Redox chemistry of bipyrroles: Further insights into the oxidative polymerization mechanism of pyrrole and oligopyrroles

The oxidation of 2,2'-bipyrrole, 5-methyl-2,2'-bipyrrole, and 5,5'-dimethyl-2,2'-bipyrrole has been investigated by means of electrochemistry, flash photolysis, and pulse radiolysis. The bipyrrole cation radical was found to give polypyrrole or oligopyrrole under electrochemical and chemical oxidation and also under UV-light irradiation of the solution in the presence of CCl4 as an electron acceptor, The cation radicals have been characterized by their optical absorption spectra, and their decay processes have been followed. In all processes (chemical, electrochemical, and photochemical), the first step involves the reaction between two cation radicals. The cation radical does not react on starting bipyrrole nor on pyrrole monomer, Depending on pH, the cation radical can deprotonate to form a neutral radical. It was found that only the cation radicals, but not the neutral radicals, produce higher oligomers, which explains the inhibition of polymerization by strong bases.