Heterogeneous Composition and Microstructure of Elastomeric Polypropylene from a Sterically Hindered 2-Arylindenylhafnium Catalyst

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.