Abstract: The process, and product thereof, in which castable compositions comprising mixtures of 5-15% (by weight) of (1) a monomer or prepolymer mixture of castor oil and a dicarboxylic acid or urethane and (2) a vinyl copolymer mixture such as styrene and divinyl benzene with suitable cross-linking agents are made into highly useful products composed of simultaneously formed interpenetrating polymer networks. This process consists of first raising the temperature of the mixture to initiate polymerization of the vinyl copolymer, then, following phase separation and phase inversion but before gelation occurs, forming a product from the composition and thereafter heating the formed product to effect completion of the polymerization and crosslinking reactions.