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

Primary and secondary phosphines are investigated as chain-transfer agents for organolanthanide-mediated olefin polymerization. Ethylene polymerizations were carried out with [Cp‘₂LnH]₂ and Cp‘₂LnCH(SiMe₃)₂ (Cp‘ = η⁵-Me₅C₅; 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 10⁷ g of polymer/(mol of Ln·atm ethylene·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(SiMe₃)₂ protonolysis by the same phosphines and follows the trend H₂PPh ≫ H₂PCy > HPPh₂ > HPEt₂ ≈ HPⁱBu₂ > HPCy₂. Under the conditions investigated, dicyclohexylphosphine is not an efficient chain-transfer agent for Cp‘₂LaPCy₂- and Cp‘₂YPCy₂-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 cyclohexylphosphine-capped oligomers, respectively. For Cp‘₂YPPh₂-mediated ethylene polymerizations, a linear relationship is observed between M_n and [diphenylphosphine]^-1, consistent with a phosphine protonolytic chain-transfer mechanism.