AbstractRadical copolymerization of cyclopentene with maleic anhydride was investigated under high pressure up to 6000 kg/cm2at 60°C.The rate of copolymerization increased markedly with pressure and the conversion at 6000 kg/cm2attained a maximum constant value in a few minutes. This behavior indicated very large absolute values for the activation volume compared with other vinyl polymerization systems.The copolymer composition was affected by pressure, for a given monomer feed the content of cyclopentene in the copolymer increased with pressure. This is an explanation for the penultimate effect due to cyclopentene. High pressure is presumed to be effective for eliminating the penultimate effect in copolymerization