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Organo-fn,d0-Mediated Synthesis of Amine-Capped Polyethylenes. Scope and Mechanism

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journal contribution
posted on 09.06.2008 by Smruti B. Amin, SungYong Seo, Tobin J. Marks
Amines of varying Brønsted acidity and steric encumberance are investigated as chain-transfer agents to functionalize polyolefins via organolanthanide-mediated olefin polymerization processes. Ethylene homopolymerizations are carried out with activated Cp′2LnCH(Si(CH3)3)2 (Cp′ = η5-Me5C5; Ln = La, Sm, Y, Lu) precatalysts in the presence of aniline, n-propylamine, N,N-bis(trimethylsilyl)amine, di-sec-butylamine, N-tert-butyl(trimethylsilyl)amine, di-isopropylamine, and dicyclohexylamine. In the presence of these amines, polymerization activities up to 104 g polymer/(mol of Ln·atm ethylene·h) and narrow product polymer polydispersities are observed, consistent with single-site polymerization processes. Amine chain-transfer efficiency follows the trend C6H5NH2nC3H7NH2 << [Si(CH3)3]2NH ≈ secBu2NH < N-tBu[Si(CH3)3]NH ≈ iPr2NH < Cy2NH to yield polyethylenes of the structure H(CH2CH2)nNRR′, where an efficient chain-transfer agent is defined as a reagent that both terminates polymer chain growth and facilitates reinitiation of polymer chain growth. Under the conditions investigated, primary amines are found to be the most inert toward Cp′2La-mediated polymerizations, affording no detectable insertion products, while di-sec-butylamine and N,N-bis(trimethylsilyl)amine are marginally efficient and produce monoethylene insertion products. In contrast, N-tert-butyl(trimethylsilyl)amine and di-isopropylamine afford amine-capped oligoethylenes, while dicyclohexylamine is the most efficient chain-transfer agent investigated, producing high molecular weight amine-terminated polyethylenes. For these Ln catalysts, dicyclohexylamine chain transfer exhibits a linear relationship between product Mn and [dicyclohexylamine]−1, consistent with a well-behaved aminolysis chain termination pathway. In all of the above systems, protonolysis appears to be the dominant chain-transfer pathway. Organotitanium-mediated ethylene and propylene polymerizations in the presence of secondary amines result in modest polymerization rates with activities of 104 g polymer/(mol of Ti·atm ethylene·h).