High-resolution C-13 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 determin ed by means of high-resolution C-13 NMR and analyzed in terms of statistica l models of increasing sophistication. Two-site models, including the one n ormally used for the interpretation of "routine" C-13 NMR data at pentad le vel, 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 a s a mixture of highly isotactic, weakly isotactic ("isotactoid") and syndio tactic sequences. According to such model, the two fractions comprise the s ame three building blocks (the configuration of the three different types o f stereosequences being almost invariant) and differ merely in their relati ve amounts (which indicates a stereoblock nature). The correlations with th e physical properties of the materials and the implications on the nature o f the catalytic species are also briefly discussed.