A new mechanism of benzophenone photoreduction in photoinitiated crosslinking of polyethylene and its model compounds

Abstract

The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, 13C and 1H NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K•) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K• radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1-phenyl-hydroxymethylene-4-diphenyl-hydroxymethyl-2,5-cyclohexa-diene and three kinds of α-alkylbenzhydrols have been detected and identified for the first time by 13C and 1H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K• radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state 3(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 999–1005, 2000