In the first part of this study, simultaneous addition of bromine and acrylate to the double bonds of castor oil was achieved. In the second part of the study, bromoacrylated castor oil (BACO) was reacted with toluenediisocyanate (TDT), to form a prepolyurethane (BACOP). The prepolyurethanes were reacted with styrene (STY), 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (M AA), and 3-(acry1oxy)-2-hydroxy propyl methacrylate (AHPMA) free radically, using the acrylate functional group to prepare the simultaneous interpenetrating polymer networks (SINs). 2,2'-Azobis (isobutyronitrile) (AIBN) was used as the initiator and diethylene glycol dimethacrylate (DEGDMA) was used as the crosslinker. BACO and BACOP were characterized by IR, H-1-NMR, and C-13-NMR techniques. Synthesized polymers were characterized by their resistance to chemical reagents, thermogravimetric analysis, and dynamic mechanical thermal analyzer (DMTA). All the polymers decomposed with 6-10% weight loss in a temperature range of 25-240 degrees C. MMA-type SIN showed the highest T-g (126 degrees C), while STY-type SINs showed the highest storage modulus (8.6 X 10(9) Pa) at room temperature, with respect to other synthesized SINs. (c) 2006 Wiley, Periodicals, Inc.