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 started with either DMP itself, or with its C-O coupled dimer, 4-(2',6'-dimethylphenoxy)-2,6-dimethylphenol, under both aerobic and anaerobic conditions. These measurements have provided information on how the actual phenol coupling step takes place and how the polymerization reaction proceeds. In reactions started with the monomer, no or at most very small amounts of oligomers of DMP are detected, apart from a precipitate of polymeric material in the aerobic experiments. Reactions started with the dimer only result in swift formation of significant amounts of monomer in addition to oligomers. This difference in behavior can be ascribed to the higher reactivity of oligomeric phenols compared to the monomer. The fact that monomer phenol is formed from dimer phenol is strong evidence for a reaction pathway in which a quinone ketal is formed by C-O coupling of two phenolic moieties. It is believed 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 generate two (new) species, de facto resulting in redistribution of the oligomers. Once the quinone dissociates from the copper catalyst, a rearrangement may occur to afford the C-O coupled phenol. Methylated phenols (anisoles) do not take part in either redistribution or rearrangement reactions, since a quinone ketal can only be formed from species that can be deprotonated, i.e., from phenols.