Conducting polymers as free radical scavengers

The potential antioxidant activity of conducting polymers in biomedical applications has been evaluated by determining radical scavenging ability using the stable ,-diphenyl-β-picrylhydrazyl (DPPH) free radical and correlating this with the reducing strength of the polymer. Commercial soluble polyaniline grafted to lignin, poly(anilinesulfonic acid), and polypyrrole, were found to be very efficient scavengers of DPPH radicals, reacting with 2-4 DPPH radicals per aniline or pyrrole monomer unit. Shifts in IR bands in the case of a polyaniline powder pointed to polymer oxidation and methoxy-substitution as likely mechanisms. All of the conducting polymers had low formal potentials (ca. 150 mV (Ag/AgCl) at pH 7), similar to those of catechin-type polyphenol antioxidants. On the other hand, the formal potentials of pyrrole and aniline were quite high (ca. 700 mV), and the reaction of aniline with DPPH radicals was limited, while o-methoxyaniline with a lower formal potential (ca. 500 mV) neutralised DPPH radicals in a 1:1 ratio.