- Probing the reactivity of photoinitiators for free radical polymerization: Time-resolved infrared spectroscopic study of benzoyl radicals
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A series of substituted benzoyl radicals has been generated by laser flash photolysis of α-hydroxy ketones, α-amino ketones, and acyl and bis(acyl)phosphine oxides, all of which are used commercially as photoinitiators in free radical polymerizations. The benzoyl radicals have been studied by fast time-resolved infrared spectroscopy. The absolute rate constants for their reaction with n-butylacrylate, thiophenol, bromotrichloromethane and oxygen were measured in acetonitrile solution. The rate constants of benzoyl radical addition to n-butylacrylate range from 1.3 x 105 to 5.5 x 105 M-1 s-1 and are about 2 orders of magnitude lower than for the n-butylacrylate addition to the counterradicals that are produced by α-cleavage of the investigated ketones. Density functional theoretical calculations have been performed in order to rationalize the observed reactivities of the initiating radicals. Calculations of the phosphorus-centered radicals generated by photolysis of an acyl and bis(acyl)phosphine oxide suggest that P atom Mulliken spin populations are an indicator of the relative reactivities of the phosphorus-centered radicals. The α-cleavage of (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 occurs from the triplet excited state that has a lifetime less than or equal to the singlet excited state.
- Colley, Christopher S.,Grills, David C.,Besley, Nicholas A.,Jockusch, Steffen,Matousek, Pavel,Parker, Anthony W.,Towrie, Michael,Turro, Nicholas J.,Gill, Peter M. W.,George, Michael W.
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p. 14952 - 14958
(2007/10/03)
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- EPR Studies on the Photofragmentation of 2,2-Dialkyl-2-alkylaminoacetophenones
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Free radical reactions induced by photolysis of 2,2-dialkyl-2-aminoacetophenone photocuring agents have been studied by continuous wave (CW) and time-resolved EPR spectroscopy.In all solvents α-cleavage from the triplet state is the major process.In hydrogen donor solvents it is accompanied by photoreduction followed by a rapid amine elimination from the ketyl radical intermediate.The benzoyl and α-aminoalkyl radicals resulting from α-cleavage readily add to acrylonitrile whereas radicals formed by photoreduction do not.
- Leopold, Detlev,Fischer, Hanns
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p. 513 - 518
(2007/10/02)
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