POLYMERIZATION PHOTOINITIATED BY CARBONYL-COMPOUNDS .4. ALPHA-DIKETONES AND ALPHA-SUBSTITUTED ALKANONES

In the reported experiments, the photoinitiation efficiency of the free-radical polymerization of methyl methacrylate and styrene by several carbonyl compounds has been determined. The compounds considered were alpha -substituted ketones and alpha -dicarbonyl compounds. For the ketones, the initiation efficiency employing methyl methacrylate depends on the alpha substitution; the values obtained change from less than 10** minus **3 (acetone) to 0. 65 (3-hydroxy-3-methyl-2-butanone). All ketones are more efficient towards methyl methacrylate than styrene. This result can be explained in terms of triplet quenching by the last monomer. The results obtained employing alpha -dicarbonyl compounds do not conform to a simple pattern. In particular, benzil shows a considerably larger efficiency towards styrene than for methyl methacrylate. Since benzil is efficiently quenched by styrene, the initiation must involve the interaction of an excited benzil molecule and the monomer.