Isocyanate-terminated urethane prepolymer was prepared by reacting 2 mol of tolylene diisocyanate (TDI) with 1 mol of hydroxy-terminated polybutadiene (HTPB, mol. wt. = 3000) at room temperature (30 ± 1°C) in the presence of stannous octoate as the catalyst. Urethane prepolymer was subsequently mixed with 1,4-butane diol: glycerol (6 : 1 ratio), methyl methacrylate (MMA), and ethylene glycol dimethacrylate (EGDMA) to give an interpenetrating polymer network. Stannous octoate was used as a catalyst to give network I, and azobisisobutyronitrile (AIBN), as an initiator to give network II. Different proportions of MMA were added to the prepolymer for studying the effect of composition on mechanical properties, thermal behavior, and optical properties. The mechanothermal behavior was analyzed using a dynamic mechanical thermal analyzer. Tensile strength and modulus decreased with decreasing MMA content, whereas percent elongation at break increased. Thermal stability of the sample increased with increasing urethane content. © 1993 John Wiley & Sons, Inc.