Radical copolymerization of N-phenylmaleimide (PhMI) is carried out with various diene monomers including naturally occurring compounds and the copolymers are efficiently produced by the suppression of Diels-Alder reaction as the competitive side reaction. Diene monomers with an exomethylene moiety and a fixed s-trans diene structure, such as 3-methylenecyclopentene and 4-isopropyl-1-methyl-3-methylenecyclohexene, exhibit high copolymerization reactivity to produce a high-molecular-weight copolymer in a high yield. The copolymerization of sterically hindered noncyclic diene monomers, such as 2,4-dimethyl-1,3-pentadiene and 2,4-hexadiene, also results in the formation of a high-molecular-weight copolymer in a moderate yield. The NMR spectroscopy reveals that the obtained copolymers consist of predominant 1,4-repeating structures for the corresponding diene unit. The copolymers have excellent thermal stability, that is, an onset temperature of decomposition over 330 degrees C and a glass transition temperature over 130 degrees C. The copolymerization reactivity of these diene monomers is discussed based on the results of the DFT calculations. The efficient copolymer formation in competition with Diels-Alder addition is investigated under various conditions of the temperature, solvents, and initiators used for the copolymerization. (C) 2016 Wiley Periodicals, Inc.