The solid-solid crystalline transformation of isotactic polybutene-1 (iPB-1) from tetragonal form II to hexagonal form I could be accelerated by supercritical carbon dioxide (scCO(2)). In this study, in-situ Fourier transform infrared spectroscopy (FTIR) and two dimensional correlation spectroscopy (2DIR) is used to observe and investigate the crystallization behaviour of iPB in scCO(2) and compressed CO2. Based on the transform sequence given by 2DIR analysis, this transformation of helical chain structures is found to be initiated with the motion of side chains and followed by the movement of main chains. It is speculated that the motion of polymer chains was enhanced with the diffusion Of CO2. Also this crystalline transition is observed even in compressed CO2, suggesting that CO2 could also diffused into polymer under high pressure near the critical pressure. This diffusion Of CO2 is indicated by the growth of IR bands being assigned to the stretching vibration of C-O. A further investigation on the mechanically heating and freely cooling of iPB provides more evidences on the process of structure transition. The result implies that the nucleus of tetragonal form 11 formed in the melt is not affected by the existence of scCO(2), but the crystallization temperature become obviously lower. (C) 2009 Elsevier Ltd. All rights reserved.