Site control isotactic polymers

The rac-isomers have a twofold axis and therefore C2-symmetry. The meso-isomer has a mirror plane as the symmetry element and therefore Cs-symmetry. For polymerisation reactions the racemic mixture can be used since the two chains produced by the two enantiomers are identical when begin- and end-groups are not considered. Note When catalysts of this type are to be used for asymmetric synthesis, e.g. as Lewis acids in Diels-Alder reactions, separation of the enantiomers is a prerequisite [25], 

The formal view. The formal view is much simpler. The racemic catalysts have a twofold axis and therefore C2-symmetry. Both sites of the catalysts will therefore preferentially co-ordinate to the same face (be it re or si) of propene. Both sites will show the same enantiospecificity the twofold axis converts one site in the other one. Subsequently, insertion will lead to the same enantiomer. According to the definition of Natta, this means that isotactic polymer will be formed. If the chain would move from one site to the other without insertion of a next molecule of propene, it will continue making the same absolute configuration at the branched carbon atom. Hence, no mistake occurs when this happens. 

When the chain is transferredfrom one zirconium atom to another, there is a 50% chance in the racemate that the chain will continue to grow producing the opposite absolute configuration. 

If the zirconium complex racemises once in a while a blocky isotactic polypropylene will be obtained. 

6 — HOMOGENEOUS CATALYSIS WITH TRANSITION METAL COMPLEXES 

For polymerization to occur the X atoms in Fig. 6.18 are replaced by an alkyl chain and a coordinating propene molecule. Coordination of propene introduces a second chiral element and several diastereomers can be envisaged. The step-by-step regulation of the stereochemistry by the site can be most clearly depicted by drawing the molecule as shown in Fig 6.19. 

The next molecule of propene will coordinate onto the complex with the methyl group pointing upwards (6.19.d) migration of the new alkyl anion gives e with the stereochemistry shown. When this process is repeated several times, forming the polymer carbon chain in the plane of the figure, structure f is obtained. From this structure we cannot immediately deduce what the microstructure is, since the polymer chain is not stretched as shown before (Fig. 6.11). By rotating 

6 — HOMOGENEOUS CATALYSE WITH TRANSITION METAL COMPLEXES 

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These two mechanisms can be distinguished by observation of the stereoerrors found in the polymers. Catalytic-site controlled isotactic polymers have an error described in Fig. 4. One misinsertion has little effect on the face selectivity of the next insertion, because it is governed by the catalyst structure. It results in a sequence of mmmrrmmm. Thus mmmr, mmrr and mrrm pentads are found besides mmmm. On the other hand, in chain-end control, once a monomer is misinserted, the opposite chirality governs the next monomer insertion. It gives an mmmrmmm sequence, as shown in Fig. 4, and mmmr and mmrm appear in the spectra. 

On the basis of the microstructure of the prevailing isotactic polymer chains, it is well established that the steric control of the heterogeneous Ziegler-Natta catalysts is due to the chirality of the catalytic site and not to the configuration of the last inserted monomer unit.28,29,95... 

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... 

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... 

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-... 

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...

We now turn to the actual polymerization process and we will try to present a series of pictures that clarifies how chain-end control can be used to obtain either syndiotactic or isotactic polymers. Subsequently we will see how a chiral site can influence the production of syndiotactic or isotactic polymers. Finally, after the separate stories of chain-end control and site control, the reader will be confused by introducing the following elements (1) pure chain-end control can truly occur when the catalyst site does not contain chirality (2) but since we are making chiral chain ends in all instances, pure site control does not exist. In a polymerization governed by site control there will potentially always be the influence of chain-end control. This does not change our story fundamentally all we want to show is that stereoregular polymers can indeed be made, and which factors play a role but their relative importance remains hard to predict. 

When activated with MAO complex 6, under pressure, polymerizes propylene producing isotactic polymer (Table 5). The fact that in the 13C-NMR spectrum of the polymers no mrmm signals were found, can testify about the formation of the polymers exclusively by the site control mechanism. Increasing the temperature induces an increase of the reaction rate, but the molecular weight... 

The first chiral bridged zirconocene synthesized in 1984 by Brintzinger and used as an isospecific polymerization catalyst was racemic ethylenebis-(4,5,6,7-tetrahydro-l-indenyl)zirconium dichloride (see Structure 9) [45]. Ewen showed that the analogous ethylenebis(l-indenyl)titanium dichloride (a mixture of the meso form and the racemate) produces a mixture of isotactic and atactic polypropylene [46]. The chiral titanocene as well as the zirconocene were shown to work by enantiomorphic site control in the case of the titanocene, the achiral meso structure causes the formation of atactic polymer. 

This brings us to double stereoselection and reinforcement of the mechanisms. If the site (a)symmetry were to control the orientation of the chain, and if, then, the orientation of the incoming propene is controlled by both the chain and the site, the highest stereoselection is obtained when the two influences reinforce one another. For 1,2-insertion this can be done most effectively for isotactic polymerization, since chain-end control naturally leads to isotactic polymer and this we can reinforce by site control with ligands of the bis(indenyl)ethane type. The chain-end influence of short chains is smaller than that of longer polymer chain and therefore short chain ends lead to lower selectivities. It may also be irrferred that making syndiotactic polymer via a 1,2-insertion mechanism on Ti or Zr complexes is indeed more difficult than making an isotactic polymer, because the two mechanisms now play a counterproductive role. 

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