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

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 lambda similar to 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 (132) 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 lambda similar to 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 similar to 720 nm against oxidation time (t) data. The results obtained indicate that the rate controlling process are governed by the two-dimensional (132) diffusion. Good linearity (r similar to 0.985) between DeltaH(#) and DeltaS(#) for the series of acids doped PANT matrix was obtained, which suggests the existence of a compensation (or isokinetic) effect. The proposed mechanism for the oxidation of ANT is also supported by MM + calculation. Activation parameters for the rate of oxidation process of ANT have been computed and discussed. (C) 2004 Elsevier Ltd. All rights reserved.