STABLE FREE-RADICALS PRODUCED IN ACRYLATE AND METHACRYLATE FREE-RADICAL POLYMERIZATION - COMPARATIVE EPR STUDIES OF STRUCTURE AND THE EFFECTS OF CROSS-LINKING

Electron paramagnetic resonance (EPR) examinations of the stable free radicals from the polymerizations of a series of monofunctional, difun ctional, and trifunctional acrylates and methacrylates have been made. The continuous-wave (CW) EPR of polymers from selectively deuterated monofunctional monomers and the pulsed EPR of the nondeuterated polyme rs were also examined. The aim of the experiments was to attempt to cl arify several points of disagreement concerning the identity and assig nment of polymer free radicals. The EPR results on the polymethacrylat es are in general agreement with past results and interpretations, in which isotropic hyperfine interactions are assigned to the methyl and methylene protons beta to the central carbon of the propagating polyme r radical. The rate of propagation of the observed radicals has been a rrested by the high viscosity achieved in the polymer. The CW EPR spec tra of the polyacrylates are well-simulated with two different models. In one model, the spectra are assigned to the propagating free radica l with isotropic beta-methylene proton interactions and an anisotropic alpha-proton coupling. In the other model, it is assumed that the sec ondary propagating radical abstracts a hydrogen from the polymer chain to form a tertiary radical flanked by two isotropically coupled methy lene groups. The latter model gives a slightly better fit to the data. One of the unresolved problems with the EPR spectra of acrylate and m ethacrylate polymers has been the absence or selective broadening of c ertain expected hyperfine lines. One hypothesis is a dynamic polymer m otion that exchanges methylene proton positions, resulting in an alter nating homogeneous line width [Sakai, Y.; Iwasaki, M. J. Polym. Sci. A -1 1969, 7, 1719]. The spin-spin relaxation times, T-2, measured with pulsed EPR fail to support this hypothesis. We show that a static orie ntation distribution of the methylene protons with respect to the axis of the orbital of the odd electron [Best, M. E.; Kasai, P. H. Macromo lecules 1989, 22, 2622] successfully leads to an alternating heterogen eous line width.