##STR1## wherein R represents an alkylene group having 2-8 carbon atoms, R1 is H or CH3, R2 is a residue of diisocyanate, R3 is a residue of dihydric alcohol from which at least one or more hydroxyl group is removed, X is a moiety of dienic liquidus rubber having at least one or more hydroxyl groups from which at least one or more hydroxyl groups is removed, l is a number of 1-4, m is a number of