Suppression of β-Hydride Chain Transfer in Nickel(II)-Catalyzed Ethylene Polymerization via Weak Fluorocarbon Ligand-Product Interactions

The synthesis and characterization of two neutrally charged Ni(II) ethylene polymerization catalysts, [2-tert-butyl-6((2,6-(3,5-dimethylphenyl)phenylimino)methyl)phenolato]nickel(II) methyl trimethylphosphine ((CH3)FI-Ni) and [2-tert-butyl-6((2,6-(3,S-bis(trifluoromethyl)phenyl)phenylimino)methyl)phenolato]nickel(II) methyl trimethylphosphine ((CF3)FI-Ni) are reported. In the presence of a Ni(COD)(2) cocatalyst, these catalysts produce markedly different polyethylenes: densely branched oligomers with M-w = 1.4 x 10(3) g mol(-1) for (CH3)FI-Ni vs lightly branched polyethylenes with M-w = 92 x 10(3) g mol(-1) for (CF3)FI-Ni and with similar to 6.5x the polymerization activity and with much greater performance thermal stability. HOESY 2D F-19,H-1 NMR spectra of a model Ni-ethyl compound, [2-tert-butyl-6((2,6-(3,5-bis(trifluoromethyl)phenyl)phenylimino)methyl)phenolato]nickel(II) ethyl 2,4-lutidine ((CF3)FI-Ni-Et), indicate non-negligible C-beta-H-beta center dot center dot center dot F3C through-space dipolar interactions, and molecular modeling reveals that C-beta-H-beta center dot center dot center dot F(C) distances can be as small as similar to 2.61 angstrom. during the polymerization process. Furthermore, there is no structural or spectroscopic evidence for fluorocarbon inductive effects on the structure, bonding, and reactivity of these complexes, and a catalyst with CF3 introduced beta to the imino N produces only low-M-w oligomers with low activity. These results argue that weak (ligand)C-F center dot center dot center dot H-C(polymer) interactions can significantly influence the chain transfer characteristics of these catalysts.