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

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, C -13 and H-1 NMR spectroscopy. The fluorescence spectra from the PE and MD s ystems demonstrate that the main photoreduction product of BP (PPB) is benz pinacol formed by the recombination of two diphenylhydroxymethyl (K-.) radi cal 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-hydroxymethylen e-4-diphenyl-hydroxymethyl-2,5-cyclohexa-diene and three kinds of alpha-alk ylbenzhydrols have been detected and identified for the first time by C-13 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 abst raction of the excited triplet state (3)(BP)* from polyethylene or its mode l compounds. These results provide new experimental evidence for elucidatin g the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. (C) 2000 John Wiley & Sons, Inc.