The radical species produced during the photopolymerization and photocrosslinking of multifunctional acrylic, methacrylic and allylic monomers were studied by Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) spectroscopies. From the analysis of the EPR spectra, recorded both under irradiation and during post-irradiation heat treatments, a unifying model of interpretation was proposed, by taking into account the line-broadening due to dynamic exchange between two conformations of the β-methylene groups and the electron spin-spin interaction between radicals surrounded by rigid polymeric environments. All investigated radicals gave matrix ENDOR spectra, due to nuclear spins (protons) interacting with the unpaired electron by hyperfine coupling. A signal, attributed to the freely rotating methyl group, was also present in the ENDOR spectra of methacrylate systems. Radical decay was interpreted according to a dispersive kinetic model, in terms of a rate parameter and a dispersion parameter, both reflecting the properties of the photopolymerized networks, in relation to the monomer structure and to photopolymerization conditions. Trapped radicals could thus be envisaged as probes for the characterization of photopolymers.
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