Stereoisomerism of a-Olefin Polymers

In syndiotactic polypropylene the relative configuration of stereogenic tertiary carbon atoms alternates and thus, as in the isotactic polymer, provides regularity of structure although in a different form. Syndiotactic polypropylene 

It is worth noting that the absolute configuration of the tertiary carbon atoms in isotactic or syndiotactic polypropylene molecules has, in principle, no practical significance for the polymer properties, which might result from the presence of these carbon atoms in the main chain of the macromolecule. 

Whether or not the stereogenic tertiary carbon atoms in isotactic and syndiotactic polypropylene are chirotopic depends on the model chosen for their representation [16]. Stereochemical analysis, which followed Natta s discovery, involved two models of the polymer chain. Stereoregular polymers were considered, on the one hand, as the extrapolation towards high molecular weights of well-studied organic molecules, e.g. such as trihydroxyglutaric acid  

Therefore, a stereochemical model consisting of a finite chain with equal end-groups was used [17], Isotactic polypropylene is described as a meso-compound with the two half-molecules being a mirror image of each other. 

On the other hand, the concept of the infinite chain used by crystallographers does not take into account the effect of the end-groups but is only concerned with the symmetry elements of the chain itself. An attempt to reconcile the two concepts was made [18] small cyclic molecules are suitable models for linear macromolecules and have the advantage that they can be studied by means of point symmetry, much better known than the line symmetry required for infinite chains. From this point of view, 1,3,5-m-trimcthylcyclohcxane is a better configurational model of isotactic polypropylene than 3,5,7-trimethylno-nane and its homologues  

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