Photolytically generated radicals (at a wavelength of 351 nm) derived from the acetophenone-type photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone) and 2,4,6-trimethylbenzoin (2-hydroxy-1-mesityl-2-phenylethanone, TMB) (specifically the benzoyl and mesitoyl radical) are quantified in their ability to serve as initiating species in methyl methacrylate (MMA), ethyl methacrylate (EMA), and butyl methacrylate (BMA) bulk free radical polymerizations under optimized conditions. Herein, 2,4,6-trimethylbenzoin is employed for the first time as photoinitiator in pulsed laser polymerizations (PLP) employing a high-frequency excimer laser, constituting a new source for mesitoyl radicals. The current work presents an improved method for quantifying radical efficiency of photoinitiation processes using coupled online size exclusion chromatography-electrospray ionization mass spectrometry (SEC/ESI-MS) to analyze the obtained polymers. Because of the occurrence of side reactions during the benzoin-initiated MMA polymerization, reduced laser energies (∼0.35 mJ/pulse) as well as low polymerization temperatures (∼-5 °C) were employed, which avoids side product formation. A plot of the ratio of benzoyl to mesitoyl (derived from 2,4,6-trimethylbenzoin) end groups vs the ratio of both initiators in the reaction mixture indicates that the benzoin-derived benzoyl radical is 3.0 (2.6, 2.4) times more likely to initiate the polymerization process of MMA (EMA, BMA) than the TMB-derived mesitoyl fragment. This observation is in sharp contrast to the case when mesitil is employed as a source of mesitoyl radicals (8.6 times higher likelihood of benzoyl incorporation). These results clearly support the notion that the origin of a radical species significantly determines its propensity to be incorporated at a polymer chain's terminus. The cause of such an origin dependence is tentatively assigned-at least in part-to different triplet lifetimes or intersystem crossing efficiencies (ΦISC) or both of TMB and mesitil.