ELECTRON-PARAMAGNETIC-RESONANCE INVESTIGATION OF THE NATURE OF THE PROPAGATING SPECIES IN METHYL-METHACRYLATE POLYMERIZATION

The electron paramagnetic resonance (EPR) spectrum of methyl methacryl ate polymerizing in a range of systems is simulated using a consistent set of parameters for two rotamers of a single free radical. The 9-li ne spectrum observed in bulk and emulsion polymerizations was fitted b y a superposition of two spectra corresponding to two rotamers of a lo ng-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 ob served at low conversion with very high radical flux was simulated as a superposition of the spectra of two very similar rotamers, the EPR s pectra of which are indistinguishable; either rotamer is part of a ver y short chain (primarily an initiator fragment which has propagated on ce) undergoing free rotation in a low-viscosity medium. Thus the EPR s pectra can be explained without having to invoke earlier suggestions t hat 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 un der conditions of high radical flux is supported by quantitative calcu lations of the radical chain-length distribution. It is suggested that the two rotamers have significantly different propagation rate coeffi cients because of differences in the hindered rotations in their trans ition states.