Copolymers by Electrochemical Reactions

In conventional polymer synthesis copolymerization is a common strategy for modifying polymer properties. In electrochemical polymerizations of the kind used to make conducting structures, it is expected to be difficult to make good copolymers unless the oxidation potentials of the two monomers are sufficiently close that one is not significantly preferred over the other 190). 

Electrochemical polymerization of phenols to non-conducting poly(phenylene ethers) has been described 112) and Oyama et al. recently claimed that phenol 191) and p,p -biphenol192) can be polymerized to uncharacterized conducting films. Kumar et al.193) claimed that it is possible to copolymerize phenol with pyrrole 

Because their oxidation potentials are similar, substituted pyrroles can be copolymerized with pyrrole, allowing the limiting conductivity of the fully-doped polymer to be varied 194,19S). The oxidation potentials of the monomers, and hence their reactivity ratios, are sensitive to the substituent196). Inganas et al.197) reported the synthesis of pyrrole-thiophene copolymers starting from terthiophene, whose oxidation potential is similar to that of pyrrole. Sundaresan et al.198) copolymerized pyrrole with 3-(pyrrol-l-yl)propanesulphonate to give a polymer in which the sulphonate counter-ion is a part of the polymer structure. 

Because of the problems associated with copolymerizations of monomers of very different reactivity, many authors have looked at an alternative approach which is to synthesise appropriate sections of the desired polymer chain, then couple them electrochemically to get the final polymer. Naitoh et al.199) synthesised the dimer, 2,2 -thienylpyrrole and used this as a monomer to prepare the alternating pyrrole-thiophene copolymer. They claimed that the copolymer film obtained with HSO as the counter-ion is more conductive than either of the corresponding homopolymers by a factor of 10 to 20. McLeod et al. 200) synthesised 2,5-dithienylpyrrole and polymerized it electrochemically with silver p-toluenesulphonate as the electrolyte. They obtained films of polymer whose conductivity could be varied in the range 10 8 to 0.1 Scm-1. Surprisingly, some low conductivity films were soluble in acetone or acetonitrile and evaporation of the solvent gave a powder of similar conductivity. Based on the shift in the absorption maximum in the visible spectrum on polymerization, it was concluded that the soluble films were polymers with molecular weights of 4000. 

Ferraris and Skiles 201) have also described a range of thiophene copolymers synthesised in a similar way. 

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