Organolanthanide-Catalyzed Synthesis of Phosphine-Terminated Polyethylenes. Scope and Mechanism
2005-05-04T00:00:00Z (GMT) by
Primary and secondary phosphines are investigated as chain-transfer agents for organolanthanide-mediated olefin polymerization. Ethylene polymerizations were carried out with [Cp‘<sub>2</sub>LnH]<sub>2</sub> and Cp‘<sub>2</sub>LnCH(SiMe<sub>3</sub>)<sub>2</sub> (Cp‘ = η<sup>5</sup>-Me<sub>5</sub>C<sub>5</sub>; 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<sup>7</sup> 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(SiMe<sub>3</sub>)<sub>2</sub> protonolysis by the same phosphines and follows the trend H<sub>2</sub>PPh ≫ H<sub>2</sub>PCy > HPPh<sub>2</sub> > HPEt<sub>2</sub> ≈ HP<i><sup>i</sup></i><sup></sup>Bu<sub>2</sub> > HPCy<sub>2</sub>. Under the conditions investigated, dicyclohexylphosphine is not an efficient chain-transfer agent for Cp‘<sub>2</sub>LaPCy<sub>2</sub>- and Cp‘<sub>2</sub>YPCy<sub>2</sub>-mediated ethylene polymerizations. Diisobutylphosphine and diethylphosphine are efficient chain-transfer agents for Cp‘<sub>2</sub>La-mediated polymerizations; however, phosphine chain transfer does not appear to be competitive with other chain-transfer pathways in Cp‘<sub>2</sub>Y-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‘<sub>2</sub>Y-mediated ethylene polymerizations conducted in the presence of phenylphosphine and cyclohexylphosphine produce low-molecular-weight phenylphosphine- and cyclohexylphosphine-capped oligomers, respectively. For Cp‘<sub>2</sub>YPPh<sub>2</sub>-mediated ethylene polymerizations, a linear relationship is observed between <i>M</i><sub>n</sub> and [diphenylphosphine]<sup>-1</sup>, consistent with a phosphine protonolytic chain-transfer mechanism.