Fluorinated cobalt catalysts and their use in forming narrowly dispersed polyethylene waxes of high linearity and incorporating vinyl functionality†
Abstract
A family of six bis(arylimino)pyridine-cobalt(II) chloride complexes, [2-{CMeN(2-F-4,6-((4-FC6H4)2CH)2C6H2)}-6-(CMeNAr)C5H3N]CoCl2 (Ar = 2,6-Me2C6H3Co1, 2,6-Et2C6H3Co2, 2,6-i-Pr2C6H3Co3, 2,4,6-Me3C6H2Co4, 2,6-Et-4-MeC6H2Co5) and [2-[CMeN{2-{(4-FC6H4)2CH}-4-{CH(C6H5)ArF}-6-F}]-6-(CMeN(2,6-i-Pr2C6H3))C5H3N]CoCl2 (ArF = (3-(4-FC6H4)2CH-4-NH2-5-F)C6H2) (Co6), each incorporating one ortho-fluoride and one ortho-bis(4-fluorophenyl)methyl group, has been synthesized in good yield from the corresponding unsymmetrical N,N,N′-ligands, L1–L6. All complexes have been well characterized including by 19F NMR spectroscopy; a variable temperature 19F NMR study has also been performed on Co6 to investigate a dynamic isomerization process. The molecular structures of Co1 and Co5 emphasize the variation in steric protection of the metal center imparted by the inequivalent N-aryl groups. In the presence of methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), Co1–Co5 exhibited high activities of up to 11.7 × 106 g of polyethylene (PE) (mol of Co)−1 h−1 at 70 °C, delivering strictly linear, low molecular weight PE waxes (between 924 and 5550 g mol−1) of narrow dispersity and terminated with vinyl functionality. By comparison, Co6, possessing a more extended fluorinated para-CH(C6H5)ArF group, was the least active but afforded the highest molecular weight polyethylene (up to 10 027 mol−1). In addition, DFT studies of the model Co(I)-propyl species, Co1′, Co3′ and Co6′, have been performed in an attempt to explain the favorable formation of the low molecular weight polymers.