Ring-opening Ziegler polamerization of methylenecycloalkanes catalyzed by highly electrophilic d0/f(n) metallocenes. Reactivity, scope, reaction mechanism, and routes to functionalized polyolefins

Li Jia, Xinmin Yang, Affif M. Seyam, Israel D L Albert, Peng Fei Fu, Shengtian Yang, Tobin J Marks

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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)2LuH]2 and [(Me5Cp)2LuH]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, MCP-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 13CH2=13CH2 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(N(t)Bu)]ZrMe+ B(C6F5)4-. The activity of d0/f(n) catalysts in incorporating MCP into the polymer follows the order: [(Me4CpSiMe2(N(t)Bu)]ZrMe+B(C6F5)4->[(Me5Cp)2LuH]2>[(Me5Cp) 2SmH]2>[(Me5Cp)2LaH]2.

Original languageEnglish
Pages (from-to)7900-7913
Number of pages14
JournalJournal of the American Chemical Society
Volume118
Issue number34
DOIs
Publication statusPublished - Aug 28 1996

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ASJC Scopus subject areas

  • Chemistry(all)

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