Approaches to a cathodic route to polypyrrolylvinylenest

Abstract

Cathodic hydrodimerisation and cleavage reactions of 5-substituted pyrrole-2-carboxaldehydes have been explored with a view to developing an electroreductive route to polypyrrolylvinylenes. Polymers are formed by controlled potential reduction at mercury cathodes. Cyclic voltammetric and coulometric experiments together with detailed product analyses indicate that both hydrodimerisation and cleavage to highly reactive quinodimethane analogues are involved. In some cases carbanions are formed by cleavage, rather than quinodimethanes, and these undergo reaction with their precursor either via self-protonation or by nucleophilic attack.

The key structural features of some of the pyrrolic polymers have been determined, notably using direct pyrolysis mass spectrometry (DPMS). They are highly coloured (red-black), paramagnetic, soluble in organic solvents, probably linear and with molecular weights (M_w) in the region of 5500. Combustion microanalysis combined with DPMS suggests repeat units such as C₇H₇NO₂, C₆H₆NO₂ and C₇H₈NO₂, i.e. reductive cleavage is not complete.