Probing the reactivity of photoinitiators for free radical polymerization: Time-resolved infrared spectroscopic study of benzoyl radicals

Christopher S. Colley, David C. Grills, Nicholas A. Besley, Steffen Jockusch, Pavel Matousek, Anthony W. Parker, Michael Towrie, Nicholas J. Turro, Peter M.W. Gill, Michael W. George

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104 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)14952-14958
Number of pages7
JournalJournal of the American Chemical Society
Volume124
Issue number50
DOIs
Publication statusPublished - Dec 18 2002

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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    Colley, C. S., Grills, D. C., Besley, N. A., Jockusch, S., Matousek, P., Parker, A. W., Towrie, M., Turro, N. J., Gill, P. M. W., & George, M. W. (2002). Probing the reactivity of photoinitiators for free radical polymerization: Time-resolved infrared spectroscopic study of benzoyl radicals. Journal of the American Chemical Society, 124(50), 14952-14958. https://doi.org/10.1021/ja026099m