Polymerization reaction coupled to charge transfer: propagation versus termination as a source of permanent travelling waves and multi-peak voltammograms

The voltammetric response for an RRC (radical-radical coupling)-initiated p olymerization reaction coupled to an irreversible charge transfer has been considered. Only the propagation and termination steps of the polymeric cha ins in solution have been taken into account, with the exclusion of any ads orption and hindered diffusion effects. A set of nonlinear partial differen tial equations is derived. It is demonstrated that this set of equations po ssesses permanent travelling wave solutions as soon as a newly defined prop agation versus termination balance (PTB) parameter gamma is greater than un ity. These permanent travelling waves feature the building of a uniformly m oving polymer I solution boundary as the polymer film is being constructed on the surface under gamma >1 conditions. Finally, it is found that under g amma > 1 conditions the permanent travelling wave is responsible for the oc currence of a sharp pre-peak on the resulting voltammograms, which is thus directly connected to the abundant consumption of the monomer by the polyme rization. All these features are confirmed by a direct numerical resolution of the initial set of nonlinear partial differential equations. The result s are in line, qualitatively, with some results which were still uninterpre ted, and which were obtained in the electropolymerization of vinylic monome rs in anhydrous acetonitrile. In particular, it is demonstrated that the pr e-peak can most probably not be assigned either to passivation or to precip itation effects, but is a mere dynamical effect due to the polymerization i tself. (C) 1999 Published by Elsevier Science S.A. All rights reserved.