Ring-Opening Ziegler Polymerization of Methylenecycloalkanes Catalyzed by Highly Electrophilic d0/fn Metallocenes. Reactivity, Scope, Reaction Mechanism, and Routes to Functionalized Polyolefins

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Abstract

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-Me2Cp)2ZrMe+MeB(C6F5)3-, and MCP (methylenecyclopropane) affords the polymer [CH2CH2C(CH2)]n under the catalytic action of [(Me5Cp)2LuH]2. Reversible deactivation of the [(Me5Cp)2LuH]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 [(Me5Cp)2SmH]2 and [(Me5Cp)2LaH]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. (Me5Cp)2ZrMe+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 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 {[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: Cp2ZrMe+ > (1,2-Me2Cp)2ZrMe+ ≫ (Me5Cp)2ZrMe+, regardless of the counteranion identity. Labeling experiments with 13CH213CH2 confirm that MCB ring-opening occurs with C2−C3, C2−C5 bond scission. MCP-ethylene copolymerization to yield high molecular weight {[CH2CH2]x[CH2CH2C(CH2)]y} having an exclusively ring-opened microstructure is catalyzed by [(Me5Cp)2LuH]2 and [(Me5Cp)2SmH]2. When [(Me5Cp)2LaH]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 [(Me4CpSiMe2(NtBu)]ZrMe+ B(C6F5)4-. The activity of d0/fn catalysts in incorporating MCP into the polymer follows the order: [(Me4CpSiMe2(NtBu)]ZrMe+B(C6F5)4- > [(Me5Cp)2LuH]2 > [(Me5Cp)2SmH]2 > [(Me5Cp)2LaH]2.

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