Polypropylene (PP) prepared with the MgCl2/TiCl4-AlEt(3)/Ph(2)Si(OMe)(2) catalyst system was fractionated by a preparative temperature rising elution fractionation (TREF) technique in the wide temperature range of room temperature to 130 degrees C. The results of fractionation showed that there were plural active sites in this heterogeneous catalyst. 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 concluded that the polymerization mechanism proceeding on highly stereospecific 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.