Polyethylene Waxes with Short Chain Branching via Steric and Electronic Tuning of an 8-(Arylimino)-5,6,7-trihydroquinoline-nickel Catalyst

ABSTRACT: 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)2-6RC6H2N)}C9H8N]NiCl2 [R = Me (Ni6), Et (Ni7), iPr (Ni8), Cl (Ni9), or F (Ni10)], each incorporating a sterically encumbered 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(mu-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 x 106 g of PE (mol of Ni)-1 h-1 for Ni6/EtAlCl2 at 20 degrees 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 beta-H elimination and isomerization.

Automated summary

This study reports ten nickel(II) halide complexes incorporating a sterically encumbered 5,6,7-trihydroquinoline ligand. It was found that catalysts with alkyl substituents displayed higher activity and formed high-molecular weight polymers with narrower dispersity compared to those with halide substituents in ethylene polymerization reactions.