The isothermal crystallization behavior of poly(beta-propiolactone) (PPL) blocks starting from lamellar microdomain structures (LMS) formed in molten PPL-block-polyethylene (PPL-b-PE) copolymers has been examined using time-resolved synchrotron small-angle X-ray scattering and Fourier transform infrared spectroscopy as a function of the crystallinity of PE blocks chi(PE). For getting PPL-b-PE copolymers with varying chi(PE), the amount of ethyl branches in PE blocks was controlled during polymerization. The crystallizable temperature of PE blocks was slightly higher than that of PPL blocks and the crystallization rate of PE blocks was extremely fast, so the crystallization of PE blocks occurred first on quenching to form the crystallized LMS, in which the PPL block crystallized. The time evolution for the crystallinity of PPL blocks chi(PPL) showed an exponential-type increase with no induction time. In contrast, LMS was replaced with the crystallized lamellar morphology and chi(PPL) showed a sigmoidal time evolution in PPL-b-PE with a higher amount of ethyl branches, where the PE block did not crystallize (i.e., chi(PE) = 0) prior to the crystallization of PPL blocks. It was concluded that the prompt crystallization of PE blocks just after quenching reinforced the existing LMS to make it stable against subsequent crystallization, yielding the confined crystallization of PPL blocks. (C) 2012 Elsevier Ltd. All rights reserved.