• free-radical polymerization of methyl methacrylate;
  • multiphotoinitiated pulsed laser polymerization;
  • pulsed laser polymerization–online size-exclusion chromatography–electrospray ionization–mass spectrometry;
  • quantifying initiator efficiencies;
  • substituted benzoins


Online size exclusion chromatography–electrospray ionization–mass spectrometry (SEC/ESI–MS) is employed for quantifying the overall initiation efficiencies of photolytically generated radical fragments. In a unique experiment, we present the first quantitative and systematic study of methyl-substituted acetophenone-type photoinitiators being employed in a single cocktail to initiate the free-radical polymerization of methyl methacrylate (MMA) in bulk. The photoinitiators are constituted of a set of two known and four new molecules, which represent an increasing number of methyl substituents on their benzoyl fragment, that is, benzoin, 4-methylbenzoin, 2,4-dimethylbenzoin, 2,4,6-trimethylbenzoin, 2,3,5,6-tetramethylbenzoin, and 2,3,4,5,6-pentamethylbenzoin. The absolute quantitative evaluation of the mass spectra shows a clear difference in the initiation ability of the differently substituted benzoyl-type radical fragments: Increasing the number of methyl substituents leads to a decrease in incorporation of the radical fragments.