Polymerization 2- propene, stereochemistry

Insertion of the next propene by a repeat of the previous step now starts the polymerization. Each new C-C bond is formed on the coordination sphere of the Ti atom by transformation of a n complex into a O complex. Repetition of this process leads to polymerization. We have shown the polymer with isotactic stereochemistry, and this control over the stereochemistry reflects the close proximity of the new propene molecule and the growing polymer. 

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. 

Some Common Stereochemistries Resulting from Z-N Polymerization of Propene and Other 1-Alkenes... 

Draw structures of each of the metallocene catalysts given below and indicate (a) type of symmetry and (b) stereochemistry of the polymer produced from polymerization of propene. 

Sacchi, M. C. Barsties, E. Tritto, L Locatelli, R Brintzinger, H.-H. Stehling, U. Stereochemistry of first monomer insertion into metal-methyl bond A tool for evaluating Hgand-monomer interactions in propene polymerization with metallocene catalysts. Macromolecules 1997, 30, 3955-3957. 

Erker, G Nolte, R. Aul, R. Wilker, S. Kruger, C. Noe, R. Cp-substituent additivity effects controlling the stereochemistry of the propene polymerization reaction at conformationaUy unrestricted (Cp-CHRiR2)2ZrCl2/methylalumoxane catalysts. J. Am. Chem. Soc. 1991,113, 7594—7602. 

In view of the stereochemistry, Natta managed to synthesize isotactic crystalline polypropene with the combination catalysts that have previously been discovered by Ziegler [230]. He thus achieved a breakthrough for a technical application of polypropene. The most widely used catalyst for the stereospecific polymerization of propene still consists of titanium halogenides and alkylaluminum compounds. In addition to this catalyst, a large number of other systems have been tested. Table 14 lists important heterogeneous systems. 

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