Sterically and Electronically Modified Aryliminopyridyl-Nickel Bromide Precatalysts for an Access to Branched Polyethylene with Vinyl/Vinylene End Groups

A series
of 2-((arylimino)­ethyl)­pyridine derivatives (L1–L5), each containing N-2,4-bis­(dibenzocycloheptyl)
groups with variations in the steric/electronic properties of the
ortho-substituent in the aryl ring, and the corresponding nickel bromide
precatalysts [2-N­{2,4-(C15H13)-6-R-C6H2}­C7H7N]­NiBr2 (R = Me
(Ni1), Et (Ni2), i–Pr (Ni3), Cl (Ni4), or F
(Ni5)), have been prepared in high yield. All the precatalysts
are air-stable and characterized by Fourier transform infrared spectroscopy
and elemental analysis. The molecular structures of Ni2 and Ni5 were proved through single-crystal X-ray diffraction
analysis. The steric/electronic impact of the catalysts on ethylene
polymerization and the resulting polymer properties were studied.
Upon activation with either MAO or EASC, all the complexes displayed
higher activities (up to 7.93 × 106 g of PE (mol of
Ni)−1 h–1 with MAO) in ethylene
polymerization and produced moderate to highly branched unsaturated
polyethylene with a molecular weight of up to 16.55 kg/mol with narrow
dispersities (1.6–2.4). Significantly, the generated polyethylenes
are branched and unsaturated with a major class of internal double
bond (−CHCH−) as compared to the terminal double
bond (−CHCH2) (vinylene/vinyl = 9.8:1 to
1.8:1). Notably, their catalytic activities, types of unsaturation,
and branches are highly affected by the nature of the ortho-substituent
and reaction temperature. Moreover, the precatalysts Ni4 and Ni5 (with N-ortho = Cl and F) exhibited lower catalytic
activities, produced low-molecular-weight polyethylene with a high
melt temperature and the least number of branches with an increased
level of terminal double bonds.