Growing chain chiral orientation

Figure 1.23 Transition states for secondary insertion of styrene into secondary growing chain presenting si chirality (that is, generated secondary insertion of. si-coordinated styrene), (a) Model for unlike (syndiospecific) propagation includes fluxional site of R chirality at metal atom, which imposes re-propene coordination, while (b) model for like (isospecific) propagation includes fluxional sites of S chirality at metal, which imposes. si-propene coordination. Syndiospecific transition state (a) is favored because smallest substituent on C atom of chain, the H atom, can be pointed toward Cp ligand, whereas isospecific transition state (b) is of higher energy because Cp of growing chain is oriented toward Cp ring.

Stereoselectivity Mechanism of Chiral Orientation of Growing Chain... 

A large part of the stereospecific behavior of polymerization catalysts presented in this review can be rationalized in the framework of a stereoselectivity mechanism involving a chiral orientation of the growing chain. The discovery... 

Hence, this analysis indicates that the stereoselectivity of these models is due, not to direct interactions of the tt-ligands with the monomer, but to interactions of the n-ligands with the growing chain, determining its chiral orientation (0i —60° preferred to 0i +60°), which in turn discriminates between the two prochiral faces of the propene monomer.15,37... 

Molecular modeling studies relative to both preinsertion intermediates and insertion states indicate that for all the metallocenes from 1 to 39 of Scheme 1.2 (independent of their structure and symmetry), when a substantial stereoselectivity is calculated for primary monomer insertion, this is mainly due to nonbonded energy interactions of the methyl group of the chirally coordinated monomer with the chirally oriented growing chain. 

It is worth noting that the lower syndiospecificity of catalytic systems based on 31, with respect to those based on 30,9 is accounted for by these calculations. This is easily rationalized in the framework of the enantioselective mechanism which imposes to the growing chain (both in the preinsertion intermediate and in the approximated transition state) a chiral orientation toward... 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

The experimental observation was that C2-symmetric metallocene complexes of zirconium (Fig. 6) produced isotactic polymers, while Cs-symmet-ric metallocene complexes (Fig. 6) produced syndiotactic polymers. Pure MM calculations with frozen core showed that the stereoselectivity is not related to direct interactions of the -ligands of the chiral metallocene with the entering monomer, but to interactions of the -ligands with the growing chain. It is therefore the chirally oriented growing chain which discriminates between the prochiral faces of the propene monomer. For C2-symmetric complexes, identical enantiofacial orientation in all insertion steps results in isotactic polymer formation for Cs-symmetric complexes the enantiofacial orientation alternates between insertion steps and leads to syndiotactic polymers. 

In conclusion, in site-controlled stereoselective polymerizations, it is accepted and proved that the site chirality is unable to select directly between the two enantiofaces of the inserting monomer. Instead, it is accepted and proved that the site chirality can force a chiral orientation of the growing chain, which in turn is able to select between the two enantiofaces of the inserting monomer. Thus, the growing chain acts as a messenger to transfer the chiral information from the catalytic site to the monomer.172... 

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