Oligomer-oligomer versus oligomer-monomer C-2-C-2 ' coupling reactions in polypyrrole growth

The C-2-C-2' coupling reactions of oligopyrrole radical-cations of increasi ng length generated by electrochemical oxidation have been modeled by trans ition state calculations. The modeling approach takes into account solvent effects and (i) shows that the coupling distance in the transition state de creases with oligomer length, (ii) demonstrates that dimerization rates in the gas phase decrease with oligomer length but increase in water, (iii) su ggests that in a less solvating medium the dimerization rates could be equi valent, (iv) indicates that in all solvents quaterpyrrole and sexipyrrole f ormation is faster through a coupling reaction between oligomer and monomer radical-cations than two oligomer radical-cations, and (v) suggests that f or the formation of a long oligopyrrole from oligopyrrole-pyrrole reactions the mechanism might involve the coupling of the oligopyrrole dication with a non-oxidized pyrrole unit instead of the coupling of two radical-cations or that of the oligopyrrole dication with a pyrrole radical-cation.