Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification

A collection of five related 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)(2))(2)-4- RC6H2)N=CMe}C5H4N, each ortho-substituted with 4,4 '-difluorobenzhydryl groups but distinct in the electronic properties of the para-R substituent (R = Me L1, Et L2, i-Pr L3, F L4, OCF3 L5), were prepared and combined with (DME)NiBr2 to form their corresponding LNiBr2 complexes, Ni1-Ni5, in high yields. All the complexes were characterized by FT-IR, F-19 NMR spectroscopy and elemental analysis, while Ni5 was additionally the subject of an X-ray determination, revealing a bromide-bridged dimer. The molecular structure of bis-ligated (L4)(2)NiBr2 (Ni4') was also determined, the result of ligand reorganization having occurred during attempted crystallization of Ni4. On activation with either EtAlCl2 or MMAO, Ni1-Ni5 exhibited high catalytic activities (up to 4.28 x 10(6) g of PE (mol of Ni)(-1) h(-1) using EtAlCl2) and produced highly branched polyethylene exhibiting low molecular weight (M-w range: 2.50-6.18 kg center dot mol(-1)) and narrow dispersity (M-w/M-n range: 2.21-2.90). Notably, it was found that the type of para-R group impacted on catalytic performance with Ni5 > Ni4 > Ni3 > Ni1 > Ni2 for both co-catalysts, underlining the positive influence of electron withdrawing substituents. Analysis of the structural composition of the polyethylene by H-1 and C-13 NMR spectroscopy revealed the existence of vinyl-end groups (-CH=CH2) and high levels of internal unsaturation (-CH=CH-) (ratio of vinylene to vinyl, range: 3.1:1-10.3:1) along with various types of branch (Me, Et, Pr, Bu, 1,4-paired Me, 1,6-paired Me and LCBs). Furthermore, reaction temperature was shown to greatly affect the end group type, branching density, molecular weight and in turn the melting points of the resulting polyethylenes.