Metallocene catalysts classes

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

Polymerization Reactions. Polymerization addition reactions are commercially the most important class of reactions for the propylene molecule and are covered in detail elsewhere (see Olefin polymers, polypropylene). Many types of gas- or liquid-phase catalysts are used for this purpose. Most recently, metallocene catalysts have been commercially employed. These latter catalysts requite higher levels of propylene purity. 

The studies summarized above clearly bear testimony to the significance of Zr-based chiral catalysts in the important field of catalytic asymmetric synthesis. Chiral zircono-cenes promote unique reactions such as enantioselective alkene alkylations, processes that are not effectively catalyzed by any other chiral catalyst class. More recently, since about 1996, an impressive body of work has appeared that involves non-metallocene Zr catalysts. These chiral complexes are readily prepared (often in situ), easily modified, and effect a wide range of enantioselective C—C bond-forming reactions in an efficient manner (e. g. imine alkylations, Mannich reactions, aldol additions). 

Metallocene catalysts are most common. Complexes of chromium, iron, cobalt and copper are rarely used, as well as nickel and palladium catalysts. However, the latter class exhibits a high activity. 

Fig. 1.8 Structural comparisons of i-PP metallocene catalyst precursors and two recently reported new classes of competitive single-site systems discovered using HT-R. D.

The number of classes, equal to 5, is derived considering all the possible conditions for chirotopicity of the catalytic sites corresponding to L. If they are not chirotopic, i.e. if they are achirotopic (e.g. they are bisected by a horizontal mirror plane), there are two possibilities only the two sites are equal (class I catalysts) or different from each other (class II catalysts). If, on the contrary, they are chirotopic, three possibilities exist the two catalytic sites are homotopic (equal) - related by a twofold symmetry axis (class III catalysts), enan-tiotopic - related by a vertical mirror plane (class IV catalysts) or diastereotopic (different from each other) - no symmetry element is present (class V catalysts). As a consequence, only five classes of metallocene catalysts may exist if interconversion among stereoisomers is not taken into account [122]. 

Figure 3.9 Schematic Fischer projections for the bent metallocene catalysts representing five classes (I-V). The grey areas correspond to the cyclopentadienyl-like ligands...

Pathways (a)-(b)-(c)-(d) and (a )—(b )—(c )—(d ) correspond to the original mechanism proposed by Cossee [268,276,277] and are still valid, apart from some minor modifications [1], for heterogeneous catalysts. For metallocene-based catalysts of classes II and partially V, this mechanism gives rise to successive additions at the same site (from a configurational point of view) and is known as the chain stationary insertion mechanism ( chain skipped insertion or site isomerisation without insertion mechanism) [143, 146, 345], The (a)-(b)-(c)-(a )—(b )—(c ) pathway corresponds to the chain migratory insertion mechanism found in the case of metallocene catalysts of classes I, III, IV and partially V [143, 146]. 

In catalysts obtained from achiral non-bridged metallocenes of class I with C2v molecular symmetry (double helical), such as Cp2MtX2, the positions of the coordinated monomer and of the alkyl ligand are not chirotopic and, therefore, the catalyst control is completely lacking (Table 3.1) [68],... 

In catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry (helical), such as racemic isomers of ansa-metallocenes, e.g. rac.-(IndCH2)2MtX2 (Table 3.1) and rac.-(ThindCH2)2MtX2, the two coordination positions available for the incoming monomer and the growing... 

The importance and relevance of homogeneous catalysis in polymerization reactions have increased tremendously in the past few years for two reasons. First, from about the beginning of the early 1990s a special class of sandwich complexes has been used as homogeneous catalysts. These catalysts, often referred to as metallocene catalysts, can effect the polymerization of a wide variety of alkenes to give polymers of unique properties. Second, the molecular mechanism of polymerization is best understood on the basis of what is known about the chemistry of metal-alkyl, metal-alkene, and other related complexes. 

Figure 2 shows some of the classes of metallocene catalysts used for the polymerization of ethene. In order to compare the reactivities and molecular masses, the polymerizations are carried out under the same conditions (30 °C, 2 bar ethene pressure, toluene as solvent) or calculated to these parameters by data from the literature [57-60]. 

The most successful classes of metallocene catalysts studied for low-tacticity iPP are (i) the fluxional bis(2-arylindenyl) metallocenes first conceived and demonstrated by Waymouth and Coates748 and recently reviewed 749,750 (ii) a few examples of C2-symmetric, 3-alkyl-substituted ansa-bis(indenyl) zirconocenes 222,709,751 and (iii) several types of -symmetric catalysts. 

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

This class of catalysts, being so much more versatile in terms of ligand variation, is likely going to improve the performance of the current best metallocene catalysts and expand the range of achievable polypropene microstructures. A further step toward this direction has been recently taken by Ostoja Starzewsky and co-workers, who introduced the concept of donor/acceptor bridges. ... 

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