Microtacticity of polypropylene fractions produced by different active sites of heterogeneous Ziegler–Natta catalyst

Polypropylene (PP) prepared with the MgCl₂/TiCl₄–AlEt₃/Ph₂Si(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°C. The results of fractionation showed that there were plural active sites in this heterogeneous catalyst. Some key fractions were selected for characterization of ¹H- and ¹³C-NMR. ¹H-NMR results suggested that the fractionation takes place according to tacticity. Based on analysis of ¹³C-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 syn-diotactic block. © 1996 John Wiley & Sons, Inc.