High-molecular-weight diene copolymers with a regiospecific repeating structure were produced in a high yield during the alternating radical copolymerization of N-substituted maleimides (RMIs) and 3-methylenecyclopentene (MCP) as the cyclic 1,3-diene monomer including a reactive exomethylene reaction of isoprene with the RMIs rather than copolymeriz contrast to the predominant occurrence of the Diels Alder tion. The highly alternating structure of the copolymers was confirmed based on the monomer reactivity ratios for the copolymerization of MCP (M-1) and N-phenylmaleimide (PhMI, M-2), r(1) = 0.010 and r(2) = 0.0080. A mechanism for the highly controlled 1,4-regiospecific propagation, which consists of the addition of an RMI radical to the exomethylene group of MCP and subsequent 1,4-regiospecific propagation, was supported by the DFT. calculations using model reactions as well as the precise structure determination of oligomers produced during telomerization in the presence of 1-butanethiol as a chain transfer agent. The resulting copolymers exhibited no weight-loss under 340 degrees C during heating in a nitrogen atmosphere and their glass transition temperature was over the wide temperature range of 66-159 degrees C, depending on the structure of the N-alkyl substituents. The transparent and flexible films were fabricated by a casting method. The optical properties of the films were as follows: the visible light transmittance over 95% at 380 rim, the refractive indices of 1.54-1.58, and the Abbe number of 42-45.