title: Probing the reactivity of photoinitiators for free radical polymerization: time-resolved infrared spectroscopic study of benzoyl radicals.
abstract: A series of substituted benzoyl radicals has been generated by laser flash photolysis of alpha-hydroxy ketones, alpha-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 10(5) to 5.5 x 10(5) 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 alpha-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 alpha-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 alpha-cleavage occurs from the triplet excited state that has a lifetime less than or equal to the singlet excited state.
authors.affiliation: School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom.
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Colley
authors.forename: David C
authors.initials: DC
authors.lastname: Grills
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Besley
authors.forename: Steffen
authors.initials: S
authors.lastname: Jockusch
authors.forename: Pavel
authors.initials: P
authors.lastname: Matousek
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Parker
authors.forename: Michael
authors.initials: M
authors.lastname: Towrie
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Turro
authors.forename: Peter M W
authors.initials: PM
authors.lastname: Gill
authors.forename: Michael W
authors.initials: MW
authors.lastname: George