Stereoregularity of Polypropylene

The properties of polymers depend not only on overall chain length, but also on the degree to which the monomers are ordered along the chain. Different methods of preparation lead to vastly different degrees of ordering. A good example is found in the polymerization of propylene. This polymerizes predominantly head-to-tail , and leads to a stereocenter ( ) at every other atom in the polymer chain. 

Polypropylene made by free-radical polymerization is generally atactic , that is to say, there is no pattern to the stereochemistry. On the other hand, both isotactic polypropylene (in which all the stereocenters are the same) and syndiotactic polypropylene (in which the stereocenters alternate) may be made via the Ziegler-Natta process (see Chapter 18, Problem 4). Experimentally, both isotactic and syndiotactic polypropylene generally have higher melting points than atactic polypropylene. 

Examine three different strands ofpolypropylene. For each strand, assign R/S stereochemistry to each stereocenter. (All three strands have as their terminal monomer perfluoropropane in order to facilitate assignment of stereochemistry.) Which of the three strands corresponds to atactic polypropylene, isotactic polypropylene and syndiotactic polypropylene  

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Fig. 20. Stereoregularity of polypropylene plotted against grinding time of the support. (—A—A—), MgCij/TiCl (— — —), TiCh on MgClj/EB ground together (—0—0—), TiCl on ground MgClj + EB 46t. By permission of Butterworth Scientific Ltd.

Thble 9.1 Stereoregularity of Polypropylene Produced with Different Catalyst Systems... 

Ihble 9.2 Influence of the Transition Metal on the Stereoregularity of Polypropylene... 

Figure 3.14 Correlation of crystallinity with stereoregularity of polypropylene prepared with a series of thiophene Cl/istj-bridged metallocenes. Reproduced from Ref. [90].

Busico et al., on the other hand, came to a conclusion [299] that the stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts is a result of the interplay of two competing reactions. These are isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end. That makes this class of homogenous catalysts different from the typical Ziegler-Natta catalyst, because with these catalysts enantioselectivity and stereoselectivity are not necessarily coincidental [96]. 

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