Polyethylene Waxes with Short Chain Branching via Steric and Electronic Tuning of an 8-(Arylimino)-5,6,7-trihydroquinoline-nickel Catalyst
Ten examples of nickel(II) halide complexes [8-{N(2,4-(C15H13)2-6-RC6H2N)}C9H8N]NiBr2 [R = Me (Ni1), Et (Ni2), iPr (Ni3), Cl (Ni4), or F (Ni5)] and [8-{N(2,4-(C15H13)2-6-RC6H2N)}C9H8N]NiCl2 [R = Me (Ni6), Et (Ni7), iPr (Ni8), Cl (Ni9), or F (Ni10)], each incorporating a sterically encumbered N,N-chelating 8-[2,4-bis(dibenzocycloheptyl)-6-R-phenylimino]-5,6,7-trihydroquinoline ligand, are disclosed. Full details for the preparation of these complexes as well as the zinc-mediated synthesis of precursor ligands L1–L5 are given. Structural characterization of Ni3(H2O), Ni6, Ni7(H2O), and Ni8(H2O) reveals halide-bridged dinuclear structures of the type (N,N)NiX(μ-X)2NiX(N,N) with the ortho-substituted dibenzocycloheptyl groups providing steric protection to each metal center. In the presence of ethylaluminum dichloride (EtAlCl2) or methylaluminoxane (MAO), Ni1–Ni10 displayed high activities for ethylene polymerization [≤6.17 × 106 g of PE (mol of Ni)−1 h–1 for Ni6/EtAlCl2 at 20 °C], generating low-molecular weight polyethylene waxes. Notably, catalysts incorporating a 6-alkyl group (Me, Et, or iPr) as the N-aryl substituent proved to be more active than their 6-halide (Cl or F) counterparts and formed relatively higher-molecular weight polymers (Mw range of 9.1–22.9 kg mol–1) with a narrow dispersity. Moreover, analysis of these waxes by 13C nuclear magnetic resonance spectroscopy showed that they typically possessed short chain branching (>83% methyl) with branching densities of 79–90 branches per 1000 Cs. In addition, chain end analysis revealed the presence of both vinyl (−CH═CH2) and internal vinylene (−CH═CH−) functional groups, highlighting the role of both β-H elimination and isomerization.
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