Electrochemical synthesis of linear polyaniline in aqueous solutions

An attempt was undertaken to prepare polyaniline with a fine linear structure in aqueous media. Comparable electrochemical polymerization was carried out in solutions of the following acids: hydrochloric, sulfuric, perchloric and tetrafluoroboric. Spectroelectrochemical measurements have revealed the presence of an intermediate product (nitrenium cation) in the initial stage of aniline polymerization carried out in these media. The polymerization starts after an induction period which depends on the final potential for cyclic voltammetry and the nature of the medium. The number of spins as a function of the applied potential was a criterion to determine the linearity of the polymer chain. In polymers with linear structure (without ortho-bindings and cross-linking effects), polarons formed during the doping process in the presence of double-charged anions undergo a recombination process to form bipolarons. As a result no EPR signal is observed between the two redox systems of polyaniline. This kind of polymer can be formed only by voltammetric polymerization in a 5 M solution of tetrafluoroboric acid. The other media mentioned lead to formation of more or less cross-linked chains. The measurements performed suggest that the mechanism of aniline polymerization in aqueous media is analogous to that in non-aqueous solutions for a eutectic mixture of NH₄F; 2.35 HF.