Mechanistic investigation of the oxidative polymerization of aniline hydrochloride in different media

Abstract

Oxidative polymerization reaction of the protonated form of aniline (ANI-HCl) was investigated experimentally in acidic solutions with in situ spectrophotometric technique and computationally using molecular mechanics (MM+) calculations. Well-defined absorbance peaks of an intermediate and of an end product were observed after first scan of the spectrogram, using zero- and first-derivative spectroscopic treatment. Increases in the absorbance at λ∼420 nm with increasing the oxidant and/or anionic surfactant (sodium dodecyl sulfate, SDS) content in the polymerization medium were observed. This indicates that the presence of nitrenium cations and suggests that the polyaniline evolution goes towards a two-dimensional (D2) polymer with phenazine rings, which can be formed by a cross-linking reaction. It was confirmed by kinetic studies, as well as verified by MM+ calculations. Absorbance peak at λ∼520 nm (dark brown), in the first scan of the spectrogram was observed. This peak increases with increasing the acid surfactant [poly(acrylic acid)] (PAA) matrix content in the polymerization medium. Kinetic parameters of the oxidative polymerization reaction were deduced employing a computer-aided kinetic analysis of the absorbance ( A) at ∼720 nm against oxidation time ( t) data. The results obtained indicate that the rate controlling process are governed by the two-dimensional (D2) diffusion. Good linearity ( r∼0.985) between Δ H # and Δ S # for the series of acids doped PANI matrix was obtained, which suggests the existence of a compensation (or isokinetic) effect. The proposed mechanism for the oxidation of ANI is also supported by MM+ calculation. Activation parameters for the rate of oxidation process of ANI have been computed and discussed.