We successfully prepared a series of transparent materials with semi-interpenetrating polymer networks (semi-IPNs) from castor-oil-based polyurethane (PU) and benzyl starch (BS). The miscibility, morphology, and properties of the semi-IPN films were investigated with attenuated total reflection/Fourier transform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, scanning electron microscopy, wide-angle X-ray diffraction, electron spin resonance (ESR), ultraviolet-visible spectroscopy, and tensile testing. The results revealed that the semi-IPN films had good or certain miscibility with BS concentrations of 5-70 wt % because of the strong intermolecular interactions between PU and BS. With an increase in the concentration of BS, the tensile strength and Young's modulus of the semi-IPN materials increased. The ESR data confirmed that the segment volume of PU in the semi-IPNs increased with the addition of BS; that is, the chain stiffness increased as a result of strong interactions between PU and BS macromolecules. It was concluded that starch derivatives containing benzyl groups in the side chains more easily penetrated the PU networks to form semi-IPNs than those containing aliphatic groups, and this led to improved properties. (C) 2005 Wiley Periodicals, Inc.