Ring-Opening Ziegler Polymerization of Methylenecycloalkanes Catalyzed by Highly Electrophilic d 0 /f n Metallocenes. Reactivity, Scope, Reaction Mechanism, and Routes to Functionalized Polyolefins

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 [CH 2 CH 2 CH 2 C(CH 2 )] n under the catalytic action of (1,2-Me 2 Cp) 2 ZrMe + MeB(C 6 F 5 ) 3 -, and MCP (methylenecyclopropane) affords the polymer [CH 2 CH 2 C(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 D 2 O 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(C 6 F 5 ) 3 - catalyzes the polymerization of MCP to a polyspirane consisting of 1,3-interlocked five-membered rings (poly(1,4:2,2-butanetetrayl), (C 4 H 6 ) n ). From end group analysis, the reaction pathway is proposed to consist of β-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 ∼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 {[CH 2 CH 2 ] x [CH 2 CH 2 CH 2 C(CH 2 )] 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 + ≫ (Me 5 Cp) 2 ZrMe +, regardless of the counteranion identity. Labeling experiments with 13 CH 2 13 CH 2 confirm that MCB ring-opening occurs with C2−C3, C2−C5 bond scission. MCP-ethylene copolymerization to yield high molecular weight {[CH 2 CH 2 ] x [CH 2 CH 2 C(CH 2 )] 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(C 6 F 5 ) 4 - . The activity of d 0 /f n catalysts in incorporating MCP into the polymer follows the order: [(Me 4 CpSiMe 2 (N t Bu)]ZrMe + B(C 6 F 5 ) 4 - > [(Me 5 Cp) 2 LuH] 2 > [(Me 5 Cp) 2 SmH] 2 > [(Me 5 Cp) 2 LaH] 2 .