The polymorphic transformations associated with the plastic deformation of isotactic polypropylene samples of different stereoregularity, prepared with different metallocene catalysts, have been studied. Crystals of alpha or gamma forms, present in the unstretched samples, transform into the mesomorphic form by stretching. The formation of the mesophase facilitates successive further deformation up to very high strains and produces development of outstanding unusual properties of high flexibility and elasticity. The phase diagram of isotactic polypropylene, where the regions of stability of the different polymorphic forms are defined as a function of stereoregularity and degree of deformation, is reported.