High-resolution13C NMR configurational analysis of polypropylene made with MgCl2-supported Ziegler-Natta catalysts. 1. The 'model' system MgCl2/TiCl4-2,6-dimethylpyridine/Al(C2H5)3

The stereosequence distribution of the "atactic" and "isotactic" fractions of a polypropylene sample made with a MgCl2-supported catalyst was determined by means of high-resolution C-13 NMR and analyzed in terms of statistical models of increasing sophistication. Two-site models, including the one normally used for the interpretation of "routine" C-13 NMR data at pentad level, were shown to be inconsistent with the much finer high-resolution data. A good agreement between experimental and calculated distributions could be obtained only in terms of a three-site model, describing each fraction as a mixture of highly isotactic, weakly isotactic ("isotactoid") and syndiotactic sequences. According to such model, the two fractions comprise the same three building blocks (the configuration of the three different types of stereosequences being almost invariant) and differ merely in their relative amounts (which indicates a stereoblock nature). The correlations with the physical properties of the materials and the implications on the nature of the catalytic species are also briefly discussed.