Abstract: A method for producing a prepolymer for the production of an integral polyurethane foam is provided. The prepolymer is or can be obtained by reaction of a composition that contains the following components: component A containing a polyoxymethylene-polyoxyalkylene block copolymer having a hydroxyl number of 20 mg KOH/g to 200 mg KOH/g as component Al, component B containing di- and/or polyisocyanates with an NCO content of 15 to 45 wt.-% relative to component B, 0.04 to 1.0 wt.-%, relative to the composition, a proton acid as component C, and optionally a component D that contains auxiliary agents, at a characteristic number of 450 to 850. The invention further relates to the prepolymer obtained by the method, to an integral polyurethane foam based on the prepolymer, and to the use thereof.