SHOP catalyst

Another example for a defined pincer carbene chromium(III) catalyst for the oligomerisation of ethylene also comes from McGuinness et al. [482]. It is an example of a nonmetallocene polymerisation catalyst [483] that traditionally falls into the categories of chromium based Phillips and Union Carbide systems [484], nickel based SHOP catalysts [485] and titanium/aluminium based Ziegler-Natta catalysts [29], The chromium(III) pincer carbene catalysts are highly active for the oligomerisation of ethylene and produce mainly a-oleflns. 

The Ni complex incorporating mixed-donor P/O-ligands find industrial application in the Shell Higher Olefin Process (SHOP) to yield a-alkenes, while the complex (71) has been shown to be active in the polymerization of olefins and will also tolerate functionalized monomers such as methyl methacrylate. Grubbs and coworkers have adapted the standard SHOP catalyst to yield a highly active family of catalysts (eg. 72) for the polymerization of low-branched polyethylene. This is in contrast to the diimine catalysts (Section 5.2) which lead to a more highly branched polyethylene. 

The X-ray diffraction analysis indicated that the SHOP catalyst calcined at 500° for 2 hours was an amorphous compound of ferric oxide and alumina or a mixture of this compound and the a-ferric oxide crystals that formed a solid solution with alumina. 

The chemical modification of SHOP-catalysts was the center of interest of Klabunde and Ittel who intensively focused on the synthesis of high molecular weight products and on the copolymerization of ethene with a-olefms. Therein, the combination of ethene with polar monomers and carbon monoxide depicts the major concern [14]. They realized the importance of the nature of donor ligands coordinating to the nickel center as the primary factor limiting the polymer molecular weight (Fig. 2.3, C). 

As with ethylene polymerization, the metal-mediated conversion of ethylene to short chain Hnear a-olefins (range C4-C20) represents an important industrial process. Such oHgomers find considerable use in the manufacture of detergents, and plasticizers and in the production of linear low density polyethylene (LLDPE). An additional benefit of bis(imino)pyridine iron and cobalt catalysts is the ability to tune the Hgand environment to allow the formation of exclusively Unear a-olefins with activities and selectivities comparable with other weU-known late transition metal catalysts (e.g., the SHOP catalyst) [115]. 

The p elimination of the linear alkyl gives a 1-alkene as the kinetic product. Clearly, the SHOP catalyst cannot be a good isomerization catalyst otherwise the 1-alkene would be isomerized. 

Important for the polymerization activity is the substituent 1 which has to be a bulky aryl group. The task of this substituent is to fill up the coordination spheres below and above the square plane of the complex and thus enable the growing polymer chain to stay coordinated to the metal center. This is one of the main differences to the well-known SHOP catalysts invented by Keim et al. [164] and Ostoja-Starzewski and Witte [165] which produces mainly ethene oligomers. 

Neutral aryl and alkyl complexes have proved a very fertile ground for new catalytic developments. In spite of the attention devoted to new catalysts based on hard donor ligands, phosphinoenolate complexes (SHOP catalysts) 193,194,212... 

---