Electrochemistry of poly(3-thiopheneacetic acid) in aqueous solution: evidence for an intramolecular chemical reaction

Electrochemical oxidation of 3-thiopheneacetic acid in dry acetonitrile leads to the formation of a conducting polymeric film. These films can be cycled between oxidised and reduced forms in acetonitrile, but on oxidation in water or methanol are converted to a passive film. This process is accompanied by the passage of approximately two electrons for every monomer unit within the film. Based on the electrochemistry and FTIR studies of the process, a mechanism for the electrochemical passivation of the polymer is proposed involving the formation of an intermediate cyclic lactone and subsequent breakdown by reaction with the solvent. Electropolymerised films of the corresponding methyl ester, methyl 3-thiopheneacetate, are not subject to the same electrochemically driven passivation reaction in water.