MICROTACTICITY OF POLYPROPYLENE FRACTIONS PRODUCED BY DIFFERENT ACTIVE-SITES OF HETEROGENEOUS ZIEGLER-NATTA CATALYST

Polypropylene (PP) prepared with the MgCl2/TiCl4-AlEt(3)/Ph(2)Si(OMe)( 2) catalyst system was fractionated by a preparative temperature risin g elution fractionation (TREF) technique in the wide temperature range of room temperature to 130 degrees C. The results of fractionation sh owed that there were plural active sites in this heterogeneous catalys t. Some key fractions were selected for characterization of H-1- and C -13-NMR. H-1-NMR results suggested that the fractionation takes place according to tacticity. Based on analysis of C-13-NMR, it was conclude d that the polymerization mechanism proceeding on highly stereospecifi c active sites was active site-controlled and nonstereospecific active sites were able to convert into syndiotactic active sites during the polymerization, which led to the formation of a syndiotactic block. (C ) 1996 John Wiley & Sons, Inc.