The course of the copper-catalyzed oxidative polymerization of 2,6-dimethylphenol. Analysis of oligomeric phenols during the coupling reaction

The course of the copper-catalyzed oxidative coupling of 2,6-dimethylphenol (DMP) has been studied by HPLC-analysis of reaction mixtures which had sta rted with either DMP itself, or with its C-O coupled dimer, 4-(2',6'-dimeth ylphenoxy)-2,6-dimethylphenol, under both aerobic and anaerobic conditions. These measurements have provided information on how the actual phenol coup ling step takes place and how the polymerization reaction proceeds. In reac tions started with the monomer, no or at most very small amounts of oligome rs of DMP are detected, apart from a precipitate of polymeric material in t he aerobic experiments. Reactions started with the dimer only result in swi ft formation of significant amounts of monomer in addition to oligomers. Th is difference in behavior can be ascribed to the higher reactivity of oligo meric phenols compared to the monomer. The fact that monomer phenol is form ed from dimer phenol is strong evidence for a reaction pathway in which a q uinone ketal is formed by C-O coupling of two phenolic moieties. It is beli eved that as long as this quinone remains coordinated to the copper, it can decompose by (probably heterolytic) fission of one of the ether bonds to g enerate two (new) species, de facto resulting in redistribution of the olig omers. Once the quinone dissociates from the copper catalyst, a rearrangeme nt may occur to afford the C-O coupled phenol. Methylated phenols (anisoles ) do not take part in either redistribution or rearrangement reactions, sin ce a quinone ketal can only be formed from species that can be deprotonated , i.e., from phenols.