N. Allen, T. Corrales, M. Edge
1998
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0
Influential Citations
11
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Journal
European Polymer Journal
Abstract
The photoinduced polymerisation activities of five novel alkylthio substituted benzophenones have been determined and compared to that of the commercial 4-methylthio derivative and benzophenone itself in different monomers and prepolymers using RTIR and pencil hardness methods. Absorption, fluorescence and phosphorescence analysis, as well as photoreduction/photolysis studies have been undertaken on the compounds and the data are interrelated to their photopolymerisation activities. Fluorescence and phosphorescence analysis indicates a high rate of intersystem crossing to the triplet state. The latter is essentially a mixed excited nπ/ππ * state in nature, as indicated by the phosphorescence lifetime data and quantum yields of emission. Compared with benzophenone all the alkylthio derivatives exhibit higher activity using commercial pencil hardness curing. This is also reflected in the RTIR data apart from the sulphoxide derivative, which displays a lower activity than benzophenone. The differences in activity are accounted for by differences in oxygen quenching effects during the two curing studies. The enhanced photoactivities of the thio derivatives are associated with their higher extinction coefficients and longer wavelength absorptions in the near UV region above 300 nm. Microsecond flash photolysis showed weak ketyl radical formation in the region 500-600 nm due to hydrogen atom abstraction by the excited triplet state of the benzophenone chromophore. Significant transient absorption in the region 300-400 nm is suggestive of side chain scission of the phenyl-S bond while the sulphoxide group is also prone to scission of the alkyl-S bond.