Catalytic Effects of Aniline Polymerization Assisted by Oligomers

Polyaniline was first confirmed as a dark green precipitate on an electrode during the electrochemical polymerization of aniline in 1862. Since then, scientists have been studying the kinetics and growth mechanisms of polyaniline through the electrochemical approach. Studies have shown that <i>p</i>-phenylenediamine, <i>p</i>-aminodiphenylamine, and other aromatic small molecules may serve as initiators for accelerating the polymerization reaction due to the autocatalytic effect of polyaniline. However, little research has been focused on the catalytic effects of introducing oligoanilines. In this paper, quantitative rate constants for the electrochemical polymerization of aniline in both HCl and acetonitrile/HCl solutions with 0.5 mol % of added oligoanilines including diphenylamine, <i>N</i>-phenyl-<i>p</i>-phenylenediamine, 1,4-phenylenediamine, <i>N</i>,<i>N</i>′-diphenyl-1,4-phenylenediamine, 4,4′-diaminodiphenylamine, and tetraaniline in both emeraldine and leucoemeraldine states are reported. Among all the rate constants, <i>N</i>-phenyl-<i>p</i>-phenylenediamine, 1,4-phenylenediamine, and 4,4′-diaminodiphenylamine are shown to be the most effective catalysts for aniline polymerization. Tetraaniline is likely the intermediate species where the polymerization process starts to slow down, while diphenylamine and <i>N</i>,<i>N</i>′-diphenyl-1,4-phenylenediamine decelerate the reaction. Additionally, adding in oligothiophenes is confirmed to reduce the reaction rate. It is also shown that the rate constants measured are consistent with two other methods: (1) monitoring the open-circuit potential and (2) measuring the temperature of the solution. These methods were used previously to qualitatively compare the speed of the polymerization reactions. Additionally, the existence of both agglomerated and nanofibrillar polymer morphologies for reactions with slow rate constants is revealed.