On the origin of the so-called nucleation loop during electropolymerization of conducting polymers

During the potentiodynamic preparation of conducting polymers, cyclic voltammograms of many pi-conjugated monomers and oligomers often show a marked crossing or loop effect. The so-called nucleation loop of the first cycle has been ascribed to the nucleation process requiring an activation energy provided by an overpotential. This paper presents cyclic voltammograms of pi-systems with trace crossing as well as loop effects that suggest that the homogeneous formation of oligomeric redoxactive follow-up products from the starting species is responsible for this occurrence. As the investigated species are typical starting components of resulting oligomers or polymers, all these findings are evidence that similar mechanisms also hold for the formation of many other classical polymers with a nucleation loop like polypyrrole, and that the true reason for the nucleation loop is the comproportionation reaction between an oligomeric follow-up product and the starting monomer.