Conformation of Stereoregular Polymers

Many of the stereoregular polymers prepared are highly crystalline, and the tendency to form ordered structures increases as the stereoregularity becomes more pronounced. We shall see later that crystalline order is usually associated with regular symmetrical polymer structures, whereas the asymmetric monomers form highly unsymmetrical chains. Some other factors must aid crystallite formation. 

FIGURE 6.2 A 3i-helix formed by a poly a-olefin, in this case polypropylene. This structure is also seen to fit a triangular template. 

The automatic identification of crystallinity with stereoregularity should be avoided however, as they are not necessarily synonymous, and whereas highly stereoregular polymers tend to be crystalline, the existence of any polymer in a crystalline state does not automatically mean the sample is markedly stereoregular. 

FIGURE 6.3 A poly a-olefin in the syndiotactic configuration showing the ttgg sequence along the chain, and the twofold helix, which fits a square template. 

FIGURE 6.4 A diagrammatic representation of various other ordered helical structures adopted by isotactic polymers. (From Natta, G. and Corradird, V., Rubber Chem. TechnoL, 33,703,1960. With permission.) 

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This point of view implies that in the comparison between ORD and CD of a polymer, one must preferentially consider similarity in shape and in behavior (even if magnitudes are different) instead of differences often used to prove contributions of preferential conformations of stereoregular polymers in solution. Let us consider now a few examples of non-regular optically active polymers undergoing chemical reactions or interactions. 

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