The crystalline morphology of a double crystalline polyethylene-block-poly((beta-propiolactone) diblock copolymer (PE-b-PPL) was examined using synchrotron small-angle X-ray scattering (SR-SAXS), where the PPL block crystallized into delta- or beta-form according to the thermal history during sample preparation. Furthermore, the isothermal crystallization process of PE-b-PPL was pursued using simultaneous time resolved SR-SAXS and wide-angle X-ray diffraction (SR-WAXD). The crystallization behavior of delta PPL blocks (i.e., PPL blocks to crystallize into delta-form) was similar to that of PE blocks without any detectable induction time, so simultaneous crystallization of PE and delta PPL blocks was observed on quenching from a lamellar microdomain structure (LMS), yielding a largely distorted LMS (with the standard deviation of each lamella thickness similar to 36%). On the other hand, the beta PPL blocks (i.e., PPL blocks to crystallize into (3 form) started to crystallize after an appreciable induction time, showing sequential crystallization (i.e., advance crystallization of PE blocks and subsequent crystallization of (beta PPL blocks) to form a slightly distorted LMS (18%). Consequently, the final crystalline morphology was significantly different depending on the crystal structure of PPL blocks, though the crystallization started from an identical LMS. The crystallization mechanism of PE-b-PPL was discussed by considering the difference in crystallization kinetics between PE blocks and PPL (beta PPL or beta PPL) blocks. (C) 2017 Elsevier Ltd. All rights reserved.