Organo-fn,d0-mediated synthesis of amine-capped polyethylenes. scope and mechanism

Amines of varying Bronsted 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(CH3)(3))(2) (Cp' = eta(5)-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 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 C6H5NH2 approximate to (C3H7NH2)-C-n < < [Si(CH3)(3)](2)NH approximate to (Bu2NH)-Bu-sec < N-Bu-t[Si(CH3)(3)]NH approximate to (Pr2NH)-Pr-i < Cy2NH to yield polyethylenes of the structure H(CH2CH2)(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'La-2-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 [dicyclohexyl-amine](-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).