A mechanism for electropolymerization of 2-vinylpyridine coatings on metalsurfaces

The mechanism of coating formation by electropolymerization of poly(2-vinyl pyridine) has been studied on a copper electrode by surface-enhanced Raman scattering spectroscopy and cyclic voltammetry. The experimental results ar e consistent with a mechanism in which 2-vinylpyridine molecules become pro tonated in acidic aqueous solutions and are selectively adsorbed on cathodi c surfaces. The adsorbed 2-vinylpyridinium ions then undergo electrochemica l reduction to free radicals, which initiate polymerization by combining wi th neutral 2-vinylpyridine molecules also present in solution. The pH of th e electrolyte has been found to be critical for this process. On the basis of the voltammetric experiments, an additional mechanism for polymer growth on the cathodic surface at more negative potentials is proposed. This invo lves protonation of nitrogen sites along the newly formed poly(2-vinylpyrid ine) chains, followed by their reduction to form polymeric radicals that ca n initiate chain branching. In this way, inactive polymer chains can be rea ctivated and highly branched and cross-linked poly(2-vinylpyridine) coating s can be formed, leading to the relatively low solubility of these coatings in typical organic solvents.