Electron spin resonance studies of propagating type radicals of vinyl esters and butadiene

Electron spin resonance spectra of short‐lived free radicals formed by addition of HO. to vinyl acetate, vinyl propionate, and 1,3‐butadiene in aqueous solution were studied by means of a rapid flow technique. The observed spectra were assigned to be due to free radicals, CH 2 CHOCOCH 3 , CH 2 CHOCOC 2 H 5 , and CH 2 CH(CHCHS 2 ), respectively. The observed hyperfine coupling constants give the spin density of 0.855, 0.848, and 0.629. Taking previously reported spin densities of propagating type free radicals of acrylic acid, methyl acrylate, and ethylene, as well as the present results into consideration, the order of the spin density is ethylene > methyl acrylate ≈ acrylic acid ≈ vinyl acetate > butadiene, which is in agreement with the order of the reactivity of the propagating radicals in addition polymerization. This is interpreted as an effect of resonance stabilization due to the side‐chain substituent of the vinyl monomers. In addition, there are differences between vinyl ester and acrylic ester radicals which tentatively are interpreted as related to the hindered rotation of the endgroups of the growing chain radicals.