A chain extending agent for reaction injection molding (RIM) comprising a normally liquid blend of butanediol and a toluene diamine mono-substituted with an electron withdrawing group and having the formula ##STR1## wherein X is vicinal to an amino radical and is selected from the group of halogen, CN, NO.sub.2, acyl, carboxylate and isomeric mixtures thereof, and wherein the butanediol and diamine are combined in a critical mole ratio of from 4:1 to about 19:1. The above blend is particularly useful in the preparation of large moldings of polyurethane-polyurea elastomer by the RIM or RRIM (reinforced reaction injection molding) process wherein instant preformed blend is added to a mixture of polyol reactant and catalyst before polymerization in the mold with isocyanate to produce a urethane/urea polymer product of improved properties. Accordingly, the intention also concerns an enhanced RIM or RRIM process and improved product by the use of a mixture of the present chain extender blend and polyol reactant maintainable in the liquid state at ambient temperature.