Various elastomeric polypropylenes (PPs) are synthesized through homogeneous propylene polymerization with metallocene catalyst (2-PhInd)(2)ZrCl2 in the presence of different cocatalyst mixtures: triethylaluminum (TEA)/methylaluminoxane (MAO) or triisobutylaluminum (TIBA)/MAO in the range of Al-AIR3/Al-MAO = 0.0-0.9. The cocatalyst formulation impacts the resultant polymer microstructure and the thermal and dynamic mechanical properties of the produced PPs. C-13 NMR analysis of the polymers reveals essentially atactic PP, with mmmm = 7.9%, when Al-AIR3/Al-MAO = 0.0. The mmmm pentad content is maximized when Al-AIR3/Al-MAO = 0.8; for TIBA, mmmm 23.5%; and for TEA, mmmm = 17.6%. Differential scanning calorimetry analysis and dynamic mechanical thermal analysis corroborate these findings. Specifically, T-m, Delta H-m, and T-g are essentially maximized under these conditions, and the minimum damping is observed for Al-AIR3/Al-MAO = 0.6-0.8. H-1 NMR analysis of the mixtures of catalyst and cocatalysts (without monomer) shows very minor differences for [ Zr]: Al-AIR3 in the range of 1: 1-1: 5. (C) 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 724-731