Reactivity of carbon-centered radicals toward acrylate double bonds:: Relative contribution of polar vs enthalpy effects

The different factors controlling the reactivity of a large series of carbon-centered radicals toward the methyl acrylate monomer unit were examined in detail by using molecular orbital calculations. In agreement with the state correlation diagram, the energy barrier is governed for a large part by the enthalpy term, as supported by an increase of reactivity with increasing exothermicity of the reaction. However, important polar effects, as evidenced by molecular calculations on the transition states, were also highlighted: they dramatically enhance the reactivity of the nucleophilic radicals (aminoalkyl or dialkylketyl radicals) as well as the electrophilic radicals (malonyl radical). Their contribution to the decrease of the barrier was evaluated by using a model based on chemical descriptors. This allows a clear separation of the relative role of the polar and enthalpy effects for 22 radicals.