Electrochemistry of conductive polymers VII, Autocatalytic rate constant for polyaniline growth, Synth. Met

The earlier stage of the polymerization reaction of polyaniline has been studied employing in situ spectroelectro-chemical techniques. The r sults indicate that the nitrene cation appears to be a key intermediate species, which leads to all three possible dimers including a head-to-tail dimer (N-phenyl-p-phenylenediamine), a tail-to-tail dimer (benzidine), and a head-to-head dimer (hydrazobenzene). The oxidized forms of these dimers were all shown to be capable of growing polyaniline in the presence of aniline, even though aniline was not oxidized. The cyclic voltammetric peaks observed ur-ing the PA synthesis in the middle potential regions were shown to arise from the redox reactions of these dimers, oligo-mers, and degradation products of polyaniline including quinoneimines and p-benzoquinone. Polyaniline (PA) has received a great deal of interest as one of the conductive polymers due to its straightforward methods of preparation and the stability of its conductive form in air (1-18). PA can be prepared chemically (2, 4) or electrochemically (1, 3, 7-18) in aqueous or nonaqueous media. Only those prepared in acidic aqueous media have a conductive form. Its applications as energy storage media (2-4) and as electrochromic devices (10, 11) have been discussed. Despite much work on PA preparation methods and its various properties, its growth mechanisms in the earlier phase of the polymerization reaction still remains unclear. Recently, Stilwell and Park have reported the autocataly-tic nature of the polymerization reaction and its related ki-netic data (16). From the extensive studies, a conclusion is reached that the PA growth reaction competes with degra-dation reaction paths (16, 19). The rate of autocatalytic re-action has also been determined employing normal pulse voltammetric techniques (18). In the early studies by Mohilner et al. (1) and Bacon and Adams (20), it was shown that the formation of a radical cation upon oxidation of aniline is the rate-limiting step. The radical cation thus formed leads to dimerization prod-ucts such as benzidine and p-aminodiphenylamin