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

Abstract

Polypropylene (PP) prepared with the MgCl 2 /TiCl 4 -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°C. The results of fractionation showed that there were plural active sites in this heterogeneous catalyst. Some key fractions were selected for characterization of 1 H- and 13 C-NMR. 1 H-NMR results suggested that the fractionation takes place according to tacticity. Based on analysis of 13 C-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.