A quantum-chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron-withdrawing groups as well as for a nonactivated monomer containing an electron-donor group. Two examples of sulfonyl and N-halide initiators are also reported. The homolytic inner-sphere electron-transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET-LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron-withdrawing groups in SET-LRP. (c) 2007 Wiley Periodicals, Inc.