Polymerization of propylene with bis(2-phenylindenyl)zirconium dichloride and bis(2-[3,5 -bistrifluoromethyl)indenyl]zirconium dichloride produces elastomeric polypropylene. The elastomeric properties of these polymers have been interpreted in terms of a stereoblock microstructure. Analysis of the microstructure by C-13 NMR reveals isotactic pentad contents [mmmm] ranging from 6 to 74%. The hafnium derivatives were investigated to probe the influence of the transition metal on the polymerization behavior. The hafnium-based catalysts yield polypropylenes that are significantly less isotactic than the corresponding zirconium compounds, although molecular weights and productivities were similar for hafnium and zirconium derivatives. The X-ray crystal studies of these catalysts show nearly identical structures for corresponding zirconium and hafnium compounds. Variable temperature NMR of the metallocene dibenzyl analogues showed behavior consistent with rotation of indenyl ligands, where rotation of the indenyl ligand of bis(2-phenylindenyl)zirconium dibenzyl was 6800 Hz and that of hafnium was 6700 Hz at 20 degrees C. Based on our proposed mechanism of polymerization, the origin of the different microstructures can be ascribed to a faster propagation of the isospecific zirconium site relative to that of the isospecific hafnium site.