Cationic ring-opening copolymerization behavior of 1,5,7,11-tetraoxaspiro[5.5]undecane (SOC1) and epsilon-caprolactone (CL), and the thermal behavior of the obtained copolymers are described. When SOC1 and CL were cationically copolymerized under various feed ratios using BF3OEt2 as the initiator in CH2Cl2 at 25 degrees C, the corresponding copolymers were obtained in 77-99% yields. The H-1 NMR spectroscopic analysis of the copolymers revealed that the copolymer compositions were almost identical to the feed ratios, and the diad ratios of SOC1-SOC1/SOC1-CL and CL-SOC1/CL-CL are 48.0/52.0 and 54.3/45.7. These observations proved the random structures of the copolymers without containing the long blocks of the homopolymer sequences. Differential scanning calorimetric (DSC) analysis revealed that the melting points and melting entharpies decreased with the increase of the SOC1 unit compositions, suggesting that the copolymers gain flexibility as the SOC1 unit increases. (c) 2006 Wiley Periodicals, Inc.