Multi-site type catalyst

For so-called multi-site catalysts, Ziegler catalysts for example, kinetic constants might vary with the type of active sites, see [10], In such a case, one should summarize Eqn.(5.4-1) over all contributions of different active sites, or kp is interpreted as an average value of all contributing sites. 

For example, prior to the discovery of this new single-site catalyst type, commercial grades of polyethylene were primarily manufactured over the compositional range of 0-4 mol% of comonomer (1-butene, 1-hexene or 1 -octene) that provided ethylene copolymers over the density range of 0.915-0.970 g/cc. Commercial catalysts were primarily the Cr-based Phillips-type of catalyst or a Ti-based Ziegler catalyst with the xmderstand-ing that both types of catalyst consisted of many different types of active sites. Each type of active site produced a different composition of polyethylene (different molecular weight and branching content) which resulted in a final polyethylene material with a complex molecular structure. These multi-site catalysts limited the composition of the polyethylene that was commercially available due to both process and product constraints imposed by such catalysts. 

As discussed in this chapter, metallocene catalysts have unique advantages over conventional Ziegler-Natta catalysts. This new type of catalyst brought us tailor-made polymers which have not been produced by conventional catalysts. The innovation of single-site catalyst by metallocene catalyst and other single-site catalyst technologies (new organometallic catalysts) from multi-site catalysts was a remarkable event in polyolefin industry, as shown in Fig. 21. 

The high level of stereocontrol in the formation of complexes 25 and 27 suggests that compounds of this type may be useful as chiral catalysts. Indeed, several examples of enantioselective catalytic reactions carried out with half-sandwich complexes have been published recently [23, 25]. However, it seemed desirable to have access to complexes of the [21 Ru(solv)2] type, which have two easily removable solvent molecules coordinated to the central metal, in order to provide coordination sites for a substrate to be transformed. Although the chloride ligand could be easily removed from 23 and 25 all attempts to strip off the PPhs were unsuccessful. Therefore a new reaction scheme was developed which precluded the use of phosphine ligands, and the bis (acetonitrile) complex 28 could be obtained in a multi-step protocol via the T1 salt T1 21 (Scheme 1.5.12) [26]. 

Let us emphasize a simple but important circumstance. If multi-route reactions are carried out on a catalyst with an active site of the same type, they must necessarily be characterized by either kinetic or thermodynamic matching. The problem of matching will be discussed in more detail in the next paragraph. 

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