Monomers 3-methylcyclobutene (1) and 3,3-dimethylcyclobutene (2) can be polymerized by well-defined alkylidene complexes of the type M(CHC(CH3)(3))(NAr)(OC(CH3)(n)(CF3)(3-n))(2) (where Ar = 2,6-diisopropylphenyl; M = Mo or W; n = 2 or 3) to give polymers, which upon hydrogenation are equivalent to alternating copolymers of poly(ethylene-alt-propylene) and poly(ethylene-alt-isobutylene), respectively. Monomer 2 can be polymerized in a highly regioselective fashion to give polymers with exclusively trans and head-to-tail configuration. The high regioselectivity of the polymerization is attributed to the steric directing effect of the bulky substituent on the 3-position of the cyclobutene as the monomer approaches the propagating alkylidenes. In the presence of PPhMe(2) narrowly dispersed poly(3-methylcyclobutene) can be obtained.