Synthesis of Branched Polyethylene by Tandem Catalysis

Linear low-density polyethylene (LLDPE) can be prepared by addition of ethylene to a mixture of two catalysts. In this “tandem catalysis” scheme one catalyst dimerizes or oligomerizes ethylene to α-olefins while the second site incorporates these α-olefins into a growing polyethylene chain. A variety of classical catalyst combinations are available for this purpose. Better control over the polymerization process, and therefore product properties, is attained by the use of homogenous “single site” catalysts. The best-behaved tandem processes take advantage of well-defined catalysts that require stoichiometric quantities of activators. One such system employs [(C6H5)2PC6H4C(OB(C6F5)3)O-κ2P,O]Ni(η3-CH2CMeCH2) and {[(η5-C5Me4)SiMe2(η1-NCMe3)]TiMe}{MeB(C6F5)3}. The nickel sites are responsible for converting ethylene to 1-butene or mixtures of 1-butene with 1-hexene. These olefins are copolymerized with ethylene at the titanium sites. It is possible to obtain a linear correlation between the branching content in the polymer product and the Ni/Ti ratio. The effect of ligand substitution at nickel has also been investigated. When the benzyl derivative [(C6H5)2PC6H4C(O-B(C6F5)3)O-κ2P,O]Ni(η3-CH2C6H5) is used instead of the methallyl counterpart [(C6H5)2PC6H4C(OB(C6F5)3)O-κ2P,O]Ni(η3-CH2CMeCH2), one obtains, at a constant Ni/Ti ratio, considerably more branching in the final polymer structure. These results are rationalized in terms of a more efficient initiation when the more labile benzyl ligand is used.