Conjugated polyaniline as a result of the benzidine rearrangement

The mechanism of the oxidative polymerization of aniline is reviewed on the basis of the experimental evidence and reaction schemes proposed in the literature. It is demonstrated that the balance between the non‐protonated and protonated forms of the monomer and the growing chain is responsible for the diversity of the molecular structure, morphology and properties of aniline oxidation products. Various forms are oxidized by two oxidation mechanisms: (1) chain reaction of electrophilic substitution and (2) coupling of cation‐radical centres. At pH > 2.5, the non‐protonated reagents are oxidized with a chain reaction controlled by electrophilic substitution of the aromatic ring. The reaction leads to the formation of non‐conducting aniline oligomers with heterogeneous molecular structures. At pH = 2.5–4, the electrophilic substitution is reduced because of the protonation of the aniline, and the cyclic dimer, phenazine, becomes the main product of the oxidation. The growth of the conjugated chain proceeds at pH < 2.5, when the reactants are protonated. The reaction proceeds through coupling between the terminal cation radical and the monomer cation radical with formation of a π ‐complex. The transformation of the π ‐complex into a para ‐substituted monomer unit is thermodynamically controlled and is produced through an intramolecular benzidine (semidine) rearrangement. The regular structure of the growing chains is a result of the high regioselectivity of the sigmatropic rearrangements and significant energy gain attained by the formation of protonated polyconjugated chains in the agglomerated state.