The control of the structural organization of the matter at meso- and nano-scale at molecular level as a tool for obtaining a range of materials with tailored properties is still a challenge. We show that in the case of isotactic polypropylene (iPP) the material properties can be finely tuned at molecular level via formation of a solid mesophase, characterized by an intermediate degree of order between amorphous and crystalline state. The effect of different degree of stereoregularity on the mesophase formation, thermal stability, morphology, and polymorphic transformations induced by stress is analyzed at different length scales, using different technique including wide and small angle X-ray scattering, and atomic and optical microscopy. In contrast to the lamellar morphology of crystals normally obtained from the melt, the solid mesophase show a nodular morphology. The mesophase transforms by thermal treatments into the α form preserving the nodular morphology, with increase of strength while maintaining the ductility typical of the mesophase. We show that annealing of the mesophase permits a precise adjustment of crystallinity and size of the nodular crystals offering unprecedented options to modify the mechanical properties. Preliminary results of neutron scattering experiments performed in an extended q range (0.1< q < 400 nm-1) are also shown. These experiments allow achieving simultaneous information about transformations occurring in the mesophase at the three key length scales of polymers, i.e. the change involving the chain conformation (0.1 nm), the packing mode of the helical chains in mesomorphic aggregates up to crystallization (1 nm) and the structural rearrangement of nodules in more regular super-structures embedded into the amorphous phase (~10nm).