A number of propylidene- and dimethylsilylene-bridged metallocene complexes of titanium, zirconium, and hafnium were prepared and employed in a study of propylene polymerization in the presence of methylaluminoxane. This work has revealed that catalysts based on hafnium are desirable for the production of elastomeric poly(propylene) in that they polymerize propylene to high molecular weight polymer and are indefinitely stable under typical polymerization conditions. Two of the zirconium complexes prepared have been characterized by X-ray crystallography. [2-Propylidene(eta(5)-indenyl)(eta(5)-cyclopentadienyl)]zirconium dichloride 3 : Space group Pna2(1); a = 9.792 (2), b = 12.156 (3), c = 12.751 (3) Angstrom; V = 1517.8 (5) Angstrom(3); Z = 4; R = 0.0176, R(w) = 0.0196 for 2188 reflections with F > 6.0 sigma(F). [Dimethylsilylene(eta(5)-indenyl)(eta(5)-cyclopentadienyl)]zirconium:dichloride 5 : Space group Pbca; a = 13.174 (4), b = 15.873 (5), c = 15.353 (4) Angstrom; V = 3210.6 (15) Angstrom(3); Z = 8; R = 0.0351, R(w) = 0.0349 for 2538 reflections with F > 6.0 sigma(F). The microstructure of the poly(propylene) produced was sensitive not only to changes in catalyst structure but also to polymerization conditions such as temperature. The stereoregularity of the polymer was surprisingly sensitive to changes in monomer concentration (i.e., pressure). By varying both catalyst structure and polymerization conditions, it is possible to produce elastomeric poly(propylene) that exhibits excellent elastic properties.