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The oxygenation of 2,6‐dimethylphenyl phenyl ether (I) at 260°C has resulted in the formation of 4‐methylxanthone (II), 2‐hydroxy‐3‐methylbenzophenone(III), 2‐phenoxy‐3‐methylbenzaldehyde (IV), 2‐phenoxy‐3‐methylbenzoic acid (V), 2‐phenoxy‐3‐methylbenzyl o‐cresotinate (VI), 2‐phenoxy‐3‐methylbenzyl alcohol (VII), and 2‐phenoxy‐3‐methylbenzyl 2‐phenoxy‐3‐methylbenzoate (VIII), The photochemical oxidation at 75° produced compounds II, III, IV, VII, and VIII. Oxidation of poly(2,6‐dimethyl 1,4‐phenylene oxide) film at 200°C and photochemically at 50°C produced a carbonyl band at ca. 1730 cm−1. The gel content of the photochemically aged film could be significantly reduced and the 1730 cm−1 peak in the thermally aged specimen could be moved to longer wavelength by base treatment. The isolation of compound VIII in both processes with the model compound and the results with the polymer allows us to propose an ester group as a crosslinking unit in thermally and photochemically aged polymer film.

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