Mesophase separation between the incompatible blocks in multiblock copolymers OBC (Olefin Block Copolymers), coupled with the macrophase separation in its blends with iPP (isotactic polypropylene), makes iPP/OBC a unique blend system to understand the interaction between multiple phase separations and the subsequent or simultaneous crystallization in multi-scales. The role played by OBC content as well as its molecular structure on phase morphology and crystallization kinetics in blends of iPP/OBC has been investigated systematically to demonstrate synergistic coupling between thermodynamics and kinetics. The total isothermal crystallization rate, nucleation density (the nuclei number per unit volume) and linear growth rate of iPP are found to be composition and component dependent, which can be attributed to the interplay among the enhanced mobility due to the presence of a more mobile component (lower glass transition temperature and lower melting temperature) serving as an "accelerant", the associated composition dependent inter-diffusion coefficient, and the obstruction effect caused by the OBC aggregates with increasing the diluent concentration. Furthermore, the distinct phase separation morphology determined by the compatibility between iPP and various OBC influenced the growth rate of iPP spherulite as well. (c) 2013 Elsevier Ltd. All rights reserved.