Unbridged 2-arylindenylmetallocene complexes such as bis(2-phenylindenyl)zirconium dichloride in the presence of methylaluminoxane (MAO) are catalyst precursors for the synthesis of elastomeric polypropylenes. These catalysts are also active for the copolymerization of ethylene and propylene. Addition of small amounts of ethylene to a polymerization system derived from the 2-arylindene metallocenes results in a large and nonlinear increase in the polymerization rate. This increase in rate cannot be explained by the faster rate of ethylene insertion but is likely the result of the activation of dormant catalyst sites by ethylene. This "ethylene effect" is much larger for bis(2-phenylindenyl)zirconium dichloride/MAO ((2PhInd)(2)ZrCl2/MAO, 1/MAO) and bis[2-(3', 5'-bis(trifluoromethyl)phenyl)indenyl]zirconium dichloride/MAO ((CF3)(2)PhInd)(2)ZrCl2/MAO, 2/MAO) metallocene catalysts than for bridged rac-ethylene-bis(indenyl)zirconium dichloride/MAO (EBIZrCl2/MAO, 3/MAO) metallocene. Copolymerization parameters for 1-3/MAO reveal that the unbridged 2-arylindene catalysts show a tendency toward random or slightly blocky incorporation of comonomers (r(e)r(p) = 1.0-1.9 greater than or equal to 1), Whereas the ansa-metallocene 3/MAO exhibits a tendency toward alternating comonomer distribution (r(e)r(p) = 0.5 < 1).