The oxidative coupling of 2,6‐disubstituted phenols with PbO 2 and other metal oxides

The main product in the oxidative coupling of 2,6‐dimethyl phenol with PbO 2 and other metal oxides is poly‐2,6‐dimethyl phenylene oxide (M n up to 20,000). The influence of the temperature, the solvent, and the addition of small amounts of various substances on the course of the reactions is discussed. The addition of an amine to the reaction mixture suppresses the diphenoquinone formation, and polymers with much higher molecular weights may be obtained. Infrared and NMR endgroup analyses show that the polymers prepared with PbO 2 and in the absence of amines are branched. On the addition of increasing amounts of amines to the reaction mixtures the degree of branching gradually decreases. Mechanisms for the branching reaction are suggested. The effect of soluble onium salts, such as trimethylcetylammonium bromide, on the course of the oxidative coupling is mentioned (slow reactions giving polymers with high molecular weights). Aromatic polyethers are also obtained in the metal oxide oxidations of 2,6‐disubstituted phenols with bulky substituents, such as: 2,6‐diethyl phenol, 2,6‐diisopropyl phenol, and 2,6‐dimethoxy phenol. Some properties of the polymers are given. There are indications that the reactions go by way of metal phenolates.