Editing Post-metallocene catalyst

{{Short description|Catalyst for the industrial production of plastics}}
{{Use American English|date=December 2013}}
A '''post-metallocene catalyst''' is a kind of catalyst for the [[polymerization]] of [[olefin]]s, i.e., the industrial production of some of the most common plastics. "Post-metallocene" refers to a class of homogeneous catalysts that are not [[metallocene]]s. This area has attracted much attention because the market for polyethylene, polypropylene, and related [[copolymer]]s 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.<ref name=Chum/>

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==
<gallery caption="Early metal post-metallocene catalyst designs" widths="180px" heights="120px" >
File:VersifyCats.png|Generic structure of a post-metallocene catalyst based on Dow's pyridyl-amido design.
File:Zirconium bisanionic.png|Early examples of postmetallocene catalysts included [[Schiff base]] ligands.
</gallery>
Homogeneous metallocene catalysts, e.g., derived from or related to [[zirconocene dichloride]] introduced a level of microstructural control that was unavailable with heterogeneous systems.<ref>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}}</ref>  Metallocene catalysts are [[wiktionary:Homogeneous|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 (R<sub>2</sub>N<sup>−</sup>).  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]].<ref name=Chum>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}}</ref>

==Catalysts based on late transition metals==
The copolymerization of ethylene with polar monomers has been heavily studied.  The high [[oxophilic]]ity of the early metals precluded their use in this application.<ref name=Domski>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}}</ref>
<gallery widths="180" heights="120" caption="Late metal post-metallocene catalyst designs">
File:Nickel bidentate.png|Catalyst supported by charge-neutral alpha-diimine ligands.
File:Trippy-nickel-catalyst.png|Catalyst supported by highly electron-withdrawing substituted ligand.<ref>{{Cite journal|last1=Janeta|first1=Mateusz|last2=Heidlas|first2=Julius X.|last3=Daugulis|first3=Olafs|last4=Brookhart|first4=Maurice|date=2021|title=2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202013854|journal=Angewandte Chemie International Edition|language=en|volume=60|issue=9|pages=4566–4569|doi=10.1002/anie.202013854|pmid=33230900 |osti=1755772 |s2cid=227159941 |issn=1521-3773}}</ref>
File:Nickel monoanionic.png|Catalyst supported by anionic [[Schiff base]] ligand
File:Iron tridentate.png|Catalysts supported by tridentate [[diiminopyridine]] ligand.
</gallery>

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.<ref name=Domski/>   DuPont commercialized the Versipol olefin polymerization system.<ref>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.</ref>  Eastman commercialized the related Gavilan technology.<ref>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.</ref>  These complexes catalyze the homopolymerize [[ethylene]] to a variety of structures that range from high density [[polyethylene]] through hydrocarbon [[plastomer]]s 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-olefin]]s), similar to those produced by more tradition nickel(II) oligo/polymerization catalysts.  As opposed to [[metallocene]]s, 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 ligand]]s.<ref>C. Wang, S. Friedrich, T. R. Younkin, R. T. Li, R. H. Grubbs, D. A. Bansleben, M. W. Day, ''[[Organometallics]]'', '''17''', 3149 (1998).</ref> and DuPont.<ref>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.</ref><ref>{{Cite journal|last1=Janeta|first1=Mateusz|last2=Heidlas|first2=Julius X.|last3=Daugulis|first3=Olafs|last4=Brookhart|first4=Maurice|date=2021|title=2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202013854|journal=Angewandte Chemie International Edition|language=en|volume=60|issue=9|pages=4566–4569|doi=10.1002/anie.202013854|pmid=33230900 |osti=1755772 |s2cid=227159941 |issn=1521-3773}}</ref> 

The concept of bulky bis-imine ligands was extended to iron complexes<ref name=Domski/>  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.<ref name=Domski/>

A salicylimine catalyst system based on zirconium exhibits high activity for [[ethylene]] polymerization.<ref>S. Matsui, Y. Tohi, M. Mitani, J. Saito, H. Makio, H. Tanaka, M. Nitabaru, T. Nakano, T, Fujita, ''Chem. Lett.'', 1065 (1999).</ref>  The catalysts can also produce some novel [[polypropylene]] structures.<ref>[[Steven Ittel|Steven D. Ittel]] and Lynda K. Johnson and [[Maurice Brookhart]], Late-Metal Catalysts for Ethylene Homo- and Copolymerization, Chem. Rev. 2000, 100, 1169-1203.</ref>  Despite intensive efforts, few catalysts have been successfully commercialized for the copolymerization of polar monomers.

==References==
{{Reflist}}

{{DEFAULTSORT:Post-metallocene catalyst}}
[[Category:Catalysts]]
[[Category:Coordination chemistry]]
[[Category:Polymer chemistry]]