Web Release Date: January 3,
Heterogeneous Composition and Microstructure of Elastomeric Polypropylene from a Sterically Hindered 2-Arylindenylhafnium Catalyst
and
Departments of Chemistry and Chemical Engineering, Stanford University, Stanford, California 94305-5080, and Phasex Corporation, 360 Merrimack Street, Lawrence, Massachusetts 01843
Received March 4, 2002
Revised Manuscript Received November 11, 2003
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.
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