Syndiotactic propagation, propylene

The syndiotactic propagation of propylene has been accounted for in terms of a steric control by the asymmetric carbon of the last propylene unit of the growing chain end 10 12). In the last unit control of the growing chain end, the steric triad fractions are represented by the following symmetric Bemoullian statistics89). 

On the basis of 13C NMR analysis on ethylene-propylene copolymers obtained with the soluble VCiy A1(C2H5)2C1 catalyst, Zambelli et al. 90,91> have shown that the syndiotactic propagation of propylene occurs by secondary insertion, i.e. in the 1 unit sequence blocks, whereas the primary insertion in the 0 unit blocks results in either atactic or isotactic propagation. 

Syndiotactic propagation of propylene is know to be catalyzed by homogeneous vanadium catalyst (1 ). In the polypropylene samples prepared with the homogeneous catalysts, the relative population of iso-, hetero- and syndiotactic triads is in accordance with that predicted from the first order Markov model (25, 26). There is no chiral structure around the homogeneous vanadium species. The stereochemistry of the entering monomer is controlled by the chirality of the growing chain end, in contrast with the isotactic propagation. 

To summarize, several mechanistic differences exist between isotactic propagation and syndiotactic propagation for the coordination polymerization of propylene... 

Natta et al. (63% Miyazawa cr al. (64) and Zambelli et cd. (65) determined the structure of isotactic polymers of cis and treats 1-dj propylene and of syndiotactic copolymers of dg-propylene with cis and trans 1-di-propyIene. In so doing, they succeeded in establishing that cis addition is common to both isotactic and syndiotactic propagation. 

The propagating polymer then terminates, producing an isotactic polypropylene. Linear polyethylene occurs whether the reaction takes place by insertion through this sequence or, as explained earlier, by ligand occupation of any available vacant site. This course, however, results in a syndiotactic polypropylene when propylene is the ligand. 

Figure 3.4 Origin of enantioface selectivity for primary, isotactic (A and B), and secondary, syndiotactic (C) propagation of propylene.

The best way to obtain syndiotactic polypropylene is the low temperature polymerization in the presence of homogeneous vanadium based catalysts (29)- Also, in this case the overall stereochemical mechanism of addition to the double bond is cis (30) as for isotactic propagation, but this seems to be the only common feature of the two types of stereospecific polymerization of propylene. 

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