Photoinitiated polymerization of methyl methacrylate and methyl acrylate with 14C‐labeled benzoin methyl ethers

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Abstract
Three 14C‐labeled benzoin methyl ether (α‐methoxy‐α‐phenylacetophenone) derivatives were utilized as photoinitiators in the polymerization of methyl methacrylate (MMA) and methyl acrylate (MA). The results of polymer end‐group analysis are in accord with a mechanism of benzoin ether photocleavage into initiator radicals and dispute earlier labeling studies which were interpreted as evidence for copolymerization of excited‐state benzoin ethers with reactive monomers. In MMA polymerization, the results indicate a preference for termination by disproportionation (∼60%) and provide evidence for primary radical termination at 0.041M photoinitiator (optically dense solutions) in neat MMA. Evidence for chain branching by initiator radical hydrogen abstraction from poly(methyl acrylate) (PMA) is also presented. The benzoyl and α‐methoxybenzyl radicals, produced on photolysis of benzoin methyl ether, appear to be equally effective in both initiation and hydrogen‐abstraction processes. Quantum yields at 366 and 313 nm indicate the absence of a wavelength effect.