Formation and evolution of chain-propagating species upon ethylene polymerization with neutral salicylaldiminato nickel(II) catalysts

Abstract

Formation of Ni–polymeryl propagating species upon the interaction of three salicylaldiminato nickel(II) complexes of the type [(N,O)Ni(CH3)(Py)] (where (N,O)=salicylaldimine ligands, Py=pyridine) with ethylene (C2H4/Ni=10:30) has been studied by 1H and 13C NMR spectroscopy. Typically, the ethylene/catalyst mixtures in [D8]toluene were stored for short periods of time at +60 °C to generate the [(N,O)Ni(polymeryl)] species, then quickly cooled, and the NMR measurements were conducted at −20 °C. At that temperature, the [(N,O)Ni(polymeryl)] species are stable for days; diffusion 1H NMR measurements provide an estimate of the average length of polymeryl chain (polymeryl=(C2H4)nH, n=6–18). At high ethylene consumptions, the [(N,O)Ni(polymeryl)] intermediates decline, releasing free polymer chains and yielding [(N,O)Ni(Et)(Py)] species, which also further decompose to form the ultimate catalyst degradation product, a paramagnetic [(N,O)2Ni(Py)] complex. In [(N,O)2Ni(Py)], the pyridine ligand is labile (with activation energy for its dissociation of (12.3±0.5) kcal mol−1, ΔH≠298=(11.7±0.5) kcal mol−1, ΔS≠298 =(−7±1) cal K−1 mol−1). Upon the addition of nonpolar solvent (pentane), the pyridine ligand is lost completely to yield the crystals of diamagnetic [(N,O)2Ni] complex. NMR spectroscopic analysis of the polyethylenes formed suggests that the evolution of chain-propagating species ends up with formation of polyethylene with predominately internal and terminal vinylene groups rather than vinyl groups.