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

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′ 2 LnCH(Si(CH 3 ) 3 ) 2 (Cp′ = η 5 -Me 5 C 5; 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 10 4 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 C 6 H 5 NH 2 ≈ n C 3 H 7 NH 2 << [Si(CH 3 ) 3 ] 2 NH ≈ sec Bu 2 NH < N - t Bu[Si(CH 3 ) 3 ]NH ≈ i Pr 2 NH < Cy 2 NH to yield polyethylenes of the structure H(CH 2 CH 2 ) n NRR′, 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′ 2 La-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 M n 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 10 4 g polymer/(mol of Ti·atm ethylene·h).