Metallocene active sites, symmetry

Active sites located on crystal surfaces different from the basal (001) ones were also proposed by Allegra14 for which the Ti atom at the surface of the cut would be bonded to four Cl atoms only (bridged to further metal atoms) (Figure 1.14). In this case both octahedral sites of coordination for the monomer and growing chain (indicated by arrows in Figure 1.14) are equivalent because the surface atoms with relevant nonbonded interactions at the catalytic site are locally related by a twofold axis (dashed line in Figure 1.14). It is worth noting that this model site presents a local C2 symmetry as the isospecific metallocenes of the previous section. 

The C2-symmetric ansa metallocenes possess a C2 axis of symmetry, are chiral, and their two active sites are both chiral. The two sites are equivalent (homotopic) and enantioselective for the same monomer enantioface. The result is isoselective polymerization. C2 ansa metallocenes are one of two classes of initiators that produce highly isotactic polymer, the other class being the C ansa metallocenes (Sec. 8-5e). C2 ansa metallocenes generally produce the most isoselective polymerizations. 

One major drawback of rflc-C2-symmetric ansa-metallocene catalysts is that either during the synthesis of the precursors or by subsequent epimerization the meso-form with Cs-symmetry can also be obtained. The active sites of catalytic species formed from meso-Cs-symmetric precursors are obviously nonchiro-topic, and cannot exert any stereocontrol on the chain propagation. As a result, some atactic PP is invariably obtained. ... 

Unbridged metallocenes, in which the backbone of the catalyst is loosely bound, were the driving force for the development of the next generation of polymerization catalysts because of dilferent microstructures produced due to the change in the symmetry of the active site of the metal center involved. Thus, a new horizon was bom toward the synthesis of controlled microstructures. 

The symmetry of the metallocene and also the kind of procatalyst metal atom, the nature of the catalyst activator and the polymerisation temperature determine the polypropylene tacticity. The general stereoregulation behaviour of metallocene catalysts may be explained in terms of the local chirality, or chirotopicity, of the catalytic sites bonded to the same metal atom. For this analysis, the structure of metallocenes as catalysts should be considered. 

Single site catalysts, such as metallocene compounds, CGCs, and nickel or palladium diimine complexes, used in combination with MAO or borate cocatalysts, are highly active for the homopolymerization of norbornene and its copolymerization with ethylene. The structure of the norbornene homo- and copolymers can be widely influenced by the symmetry and structure of the ligands on the transition metal complexes. 

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