Catalysis of the isocyanate‐hydroxyl reaction

Abstract The reaction of toylene diisocyanate (80/20 mixture of 2,4 and 2,6‐isomer) with a secondary hydroxyl‐containing polyoxypropylene triol was studied in the presence of numerous compounds to test their catalytic effect on the reaction. The reaction was studied by determination of the gelation time of the reaction mixture. Many metallic compounds were found to be good catalysts for the isocyanate‐hydroxyl reaction. Compounds in roughly descending catalytic activity are: Bi, Pb, Sn, triethylenediamine, strong bases, Ti, Fe, Sb, U, Cd, Co, Th, Al, Hg, Zn, Ni, trialkylamines, Ce, Mo, V, Cu, Mn, Zr, trialkyl phosphines. Similar gelation tests were made with m ‐xylene diisocyanate and the same triol and also with hexamethylene diisocyanate and the triol. In these tests the order of strength of the catalysts was found to be different than when tolylene diisocyanate was used. In decreasing order of activity: Bi, Fe, Sn, Pb, Ti, Sb, strong bases, Co, Zn, triethylenediamine, trialkyl amines. A mechanism is proposed for the way in which a metal compound may catalyze the isocyanate‐hydroxyl reaction. Steric hinderance is suggested to explain the differing catalytic activity observed with tolylene diisocyanate and the unhindered aliphatic diisocyanates.