The ringopening polymerization (ROP) of ε-caprolactone (CL) in 1-butylmethylimidazolium chloride (［bmim］［Cl］) ionic liquid (IL) and styrene oxide (SO) is reported without using any metal compounds.Both ［bmim］［Cl］ and SO were essential in the ROP of CL.No polymerizations were found without ［bmim］［Cl］ or SO or both of them under the same conditions.No poly(styrene oxide) was found by heating IL and SO indicating that the ROP of SO was absent or too slow to be detected.IL acted as both solvent and catalyst.SO was regarded as initiator since a ring-opened SO unit was found at one end of PCL chain according to the 1H-NMR analysis.The molecular weights of the obtained PCLs were between 4.22 and 6.86 kDa with the molecular weight distributions less than 1.3. The ［bmim］［Cl］ IL could be recycled and reused.The polymerization was quenched and the product was precipitated by the addition of ethanol.After PCL was isolated by suction

the filtrate was a homogeneous mixture of IL and ethanol.The latter was collected by distillation under reduced pressure and thus IL was recovered.The yield and the molecular weight of PCL obtained in the recycled IL were similar to those in the original one.