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Keywords:

  • GPC analysis, PP;
  • poly(propylene) (PP);
  • stereospecificity;
  • Tref analysis, PP;
  • Ziegler-Natta catalysts

Abstract

Thumbnail image of graphical abstract

Resolution of the Tref curve of the highly crystalline fraction of poly(propylene) prepared with catalyst system 3.

Summary: This article describes the quantitative aspects of the isoselectivity distribution of isospecific centers in five supported titanium-based Ziegler–Natta catalyst systems containing different internal and external organic donors. Highly crystalline poly(propylene) fractions of the polymers (insoluble in o-xylene at 95 °C) were analyzed by analytical Tref and gel permeation chromatography (GPC) methods. Separation of the Tref curves into the peaks of sterically uniform components (each represented by the Lorentz function) showed that all these “isotactic” poly(propylene) fractions are nonuniform materials. Each highly crystalline fraction contains several distinct components that differ in stereoregularity. The [mmmm] values for different components (which have approximately the same isotacticity in different polymers) range from as high as 0.986–0.993 to much lower values, in the 0.935–0.955 range. The number of Tref components and their relative contents are substantially different for the crystalline fractions of the polymers produced with different catalyst systems; these parameters depend on the nature of the internal and the external electron donors in the catalysts. GPC analysis of the crystalline poly(propylene) fractions showed that each consists of four or five Flory components (the components produced by kinetically uniform active centers) with widely different average molecular weights, from ≈1 × 104 to ≈1.3 × 106. In general, the probabilities of chain transfer reactions for different active centers decrease with the increase in their isospecificity.