Theoretical study of the oxidative polymerization of aniline with peroxydisul fate:: Tetramer formation

Semi-empirical quantum chemical study of the oxidative polymerization of aniline with ammonium peroxydisulfate, in aqueous solutions without added acid, has been based on the MNDO-PM3 computations of thermodynamic, redox, and acid-base properties of reactive species and the intermediates, combined with the MM2 molecular mechanics force-field method and conductor-like screening model of solvation. The main reaction routes of aniline tetramerization are proposed. The regioselectivity of the formation of aniline tetramers by redox and electrophilic aromatic substitution reactions is analyzed. It was proved that the linear N-C4 coupled tetra-aniline is formed as a dominant product by three different pathways: comproportionation redox reaction between N-phenyl-1,4-benzoquinonediimine and 4-aminodiphenylamine, the one-electron oxidation of aniline with its half-oxidized N-C4 coupled trimer, and the electrophilic aromatic substitution reaction of aniline with fully oxidized N-C4 coupled trianiline nitrenium cation. The electrophilic aromatic substitution reaction of the N-C4 coupled aniline trimer with aniline nitrenium cation, as well as the oxidation of aniline with half-oxidized branched trimer, lead to the branched aniline tetramers. The competing character of different tetramerization routes is highlighted. The oxidative intramolecular cyclization of branched oligoanilines and polyaniline, leading to the generation of substituted phenazine units, has been predicted to accompany the classical routes of the polymerization of aniline. Various molecular (branched vs. linear) oligomeric structures produced at different level of acidity during the course of polymerization and their impact on the formation of supramolecular structures of conducting polyaniline (nanorods and nanotubes vs. granular morphology), are discussed. (C) 2007 Wiley Periodicals, Inc.