A post-metallocene catalyst is a kind of catalyst for the polymerization of olefins, i.e., the industrial production of some of the most common plastics. "Post-metallocene" refers to a class of homogeneous catalysts that are not metallocenes. This area has attracted much attention because the market for polyethylene, polypropylene, and related copolymers is large. There is a corresponding intense market for new processes as indicated by the fact that, in the US alone, 50,000 patents were issued between 1991-2007 on polyethylene and polypropylene.[1]

Many methods exist to polymerize alkenes, including the traditional routes using Philips catalyst and traditional heterogeneous Ziegler-Natta catalysts, which still are used to produce the bulk of polyethylene.

Catalysts based on early transition metals

Homogeneous metallocene catalysts, e.g., derived from or related to zirconocene dichloride introduced a level of microstructural control that was unavailable with heterogeneous systems.[2] Metallocene catalysts are homogeneous single-site systems, implying that a uniform catalyst is present in the solution. In contrast, commercially important Ziegler-Natta heterogeneous catalysts contain a distribution of catalytic sites. The catalytic properties of single-site catalysts can be controlled by modification of the ligand. Initially ligand modifications focused on various cyclopentadienyl derivatives, but great diversity was uncovered through high throughput screening. These post-metallocene catalysts employ a range of chelating ligands, often including pyridine and amido (R2N−). These ligands are available in great diversity with respect to their steric and electronic properties. Such postmetallocene catalysts enabled the introduction of Chain shuttling polymerization.[1]

Catalysts based on late transition metals

The copolymerization of ethylene with polar monomers has been heavily studied. The high oxophilicity of the early metals precluded their use in this application.[3]

Efforts to copolymerize polar comonomers led to 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.[3] DuPont commercialized the Versipol olefin polymerization system.[5] Eastman commercialized the related Gavilan technology.[6] 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 modifying the bulk of the alpha-diimine, 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 catalysts feature mono-anionic bidentate ligands related to salen ligands.[7] and DuPont.[8][9]

The concept of bulky bis-imine ligands was extended to iron complexes[3] Representative catalysts feature diiminopyridine ligands. These catalysts are highly active but do not promote chain walking. The give very linear high-density polyethylene when bulky and when the steric bulk is removed, they are very active for ethylene oligomerization to linear alpha-olefins.[3]

A salicylimine catalyst system based on zirconium exhibits high activity for ethylene polymerization.[10] The catalysts can also produce some novel polypropylene structures.[11] Despite intensive efforts, few catalysts have been successfully commercialized for the copolymerization of polar monomers.

References

  1. 1 2 Chum, P. S.; Swogger, K. W., "Olefin Polymer Technologies-History and Recent Progress at the Dow Chemical Company", Progress in Polymer Science 2008, volume 33, 797-819. doi:10.1016/j.progpolymsci.2008.05.003
  2. ↑ Brintzinger, H. H.; Fischer, D.; Muelhaupt, R.; Rieger, B.; Waymouth, R. M., "Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts", Angew. Chem. Int. Ed. Engl. 1995, 34, 1143-1170. doi:10.1002/anie.199511431
  3. 1 2 3 4 Domski, G. J., Rose, J. M., Coates, G. W., Bolig, A. D., Brookhart, M., "Living alkene polymerization: New methods for the precision synthesis of polyolefins", Prog. Polymer Sci. 2007, volume 32, p.30. doi:10.1016/j.progpolymsci.2006.11.001
  4. ↑ Janeta, Mateusz; Heidlas, Julius X.; Daugulis, Olafs; Brookhart, Maurice (2021). "2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts". Angewandte Chemie International Edition. 60 (9): 4566–4569. doi:10.1002/anie.202013854. ISSN 1521-3773. OSTI 1755772. PMID 33230900. S2CID 227159941.
  5. ↑ US 5,866,663 "Process of Polymerizing Olefins," Samuel David Arthur, Alison Margaret Anne Bennett, Maurice S. Brookhart, Edward Bryan Coughlin, Jerald Feldman, Steven Dale Ittel, Lynda Kaye Johnson, Christopher Moore Killian; Kristina Ann Kreutzer, Elizabeth Forrester McCord, Stephan James McLain, Anju Parthasarathy, Lin Wang, Zhen-Yu Yang; February 2, 1999. WO 9623010 A2 960801.
  6. ↑ MacKenzie, P. B.; Moody, L. S.; Killian, C. M.; Ponasik, J. A.; McDevitt, J. P. WO Patent Application 9840374, Sept. 17, 1998 to Eastman, priority date Feb 24, 1998.
  7. ↑ C. Wang, S. Friedrich, T. R. Younkin, R. T. Li, R. H. Grubbs, D. A. Bansleben, M. W. Day, Organometallics, 17, 3149 (1998).
  8. ↑ US 6,174,975, “Polymerization of Olefins,” Lynda Kaye Johnson; Alison Margaret Anne Bennett, Lin Wang, Anju Parthasarathy, Elisabeth Hauptman, Robert D. Simpson, Jerald Feldman, Edward Bryan Coughlin, and Steven Dale Ittel. January 16, 2001.
  9. ↑ Janeta, Mateusz; Heidlas, Julius X.; Daugulis, Olafs; Brookhart, Maurice (2021). "2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts". Angewandte Chemie International Edition. 60 (9): 4566–4569. doi:10.1002/anie.202013854. ISSN 1521-3773. OSTI 1755772. PMID 33230900. S2CID 227159941.
  10. ↑ S. Matsui, Y. Tohi, M. Mitani, J. Saito, H. Makio, H. Tanaka, M. Nitabaru, T. Nakano, T, Fujita, Chem. Lett., 1065 (1999).
  11. ↑ Steven D. Ittel and Lynda K. Johnson and Maurice Brookhart, Late-Metal Catalysts for Ethylene Homo- and Copolymerization, Chem. Rev. 2000, 100, 1169-1203.
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