A new cyclobutene monomer, methyl 3-methyleyelobutene-1-carboxylate (MMCB), has been synthesized and polymerized by radical and anionic initiators. The synthesis started from a [2 + 2] cycloaddition of N-(l-propenyl)piperidine to methyl acrylate, followed by methylation and treatment with base to yield the monomer. Free radical polymerization of MMCB led to low yields of low molecular weight polymers, probably due to chain transfer at the allylic hydrogen. However, p-methoxystyrene, styrene and methyl methacrylate gave high molecular weight copolymers with MMCB in high yields. Anionic homopolymerization with tert-butyllithium (t-BuLi), t-BuLi/bis(2,6-di-tert-butylphenoxy)ethylaluminum, lithium bis(trimethylsilyl)amide, and potassium bis(trimethylsilyl)amide in toluene at 0 and -78 degreesC proceeded smoothly and gave polymers in high yields. NMR and IR analyses of the polymers suggested that the polymerization proceeds via addition mechanism without ring-opening. Thermal properties of the polymers are also described briefly. In contrast to MMCB, methyl 3,3-dimethylcyclobutene-1-carboxylate MDCB did not polymerize due to excessive steric hindrance.