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The photolysis of copolymers of methyl acrylate and methyl methacrylate, covering the entire composition range, is studied with the copolymers in the form of film and in solution, under vacuum and in an atmosphere of oxygen. In film under vacuum, copolymers with high acrylate contents become insoluble, and cross‐linking and chain scission may be separated by application of the Charlesby‐Pinner equation to sol‐gel separation data. Otherwise, the polymer remains soluble, chain scission predominates and may be measured by the changes in number average molecular weight. Differences in the characteristics of chain scission under the four sets of conditions are accounted for in terms of physical factors, such as glass transition temperatures and “cage” recombination of radicals, and chemically in terms of the alternative reactions available to the primary radicals involved.

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