To augment the reactivity of a urethane prepolymer of allophanate- modified diphenylmethane diisocyanate, thereby reducing the use quantity of catalyst.

This prepolymer is a mixture of (A) an MDI-based urethane prepolymer, which is a stable liquid containing 5 to 30 wt.% of NCO and allophanate-modified MDI and (B) an epoxide, the former being present at 90 to 99.5 wt.% and the latter at 0.5 to 10 wt.%, the prepolymer being composed of a reaction product prepared by reacting (1) an allophanate-modified MDI containing 12 to 32.5 wt.% of NCO prepared by reacting (a) a compound selected from the group consisting of an alcohol, diol and triol with (b) a composition of 3 types of diphenylmethane diisocyanates, mainly comprising approximately 0 to 60 wt.% of 2,4'-diphenylmethane diisocyanate, with (2) an organic compound containing 2 to 6 hydroxyl groups and having a molecular weight of 76 to 6,000.