A comparative analysis of the structure and mechanical properties of two elastomeric amorphous and semicrystalline polypropylenes samples, produced with C-2v-symmetric and chiral C-2-symmetric ansa-zirconocene catalysts, respectively, is presented. The poorly isotactic polypropylene (iam-PP) of medium molecular weight, prepared with the chiral C-2-symmetric catalyst, slowly crystallizes, by aging at room temperature or by stretching, in a continuum of disordered modifications intermediate between alpha and gamma forms of isotactic polypropylene. Structural and morphological transformations occur during stretching. Disordered alpha/gamma modifications, containing a high fraction of perpendicular chains, present in the unoriented film, transform by stretching at high deformations into structures more similar to the a. form with a high content of parallel chains. The development of the small level of crystallinity induces elastic properties. The small crystalline domains in the amorphous matrix, indeed, act as physical knots of the elastomeric lattice. This poorly isotactic iam-PP sample shows, instead, poor elastic properties and viscous flow at high deformations in the amorphous state, before crystallization, because of the not high molecular weight. The unique crystallization mode of the iam-PP sample explains the remarkable enhancement of elastic properties in low stereoregular polypropylene upon development of crystallinity. The fully amorphous high molecular weight atactic polypropylene (a-PP), prepared with the C-2v-symmetric catalyst, presents elastic behavior only in a small deformation range (up to 300% strain) and experiences viscous flow of the chains at high deformations. The elasticity arises from the physical network generated by the high degree of entanglement of the polymeric chains generated by the high molecular weight of the sample. The poorly isotactic iam-PP sample, before crystallization, presents much poorer elastic properties than the a-PP sample because of the lower molecular weight. After crystallization, it shows better elastic properties in a large range of deformation with higher strength, notwithstanding the lower molecular weight, due to the presence of crystallinity.