Cyclic butylene terephthalate oligomers (CBT) with ultra-low melt viscosity can be polymerized into poly (butylene terephthalate) (pCBT) via entropically-driven ring-opening polymerization (ED-ROP) in a short time (ranging from several seconds to 10 min) with no chemical emission and no heat generation during the polymerization process. Due to no heat generation, dynamic rheological measurements were used to monitor the polymerization of CBT from 220 to 250 degrees C. The polymerization was accompanied by a steep increase of the melt viscosity and modulus in isothermal rheological tests, and much faster at higher temperature. With rheological results, reptation theory and Double reptation model were adopted to determine the variation of the molecular weight and concentration with time for pCBT. According to the ED-ROP mechanism of CBT, kinetics equations were also established to simulate the polymerization process. Furthermore, using the results of variation of molecular weight with time for pCBT and kinetics equations, the polymerization rate constants for initiation and propagation steps were evaluated, and the activation energy was also obtained. It was proved that rheological method is a convenient and reliable way to investigate the kinetics of ED-ROP of CBT. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers