A new method of toughening polymerized cyclic butylene terephthalate (pCBT) with tetrahydrofuran (THF) is proposed. ThepCBT was prepared by in situ ring-opening polymerization of a commercial cyclic butylene terephthalate, a cyclic form ofpoly(butylene terephthalate), in the presence of THF. In comparison to conventionally polymerized pCBT, the resultant materialwas found to be ductile, showing a strain at break of well above 100% in tensile tests. Other matrix properties, such as tensilemodulus, tensile strength and glass transition temperature, were not significantly altered by the addition of THF. It was foundthat the presence of THF enhanced the polymerization reaction, resulting in an increased molecular weight and a narrowedmolecularweightdistribution. Moreover, remaining oligomers after polymerization were extractedby the THF anda toughenedoligomer-free pCBT was obtained. The influence of time and temperature on the long-time toughening action of THF wasstudied. The results showed that samples became brittle after 3months when subjected to a temperature of 80 ◦C, resulting ina reduction of the toughening action. c 2010 Society of Chemical Industry