Abstract
Bi-nickel-center catalysts were prepared by Schiff-base condensation of 2,3-butanedione with 2,6-diisopropylaniline; 2-5-butylaniline, 2-isopropylaniline and 1,4-phenylene diamine(I). More Schiff-base ligands were prepared by condensation of 2,3-butandione with 2,6-diisopropylaniline; 2,4-6-trimethyl aniline and 4,4-diaminophenyl (benzidine)(II), and subsequent metathesis reaction with (DME) NiBr2. Bis(alpha diimine) nickel(II) dibromide complexes are suitable catalysts and are precursors for the polymerization of ethylene after activation with methylaluminoxane (MAO). In comparison with these complex analogs, mono-nickel-center catalysts, the new catalysts have much bigger molecules, illustrated by the distance between every two active centers (II and III). For bi-nickel-center catalysts, the catalyst's structural credibility had profound influence on the catalytic activity. When the substituent was diisopropyl; t-butyl or methyl, the catalysts demonstrated much higher catalytic activity than the corresponding mononickel-center catalysts. Catalyst (II) with phenyl bridge showed high activity compared with catalyst (III) with diphenyl bridge. The catalytic properties of these complexes and the character of the obtained polymers depend on the ligand structure of the used catalysts. Substituents on the arene moiety and/or the backbone of the ligand influence the polymerization reaction. Small aryl substituents result in the formation of low molecular weight oligomers, whereas bulky aryl substituents gave high molecular weight polyethylene. Catalysts are cheap and can be prepared easily with available starting material and stable in air. The effect of Al/aNi ratio, of reaction time, variation of ethylene pressure, and the effect of temperature on catalyst performance will be discussed.