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′2-LnCH(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(SiMe 3)2 protonolysis by the same phosphines and follows the trend H2PPh ≫ H2PCy > HPPh2 > HPEt2 ≈ HP′Bu2 > 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 Mn and [diphenylphosphine] -1, consistent with a phosphine protonolytic chain-transfer mechanism.
ASJC Scopus subject areas
- Colloid and Surface Chemistry