Synthesis of hyperbranched low molecular weight polyethylene oils by an iminopyridine nickel(II) catalyst

A 6-(2,6-dimethylphenyl)-2-(2,6-diisopropylphenyl)iminopyridine dibromo nickel(II) complex was synthesized, characterized by X-ray diffraction analysis and tested in ethylene polymerization using diethylaluminumchloride as the cocatalyst. Low molecular weight (M_n ∼ 10³ g mol⁻¹) polyethylene oils were obtained under a variety of reaction conditions. Detailed NMR analysis showed the formation of hyperbranched macromolecules (branching density >100 branches per 1000 carbons) with a high fraction of “branches on branch” and one unsaturation per chain, resulting in polymer features comparable to those of polymers produced by α-diimine Pd(II) catalysts. The DFT model of the catalytic species showed that the ortho-2,6-dimethylphenyl substituent of the pyridine group destabilizes the ethylene coordination to the metal centre but does not encumber both axial coordination site. So the polymerization performance of 1 can be addressed to the catalytic pocket generated by the coordinated ligand that favors both chain transfer and chain walking over propagation.