ESR Study of the Propagating Radicals of Diene Compounds in Frozen State. Conformations, Spin Densities, and Reactivities of the Propagating Radicals

Abstract

An ESR study on the radical polymerization of diene compounds was carried out in frozen aromatic solvents, using benzoyl peroxide as a photoinitiator. The diene compounds were CH2 =CHX–CH=CHY (X=OCH3 and Y=COOCH2CH3, X=CH3 and Y=COOCH2CH3, X=H and Y=COOCH2CH3, and X=H and Y=OCOCH3). Spectra were observed at −120°C. The hyperfine splitting constants show that the propagating radical ends are allylic radicals whose unpaired electrons were completely delocalized over the three carbons of their chain ends. The conformation and spin density of the radical ends were estimated from the hyperfine splitting constants of methylene protons adjacent to the radicals and α-protons. The conformations could be explained by a single conformation. The reactivity of the propagating radicals was estimated from the propagation rate constants for radical polymerizations and reactivity ratios in the copolymeri-zations with styrene. The relation between the spin density and reactivity of the propagating radicals is discussed.