Semi-interpenetrating polymer networks (S-IPNs) were prepared by the reactions of hydroxyl-terminated four-armed, star-shaped epsilon-caprolactone oligomers with degrees of polymerization per one oligocaprolactone chain (ns) of 3, 5, and 10 and 2,4-tolylene diisocyanate (TDI) in the presence of poly(epsilon-caprolactone) (PCL). In the dynamic mechanical analysis of the S-IPN [2,4-tolylene diisocyanate bridged hydroxyl-terminated four-armed, star-shaped epsilon-caprolactone oligomer (TH4CLO)/PCL], only one tan peak was observed; its temperature increased with increasing TH4CLO content and with decreasing n value. Differential scanning calorimetric analyses of the TH4CLOs and TH4CLO/PCLs revealed that the TH4CLOs with ns of 3 and 5 were amorphous, whereas TH4CLO with an n of 10 was semicrystalline and that the crystallization of the PCL chain for TH4CLO/PCLs was more strongly disturbed with increasing TH4CLO content and decreasing n value. Although the tensile strength, modulus, and elongation at break of TH4CLO were much lower than those of PCL, those values increased with the n value. Although the tensile strength and modulus of the TH4CLO/PCLs decreased with increasing TH4CLO content, TH4CLO (n=3)/PCL 50/50 showed the highest elongation at break (314%) among the S-IPNs because of the suppression of crystallization of the polycaprolactone chain. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4229-4236, 2013