The addition of two carbon centered radicals R (benzoyl and isopropylketyl) as initiating radicals to acrylonitrile, methyl acrylate, methyl vinyl ether, and vinyl acetate as monomers M was examined in detail by using molecular orbital calculations. The influence of the radical and monomer structures on the reactivity of the various R/M systems were studied. It was found that both polar and enthalpy effects account for the activation energies for this reaction. The quantitative evaluation of these effects allows to predict quite well the change of the experimental addition rate constants.