Metallocenes stereoselectivity mechanism

It is becoming apparent that nonbonding interactions play a greater role than previously appreciated in determining the structures of both main-group and transition-metal metallocenes. Molecular mechanics are particularly efficient in describing such interactions and can be readily applied to much larger systems than traditional MO methods. The analysis of structural conformations of bis(cyclopentadienyl) compounds used in stereoselective synthesis can be expected to benefit from wider applications of MM methods. 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

The structure of ligands in metallocene complexes determines activity, stereoselectivity, and molecular weight of 1-alkene polymerizations, by controlling the preferential conformation of the growing polymer chain which in turn controls the stereochemistry of monomer coordination ( enantiomorphic site control ). The difference between this and the chain-end control mechanism mentioned earlier is that stereo errors due to misinsertions can be repaired.101,106... 

On the other hand, the above phenomena do not apply to Cj-symmetric (nor to syndiospecific (7,-symmetric) metallocenes for these, decreasing monomer concentration either increases the isotacticity and melting point of iPP or has no relevant effect. This is due to the mechanism of site epimerization (also referred to as chain backskip, Scheme 28), in which the chain, at the lower monomer concentrations, has a higher chance to migrate to the less hindered site, which is usually also the more stereoselective. For the same reason, increasing the polymerization temperature either increases the melting point of an isotactic poly(a-olefin), or has no relevant effect.725... 

With the replacement of Cp by a 9-fluorenyl moiety, the metallocene promotes syndiotactic polymerization of propylene under site control. Syndiotactic PP with (rrrr) as high as 0.77 was obtained with Me2Si(9-Flu) (N-t-Bu)ZrCl2/MAO catalyst. ° The stereoselectivity is due to the (pseudo)Cs-symmetry of the catalytic complex, and the stereocontrol mechanism is analog to that for the Cs-symmetric awM-metallocenes. 

Because of the mechanism of enantioface selectivity and the two-site, chain migratory insertion mechanism, the microstructure of a poly(l-olefin) made with a given metallocene is, to a large extent, predictable. In a series of landmark papers, Ewen and co-workers and Kaminsky and co-work-ers described a series of stereoselective metallocene catalysts which define what are now referred to as Ewen s symmetry rules . These are summarized in Chart 2. When the metallocene molecule is C v, meso Cs-symmetric, or highly fluxional, an aspecific polymerization has to be expected. 

There are a few examples of unbridged metallocenes which are stereoselective by site control. Examples have been reported with substituted cy-clopentadienyl, indenyl, and fluorenyl ligands, the latter being apparently the most stereoselective. The biscyclopentadienyl system C2-ITI (Chart 16) produces, at low polymerization temperature (—50 °C), a low molecular weight, low isotactic PP mmmm = 51%) with a double stereodifferentiating mechanism, partly site control (27%, b = 0.96) partly chain-end control (73%, p = 0.79). The related, but much bulkier, rac-[Cp-CH(Ph)CH2(9-BBN)]2ZrCl2 (Q-II-Z) produced at —50 °C a more isotactic PP mmmm = 75%), with predominance of site control (67%, b =... 

The polymers obtained with Me2Si(l-Ind)2ZrCl2 activated by either [BusNH] [B(C6Fs)4] or MAO show the same microstructure, indicating that the stereoselectivity of the metallocene catalysts does not depend on the nature of the cocatalyst and that the active species are similar in the two cases. This is a key observation, in accordance with the report of Ewen and our own experience. Nevertheless, the two catalyst systems differ considerably in the mechanism of formation of these active species. Similarly, Miilhaupt et al. found for propene polymerization with rac-Me2Si(2-Me-Benz[e]ind)2ZrX2 (X = Cl, Me)... 

It was found that polypropylene made by such complexes (cocatalyzed by MAO) tended to be isotactic, as opposed to the atactic polypropylene that had been hitherto made by metallocenes such as Cp2ZrCl2. The strong relationship between ligand structure and polymer tacticity is considered to be the most secure proof of the migratory insertion mechanism for polymerization. The lure of homogeneous stereoselective olefin polymerization is responsible for much of the diversity of ligand structures within the bis(Cp) family. 

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