Electron Paramagnetic Resonance at 4.2° K. of λ-Irradiated Polymethyl Methacrylate and Polymethacrylic Acid

Abstract

THE electron paramagnetic resonance spectrum given by polymethyl methacrylate after high-energy irradiation at room temperature consists of five lines, about 23 gauss apart, with intensities in approximately the ratios 1 : 4 : 6 : 4 : 1, with a weaker intermediate pattern of four lines, both closely centred on the electron spin g factor of 2.00^1,2. Since this spectrum is also given by free radicals trapped during the addition polymerization of methyl methacrylate^3–5, it has been postulated that it arises from trapped propagating radicals of structure, —CH₂— C(CH₃)CO₂CH₃, the detailed explanations involving the exact conformation of these radicals^2,4,6. Both of the explanations put forward by Symons^4,6 require that the methyl group bonded to the ‘radical’ carbon atom rotates rapidly, so that its three protons interact equally with the unpaired electron. If one of these explanations is correct, the hyperfine structure of the spectrum should alter when the temperature is lowered sufficiently. The rotation of the methyl group might change to a torsional oscillation, thus destroying the equivalence of the three methyl protons. If however the rotation remains free, at a temperature where only the lowest rotational level is populated, symmetry conditions will govern the occupation of the nuclear spin levels.