Reactivity/structure correlations for the electropolymerization of pyrrole: An INDO/CNDO study of the reactive sites of oligomeric radical cations

Abstract

INDO/CNDO molecular orbital methods applied to the oligomeric radical cations of pyrrole reveal that, during electropolymerization, the reactive sites in the intermediate oligomers become chemically inequivalent to those of the monomer. Thus, with increasing chain length, the unpaired electron becomes increasingly delocalized such that the α-positions, which are the most reactive sites in the monomer, no longer have the highest localization of the unpaired electron, which is taken- as a measure of reactivity. In the trimer radical cation, the spin density at the 3,3''-positions (β-positions) is approximately equal to that of the α-positions, which are, therefore, about equally reactive. A torsional energy diagram of the trimer radical cation reveals that a coplanar anti conformation is energetically favored. Extensions of this concept predict that whereas initially the oxidative electropolymerization of pyrrole might result in a rather regularly α,α'-linked polymer, a more irregular connection pattern can be expected to result during later stages of the chain growth of polypyrrole. In particular, a number of linkages involving the β-positions are predicted to increase with increasing chain length. © 1984.