Metallocene catalyst systems derived from bis(2-phenylindenyl)zirconium dichloride (1) yield elastomeric polypropylenes of low to intermediate tacticity. The sterically hindered metallocene bis(2(3,5-di-tert-butylphenyl)indenyl)zirconium dichloride (2) produces polypropylenes of much higher isotacticity than 1 ([mmmm] = 78% vs 44% in liquid propylene). Kinetic profiles of polymerizations conducted with 1/MMAO and 2/MMAO reveal that the initial rates of polymerization increase with increasing temperature; the activity of both catalysts decreases with time, but the decrease is less pronounced for 2/MMAO than for 1/MMAO. The polymer microstructure is highly dependent upon polymerization conditions: isotacticity decreases with increasing temperature and decreasing monomer concentration. Lower tacticity polypropylenes ( [mmmm] = 40%) generated with 2/MMAO at low monomer concentrations contain a high percentage of [mmrm] stereoerrors, which is indicative of a stereoblock microstructure comprised of isotactic stereosequences of opposite relative configuration. Activation of bis(2-(3,5-di-tertbutylphenyl)indenyl)zirconium dimethyl (3) with [Ph3C] [B(C6F5)(4)] (4) or B(C6F5)(3) (5) yielded catalysts with quite different polymerization behavior. A modest drop in productivity and isotacticity in propylene polymerization is observed when 3 is activated with 4, relative to MMAO, but catalysts derived from 3/B(C6F5)(3) exhibited low productivity and afforded amorphous polypropylenes of very low tacticity ([mmmm] = 12%).