Organolanthanide-Catalyzed Synthesis of Phosphide-Terminated Polyethylenes.Scope and Mechanism

摘要

Primary and secondary phosphines are investigated as chain-transfer agents for organolan-thanide-mediated olefin polymerization.Ethylene polymerizations were carried out with [Cp'_2LnH]_2 and Cp'_2-LnCH(SiMe_3)_2 (Cp' = eta~5-Me_5C_5;Ln = La,Sm,Y,Lu) precatalysts in the presence of dicyclohexyl-,diisobutyl-,diethyl-,diphenyl-,cyclohexyl-,and phenylphosphine.In the presence of secondary phosphines,high polymerization activities (up to 10~7 g of polymer/(mol of Ln centre dot atm ethylene centre dot h)) and narrow product polymer polydispersities are observed.For lanthanocene-mediated ethylene polymerizations,the phosphine chain-transfer efficiency correlates with the rate of Ln-CH(SiMe_3)_2 protonolysis by the same phosphines and follows the trend H_2PPh " H_2PCy > HPPh_2 > HPEt_2 approx= HP~iBu_2 > HPCy_2.Under the conditions investigated,dicyclohexylphosphine is not an efficient chain-transfer agent for Cp'_2LaPCy_2- and Cp'_2YPCy_2-mediated ethylene polymerizations.Diisobutylphosphine and diethylphosphine are efficient chain-transfer agents for Cp'_2La-mediated polymerizations;however,phosphine chain transfer does not appear to be competitive with other chain-transfer pathways in Cp'_2Y-mediated polymerizations involving diisobutylphosphine.Regardless of the lanthanide metal,diphenylphosphine is an efficient chain-transfer agent for ethylene polymerization.Polymerizations conducted in the presence of primary phosphines produce only low-molecular-weight products.Thus,Cp'_2Y-mediated ethylene polymerizations conducted in the presence of phenylphosphine and cyclohexylphosphine produce low-molecular-weight phenylphosphine- and cyclo-hexylphosphine-capped oligomers,respectively.For Cp'_2YPPh_2-mediated ethylene polymerizations,a linear relationship is observed between M_n and [diphenylphosphine]~(-1),consistent with a phosphine protonolytic chain-transfer mechanism.