Polyurethanes were obtained by the reaction of the hydroxyl group of castor oil with isophorone diisocyanate under different stoichiometric ratios of NCO/OH. A series of polyurethane-poly(methyl methacrylate) simultaneous interpenetrating polymer networks (PU/PMMA IPNs) of different compositions were synthesized as tough films by casting from glass molds. These IPNs were investigated using their resistance to chemical reagents, thermal behavior, tensile strength, Young's modulus, elongation at break (%), and Shore A hardness. The dielectric properties (σ, ε′, ε″ and tan δ) at different temperatures were studied. Electron microscopy confirmed the phase separation.