Post-metallocene catalyst

A post-metallocene catalyst is a kind of catalyst for olefin polymerization. "Post-metallocene" refers to the generation of catalysts following Kaminsky catalysts, which are metallocene catalysts discovered in 1980 by Walter Kaminsky, and have been highly publicized in the olefin polymerization catalyst area for the past quarter century. Nonetheless, traditional Ziegler-Natta catalysts still dominate the industry.

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Metallocene catalysts are homogeneous single-site systems, implying that there is a single, uniform type of catalyst present in the system. This is in contrast to the Ziegler-Natta catalysts that are heterogeneous catalysts and contain a range of catalytic sites. The catalytic properties of single-site catalysts can be controlled by modification of the structure of the catalyst. A large number of studies have been conducted by academia and industry to construct high-performance metallocene catalysts based on new concepts. One major untapped area was the copolymerization of ethylene with polar comonomers. The high oxophilicity of the early metals precluded their use in this application.

In an effort to copolymerize polar comonomers, Maurice Brookhart at the University of North Carolina began to investigate catalysts based upon nickel and palladium. These catalysts were referred to as post-metallocene catalysts. They were based upon complexes bearing bulky, neutral, alpha-diimine (or diketimine) ligands.^[1] The technology was further developed in the laboratories of DuPont’s Central Research. They have been commercialized as DuPont’s Versipol® olefin polymerization system.^[2] A significant effort at Eastman led to the related Gavilan® technology^[3] that has now been incorporated into the DuPont patent estate. The catalysts are able to homopolymerize ethylene to a variety of structures that range from high density polyethylene through hydrocarbon plastomers and elastomers to hyperbranched hydrocarbon oils by a mechanism referred to as “chain-walking.”. As opposed to metallocenes, they can also randomly copolymerize ethylene with polar comonomers such as methyl acrylate.

A second class of complexes were discovered simultaneously by Robert H. Grubbs^[4] at CalTech and Lynda Johnson at DuPont.^[5] These are the complexes bearing mono-anionic bidentate ligands shown to the right. Grubbs focused primarily on salicylaldimine ligands such as the one shown, while DuPont explored them more broadly. One interesting feature of these catalysts was that pendant Lewis acid functionality could be incorporated to direct the incorporation of polar comonomers. These catalysts are often better suited for the preparation of specialty oligomers and polymers incorporating reactive functionality.

The concept of bulky bis-imine ligands was extended to iron complexes by Vernon Gibson^[6], Maurice Brookhart^[7] and Alison Bennett.^[8] The catalysts incorporated a pyridyl between the two imine groups giving a tridentate ligand. These catalysts, illustrated to the left, were remarkably active and no chain walking is observed in these systems. As a result, these complexes give very linear high density polyethylene when bulky and when the steric bulk is removed, they are very active catalysts for ethylene oligomerization to linear alpha-olefins.^[9]

A salicylimine catalyst system based on zirconium, shown to the right, was developed by Terunori Fujita and provides extremely high activity for ethylene polymerization.^[10] The catalysts can also produce some novel polypropylene structures, as demonstrated by Geoff Coates.

A review of the late-metal catalysts described above is available.^[11] Despite intensive research and development of these systems, very few have been successfully commercialized due to the presence of established technologies and their relative fragility compared to metallocene catalysts.

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