Propylene Polymerization with Titanium‐Based Ziegler‐Natta Catalysts: Effects of Temperature and Modifiers on Molecular Weight, Molecular Weight Distribution and Stereospecificity

Abstract

AbstractSummary: The article discusses two subjects: (a) temperature effects on the molecular weight, molecular weight distribution and stereoregularity of crystalline fractions of propylene polymers prepared with a supported TiCl4/dibutyl phthalate/MgCl2 – Al(i‐Bu)3 catalyst system, and (b) effects of different modifiers (components of cocatalyst mixtures that either greatly increase or decrease the content of the crystalline material) on the structures of the crystalline and the amorphous fraction. The principal approach of the research is the combination of GPC data for a series of polypropylene samples prepared under different conditions, the data on the distributional stereoregularity of the polymers (from analytical temperature‐rising elution fractionation (Tref) and DSC), and the 13C NMR analysis in order to produce a coherent picture of functioning of active centers. The combination of the experimental results shows that the crystalline fractions of polypropylene are produced by several families of active centers. The centers significantly differ in the average molecular weights of the polymer components they produce and in stereospecificity. The steric control exerted by the active centers of the highest isospecificity slightly decreases with temperature. In contrast, average isotacticity parameters of the crystalline fractions (estimated by NMR) increase with temperature. The latter effect is mainly the outcome of the variation in the relative contents of polymer components of low isotacticity in the crystalline fractions prepared at different temperatures. When a silane is used as a part of a cocatalyst, it not only poisons aspecific centers but poisons the centers of reduced isospecificity as well. This effect results in the increase of the average molecular weight of the crystalline fraction, narrowing of its molecular weight distribution and the increase of its average isotacticity.Resolution of GPC curves of crystalline fractions into Flory components at 50 °C.imageResolution of GPC curves of crystalline fractions into Flory components at 50 °C.