Transition states (TSs) of radical addition homopolymerization reactions of methyl acrylate, methyl methacrylate, dimethyl itaconate, and N-methyl itaconimide were examined with two-unit radical models using MOPAC (PM3 UHF) semiempirical method. Calculated activation energies (E(alpha)s) show good correlations with experimental values. Calculated activation entropies (-Delta S double dagger s) are found to be well proportional to E(alpha)s. The entropy terms play an important role as well as E(alpha) in radical additions. E(alpha) depends on the angle (theta(rs)) between reaction points of radical and of monomer at TS. The bund length between reaction points at TS is constant regardless of monomers studied. The geometries and thermodynamical properties calculated here fur TS indicate the importance of steric effects caused by substituted group(s) rather than electronic perturbation energies reported previously.