A series of MAO-activated Cl-symmetric indenyl-ansa-dithienocyclopentadienyl-based zir-conocenes have been used to produce isotactic polypropylenes of medium to high molecular weights, of different degrees of stereoregularity, and free from regioerrors. The effect of the presence of rr defects on the polymorphic behavior and mechanical properties of polypropylene is analyzed. The presence of rr defects induces crystallization of gamma form and of disordered modifications intermediate between alpha and gamma forms. A linear relationship between the amount of gamma form and the average length of isotactic sequences has been found. Samples with low concentration of rr defects, up to 3-4%, present high melting temperatures, in the range 160-130 degreesC, and behave as stiff-plastic materials; sample with higher rr content, in the range 4-6% and melting temperatures around 115-120 degreesC are highly flexible thermoplastic materials, and, finally, samples with concentration of rr defects in the range 7-11% and melting temperatures in the range 8-110 degreesC are thermoplastic elastomers with high strength. The fine-tuning of the chain microstructure, achieved by a tailored design of new metallocene catalysts, has allowed production of new polypropylenes having desired properties, intermediate between those of stiff plastic and elastomeric materials.