Ligand Steric and Fluoroalkyl Substituent Effects on Enchainment Cooperativity and Stability in Bimetallic Nickel(II) Polymerization Catalysts

The synthesis and characterization of two neutrally charged bimetallic NiII ethylene polymerization catalysts, {2,7-di-[2,6-(3,5-di-methylphenylimino)methyl]1,8-naphthalenediolato}-bis-NiII(methyl)(trimethylphosphine) [(CH3)FI2-Ni2] and {2,7-di-[2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1,8-naphthalenediolato}-bis-NiII(methyl)(trimethyl-phosphine) [(CF3)FI2-Ni2)], are reported. The diffraction-derived molecular structure of (CF3)FI2-Ni2 reveals a Ni center dot center dot center dot Ni distance of 5.8024(5) angstrom. In the presence of ethylene and Ni(COD)2 or B(C6F5)3 co-catalysts, these complexes along with their monometallic analogues [2-tert-butyl-6-((2,6-(3,5-dimethylphenyl)phenylimino)methyl)-phenolate]-NiII-methyl(trimethylphosphine) [(CH3)FI-Ni] and [2-tert-butyl-6-((2,6-(3,5-ditrifluoromethyl-phenyl)phenylimino)methyl)phenolato]-NiII-methyl-(trimethylphosphine) [(CF3)FI-Ni], produce polyethylenes ranging from highly branched Mw=1400 oligomers (91 methyl branches per 1000?C) to low branch density Mw=92?000 polyethylenes (7 methyl branches per 1000?C). In the bimetallic catalysts, Ni center dot center dot center dot Ni cooperative effects are evidenced by increased product polyethylene branching in ethylene homopolymerizations (3 x for (CF3)FI2-Ni2 vs. monometallic (CF3)FI-Ni), as well as by enhanced norbornene co-monomer incorporation selectivity, with bimetallic (CH3)FI2-Ni2 and (CF3)FI2-Ni2 enchaining approximately three- and six-times more norbornene, respectively, than monometallic (CH3)FI-Ni and (CF3)FI-Ni. Additionally, (CH3)FI2-Ni2 and (CF3)FI2-Ni2 exhibit significantly enhanced thermal stability versus the less sterically encumbered dinickel catalyst {2,7-di-[(2,6-diisopropylphenyl)imino]-1,8-naphthalenediolato}-bis-NiII(methyl)(trimethylphosphine). The pathway for bimetallic catalyst thermal deactivation is shown to involve an unexpected polymerization-active intermediate, {2,7-di-[2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1-hydroxy,8-naphthalenediolato-NiII(methyl)-(trimethylphosphine).