Stereochemical Issues in Polymer Chemistry

Control of polymer stereochemistry is a major research area in academic and industrial laboratories. This is because polymers with different stereochemistries often have very different properties. For example, atactic polypropylene is a gummy, sticky paste sometimes used as a binder, while isotactic polypropylene is a rugged plastic used for bottle caps. Recent advances (see the Going Deeper highlight on the next page and Chapter 13) have greatly improved the ability to control polymer stereochemistry, leading to commercial production of new families of polymers with unprecedented properties. 

Another stereochemical issue is helicity, as some simple polymers can adopt a helical shape. We defer discussion of this to Section 6.8.2, in which we discuss helicity in general. 

Just for fun, calculate the mass of a sample of molecular weight 100,000 polypropylene that has just one molecule of each of the 10 possible stereoisomers. In doing so. 

The C2-symmetric Zr metallocene catalyst (top) and a highly schematic view of propylene complexing to it. The middle two structures use the same face of the propylene, and lead to the same tacticity because of the C2 symmetry of the catalyst. The bottom two structures use the opposite faceof the olefin. The adverse steric interaction of the CH3 with the aromatic ring disfavors these structures. 

One of the most exciting recent advances in organic and organometallic chemistry has been the development of new catalysts that produce polypropylene with high stereochemical purity. Both isotactic and syndiotactic polypropylene are now made commercially with a new class of metallocene catalysts, prototypes of which are shown below. The mechanism of the polymerization reaction is discussed in Chapter 17. Here we will focus on the stereochemistry, because symmetry principles of the sort we discussed above were crucial in the design of this chemistry. 

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