Enantiomorphic site

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... 

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]. 

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). 

FIGURE 2.13 The pentad distribution of polymer formed by (CH3)2C(3-fert-butyl-4-methyl-C5H2)(Ci3Hg)ZrCl2/MAO (ih-S) at Tp = 20 °C is poorly described by a one-site enantiomorphic site control model. A superior fit is obtained with a two-site model that approximates hemiisotactic polypropylene. (Reprinted with permission from Miller, S. A. Bercaw, J. E. OrganometalUcs 2006, 25, 3576-3592. Copyright 2006 American Chemical Society.)... 

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). ... 

When isoselective polymerization is promoted by nickel catalysts, such as the Ni(acac)2/Et3N/MAO system, the insertion of the monomer is mainly secondary (2,1-). The steric control of the polymerization still arises from chiral sites (enantiomorphic-site control), but the configuration of the sites apparently changes rather often during chain propagation since iPS with lower isotacticity is obtained. 

One component obeys the Bemoullian model the other two obey the enantiomorphic- site model. Similarly, the NMR data of fractionated copolymers can be used to demonstrate the presence of multiple components in the copolymers. An example is shown of ethylene-propylene copolymers where the NMR/fractionation data are used to show the presence of two or three catalytic sites. 

An excellent way to treat such data is to use reaction probability models.(1,2) In the NMR analysis of tacticity, it is frequently possible to distinguish whether the configuration is chain-end controlled or catalytic-site controlled during polymerization. Various statistical models have been proposed. The chain-end controlled models include Bemoullian (B), and first- and second-order Markovian (Ml and M2) statistics.(1) The simplest catalytic-site controlled model is the enantiomorphic site (E) model.(3) The relationship between the chain-end and catalytic-site controlled models and possible hybrid models have been delineated in a recent article.(4)... 

Sample II being someWhat more syndiotactic. Thus, the selectivity control agent (and any attendant changes in synthetic procedure) appears to change (1) the amount of polymer made at different sites (wj w2 w3 = 39 42 20), and (2) the nature of the Bemoullian polymer. It appears from this analysis that the nature of the enantiomorphic catalytic sites remains unchanged in the absence of the selectivity control agent. 

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. 

The enantiomorphic site selectivity of (R)-(—)-(263) allows highly syndiotactic PLA to be prepared via the polymerization of meso-LA (Mn = 12,030, Mn calc= 13,540, Mw/Mn= 1.05).802 The ring opening of meso-LA by (R)-(—)-(263) occurs to produce a syndiotactic propagating chain bound to the metal via an (S)-lactic acid unit.803 Attempts to produce syndiotactic PLA via the ROP of meso-LA using rac-( )-(263) instead afforded heterotactic-biased material. 

Scheme 1.1 Typical steric defects in a (mainly) isotactic poly-l-alkene chain (adapted Fisher projections) for (a) chain-end-stereocontrol (b) enantiomorphic site stereocontrol.

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... 

Audisio studied the microtacticity of the 1,4-rrans-polypentadiene [—CH2—CH=CH—CH(CH3)—] in connection with that of the poly-(methyltetramethylene) [—CH2—CH2—CH2—CHfCHs)—] obtained from the preceding compound by reduction (109, 110), and succeeded in evaluating the distribution of the triads mm, mr, and rr. He has proposed an interpretation according to a one-parameter model based on enantiomorphic catalyst sites (111) (see Table 4, column 4, 3/ = 1). 

The probabilistic aspect of error propagation in isotactic polypropylene was treated both as a second-order Markov chain (in terms of m and r dyads) (408) and, in terms of a model of enantiomorphic catalyst sites, as asymmetric Ber-... 

Table 9.9 Enantiomorph discrimination for protein maps based on the Se sites for 1JC4 triai 5...

The a vaiues are a measure of the electron-density variation in the protein and solvent regions, and the ratio of these numbers is a measure of the contrast between the two regions. Since anomalous dispersion data were used to phase the maps, the map for the correct hand will show greater contrast. In this case, the original direct-methods sites give rise to greater contrast thereby indicating that these sites do correspond to the correct enantiomorph. 

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