The polymerization and crystallization of cyclic butylene terephthalate oligomers (CBT) were followed by fiber Bragg grating (FBG) and normal force measurements under isothermal conditions at T = 170 and 190 degrees C, respectively. It was found that the FBG and normal force sense only the crystallization-induced shrinkage. The course of the FBG signal and the normal force as a function of time suggested that crystallization of the polymerized CBT (pCBT) occurs in two steps. The primary crystallization-induced shrinkage is several hundreds by contrast to the secondary one showing several tens in ppm/min unit according to the FBG results. The two-step crystallization was confirmed by normal force measurements. The studied by differential scanning calorimetry and wide-angle X-ray scattering. It was found that the crystallinity and the crystalline parameters slightly differ for the pCBTs formed at T 170 and 190 degrees C, respectively. The pCBT produced at T 190 degrees C had slightly higher crystallinity and more perfect crystals than the pCBT formed at T = 170 degrees C. The reliability of the FBG sensing was checked by thermo-mechanical analysis (TMA). A fair agreement was established between the thermal contraction and thermal expansion measured by FBG and TMA, respectively. (c) 2008 Wiley Periodicals, Inc.