OBSERVATION OF SOLUBLE PYRROLE OLIGOMERS AND THE ROLE OF PROTONS IN THE FORMATION OF POLYPYRROLE AND POLYBIPYRROLE

Combined rotating ring disk electrode (RRDE) and spectroelectrochemical evidence is presented that shows clearly that bipyrrole is formed in solution during the anodic polymerization of pyrrole in acetonitrile (ACN). Spectroelectrochemistry shows that a measurable solution concentration is built up, but the RRDE collection efficiency N(r) of 9 X 10(-3) (0.22 theoretical) shows that the yield is low. Other intermediates were observed and the presence of H+ and protonated pyrrole is discussed. The half wave potential E1/2 for the H+ reduction wave (N(r) = 0.22) at the ring shifts from -0.08 V vs. saturated sodium chloride calomel electrode (SSCE) in 0.1 M ClO4- to -0.90 V in 0.1 M (C4H9)4NBF4, and to -0.66 V in 0.1 M ClO4- salt + 0.1 M pyridine or di-t-butylpyridine for 50 mM pyrrole solutions in ACN. An absorbance peak at 242 nm formed during electropolymerization is identified as being due to protonated pyrrole. Since this species goes on to form pyrrole black it may influence the conductivity of the electropolymerized product, and this suggests why the more basic BF4- is a preferred electrolyte. Authentic bipyrrole underwent two oxidations at an RRDE, with E1/2 values of 0.55 and 1.15 V vs. SSCE in 0.1 M NaClO4. The first E1/2 was pH independent, while the second shifted to 0.55 V in 0.1 M NaClO4 + 0.1 M pyridine to give a single two-electron wave. Polybipyrrole formed from oxidation of 0.03 mM bipyrrole in 0.1 M NaClO4 + ACN has an oxidation peak at -0.35 V vs. SSCE in aqueous ClO4-, compared with more positive than -0.05 V for polypyrrole. The polybipyrrole oxidation peak is narrower and shows a better defined range of redox states than does polypyrrole, and coincides with a shoulder often observed for polypyrrole.