weight of about 200 to 500 a preferred polycarbonate diol is an oligomer reaction product of 1 6 hexanediol and a carbonate which has an average molecular weight of about 400 to 440 the polycarbonate diols are preferably synthesized in accordance with the teachings of u s pat no 4 160 853 the disclosure of which is incorporated herein by reference the triisocyanate preferably has a molecular weight greater than 500 preferably greater than 560 preferred triisocyanates are relatively nonvolatile examples of preferred triisocyanates include a triisocyanurate of isophorone diisocyanate and a triisocyanate biuret of 1 6 hexamethylene diisocyanate a preferred triisocyanurate of isophorone diisocyanate is available as t 1890s from thorson chemical company new york n y and a preferred triisocyanate biuret of 1 6 hexamethylene diisocyanate is available as desmodur n from mobay chemical corporation pittsburgh pa the aliphatic diisocyanate and polyol are preferably mixed together in a solvent under vacuum in a reaction kettle until the reaction mixture is single phase and well degassed the reaction solution further comprises a catalyst preferably the tetrabutyl titanate which is present in the polycarbonate diol synthesized by the preferred method disclosed above the catalyst is preferably present at a level of about 100 to 1000 parts per million the diisocyanate and polycarbonate diol are reacted in appropriate proportions to form an oh terminated prepolymer the prepolymer is then crosslinked to form an abrasion resistant polyurethane by means of the triisocyanate crosslinking agent in an alternative embodiment.