alpha-Methylenebenzocyclobutene (MB) was prepared by Wittig reaction of 1-(triphenylphosphonio)benzocyclobutene bromide with paraformaldehyde in 70% yield. Radical polymerization of MB afforded poly(alpha-methylenebenzocyclobutene) (PMB) in 66% yield (($) over bar M(n) = 20 000). MB, which can be regarded as a cyclic analog of a-methylstyrene, had a higher polymerizability than cx-methylstyrene. The higher polymerizability of MB was confirmed by semiempirical molecular orbital calculation by comparison with that of alpha-methyleneindane. Radical copolymerizations of MB with styrene (St), methyl methacrylate (MMA), and maleic anhydride (MANH) gave corresponding copolymers quantitatively. Especially, the copolymerization with MANH afforded the corresponding alternating copolymer. and and e values of MB were determined as 1.35 and -1.00, respectively, from the results of the copolymerizations with MMA. Thermal properties of the obtained PMB were examined, and the thermal decomposition of PMB was discussed.