Active sites enantiomorphic control

Mechanisms of Stereocontrol. Stereochemistry of the olefin insertion step can be controlled by both the steric environment of the active site (enantiomorphic-site control) as well as the growing polymer chain (chain end control). In chain end stereocontrol, stereospecificity arises from the chiral )3-carbon atom of the last enchained monomer imit, which in turn influences the stereochemistry of monomer addition. Chain-end control is usually less effective than site control and has been observed for some achiral metallocenes at low polymerization temperatures. Partially iPP resulting from chain end stereocontrol has been obtained with Cp2TiPh2/MAO (56,272). The syndiospecific polymerization of 1-butene using the Cp 2MCl2/MAO (M = Zr, Hf) catalyst systems has been described (273). Predominantly sPP has been obtained under chain end control, using Brookhart s diimine nickel catalysts (274-277). 

The isoselective polymerization of styrene can be achieved by different mechanisms depending on the catalyst used. With a Ziegler-Natta catalyst, such as HCU/TIBA/MgCh, the insertion of the monomer into the metal-carbon bond of the active site is primary (1,2-), and the stereochemistry of the insertion is controlled by the chirality of the active sites (enantiomorphic-site control). ... 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

The enantiomorphic site control model attributes stereocontrol in isoselective polymerization to the initiator active site with no influence of the structure of the propagating chain end. The mechanism is supported by several observations ... 

For stereospecific polymerization of a-olefms such as propene, a chiral active center is needed, giving rise to diastereotopic transition states when combined with the prochiral monomer and thereby different activation energies for the insertion (see Figure 2). Stereospecificity may arise form the chiral /0-carbon atom at the terminal monomer unit of the growing chain - chain end control - or from a chiral catalyst site - enantiomorphic site control . The microstructure of the polymer produced depends on the mechanism of stereocontrol as well as on the metallocene used [42-44]. 

Hence, there can be four stereospecific polymerization mechanisms in primary polyinsertion, all of which have been documented with metallocene catalysts (Scheme 13) the two originated by the chiralities of the catalyst active sites, referred to as enantiomorphic site control (isospecific and syndio-specific site control), can be relatively strong, with differences in activation energy (AA. ) for the insertion of the two enantiofaces up to 5 kcal/mol. A value of 4.8 kcal/mol has been found by Zambelli and Bovey for a Ti-based heterogeneous catalyst. 

The above means that the active sites act as templates or molds for successive orientations of the monomers. The monomers are forced to approach these site with the same face. This sort of monomer placement is called enantiomorphic site control or catalyst site control. 

A prochiral monomer such as propylene offers two faces for coordination to a metal center. The steric environment at the active site, formed by the coordinated ligands and the growing polymer chain after activation with a cocatalyst, determines the orientation of the incoming monomer. In this case, the mechanism of stereoselection is referred to as enantiomorphic site control. The stereochemistry of the polymer is thus determined by the chirality relationship of the two coordination sites of the catalyst. However, every monomer insertion generates a new stereogenic center. As a consequence, chiral induction (enantioface preference) arises from the last-inserted monomer unit in the growing polymer chain. This mechanism is referred to as chain-end control (see Chapter 1 for an introduction to chain-end and enantiomorphic site control mechanisms in iPP synthesis). 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

Ring-opening polymerization of racemic a-methyl-/J-propiolactone using lipase PC catalyst proceeded enantioselectively to produce an optically active (S)-enriched polymer [68]. The highest ee value of the polymer was 0.50. NMR analysis of the product showed that the stereoselectivity during the propagation resulted from the catalyst enantiomorphic-site control. 

Several isospecific Ci-symmetry catalysts have also been described including (12-15). When activated with [Ph3C]+ [B(C6F5)4]-, (12) affords highly regioregular i-PP (mmmm = 95%) with the stereochemical defects predominantly being isolated rr triads, consistent with a self-correcting enantiomorphic site-control pathway. 2,73 The isospecificity was therefore explained by a mechanism... 

Three stereoisomers are possible in the cholestanylindene-derived zir-conocene complexes illustrated in Scheme 67. Two are racem-like, and the other is meso-like depending on the geometry of the metallocene moiety. The stereochemistry of the reaction is controlled by both the structure of the metallocene skeleton and steroidal substituent. Polymerization of propylene with 0-C activated with MAO gave polypropylene of 240,000, about 40% mmmm approximately 70% is due to enantiomorphic site control and the rest is due to chain-end control. Use of the catalyst derived from a /3-A-B mixture produced a mixture of polymers. The a-A and a-B/MAO catalysts afforded isotactic poly-... 

The steric triad distributions of polypropylene with structure (IS) are consistent with an enantiomorphic-site propagation model based on stereochemical control by the chirality of the active center on the catalyst 132,133). It should be noted that isotactic polypropylenes are formed along both propagation models, enantiomorphic-site control and chain-end control. 

A single step of the polymerization is analogous to a diastereoselective synthesis. Thus, to achieve a certain level of chemical stereocontrol, chirality of the catalytically active species is necessary. In metallocene catalysis, chirality may be associated with the transition metal, the ligand, or the growing polymer chain (e.g., the terminal monomer unit). Therefore, two basic mechanisms of stereocontrol are possible (145,146) (i) catalytic site control (also referred to as enantiomorphic site control), which is associated with the chirality at the transition metal or the ligand and (ii) chain-end control, which is caused by the chirality of the last inserted monomer unit. These two mechanisms cause the formation of microstructures that may be described by different statistics in catalytic site control, errors are corrected by the (nature (chirality) of the catalytic site (Bernoullian statistics), but chain-end controlled propagation is not capable of correcting the subsequently inserted monomers after a monomer has been incorrectly inserted (Markovian statistics). 

The main reason for isospecificity is enantiomorphic site control, which means a chiral active center is necessary. To measure the influence of... 

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... 

In (53) PJ and Pg represent chains with active L and D ends in the case of an end-controlled polymerization such as the one we are considering (or, in the case of the enantiomorphic catalyst site model, catalyst sites preferring L- and D-monomers, respectively), P L, P D etc. are the... 

The mechanistic complexities of stereoselectivity is further evidenced by a recent report by Maudoux et a/. who describe a chiral aluminum salen catalyst that generates highly isotactic PLA from rac-lactide (Pm-0.90). In this example, the kinetics indicated a dominant chain-end control mechanism, which contrasts to other chiral aluminum salen catalysts where enantiomorphic site control is thought to predomi-nate. ° All the previously mentioned chiral aluminum salen alkoxide systems require multiple days at elevated temperatures to polymerize -200 equiv. of lactide. The low activity of chiral aluminum salen systems towards lactide polymerization is a major drawback of these systems. 

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