Highly active and thermostable camphyl α‐diimine–nickel(II) catalysts for ethylene polymerization: Effects of N‐aryl substituting groups on catalytic properties and branching structures of polyethylene

Abstract

A group of nickel bromide complexes containing the rigid bidentate bis(arylimino)camphane ligands (Ar‐BIC) with different N‐aryl substituents, [ArN═C(camphyl)C(camphyl)═NAr]NiBr2 (Aryl = 2,6‐Me2C6H3 [Ni1], 2,4‐Me2C6H3 [Ni2], 2,4,6‐Me3C6H2 [Ni3], 2,6‐Me2‐4‐{(C6H4F)2CH}‐C6H2 [Ni4], 2,4‐Me2‐6‐{(C6H4F)2CH}‐C6H2 [Ni5, with the corresponding ligand as L5], and 2‐Me‐4,6‐{(C6H4F)2CH}2‐C6H2 [Ni6]), has been synthesized and characterized. The molecular structures of L5, Ni1, Ni3, Ni4, and Ni6 were elucidated with single‐crystal X‐ray diffraction. The nickel centers displayed a four‐coordinate geometry that can be best described as a distorted tetrahedral geometry. Upon treatment with either MMAO or Me2AlCl, Ni1–Ni6 showed moderate to high activities for ethylene polymerization, producing polyethylenes (PEs) with low to high molecular weights (0.02–23.7 × 105 g mol−1) and low polydispersity indices (PDI: 1.6–2.6). In comparison with reference nickel precatalysts ligated by bis(arylimino)acenaphthene (Ar‐BIAN) and bis(arylimino)butane (Ar‐BIB), the title complexes displayed much better thermal stability and higher catalytic activities (up to 11.2 × 106 g PE (mol Ni)−1 h−1 at 70°C), delivering polyethylenes with higher molecular weights. All the resultant polyethylenes are moderately to highly branched with the branching content and type of branching strongly affected by the type of N‐aryl substituting groups. Notably, the polymers produced with ortho‐hydrogen Ni2/Me2AlCl possessed the highest branching density and unique terminal vinyl (CH═CH2) and internal vinylene (CH═CH) structure. On the other hand, ortho‐di(p‐fluorophenyl)methyl‐containing Ni5 and Ni6 just gave 30 and 19 branches per 1000 Cs due to the reduced chain walking capability as a result of the bulkier substituting groups.