Semi-interpenetrating polymer networks (semi-IPNs) were prepared by reactions of 2,4-tolylene diisocyanate (TDI) and hydroxy-terminated 4-arm star-shaped l-lactide oligomers (H4LAO(n)'s) with the degrees of polymerization of lactate unit per one arm, n=3, 5, and 10 in the presence of poly(epsilon-caprolactone) (PCL). Morphologies, thermal, and mechanical properties of the TDI-bridged H4LAO(n) (TH4LAO(n))/PCL semi-IPNs were evaluated by comparing with those of poly(l-lactide) (PLA)/PCL blends. Compatibility between the two components of the TH4LAO(n)/PCL semi-IPN with a PCL content not more than 50 wt % was much better than those of the PLA/PCL blends with the same PCL content. All the TH4LAO(n) networks were substantially amorphous and their tan peak or glass transition temperatures increased with decreasing n value. Most of the semi-IPNs did not show clear glass transition temperature related to both the components. Tensile toughness and elongation at break for all the TH4LAO(n)/PCL semi-IPNs were much higher than those for the PLA/PCL blends with the same PCL content. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1420-1428