Chemical oxidative polymerization of anilinium sulfate versus aniline: Theory and experiment

The mechanism of the oxidative polymerization of anilinium sulfate with ammonium peroxydisulfate in aqueous medium has been studied by the AM1 and MNDO-PM3 semi-empirical quantum chemical methods combined with the MM2 molecular mechanics force-field method and conductor-like screening model (COSMO) of solvation. The aniline nitrenium cation is the main reactive electrophilic species generated in the initiation phase. It is proposed that the rate and regioselectivity of dimerization is governed by the electrophilic aromatic substitution reaction of non-protonated aniline molecules with aniline nitrenium cations. 4-Aminodiphenylamine and its fully oxidized form, N-phenyl-1,4-benzoquinonediimine, are the main dimeric products. The dominant aniline trimer and tetramer are shown to be linear, N–C coupled, with the prevalence of the para-substitution pattern. The mechanism of oxidation of anilinium sulfate is compared with that of non-protonated aniline molecules; the latter yield also branched trimers and tetramers that are subsequently oxidized to phenazine constitutional units. The differences in morphology of the polyanilines obtained by the oxidation of anilinium sulfate and aniline, granules, nanotubes or nanorods, and in molecular structure reflected by infrared spectra are discussed with respect to the formulated comprehensive mechanistic scheme of the early stages of both anilinium cation and aniline oxidation.