During the investigation of the crystallization of metallocene isotactic polypropylene and copolymers with low amount of 1-butene and 1-hexene at large undercoolings, an unexpected behavior has been found. Random copolymers crystallize faster than the homopolymer between 80 and 40 degrees C, while at high temperatures the overall crystallization rates follow the expected trend. On the basis of structural and morphological evidences we suggest that the overall structuring kinetics of the homopolymer is slowed down by the concomitant formation of mesophase and monoclinic structures. This effect is absent in the copolymers because the branched counits retard the development of mesophase. (C) 2014 Elsevier Ltd. All rights reserved.