under ambient conditions as for example organo tin compounds lead salts of carboxylic acids mercuric compounds a combination of a calcium or lead salt of a carboxylic acid such as calcium or lead octoate an ionizable monoorgano mercuric compound such as phenyl mercuric acetate and lead oxide the total amount of the catalyst should not be less than about 0 1 of the reaction mixture and to hasten the setting up of hardening time desired may be adjusted upwardly to about 3 or to such higher percentage as desired before the accelerating effect is lost or undesirable side effects become apparent for the elastomer to form as a tough wear and abrasion resistant rubbery product some trifunctionality may be desired to facilitate cross linking of the reactants as well as chain extension thereof this is readily accomplished by including some triisocyanate or triol or both in the reaction mixture thus for example when the reaction mixture is comprised essentially of an aromatic diisocyanate and polypropylene glycol a certain amount of trifunctionality can be built in very readily by pre reacting from 5 to 15 of a triol such as trimethylol propane with the aromatic diisocyanate to form some triisocyanate or by including as part of the monomer charge for making the starting polymeric polyol from about 5 to 15 of a triol such as trimethylol propane glycerine or the like the.