Ring-opening radical polymerization of 1,1-bis(ethoxycarbonyl)-2-vinylcyclopropane (BEVCP) with. dimethyl 2,2'-azobisisobutyrate (MAIB) was investigated kinetically in benzene. The overall activation energy (E-a = 94 +/- 4 kJ/mol) was calculated from the polymerization rate (R-p) obtained at 30-60 degreesC. R-p (40 degreesC) is given by R-p = k[MAIB](0.6+/-0.1) [BEVCP](1.0+/-0.1), which is similar to that of conventional radical polymerizations involving bimolecular termination. The present polymerization system involves ESR-observable polymer radicals under the actual polymerization conditions. The total polymer radical concentration ([P-.]) was determined at different temperatures by ESR. Using R-p, [P-.], and the initiation rate obtained separately, the apparent rate constants of propagation (k(p)) and termination (k(t)) were estimated at different temperatures. The k(p) (71 L/mol . s, 60 degreesC) and k(t) (5.8 x 10(4) L/mol. s, 60 degreesC) values are much lower than those of typical vinyl monomers such as methyl methacrylate (MMA) and styrene. Owing to the markedly low k(t), BEVCP is more homopolymerizable than MMA. Analysis of the observed ESR spectrum suggests that the present polymerization system involves ring-opened type and cyclobutyl-type polymer radicals. This corresponds to the fact that the resulting poly(BEVCP) contains 1,5-adduct and cyclobutane structural units.