Ethylene Oligomerization Transition Metal Catalysts

Linear a-olefins are used primarily as comonomers for the production of linear low-density polyethylene, plasticizers and synthetic lubricants. The demands for a-olefins of the world are increasing at very fast speed. The a-olefins in industry are made from ethylene oligomerization mainly. Catalysts of ethylene oligomerization now used in industry include neutral Ni (II) complexes bearing bidentate P^ O ligands that are the basis for the Shell High Olefm Process SHOP, and aluminum alkyls, which find utility in the process of both Chevron and Amoco. Currently, much efforts are devoted toward the development of more efficient and selective catalysts for oligomerization and polymerization of alkenes by early and late transition metal catalysts. In recent years, the catalytic behavior of late transition metal complexes containing bi-and tri-dentate ligands for polymerization and oligomerization of ethylene to a-olefins has been attracting much attention. Compared to traditional Ziegler-Natta catalysts of early transition metals, the novel late transition metal catalysts possess advantages not only in catalytic activity and selectivity, but also in their potential for tolerating heteroatom functionalities. Brookhart and his coworkers have also reported the development of cationic Ni (II) and Pd (II) a-diimine complexes that polymerize ethylene and a-olefins. They pointed out that the key to the ability of these late-transition-metal catalysts to form polymer lied in the steric bulk of the a-diimine ligands. In ligands lacking bulky aryl ortho substituents are used, the resulting Ni (II) -based systems produce a Schulz-Flory distribution of ethylene oligomers. Similarly, they also found that by reducing the steric bulk of pyridinebisimine ligands which have been reported by both Brookhart and Gibson with surprisingly high activity, the resultant iron-based catalysts oligomerize ethylene to linear a-olefins with remarkably high activity and selectivity while maintaining desirable oligomer distributions. In order to investigate the effect of steric and electronic more closely, we have modified pyridinebisimine ligands by attaching different substitutents to the para position of aryl group. The resultant iron catalysts not only produce desirable oligomer distributions, but also show high activity in oligomerization of ethylene. Therefore, we have investigated the influence of the ligand structure on catalytic activity and oligomer distribution in detail. In addition, the effects of ethylene pressure, the ratio of Al to Fe, reaction temperature and reaction time on the behavior of ethylene oligomerization have also been in discussion.