The morphology of low-density (0.86 g/cm 3 ), low-crystallinity (10%) elastomeric polypropylene (ePP) derived from a bis(2-arylindenyl)hafnium catalyst was investigated using a combination of polarized optical microscopy (POM), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), Fourier transform infrared (FT-IR) spectroscopy, and tapping mode atomic force microscopy (TM-AFM). This thermoplastic elastomer, when crystallized isothermally from the melt, exhibits morphologies reminiscent of classical semicrystalline polymers. The presence of lamellae, crosshatching, hedrites, and spherulites was revealed by high-resolution TM-AFM. POM confirmed the presence of hedrites and spherulites. The parent ePP can be fractionated into components of different average tacticities ([mmmm]%: 21%-76%) and crystallinities (1%-40% as determined by DSC and WAXS) but more similar molecular mass (M w: 147-432 kg/mol) and polydispersity (Mw/Mn: 2.1-2.5). The analysis of the morphologies of these fractions revealed large hierarchical structures for all but the lowest crystallinity fraction and crosshatching typical of the R-modification of crystalline isotactic polypropylene for all fractions. The solubility of the corresponding fractions in ether and heptane combined with new evidence for crystals with melting temperatures higher than 100 °C in all of the fractions is most consistent with a stereoblock microstructure of atactic and isotactic sequences in ePP.

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 M w ) 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.