PROPENE POLYMERIZATION IN THE PRESENCE OF MGCL2-SUPPORTED ZIEGLER-NATTA CATALYSTS .5. MICROSTRUCTURAL CHARACTERIZATION OF THE ISOTACTIC (HEPTANE-INSOLUBLE) POLYPROPENE FRACTIONS

The stereosequence distribution in the heptane-insoluble fraction of polypropene samples prepared with a number of MgCl2-supported catalysts was analyzed in detail by means of C-13 NMR. The results indicate that what is conventionally taken as ''isotactic'' polypropene is normally a stereoblock polymer, in which at least part of the macromolecules contain long isotactic blocks spaced by much shorter ''syndiotactoid'' sequences. This implies that the active sites responsible for the formation of the two types of stereosequences are able to interconvert reversibly on the surface of the MgCl2 support in times which may be shorter than the average growth times of the polymer molecules. Possible models of active sites apt to explain the observed polymerization behavior are also briefly discussed.