Effect of sulphate ion on the electrochemical polymerization of pyrrole and N-methylpyrrole

Pyrrole and N-methylpyrrole were electrochemically polymerized in an identical manner in aqueous electrolytes containing (Et4N)BF4, LiClO4 or Na2SO4 to give films of the corresponding p-doped polymers. The polymer obtained from the aqueous Na2SO4 electrolyte, [(poly N-methylpyrrole)+y (SO4)y/22-]x, differed from the other polymers in that it exhibited a very low conductivity (≈ 10−7 S/cm), contained a high concentration of > C=O groups and had an EPR linewidth, ΔHpp, more than an order of magnitude greater than that found in the corresponding polymers obtained from the (Et4N)BF4 and LiClO4 electrolytes. A sample of [(polyN-methypyrrole)+y(BF4)y−]x electrochemically synthesized in CH3CN and subsequently converted electrochemically to the corresponding sulphate in the Na2SO4 aqueous electrolyte exhibited normal behaviour. A mechanism is proposed for the introduction of > C=O groups into the poly(N-methylpyrrole) polymer during its electrochemical polymerization in the Na2SO4 aqueous electrolyte.