The tendencies of ring-opening processes in radical ring-opening polymerizations were evaluated by AM1 and PM3 semi-empirical calculations and 6-31G*-level calculations based on the density functional theory (DFT) B3LYP models. Sixteen cyclic monomers bearing vinyl or exomethylene groups were categorized into ring-opening and no-ring-opening monomers by the evaluation of the differences of the internal energies and the lengths of the cleaving bonds between the ground states of the initial radicals and the activated states in the ring-opening processes. Although the semi-empirical calculations not parameterized to radical reactions resulted in the moderate categorization of the ring-opening monomers, the DFT calculation clearly distinguished the ring-opening and no-ring-opening monomers. The ring-opening tendencies were also evaluated with the changes in the internal energies throughout the ring-opening processes, but this method could not group the ring-opening and no-ring-opening monomers clearly. (c) 2007 Wiley Periodicals, Inc.