Polymorphic transformation from the form II to form I crystal was investigated for isotactic polybutene-1 (PB) and butene-1/ethylene copolymers (PBE) with differential scanning calorimetry (DSC) and dielectric response spectroscopy (DRS). DRS detected the so-called alpha relaxation of segments within the amorphous phase, and DSC traced a degree of form II to I transformation with time. It turned out that incorporation of the ethylene comonomers in PBE suppressed the crystallization and plasticized the amorphous phase, and the II to I transformation was greatly accelerated in particular at low T close to T-g. To normalize T-g enabled normalization of the a relaxation frequency (detected in DRS), but not the II to I transformation rate (detected in DSC): extra acceleration was noted for samples with a number fraction of ethylene monomers of 4.3% or higher. The extra acceleration is attributed to (1) heterogeneity of segmental mobility that facilitates the crystal growth and (2) crystal defects accumulating at the fold surface that cause internal stress to accelerate the nucleation. (C) 2018 Elsevier Ltd. All rights reserved.