Solvent and co-catalyst dependent pyrazolylpyridinamine and pyrazolylpyrroleamine nickel(II) catalyzed oligomerization and polymerization of ethylene

The pyrazolylamine ligands, 2-(3,5-dimethyl-1H-pyrazol-1-yl)-N-(pyridin-2- ylmethyl)ethan-amine (L1a), 2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)-N-(pyridin-2- ylmethyl)ethan-amine (L2a), 2-(3,5-dimethyl-pyrazol-1-yl)-ethyl-(1H-pyrrol-2- ylmethyl)-amine (L3a), 2-(3,5-di-tert-butyl-pyrazol-1-yl)-ethyl-(1H-pyrrol-2- ylmethyl)-amine (L4a), were prepared by reducing the corresponding imine compounds L1-L4. L1a-L4a were then reacted with [NiBr₂(DME)] to form the nickel complexes, [NiBr₂(L1a)] (1), [NiBr₂(L2a)] (2), [NiBr₂(L3a)] (3) and [NiBr₂(L4a)] (4) in high yields. When activated with aluminum co-catalysts, all four nickel complexes were able to catalyze the oligomerization or polymerization of ethylene; but the products of these reactions were co-catalyst and solvent dependent. With EtAlCl₂ as co-catalyst and in toluene, butene and hexene were formed, which subsequently alkylated the solvent selectively to butyl-, dibutyl- and tri-butyltoluene. The same catalysts in chlorobenzene produced butene, hexene and highly branched polyethylene. Changing the co-catalysts to MAO and with toluene as solvent, complexes 2 and 3 catalyzed the polymerization of ethylene to high molecular weight (447,300 Da) linear high density polyethylene (ca T_m = 135.5 C); with polydispersity indices ranging from 1.63 to 2.51.