A series of zirconium and lanthanide metallocene catalysts are active in the regioselective ring-opening polymerization of strained exo-methylenecycloalkanes to yield exo-methylene-functionalized polyethylenes. MCB (methylenecyclobutane) affords the polymer [CH2CH2CH2C(CH2)](n) under the catalytic action of (1,2-Me(2)Cp)(2)ZrMe(+)-MeB(C6F5)(3)(-), and MCP (methylenecyclopropane) affords the polymer [CH2CH2C(CH)(2)](n) under the catalytic action of [(Me(5)Cp)(2)LuH](2). Reversible deactivation of the [(Me(5)Cp)(2)LuH](2) catalyst is observed in the MCP polymerization reaction and is ascribed to formation of a Lu-allyl species based on D2O quenching experiments. In contrast, the catalysts [Me(5)Cp)(2)SmH](2) and [(Me(5)Cp)(2)LaH](2) yield the dimer 1,2-dimethylene-3-methylcyclopentane (DMP) from MCP with high chemoselectivity. The mechanism of dimerization is proposed to involve the intermediacy of 3-methylene-1,6-heptadiene (MHD) and is supported by the observation that independently synthesized MHD is smoothly converted to DMP under catalytic conditions. (Me(5)Cp)(2)ZrMe(+)MeB(C6F5)(3)(-) catalyzes the polymerization of MCP to a polyspirane consisting of 1,3-interlocked five-membered rings (poly(1,4:2,2:butanetetrayl), (C4H6)(n)). From end group analysis, the reaction pathway is proposed to consist of beta-alkyl shift-based ring-opening followed by an intramolecular insertive, ring-closing "zipping-up" process. AM1-level computations indicate that the zipping-up reaction is exothermic by similar to 16 kcal/(mol of ring closure). Under the same catalytic conditions, the monomers methylenecyclopentane, methylenecyclohexane, and 2-methylenenorbornane undergo double bond migration (to the adjacent internal position) rather than polymerization. In contrast to the relatively restrictive requirements for homopolymerization, MCB-ethylene copolymerization is catalyzed by a wide variety of zirconocenium catalysts, including those generated conveniently From MAO, to afford high molecular weight {[CH2CH2](x)[CH2CH2CH2C-(CH2)](y)}(n) copolymers with the incorporated MCB having an exclusively ring-opened microstructure. The activity of the catalysts in incorporating MCB into the polymer chain follows the order : Cp(2)ZrMe(+) > (1,2-Me(2)Cp)(2)ZrMe(+) much greater than (Me(5)Cp)(2)ZrMe(+), regardless of the counteranion identity. Labeling experiments with (CH2)-C-13=(CH2)-C-13 confirm that MCB ring-opening occurs with C2-C3, C2-C5 bond scission. MCP-ethyIene copolymerization to yield high molecular weight {[CH2CH2](x)[CH2CH2C(CH2)](y)} having an exclusively ring-opened microstructure is catalyzed by [(Me(5)Cp)(2)LuH](2) and [(Me(5)Cp)(2)SmH](2). When [(Me(5)Cp)(2)LaH](2) is used as the catalyst, more than 50% of the MCP is located at the chain ends in a dienyl structure. The only zirconium polymerization catalyst which incorporates MCP in the ring-opened form in a moderate percentage is [(Me(4)CpSiMe(2)(N(t)Bu)]ZrMe(+) B(C6F5)(4)(-). The activity of d(0)/f(0) catalysts in incorporating MCP into the polymer follows the order : [(Me(4)CpSiMe(2)(N(t)Bu)]ZrMe(+)B(C6F5)(4)(-) >[(Me(5)Cp)(2)LuH](2) > [(Me(5)Cp)(2)SmH](2) > [(Me(5)Cp)(2)LaH](2).R&A