Detection of Free Radicals in Polyacrylonitrile by Paramagnetic Resonance

Abstract

EVIDENCE has been presented¹ that, during the polymerization of vinyl monomers under conditions such that precipitation of the polymer occurs, the propagating free radicals become occluded by the polymer with a resulting decrease in their accessibility for reaction. The effective rate-coefficients for the free radical reactions, particularly that of chain termination, are thus reduced, giving rise to the characteristic kinetic features observed in heterogeneous vinyl polymerizations. In the case of acrylonitrile, extreme degrees of occlusion may be reached, leading to the presence in the polymer of trapped radicals for which the propagation and termination coefficients are practically zero. The concentration of these radicals has been estimated by reaction with αα-diphenyl β-picryl hydrazyl². We now wish to report the detection and estimation of trapped radicals by observation of the paramagnetic resonance absorption spectrum of the polymer. This method has already been employed to detect radicals in activated carbon and other carbonaceous solids^3,4, and also in gel systems prepared from cross-linked vinyl polymers⁵.