Site control, enantiomorphic

After activation using a Lewis acid cocatalyst such as MAO, catalysts A, B, and C will each have two coordination sites. In C, the two coordination sites are diastereotopic owing to the plane 

FIGURE 1.7 The three possible diastereomers of ethylene-bridged bis(indenyl)zirconocene diehloride (a) (/f,/f)-Et(lnd)2ZrCl2 (b) (5,5)-Et(lnd)2ZrCl2 (c) me5t -(/f,S)-Et(lnd)2ZrCl2. 

FIGURE 1.8 Lowest energy structures for propylene coordination to a chiral, C2-symmetric metallocene-isobutyl complex. For both enantiomers, the two sites shown for monomer coordination and chain growth are equivalent. 

Stereoselective Polymerization with Single-Site Catalysts 

In summary, the ability of C2-symmetric metallocene catalysts to produce iPP can be rationalized given the discussion above. The ligand imposes the same chiral orientation upon the growing polymer chain regardless of which of the two positions (coordination sites) on the metal is occupied by the chain. This results in the insertion of the same enantioface of propylene at both sites. The two coordination sites are therefore homotopic, and each insertion yields a new stereocenter with the same chirality as the previous monomer insertions. 

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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... 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

Site control versus chain-end control. Over the years two mechanisms have been put forward as being responsible for the stereo-control of the growing polymer chain firstly the site-control mechanism and secondly the chain-end control mechanism. In the site control mechanism the structure of the catalytic site determines the way the molecule of 1-alkene will insert (enantiomorphic site control). Obviously, the Cossee mechanism belongs to this class. As we have seen previously, propene is prochiral and a catalyst may attack either the re-face or the, v/-facc. If the catalyst itself is chiral as the one drawn in Figure 10.2, a diastereomeric complex forms and there may be a preference for the... 

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 ... 

Statistical analysis of the stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also supports the enantiomorphic site control model. 

The polymer stereosequence distributions obtained by NMR analysis are often analyzed by statistical propagation models to gain insight into the propagation mechanism [Bovey, 1972, 1982 Doi, 1979a,b, 1982 Ewen, 1984 Farina, 1987 Inoue et al., 1984 Le Borgne et al., 1988 Randall, 1977 Resconi et al., 2000 Shelden et al., 1965, 1969]. Propagation models exist for both catalyst (initiator) site control (also referred to as enantiomorphic site control) and polymer chain end control. The Bemoullian and Markov models describe polymerizations where stereochemistry is determined by polymer chain end control. The catalyst site control model describes polymerizations where stereochemistry is determined by the initiator. 

The formation of an isotactic polymer requires that insertion always occur at the same prochiral face of the propylene molecule. Theoretically, both a chiral catalytic site (enantiomorphic site control) and the newly formed asymmetric center of the last monomeric unit in the growing polymer chain (chain end control) may... 

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. 

Metal complexes which initiate rac-LA ROP with a high degree of stereocontrol are currently an area of major research interest and have the potential to produce a spectrum of different materials [19, 21], Much attention focuses on iso-selectivity as this can enable production of PLA of good thermal resistance (isotactic, stereoblock or even stereocomplex PLA). There are two mechanisms by which an initiator can exert iso-selectivity in rac-LA ROP (1) an enantiomorphic site control mechanism or (2) a chain end control mechanism. Enantiomorphic site control occurs using chiral initiators (Fig. 6) it is the chirality of the metal complex which... 

Arnold and colleagues have reported a series of chiral homoleptic yttrium and lanthanide fra(alkoxide) complexes [49, 50], These initiators (including complex 1) show high degrees of iso-selectivity and rapid rates, even at low temperatures. Thus, using the racemic mixture of the lanthanide initiator, stereoblock PLA was produced with a P, of 0.81 so far, this is the only known type of yttrium initiator able to exert such stereocontrol and a very exciting finding. Analysis of the stereoerrors indicates that an enantiomorphic site control mechanism is responsible for the iso-selectivity. 

As regards higher 7-olefins, their polymerisation with metallocene-based catalysts of class IV with Cs symmetry affords highly syndiotactic polymers as in the case of propylene [117]. This is a consequence of enantiomorphic site control over the polymerisation stereochemistry. 

Under an exclusively enantiomorphic site control mechanism, the symmetry of the catalyst would mandate homofacial insertion and cyclisation to yield cis... 

Figure 3.51 Enantiomorphic site control versus conformational control in the cyclisation step during cyclopolymerisation of a, co-diolefins 1,5-hexadiene (x=l), 1,6-heptadiene (x—2) and 1,7-octadiene (x=3) in the presence of a metallocene-based catalyst of C2 symmetry...

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... 

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