carbon atoms, X is an active hydrogen-containing functional group selected from the class consisting of a hydroxy group --OH, mercapto group --SH, carboxyl group --COOH and amino group --NH2 and n is a positive integer not smaller than 10; (b) a polyol compound having polyoxyalkylene linkages in an amount from 1 to 10 moles per moles of the moiety (a) above; and (c) a diisocyanate compound in such