Abstract
The metallocene catalyst bis(2-(3,5-di-tert-butylphenyl)indenyl)hafnium dichloride yields elastomeric polypropylene in the presence of a MAO cocatalyst at 50 °C in liquid propylene. The polymer synthesized has Mw = 201 000 and narrow polydispersity (Mw/Mn = 2.3) with an average isotacticity [mmmm] = 34%. Several methods of fractionations were carried out to investigate the heterogeneity of the elastomeric polypropylene: thermal fractionation by DSC, boiling-solvent extraction, supercritical fluid fractionation employing increasing pressure profiling (IPP), and critical isobaric temperature rising elution fractionation (CITREF). Thermal fractionation reveals a distribution of crystallizable fractions with several melting points between 40 and 100 °C and a large melting endotherm at 150 °C which is attributed to the presence of long crystallizable segments. Fractionation by boiling-solvent extraction separates the elastomeric polypropylene into three fractions where the molecular weight, tacticity, and crystallinity increase in the following order: ether-soluble < heptane-soluble < heptane-insoluble fractions. Supercritical fluid fractionations by either molecular weight (IPP) or crystallinity (CITREF) show that the tacticities, melting points, and heats of fusion increase gradually with increasing molecular weight.
Published Version
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