Isotactic addition steps

Figure 1. Schematic representation of syndiotactic and isotactic addition steps.

Recently, this type of deuterated monomer method was applied to the copolymerization of MM A and styrene in the presence of BC13 at low temperature, which gives a heterotactic-rich alternating copolymer.245,246 Using styrene-/3-dj, Goto et al. revealed that the addition of MMA to the styrene radical is syndiotactic-specific while the addition of styrene to the MMA radical is isotactic-specific, and that the alternation of these addition steps gives rise to the formation of a heterotactic-rich alternating sequence. 

Each monomer addition step interconverts the two organometallic components. The poly(methyl methacrylate) (PMMA) obtained is predominantly syndio-tactic, although isotactic PMMA has been obtained by using chiral indenyl zirconocenes in combination with non-zirconocene Lewis acids. No reports of attempted ethylene or a-olefin copolymerizations have been described. 

The special stability of the helix with three repeat units per turn for isotactic vinyl polymers in which the substituent is not too large was discussed in connection with Fig. 1.10a. Rodriguez has suggested a simple paper model which is also helpful in illustrating this structure. The following steps summarize this method additional details are available in this readily accessible reference. 

What is significant about these reactions is that only two possibilities exist addition with the same configuration (D -> DD or L LL) or addition with th< opposite configuration (D DL or L LD). We shall designate these isotactic (subscript i) or syndiotactic (subscript s) additions, respectively, and shal define the rate constants for the two steps kj and k. Therefore the rates o isotactic and syndiotactic propagation become... 

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

After the polymerization step, the reaction mixture is fed to a heated separation tank where the unreacted propylene is flashed off and recycled. The polymer slurry is then washed with alcohol to deactivate and remove the catalyst and the atactic polymer (the bad stuff.) Centrifuging the slurry removes the diluent from the isotactic PP (the good stuff.) The product is washed with acetone, dried, and stabilized with suitable additives. It is sold as a powder or can be pelletized into granules. 

Sequences of sterically identical steps lead to isotacticity of the product. Active sites with two vacancies or with one vacancy and a loosely bound chlorine characteristic of p-TiCl3 with a chainlike structure are nonstereospecific and result in polymers with increased atacticity.306 Molecular mechanics studies on the basis of this model lent additional support for the asymmetric active center being responsible for stereoregulation.341 It was concluded that the main factor determining... 

When the approaching monomer is in syndiotactic conformation and is added as such (c), the / carbon is fixed after addition, but the a carbon can rotate (d) and assume isotactic conformation after the addition of the next unit. The effect of the lithium cation is again assumed to be the driving force for the rotation of the potentially syndiotactic unit to the isotactic conformation. Repeating steps (a) and (b) leads to a threo-diisotactic chain, and repeated steps (c), (d) and (e) to an erythrodiisotac-tic chain. The addition of tetrahydrofuran reduces the number of threo-diisotactic units because it competes with carbonyl groups for coordination to Li+. This enhances the amount of monomer approaching the centre by the syndiotactic route. 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

When methyl iodide is replaced by 2-vinylpyridine in these reactions we have in effect the propagation step of polymerization or oligomerization, and indeed reaction proceeds in the same selective fashion. In the case of lithium counterion >95 % of isotactic trimer is formed by the next addition. Epimerization of the selectivity methylated product using K+OBu " in dimethylsulphoxide at 25 °C for 2 weeks yields a final mixture of approximately 1 2 1 of isotactic, heterotactic, and syndiotactic trimers. Thus the trimerization process must be kinetically controlled. 

Equation 22.11. As can be seen in this equation, insertion of the polymer during each of the two steps shown in this equation gives rise to a structure that is identical to the starting structure, except for the incorporation of an additional monomer unit. Thus, each inserted monomer has the same stereochemical relationship, and isotactic polypropylene is formed. This concept applies to catalysts other than metallocenes. The two binding sites in any catalyst possessing symmetry will be homotopic, and such catalysts should produce isotactic polypropylene if tfiey follow these steps for insertion in ttris geometry. 

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