Metallocene Symmetry

The type of symmetry present in each type of metallocene initiator (C2v, C2, Cs, Ci) is listed in Table 8-5. The symmetry elements (axis and plane) for each type is indicated. An axis is a C2 axis of symmetry when rotation of 180° about that axis yields a structure indistinguishable from the original structure. The stereoselectivity of each of the two coordination 

Metallocene Symmetry Axis Planes Sites Structure  

The ansa metallocene initiators are synthesized in a relatively straightforward manner (Eq. 8-53). Cyclopentadiene or a substituted analog such as an alkyl-substituted cyclopenta-diene, indene, 4,5,6,7-tetrahydroindene, or fluorene (CpH2) is reacted with butyllithium to 

Both bridged and unbridged C2v-symmetric metallocenes, mostly the unsubstituted biscyclopentadienyl initiators, but also others such as (CH3)2SiFlu2ZrCl2, have been studied. These initiators are achiral, and their two coordination (active) sites are both achiral and homotopic. The result is that atactic polymer is formed via chain end control. Modest tendencies toward slight isotactic or syndiotactic placement are observed for some initiators, depending on the temperature and other reaction conditions. 

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Dependence on Metallocene Symmetry of E-Z Selectivity for 2-Butene Copolymerizations. We have seen in the Section 3.1.3 that opposite enantiofaces are favored for primary and secondary propene insertion on C2-symmetric metallocenes, whereas the same enantioface is favored for primary and secondary insertion on Cv-symmetric metallocenes. In this framework, if the same steric interactions which rule the enantioselectivity of primary and secondary propene insertions hold for 2-butene, the insertion of... 

As for olefins different from propene, molecular modeling studies have also been able to rationalize the dependence on metallocene symmetry of E-Z selectivity for 2-butene copolymerization as well as the stereoselectivity of the cyclization step, which determines the cis or trans configuration of the rings, for cyclopolymerization of nonconjugated dienes. 

Figure 5 MO diagram for the half-sandwich main group metallocene symmetry).

Chart 2. Steric Control as a Function of Metallocene Symmetry (Ewen s Symmetry Rules) ... 

The metallocene symmetry is maintained by a group (the bridge) that links the two cyclopentadienyl ligands, thus blocking their rotation these compounds are usually referred to as chiral ansa-metal-locenes and have the general formula shown in Chart... 

Figure 3.12 ansa-Bridged metallocene symmetry/polypropylene microstructure relationships. 

Metallocene Symmetry Primary Chain Transfer Reaction Type of Polymer... 

Ansa-metallocenes with C2 and Cs symmetries generate random copolymers containing norbornene microblocks [43, 44, 61, 62, 64-68, 92]. Copolymers with norbomene content well above 50 mol% and Tg values as high as 220°C can be synthesized. Metallocene symmetry and ligand substituents dictate polymerizatiOTi activity, tacticity, and sequential distribution. The type of bridge has an influence oti polymerization activity, norbornene content, and molar masses (Fig. 5). 

Among the C2-symmetric metallocenes, I-l is the most active. The Cs-symmetric II-l, as already known, turned out to be the most productive catalyst. Among the C2-symmetric catalysts, 1-4 was shown to be noticeably more active than the others of the series. The microstructure was dominated by metallocene symmetry and ligand type. Precursor 1-3 produced copolymers with the highest norbomene content and the highest amount of meso-meso NNN sequences [68]. 

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