Single site catalysts polymerization

In quest of new single-site catalysts for polymerizations of cyclic esters, a series of mononuclear yttrium(lll) complexes have been synthesized which are depicted in Scheme 221. ... 

We can vary the density of very low density polyethylene from 0.90 down to 0.86 g/cm3 by varying the comonomer level from approximately 8 to 14 mole %. At the highest comonomer levels, crystallization is impeded by the branches to such an extent that only about 5% of the material crystallizes. The crystallites of very low density polyethylene are small and poorly organized. We polymerize these resins using single-site catalysts, which give us relatively narrow molecular weight and composition distributions... 

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

Future challenges for polymerization model catalysts are to study the structure of polymers below their melting point in what is called the nascent morphology. Such work can be undertaken on silica-supported chromium catalysts as discussed above, or on so-called single-site catalysts, such as metallocenes, applied on flat silica supports. 

While ROMP procedures are applicable to only a select group of monomers, the development of well-defined single site catalysts for the polymerization of a variety of different monomers have become an area of significant research... 

The general formula LnMR of a single site catalyst, where L is a ligand set, M the active metal center and R a group that may initiate polymerization can also... 

The advent of homogeneous olefin polymerization based on metallocenes has also ushered in a new era in olefin polymerization. In fact, the recent commercialization of numerous poly(olefins) prepared using metallocene strategies is testament to the importance of this field from both an academic and an industrial viewpoint. One of the main intellectual driving forces is the realization that as a specific example of single site catalysts, these systems are also amenable to... 

This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare single site catalysts for various known or new catalytic reactions [53] such as metathesis of olefins [54], polymerization of olefins [55], alkane metathesis [56], coupHng of methane to ethane and hydrogen [57], cleavage of alkanes by methane [58], hydrogenolysis of polyolefins [59] and alkanes [60], direct transformation of ethylene into propylene [61], etc. These topics are considered in detail in subsequent chapters. 

The preparation of almost single-site cationic polymerization catalysts on highly acidic supports shows that co-catalysts can be avoided. 

With these principles in mind, and the technological progress of well-characterized single-site catalysts in hand, catalyst design at the molecular level for stereoregular polymerization has become a reality. 

W. Kaminsky and M. Amdt-Rosenau, "Tactic norbomene homo- and copolymers made with early and late transition metal catalysts," in L.S. Baugh and J.A.M. Canich, eds., Stereoselective Polymerization with Single-Site Catalysts, chapter 16, pp. 413-444. CRC Press, Boca Raton, FL, 2008. 

G.G. Hlatky, Heterogeneous single-site catalysts for olefin polymerization, Chem. Rev., 100(4) 1347-76,2000. 

J.-C. Liu, Olefin polymerization with pyridine moiety-containing single-site catalysts, US Patent 7 091272, assigned to Equistar Chemicals, LP (Houston, TX), August 15,2006. 

Examples of catalytic reactions and processes relevant to hydrocarbon chemistry are numerous. The technologies of the oil refinery with extremely low (<0.1) E factors are excellent examples demonstrating the possibilities that can be achieved by the development of selective catalytic methods, particularly by the use of various solid acids (see detailed discussions in Chapter 2). Further examples of commercially highly successful processes are the oxidation catalyst TS-1 developed by Enichem researchers160 161 (see Sections 9.1.1, 9.2.1, and 9.4.1), the homogeneous aqueous-phase Rh-catalyzed hydroformylation (see Sections 7.1.3 and 7.4.1), and single-site metallocene polymerization catalysts, which allow the preparation of tailored polymers with new properties (see Sections 13.3.2).162-164... 

With a constant temperature and constant concentration of reacting components near the active site the molecular weight distribution of the polymer formed by a single-site catalyst can be described by a Flory-Schulz equation, that can be derived easily from the polymerization mechanism ... 

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