Ansa-Type Metallocene Catalysts

A great variety of modifications to the steric properties, based on Ewen s work, has been carried out in order to control the stereoselectivity some representative examples are summarized in Table 9. Using modified Me2C(Cp)(Flu)MCl2 (M = Zr, Hf), high syndiospecificity has been achieved 134-136, as reported by Fina Hoechst and Mitsui Toatsu Chemicals, inc. 

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The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

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. 

A very different type of catalyst was developed by Buchwald et al. [6] the chiral Ti complex with Brintzinger s ansa-metallocene ligand (ebthi) is extraordinarily effective for the enantioselective hydrogenation of cychc imines with high optical yields (>97% ee). Unfortunately, the activity and productivity of this Ti catalyst are relatively low compared to Rh- and Ir-diphosphine catalysts. The stereochemical outcome of the reaction can be predicted by straightforward steric arguments. Acyclic imines are reduced with lower enantioselectivity, probably due to isomerization problems and the presence of syn/anti isomers. Studies with multifunctional imines or in presence of other substrates revealed that the scope of the Ti-ebthi catalyst is rather Hmited. Partial or total catalyst inhibition is observed in presence of most functional groups, expect amines, alcohols, acetals, and halides [39]. 

A new type of enantioselective diene polymerization is found with cyclopolymerization of 1,5-hexadiene which leads to polymers with a saturated chiral main chain28,58>109. As catalyst, (—)-(7 )-[l,T-ethylenebis(4,5,6,7-tetrahydro-l-indenyl)]zirconium (/ )-binaphtholate is used in the presence of methylalumoxane to give optically active poly(methylene-1,3-cyclopentane) (3) with 68% trans configuration in the five-membered ring (diisotacticity). If the (S)-enantiomer of the ansa-metallocene with (ft)-binaphthol is used as catalyst then the opposite rotation of the polymer is observed58. 

A new type of support has also been developed by reaction of predried silica with MAO and simultaneous cross-linking with aromatic diols." The carrier material is a supported MAO network, which can be used as a cocatalyst for activating ansa-metallocene dichlorides. Additional aluminum alkyls are necessary to activate the supported catalyst and to control the polymerization profile as well as other polymer properties. 

In most cases the catalyst precursor is a metallocene dichloride complex consisting of two aromatic five-membered ring systems that can be tethered by a bridging unit ansa metallocene complexes) or not. The two aromatic ligands at the metal can be of the same type. i.e.. cyclopentadienyl. indenyl. or fluorenyl. The introduction of substituents at certain positions of the two aromatic ligands and/or the bridge modifies not only the steric and electronic conditions in the molecule but also the symmetry of such a metallocene dichloride complex. Another variable parameter is the metal M = Ti. Zr. Hf. 

Insufficiently explored is also the ubiquitous reactitMi by which a metallocene-bound polymer chain is transferred to an A1 center of the co-catalyst/activator in exchange for a methyl group. While there is little doubt that intermediates of type B (Fig. 3) are involved in such an alkyl exchange [28], rally scant experimental data concern the effects of different chain-ends and ansa-metallocene structures on the rate, extent and direction of these exchange reactions. As these can be used to transfer polymer chains between catalysts with different stereoselectivities [43], further exploration might be useful for a productirai of polypropylene that contains chain segments with different tacticities. 

During polymerization of 1-hexene by an MAO-activated ansa-zirconocene catalyst system, species of types A, B and B were detected by NMR spetroscopy but found to account for less than half of the initial metallocene concentration [31]. Major parts of the metallocene content of this catalyst system thus remain unaccounted for. 

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