Unbridged 2-arylindenyl metallocene complexes, such as bis(2-phenylindenyl)zirconium dichloride, in the presence of methylaluminoxane (MAO), are catalyst precursors for the synthesis of elastomeric polypropylene. The nature of the cocatalyst is shown to have a significant influence on the productivity and stereoselectivity of these metallocenes. Propylene polymerizations in liquid propylene in the presence of bis(2-phenylindenyl)zirconium dimethyl (5) activated by three different types of methyaluminoxane, trityl tetrakis(pentafluorophenyl)borate (1), dimethylanilinium tetrakis(pentafluoropheny)borate (2), or tris(pentafluorophenyl)borane (3), revealed a significant influence of activator on the polymerization behavior of these catalysts: both the productivity and stereoselectivity of the catalyst systems were highest with MAO and lowest with the borane 3 (MAO > 1 approximate to 2 much greater than 3). Catalysts derived from the borane 3 yielded amorphous atactic polypropylenes under conditions where those from cocatalysts MAO, 1, or 2 yielded semicrystalline, low-tacticity polypropylenes. The type of MAO also influences the stereoselectivity: MAOs containing isobutylaluminum. groups yielded higher tacticity polypropylenes than those with only methylaluminum groups. Solution polymerizations in solvents of varying polarities revealed that the polarity of the solvent influenced the productivities but had little influence on the stereoselectivity.