Allosteric Effects in Ethylene Polymerization Catalysis. Enhancement of Performance of Phosphine-Phosphinate and Phosphine-Phosphonate Palladium Alkyl Catalysts by Remote Binding of B(C6F5)3

Article abstract of DOI:10.1021/acs.organomet.7b00815

Remote binding of B(C₆F₅)₃ to (PPO)PdMeL (L = pyridine or lutidine) or {(PPO)PdMe}₂ ethylene polymerization catalysts that contain phosphine-arenephosphinate or phosphine-arenephosphonate ligands (PPO^- = [1-PAr₂-2-PR′O₂-Ph]^-: Ar = R′ = Ph (1a); Ar = Ph, R′ = OEt (1b); Ar = Ph, R′ = OⁱPr (1c); Ar = 2-OMe-Ph, R′ = OⁱPr (1d)) significantly increases the catalyst activity and the molecular weight of the polyethylene (PE) product. In the most favorable case, in situ conversion of (1d)PdMe(py) to the base-free adduct {1d·B(C₆F₅)₃}PdMe increases the ethylene polymerization activity from 9.8 to 5700 kg mol^-1 h^-1 and the M_n of the PE product from 9030 to 99 200 Da (80 °C, 410 psi). X-ray structural data, trends in ligand lability, and comparative studies of BF₃ activation suggest that these allosteric effects are primarily electronic in origin. The B(C₆F₅)₃ binding enhances the chain growth rate (R_growth) by increasing the degree of positive charge on the Pd center. This effect does not result in the large increase in the chain transfer rate (R_transfer) and concomitant reduction in PE molecular weight seen in previous studies of analogous (PO)PdRL catalysts that contain phosphine-arenesulfonate ligands, because of the operation of a dissociative chain transfer process, which is inhibited by the increased charge at Pd.