A post-metallocene catalyst is a kind of catalyst for olefin polymerization. "Post-metallocene" refers to a class of homogeneous catalysts, which are not metallocenes. This area has attracted much attention, although traditional heterogeneous Ziegler-Natta catalysts still dominate the industry.
Metallocene catalysts are homogeneous single-site systems, implying that a single catalyst is present in the solution. In contrast, commercially important Ziegler-Natta heterogeneous catalysts probably contain a distribution of catalytic sites. The catalytic properties of single-site catalysts can be controlled by modification of the structure of the ligand. For example, the copolymerization of ethylene with polar monomers has been heavily studied. The high oxophilicity of the early metals precluded their use in this application.
Efforts to copolymerize polar comonomers led to interest in catalysts based upon nickel and palladium, inspired by the success of the Shell Higher Olefin Process. Typical post-metallocene catalysts feature bulky, neutral, alpha-diimine ligands. The technology was further developed in the laboratories of DuPont’s Central Research. They have been commercialized as DuPont’s Versipol olefin polymerization system. Eastman commercialized the related Gavilan technology. These complexes catalyze the homopolymerize ethylene to a variety of structures that range from high density polyethylene through hydrocarbon plastomers and elastomers by a mechanism referred to as “chain-walking”. By reducing the bulk of the alpha-diimine used, the product distribution of these systems can be 'tuned' to consist of hydrocarbon oils (alpha-olefins), similar to those produced by more tradition nickel(II) oligo/polymerization catalysts. 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 and DuPont. These catalysts feature mono-anionic bidentate ligands related to salen ligands. 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 Representative catalysts feature diiminopyridine ligands. These catalysts, illustrated to the left, are highly active do not promote chain walking . 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.
A salicylimine catalyst system based on zirconium, shown to the right, exhibits high activity for ethylene polymerization. The catalysts can also produce some novel polypropylene structures. Despite intensive research and development of these systems, very few have been successfully commercialized due to the presence of established technologies.
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