| Abstract: | The electron paramagnetic resonance (EPR) spectrum of methyl methacrylate polymerizing in a range of systems is simulated using a consistent set of parameters for two rotamers of a single free radical. The 9-line spectrum observed in bulk and emulsion polymerization was fitted by a superposition of two spectra corresponding to two rotamers of a long-chain species (each a somewhat hindered, non-rotating macroradical), having anisotropic methylene proton hyperfine couplings which can be treated as isotropic to a good approximation. The 13-line spectrum observed at low conversion with very high radical flux was simulated as a superposition of the spectra of two very similar rotamers, the EPR spectra of which are indistinguishable; either rotamer is part of a very short chain (primarily an initiator fragment which has propagated once) undergoing free rotation in a low-viscosity medium. Thus the EPR spectra can be explained without having to invoke earlier suggestions that there are two types of free radicals (“trapped” and “untrapped”) in these systems, corresponding to the 9-line and 13-line spectra. The existence of an enormous proportion of very short free radicals under conditions of high radical flux is supported by quantitative calculations of the radical chain-length distribution. It is suggested that the two rotamers have significantly different propagation rate coefficients because of differences in the hindered rotations in their transition states. |
