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 polymerization reaction coupled to an irreversible charge transfer has been considered. Only the propagation and termination steps of the polymeric chains in solution have been taken into account, with the exclusion of any adsorption and hindered diffusion effects. A set of nonlinear partial differential equations is derived. It is demonstrated that this set of equations possesses permanent travelling wave solutions as soon as a newly defined propagation versus termination balance (PTB) parameter gamma is greater than unity. These permanent travelling waves feature the building of a uniformly moving polymer I solution boundary as the polymer film is being constructed on the surface under gamma >1 conditions. Finally, it is found that under gamma > 1 conditions the permanent travelling wave is responsible for the occurrence of a sharp pre-peak on the resulting voltammograms, which is thus directly connected to the abundant consumption of the monomer by the polymerization. All these features are confirmed by a direct numerical resolution of the initial set of nonlinear partial differential equations. The results are in line, qualitatively, with some results which were still uninterpreted, and which were obtained in the electropolymerization of vinylic monomers in anhydrous acetonitrile. In particular, it is demonstrated that the pre-peak can most probably not be assigned either to passivation or to precipitation effects, but is a mere dynamical effect due to the polymerization itself. (C) 1999 Published by Elsevier Science S.A. All rights reserved.