AbstractThe monomer reactivity ratio in copolymerization reactions has been interpreted by FUKUIand others in terms of the conjugation stabilization energy, ERM, between the monomer and attacking radical. The authors tried to resolve the stabilization energy into four terms by an equation, ERM= E R 0 M 0+ ER+ EM+ E'RM, where E R 0 M 0was the stabilization energy between polyethylene radical and ethylene monomer, ERand EMwere the increments in the stabilization energies of systems of (substituted polyethylene radical — ethylene monomer) and (polyethylene radical — substituted ethylene monomer) with respect to E R 0 M 0as standard. E'RMwas referred to as “the surplus conjugation stabilization energy”. Good correspondence of Q with EM, P with ER, and eRMwith E'RMwas observed which seemed to suggest the