With a synthesis route differing from previous methods, novel semi-interpenetrating polymer networks (semi-IPNs), coded UNK-II, were synthesized by the initial mixing of nitrokonjac glucomannan (NKGM) with castor oil in butanone and the subsequent addition of toluene diisocyanate (TDI) to begin the polymerization reaction in the presence of 1,4-butanediol (BD) as a chain extender at 60degreesC. The results from dynamic mechanical analysis, differential scanning calorimetry, and ultraviolet spectroscopy indicated that a certain degree of microphase separation occurred between soft and hard segments of polyurethane (PU) in the UNK-II sheets. The a-transition temperature, glass-transition temperature, heating capacity, and tensile strength increased with an increase in the NKGM content, and this suggested an interaction between PU and NKGM in the UNK-II sheets. In a previous method, semi-IPN materials (PUNK) were synthesized by the polymerization reaction between castor oil and TDI, and then this PU prepolymer was mixed with NKGM and cured in the presence of BD as a chain extender. The PUNK sheets had relatively good miscibility and mechanical properties. However, for UNK-II sheets prepared by the method reported in this work, NKGM mainly played a role in reinforcement. When the NKGM content was less than 10%, the UNK-II sheets exhibited good miscibility, tensile strength (26-28 MPa), and breaking elongation (130-140%), similar to those of PUNK materials. (C) 2003 Wiley Periodicals, Inc.