Trialkylaluminum reagents such as triisobutylaluminum (TIBA) and trimethylaluminum (TMA) influence the propylene polymerization behavior of methylaluminoxane (MAO)-activated dimethylsilyl[bis(2-phenylindenyl)]zirconium dichloride (1). TIBA selectively decreases the productivity of the meso diastereomer of 1 (1-meso). This is manifested in an increase in the isotacticity of polypropylene blends prepared from mixtures of 1-rac and 1-meso compared to polymer blends prepared in the absence of TIBA. In contrast, added trimethylaluminum decreases the productivity of both 1-rac and l-meso. Similar trends in reactivity with AlR3 are observed for unbridged bis(2-phenylindenyl)zirconium dichloride (2) compared to a mixture of the bridged catalysts containing 57% 1-rac. Mechanical testing indicates that polypropylene from 2 has superior elastomeric properties to reactor blends prepared from mixtures of 1-rac and 1-meso in the presence and absence of trialkylaluminum reagents. Reactor blends prepared without AlR3 exhibit poorer recovery at 100% and 300% strain than that of the elastomeric polypropylene from 2. The reactor blends prepared in the presence of TIBA exhibit pronounced yielding and necking during testing, characteristic of a softened plastic.