Oxidative polymerization of phenols revisited

Abstract

This paper presents an overview on recent developments of oxidative polymerizations of phenols, particularly focusing on the coupling selectivity. In 1959, Hay et al. discovered an oxidative polymerization of 2,6-dimethylphenol catalyzed by a copper/amine complex to produce poly(2,6-dimethyl-1,4-phenylene oxide). The reaction mechanism of the selectivity for C–O coupling, however, has not been made thoroughly clear yet. First, the following three reaction mechanisms so far proposed are discussed; (i) coupling of the free phenoxy radicals, (ii) coupling of the phenoxy radicals coordinated to catalyst complexes, and (iii) coupling through the phenoxonium cation. Next, an oxidoreductase enzyme such as peroxidase or oxidase and a peroxidase model complex that have been recently found as the catalyst for oxidative polymerization of phenols are described. The enzyme catalyst allowed chemoselective polymerization of a phenolic monomer having a reactive functional group like methacryloyl group and also induced polymerization of a new phenol monomer such as syringic acid that could not be polymerized by the conventional metal catalysts. Finally, a tyrosinase model complex behaved as a novel regioselective oxidative polymerization catalyst. The catalyst controls the coupling of phenoxy radicals from 2- and/or 6-unsubstituted phenols, which was named as a ‘radical-controlled’ catalyst. By using this catalyst, crystalline poly(1,4-phenylene oxide) was produced from 4-phenoxyphenol for the first time via oxidative polymerization, and a new crystalline polymer with a melting point above 300 °C was synthesized from 2,5-dimethylphenol. Thus, new catalysts and some new polymers have been developed; oxidative polymerization of phenols has now revisited as a clean and low-loading process for synthesis of phenolic polymers.