LLDPE-like Polymers Accessible via Ethylene Homopolymerization Using Nitro-Appended 2-(Arylimino)pyridine-nickel Catalysts

Four examples of para-nitro substituted 2-(arylimino)pyridine-nickel(II) bromide complexes of general formula, [2-{(2,6-R-4-NO2C6H2)N=CMe}C5H4N]NiBr2, but differentiable by the steric/electronic properties displayed by the ortho-groups [R = i-Pr (Ni1), Et (Ni2), CHPh2 (Ni3), CH(4-FPh)2 (Ni4)], have been prepared in good yield. For comparative purposes, the meta-nitro complex, [2-{(2,6-i-Pr2-3-NO2-4-(4-FPh)2C6H)N=CMe}C5H4N]NiBr2 (Ni5), has also been synthesized. The molecular structures of mononuclear Ni3·xH2O (x = 2, 3) and bromide-bridged dinuclear Ni4 and Ni5 are disclosed. Upon activation with either ethylaluminum dichloride (EtAlCl2) or modified methylaluminoxane (MMAO), all precatalysts displayed good catalytic performance at operating temperatures between 30 °C and 60 °C with higher activities generally seen using EtAlCl2 [up to 4.7 × 106 g PE (mol of Ni)−1 h−1]: Ni2 ~ Ni5 > Ni1 ~ Ni4 > Ni3. In terms of the resultant polyethylene (PE), Ni4/EtAlCl2 formed the highest molecular weight of the series (Mw up to 1.4 × 105 g mol−1) with dispersities (Mw/Mn) ranging from narrow to broad (Mw/Mn range: 2.2–24.4). Moreover, the melting temperatures (Tm) of the polymers generated via EtAlCl2 activation fell in a narrow range, 117.8–126.0 °C, which resembles that seen for industrial-grade linear-low density polyethylene (LLDPE). Indeed, their 13C NMR spectra revealed significant amounts of uniformly distributed long-chain branches (LCBs), while internal vinylene groups constituted the major type of chain unsaturation [vinylene:vinyl = 5.3:1 (EtAlCl2) and 9.9:1 (MMAO)].