| Abstract
| - A series of substituted benzoyl radicals has been generated by laser flash photolysis of α-hydroxyketones, α-amino ketones, and acyl and bis(acyl)phosphine oxides, all of which are used commercially asphotoinitiators in free radical polymerizations. The benzoyl radicals have been studied by fast time-resolvedinfrared spectroscopy. The absolute rate constants for their reaction with n-butylacrylate, thiophenol,bromotrichloromethane and oxygen were measured in acetonitrile solution. The rate constants of benzoylradical addition to n-butylacrylate range from 1.3 × 105 to 5.5 × 105 M-1 s-1 and are about 2 orders ofmagnitude lower than for the n-butylacrylate addition to the counterradicals that are produced by α-cleavageof the investigated ketones. Density functional theoretical calculations have been performed in order torationalize the observed reactivities of the initiating radicals. Calculations of the phosphorus-centered radicalsgenerated by photolysis of an acyl and bis(acyl)phosphine oxide suggest that P atom Mulliken spinpopulations are an indicator of the relative reactivities of the phosphorus-centered radicals. The α-cleavageof (2,4,6-trimethylbenzoyl)phosphine oxide was studied by picosecond pump−probe and nanosecond step-scan time-resolved infrared spectroscopy. The results support a mechanism in which the α-cleavage occursfrom the triplet excited state that has a lifetime less than or equal to the singlet excited state.
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