Isotactic site control

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 production of highly isotactic PPs with Zr- and Hf-FI catalysts//-Bu3Al/ Ph3CB(C6F5)4 (phenoxy-amine complexes site-controlled polymerization with 1,2-insertion) is in sharp contrast to that of highly syndiotactic PPs with Ti-FI cata-lysts/MAO (phenoxy-imine complexes chain-end controlled polymerization with 2,1-insertion), which will be described later [64]. 

Above we mentioned the results reported by Ewen [13] who found that Cp2TiPh2/alumoxane gives a polypropene with isotactic stereoblocks. Naturally, this achiral catalyst can only give chain-end control as it lacks the necessary chiral centre for site control. In the 13C NMR the stereoblocks can be clearly observed as they lead to the typical 1 1 ratio of mmmr and mmrm absorptions in addition to the main peak of mmmm pentads. These are two simple examples showing how the analysis of the 13C NMR spectra can be used for the determination of the most likely mechanism of control of the stereochemistry. Obviously, further details can be obtained from the statistical analysis of the spectra and very neat examples are known [18],... 

In a system with site control ideally the mistake leads to a single odd insertion in the chain the site enforces for instance the growth of an isotactic chain with all m configurations and after one mistake has occurred it will return to producing the same configurations. In other words, an isospecific catalyst will produce a polymer chain with all methyl groups pointing towards us (as in... 

Figure 10.12. Single mistake during a site-controlled isotactic polymerization and schematic 13C NMR spectrum of the methyl region.

Figure 10.14. Simplified view of site-controlled isotactic polymerization...

However, there are numerous reported instances of stereocontrol by a site-control mechanism involving chiral metal catalysts. That is, Nozaki and coworkers first illustrated the asymmetric alternating copolymerization of cyclohexene oxide and CO2 employing a chiral zinc catalyst derived from an amino alcohol (Fig. 2a) [13-16]. This was soon followed by studies of Coates and coworkers utilizing an imine-oxazoline zinc catalyst (Fig. 2b) [17]. Both investigations provided isotactic poly(cyclohexene carbonate) (Fig. 3) with enantiomeric excess of approximately 70%. 

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 term on the left is extremely sensitive, and this criterion should be used only with sufficiently accurate triad data. This is especially important if the polymer is very highly isotactic or syndiotactic, that is, with very small value of either (rr) or (mm). The term 4(mm)(rr)/ (mr)2 is considerably larger than one for the Markov and catalyst site control models. 

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... 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

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... 

Fig. 7 Polymer 1, chain end control polymer 2, enantiomeric site control, where i and s are the relative stereochemistry of a pairwise addition of lactide units, i isotactic enchainment, s syndiotactic enchainment...

The ratios of the minor defect tetrads resulting from stereoerror (i.e. not iii, in the case of isotactic PLA) can be used to determine which mechanism is operating. For a chain end control and enantiomeric site control, the relative integrals should be in the ratio 1 1 1 (sii/isi/iis) or 1 2 1 1 (iis/isi/sis/sii), respectively [21],... 

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