EPR investigation of radical formation and decay in photopolymerization of difunctional monomers

Abstract

The formation of trapped radicals under UV irradiation during the photopolymerization of butane-1,4-diyl diacrylate in the presence of different amounts of photoinitiator, as well as the radical decay after the end of irradiation, have been studied by EPR spectroscopy. Direct evidence that higher photoinitiator radical concentrations greatly favour radical–radical encounters is obtained. The formation of the same radical species in two different polymeric phases, one essentially rigid and the other more fluid, is at the origin of the two observed superimposed EPR patterns. The ratio R between them decreases with increasing photoinitiator concentration, owing to the formation of shorter and consequently more mobile polymer chains. In contrast, the more rigid phase seems to dominate in photopolymerized diallyl 3-oxapentane-1,5-dicarbonate. ENDOR measurements further support the correlation proposed by us between EPR spectral shape and the rigidity of the polymeric phase.