Iminopyridine-Nickel Precatalysts for Low Molecular Weight Branched Macro-Olefin: Dominant β-Elimination Following Isomerization

Abstract

The chain-walking mechanism, coupled with β-elimination reactions, establishes nickel complexes as a unique class of catalysts in olefin polymerization, giving access to a variety of polyethylene products from only ethylene monomer. In this study, a set of 8-(2,6-bis(aryl)-3,4,5-trifluorophenyl)imino-5,6,7-trihydroquinoline-nickel bromide complexes [aryl = Ph₂CH for Ni^H, (4-FPh)₂CH for Ni^F, (4-ClPh)₂CH for Ni^Cl, (4-MePh)₂CH for Ni^Me, (4-tBuPh)₂CH for Ni^tBu] was designed, prepared, and investigated for ethylene polymerization. Upon activation with MAO, these precatalysts exhibited high catalytic activity (as high as 3.1 × 10⁶ g mol⁻¹ h⁻¹) and produced polyethylene with tunable chain walking (branching degree = 58–90/1000C), chain transfer (polymer M_w = 0.7–2.8 kg mol⁻¹), and chain termination reactions (vinylene/vinyl = 83.4%–93.6%). Electron-donating groups at benzhydryl of aniline showed positive influence on the rate of monomer insertion and chain propagation, resulting in high catalytic activity and polymer molecular weights. Change of chain walking and chain termination rate with reaction temperature resulted in polyethylene with varied physical states from sticky wax to thick oil. Most importantly, β-elimination is the predominant route for chain termination reaction and produces polyethylene with a high ratio of internal double bonds (─CH═CH─/CH₂═CH─ up to 93.6%). These macromers are important for post-modification processes.