Abstract: Fatty prepolymers and fatty modidified organic polyisocyantes are provided as internal mold release agents useful in Reaction Injection Molded (RIM) parts. A terminal fatty chain-containing molecule having a primary or secondary hydroxyl group, a primary or secondary amine group, an epoxide group, or an isocyanate group is reacted with an organic aromatic polyisocyanate to yield a fatty prepolymer having a urethane linkage, a urea linkage, or an oxazolidinone linkage; or to yield a fatty modified polyisocyanate having an allophanate linkage, a biuret linkage, or a carbodiimide-uretonimine linkage. The terminal chain on the molecule contains at least 8 branched or unbranched, saturated or unsaturated, hydrogen substituted aliphatic carbon atoms in succession or at least 5 branched or unbranched, aliphatic or cycloaliphatic perfluorinated atoms in succession.