Reactivities of monomers towards the phenyl radical; a scale of phenyl affinities

Abstract

Relative rate constants are reported for the addition of phenyl radicals, generated from phenylazotriphenylmethane (PAT), to a series of vinyl monomers. The technique involves the study of mixtures of monomers with heptanethiol-t. The phenyl radicals partition between addition to the monomer and reaction with the thiol to form tritiated benzene. We have measured the amount of benzene formed for the standard conditions 4.16 × 10^–3 M PAT and 1.20 × 10³ sec as a function of the [monomer]/[thiol] mole ratio. These data allow calculation of relative values of the rate constant k_a for addition of phenyl radicals to the olefins. We have also measured the kinetic isotope effect, k_H/k_T, for the reaction of phenyl radicals with the thiol. Our data show that the phenyl radical is 0.6 times as selective as is the methyl radical and is only 0.4 times as selective as is the polystyryl radical in additions to these olefins. We discuss this in relation to the bond strengths of the R—H bonds for these R radicals. The reactivity pattern phenyl > methyl > polystyryl also agrees with the pattern of kinetic isotope effects for the reaction of these radicals with tritiated thiols.