Editing Post-metallocene catalyst (section)

==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>