MODELING THE GROWTH AND MOLECULAR-STRUCTURE OF ELECTRICALLY CONDUCTING POLYMERS - APPLICATION TO POLYPYRROLE

The growth of polypyrrole generated by electrochemical oxidation of th e monomer was studied by two modeling approaches. The first is based o n transition state calculations of successive coupling reactions to yi eld the polymer. The second evaluates the activation energy of couplin g reactions by means of the frontier orbital model. The two methods pr edict a growth trend for polypyrrole in agreement with the structure i nferred from spectroscopic investigations and provide a description of the electronic modifications induced by the growth of the highly conj ugated structure. The first approach overestimates electrostatic inter actions between the two reacting species, whereas the second neglects these Interactions but exaggerates the importance of orbital interacti ons. Combining these two approaches allows separation of the electroni c effects and leads to general rules for their evolution and their imp act on the growth of polypyrrole. A theoretical framework capable of r ationalizing solvent and counterion effects in electropolymerization i s proposed. The first approach suggests a mechanism for defect formati on which excludes reactions between a pyrrole radical cation and a non terminal monomer unit of an oligomer chain. Oxidation of the successiv e oligomers at high doping levels is shown to be a key factor for the growth of long, regular polymer structures.