EPR/ENDOR characterization of radicals produced in the photopolymerization of a dimethacrylate monomer

The formation of trapped radicals under UV irradiation of ethylene glycol dimethacrylate in the presence of different amounts of photoinitiator, as well as radical decay at different temperatures (90–150 °C) after the end of irradiation, have been studied by electron paramagnetic resonance (EPR) spectroscopy. A fitting procedure of the EPR signals revealed that the nine-line EPR spectrum can be attributed to the propagating radical, undergoing a fast exchange process between two conformations. This radical also gives an ENDOR spectrum even at room temperature, thus suggesting that the polymer structure is essentially solid. At least one different radical species is present in highly photo-cross-linked samples. At temperatures above 100 °C, the hyperfine structure of the nine-line EPR spectrum is partly washed out, owing to a spin–spin exchange phenomenon between radicals. This becomes more evident with increasing degree of polymerization and cross-linking during radical decay kinetic runs and also affects the rate of exchange between the two radical conformations.