Maleic anhydride, MAN, as an adjunct to triphenylbismuthine (TPB) yields roved benefits to the mechanical properties of isophorone diisocyanate cured hydroxy terminated polymer binders. Results from several propellant mixes indicate that both pot life and mechanical properties are affected not only by the concentration of TPB catalyst used, but also by the ratio of TPB to MAN. The ratio of TPB to MAN from about 1 to 0.5 to about 1 to 1.5 produces interesting results in the values of stress, strain, and modulus as shown in the data set forth in the specification and as graphically depicted in the drawing. The weight percent of MAN ranges from about 0.0125 percent to about 0.0375 percent, and the weight percent of TPB ranges from about 0.015 percent to about 0.035 percent.