EPR study of the photoinitiated polymerization of styrene
Using previously described continuous-flow procedures, we have studied, using electron paramagnetic resonance, the photolysis by near-ultraviolet radiation of dilute solutions of three symmetrical aliphatic azo compounds [RN : NR with R- equal to {A figure is presented} in neat styrene at ca. 25°C. For each RN:NR/styrene solution, we could fully interpret the EPR absorption observed in terms of two signals, one from the R· radical and the other from addition radicals of structure R-[-CH2 CHPh-]n CH2-, CHPh, [1], n ≥ 0. Since the spectroscopic properties of [1], n ≥ 0, radicals were independent of the identity of R-, we conclude that, for each RN : NR/styrene solution, the fraction of these addition radicals with n = 0 was probably small. We have carried out spin and relative-radical concentration measurements of R· and [1], n ≥ 0, radicals for each reaction solution and used these concentrations in a kinetic analysis to show that this conclusion is reasonable. Since radicals of type [1], n > 0, are chiral, in general, their β-CH2 protons would be expected to be magnetically inequivalent even if there were rapid rotation about the (α-C)-(β-CH2) bond. However, we found no evidence for magnetic inequivalence: for each case the two α(β-CH2) values for the addition radical component of the EPR absorption appeared equal and of size consistent with there being little hindrance to internal rotation about the (α-C)-(β-CH2) bond and there was no sign of differential line broadening (ΔHpp, 0.6 G); i.e., any magnetic inequivalence must be small. © 1978.
