Electron spin resonance and laser flash photolysis study of radical addition to vinyl acrylate and related alkenes

Addition of two radicals (diphenyl phosphinoyl and 2-hydroxy-2-propyl) to the bifunctional alkene, vinyl acrylate, was studied by both time-resolved (TR) and steady-state (SS) ESR and laser flash photolysis (LFP). The adduct radicals are predominately a result of tail addition (addition to the unsubstituted carbon atom) of the acrylate double bond. Chemical structures of the adducts were established by comparison of the observed ESR spectra with those of adducts of the same reactive radicals to structurally related alkenes, tert-butyl acrylate and vinyl pivalate, which have only one type of double bond. Adducts of bulky phosphinoyl radicals to the acrylates demonstrate hindered rotation and a cis-trans isomerization at room temperature. The structure of the adduct radicals and the reactivity of the two radicals are discussed. Absolute rate constants for the addition of the phosphinoyl radical to the alkenes were measured by LFP in ethyl acetate at 296 K. A rate constant of k(add) = (33 +/- 1) x 10(6) M(-1)s(-1) was found for vinyl acrylate. The latter value is similar to1.5 times higher than that for tert-butyl acrylate (k(add) = (22 +/- 1) x 10(6) M(-1)s(-1)) and similar to17 times higher than that for vinyl pivalate (k(add) = (2.0 +/- 0.1) x 10(6) M(-1)s(-1)). These rate constants are consistent with conclusions derived from the ESR data. The results provide some insights into free radical polymerization of vinyl acrylate and vinyl ethers.