The chemical oxidation of aniline in buffers: Oligoaniline intermediates in the aniline-peroxydisulfate system

Polyaniline is one of the most important electrically conductive polymers. The synthesis is generally done at a low initial pH (1--0). Uncharacteristic oligomers are formed when aniline is oxidized with ammonium peroxydisulfate in weakly acidic, neutral or alkaline aqueous media. These oligomers have been difficult to isolate because the chemical polymerization of aniline with ammonium peroxydisulfate will produce sulfuric acid as a side reaction, the acid will lower the pH from its initial conditions to pH ≤ 1 and the conventional synthesis of polyaniline will proceed. These oligomers affect the bulk morphology and the properties of the resulting polyaniline, decreasing its conductivity. To isolate the oligomers the synthesis were done in buffers (pH 2.5--10.0). The oligomers obtained are electrically insulating brown powders that are mixtures of Michael-type adducts of benzoquinone monoimine and aniline at various stages of hydrolysis. The products are chemically similar to products obtained the synthesis of aniline and 1,4-benzoquinone instead of ammonium peroxydisulfate, in similar aqueous reactions. The role of ammonium peroxydisulfate may be to oxidize aniline into the para-benzoquinoid intermediate, benzoquinone monoimine. The benzoquinone monoimine formed as a result of a Boyland-Sims rearrangement of aniline. The formation of benzoquinone monoimine is followed by conjugate Michael-type addition reaction with aniline or sulfated anilines. The oligomers isolated in pH 2.5--10.0 buffers consist of intermediate species at various stages of hydrolysis.