rise to further fusion processes under formation of complex polycyclic structures [47]. 

An established technique for preparing soluble derivatives of intractable polymers is to add solubilizing substituents to the monomer. Thus, insoluble polyphenylenes have been rendered soluble (75) in common solvents such as chloroform by phenyl substitution. Similarly, a wide range of substituted pyrroles and thiophenes have been investigated to improve the tractability of these relatively stable conducting polymers. The chemical and electropolymerization of N-substituted pyrroles (76) gave polymers with substantially decreased conductivities, generally by a fector of 10 compared to polypyrrole. However, monomers substituted at the 3 and 4 positions of... 

The solubility of polythiophenes has also been increased by attaching alkoxy groups to bithiophere monomers.12 Other workers131415 have been concerned with producing pyrroles and thiophenes with alkyl-sulfonated chains attached to increase the water solubility of the polymer. 

The electropolymerization of metallocene-functionalized pyrrole and thiophene has been utilized for the creation of organometallic polypyrroles and polythiophenes.181 241 242 243 Zotti et al. reported the electrochemical homo- and copolymerization of ferrocene-functionalized pyrrole, 177.242 Polymer 179 was prepared via the copolymerization of monomer 177 with pyrrole (Scheme 2.48). This polymer possessed an electrical conductivity of 1.5 X 10-2 S/cm. 

As in the cases of pyrrole and aniline polymers, both chemical and electrochemical procedures have been employed in the synthesis of polythiophene (PTH) and its derivatives. The thiophene polymers exhibit remarkable stability in air and water [119], The alkyl-substituted derivatives also exhibit a high degree of processability [120,121]. PTH has also been synthesized from bithiophane (the dimer) or terthiophene (the trimer). The resulting poly(2,2 -bithiophene) (PbTH) and polyterthiophene (PtTH) are more ordered than PTH and appear to have the same basic structure as the starting monomer [117,122],... 

A copolymer of pyrrole and thiophene nano-fibrUs was electrochemically polymerized within the pores of microporous, anodic, aluminum oxide template membranes [105]. The copolymer nucleated and grew on the pore wall of the membrane since the polymers were cationic and the membrane had anionic sites on the pore wall. The length, thickness, and diameter of the copolymer nanofibrils could be controlled and with higher applied potential, more thiophene units were incorporated into the copolymer nanofibrUs [105]. Copolymer nanofibrils of pyrrole and aniline were also electrochemically polymerized within the pores of microporous, anodic, aluminum oxide template membranes [106]. Copolymer nanofibrils of PPy and poly(3-methylthiophene) prepared chemically in the microporous aluminum oxide template showed higher conductivity than the homopolymers did [107]. 

Li, X., M. Lu, and H. LL 2002. Electrochemical copolymerization of pyrrole and thiophene nanofibrils using template-synthesis method. J Appl Polym Sci 86 2403. 

When a metalate ion, such as PtCl " or AUCI4 as well as phosphotungstate was used as electrolyte in the electrolytic polymerizations of pyrrole and thiophene, PtClJ /PPy and AuCl /PTh composites were obtained, respectively. The incorporations of Pt and Au " were confirmed by ESCA measurements. The resulting conducting polymers, incorporating highly dispersed metal, may find applications as catalysts etc. 

This chapter will consider in detail only PHCs and in particular pyrrole- and thiophene-based systems. This limitation implies that other important classes of polyconjugated polymers, such as polyacetylene, polyaiylenes, polyanilines, and related polymers, will not be treated. 

Conductivity and redox potential for pyrrole- and thiophene-based polymers are correlated by a simple relationship [81], which provides a useful predicting tool. 

The five-membered heterocycles such as pyrrole and thiophene and to a certain extent furan and their derivatives can be polymerized electrochemically. It seems that at least for pyrrole and thiophene derivatives the mechanisms of polymerization are similar. Furan is less studied, presumably because the resulting polymer is of inferior quality. The mechanism can be divided into several steps, which will be now studied in detail. In planning the polymerization of these compounds it is necessary to know the mechanism in order to be able to optimize the process and to assess beforehand the effect of different ingredients. 

The electrochemical copolymerization of pyrrole and thiophene is difficult due to the large differences in the oxidation potentials of the monomers. According to Tourillon and Gamier [192], the electropolymerization of pyrrole occurs at around 0.6 V (versus SCE), and that of thiophene occurs around 1.65 V (versus SCE). Inganas et al. [201] attempted to refine the electronic properties of conducting polymers by varying the molecular composition of the polymers. Terthiophene... 

A condensed copolymer of pyrrole and thiophene, polythieno-3,2-/ -pyrrole (Fig. 10c) polymerizes to produce films with conductivities up to 5 x 10" S cm" [208,209]. On applying a 0.6 V potential (versus Ag/Ag ) across a solution containing 10" M monomer and 0.1 M salt in acetonitrile, brittle, black polymer deposits formed on the anode after 10 to 20 min. X-ray photoelectron spectroscopy showed carbon, nitrogen, and sulfur and indicated that the polymer was formed in an oxidized state. 

Kuwabata et al. [210] have reported the direct copolymerization of pyrrole and thiophene through electrochemical oxidation [211]. Their polymerization was carried out potentiostatically in an acetonitrile solution containing 0.1 M thiophene, a pyrrole concentration as low as 2.0 X 10" M, and 0.1 M lithium perchlorate. As previously mentioned, the difference in the oxidation potentials between pyrrole and thiophene is rather large. Kuwabata et al. oxidized pyrrole under diffusion limiting conditions at potentials at which thiophene oxidation occurs. Conductivities of the polymer films prepared at potentials between 1.37 and 2.07 V (versus Ag/Ag" ) were between 0.15 and 44 S cm". Although much greater conductivities... 

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