Electron paramagnetic resonance (EPR) examinations of the stable free radicals from the polymerizations of a series of monofunctional, difunctional, 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 polymers were also examined. The aim of the experiments was to attempt to clarify several points of disagreement concerning the identity and assignment of polymer free radicals. The EPR results on the polymethacrylates are in general agreement with past results and interpretations, in which isotropic hyperfine interactions are assigned to the methyl and methylene protons to the central carbon of the propagating polymer radical. The rate of propagation of the observed radicals has been arrested by the high viscosity achieved in the polymer. The CW EPR spectra of the polyacrylates are well-simulated with two different models. In one model, the spectra are assigned to the propagating free radical with isotropic -methylene proton interactions and an anisotropic R-proton coupling. In the other model, it is assumed that the secondary propagating radical abstracts a hydrogen from the polymer chain to form a tertiary radical flanked by two isotropically coupled methylene groups. The latter model gives a slightly better fit to the data. One of the unresolved problems with the EPR spectra of acrylate and methacrylate polymers has been the absence or selective broadening of certain expected hyperfine lines. One hypothesis is a dynamic polymer motion that exchanges methylene proton positions, resulting in an alternating homogeneous line width [Sakai, Y.; Iwasaki, M. J. Polym. Sci. A-1 1969, 7, 1719. The spin-spin relaxation times, T2, measured with pulsed EPR fail to support this hypothesis. We show that a static orientation distribution of the methylene protons with respect to the axis of the orbital of the odd electron [Best, M. E.;Kasai, P. H. Macromolecules 1989, 22, 2622 successfully leads to an alternating heterogeneous line width.

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An electron spin and proton magnetic relaxation study is presented on the effects of the solvent extraction of coal on the macromolecular network of the coal and on the mobile molecular species that are initially within the coal. The eight Argonne Premium coals were extracted at room temperature with a 1:1 (v/v) IV-methylpyrrolidinone (NMP)-CS2 solvent mixture under an inert atmosphere. As much solvent as possible was removed from extract and residue by treatment under vacuum oven conditions (~10-2 Torr at 145-150 °C) until constant weight was achieved. The extraction, without further washing with other solvents, results in substantial uptake of NMP, apparently by H-bonding or acid-base interactions. The NMP uptake tends to be higher, and the NMP tends to be more tightly bound in coal matter with higher heteroatom (N, 0, S) content. The molecular material in the medium rank bituminous coals is more aromatic and heteroatom-poor than the macromolecular material and is mobilized by the extracting solvent. Likewise, the more aromatic and heteroatom-poor molecular free radicals are also extracted. However, mobilization of the molecular free radicals by solvent and the exposure of free radicals in the macromolecular matrix to solvent or species dissolved in the solvent result in preferential reactions of the more aromatic and heteroatompoor free radicals. Greater losses of extract free radicals, being the more aromatic, occur than residue free radicals. As a consequence, the surviving extract radicals exhibit a greater heteroatom content than the original whole coals, as determined from EPR g value changes. The electron paramagnetic resonance (EPR) spin-lattice relaxation (SLR) of these coal free radicals has previously been inferred (Doetschman and Dwyer, Energy Fuels 1992,6,783) to be from the modulation of the intramolecular electron-nuclear dipole interactions of the CH groups in a magnetic field by motions of the radical in the coal matrix. Such a modulation depends on the flexing amplitude and frequency and to a lesser extent upon the electron spin density at the CH groups in the radical. The observed EPR SLR rates decrease with coal rank in agreement with the smaller spin densities and the lower rocking amplitudes that are expected for increasing aromaticity with rank and increasing polycondensation at the highest ranks. The EPR SLR rates are found to be generally faster in the extracts (than residues) where the molecular species would be expected to be smaller and more flexible than in the cross-linked, polymeric, macromolecular matrix of the residue.

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