A process for producing a polymer foam comprises reacting components A to C in the presence of component D and optionally E or an isocyanate-functional prepolymer of components A and B with component C in the presence of component D and optionally component E, the total amount of which is 100 wt %, (A) 35 to 65 wt % of at least one polyisocyanate component A, wherein 10 to 100 wt % of component A is a condensation product comprising polyimide groups and resulting from condensing at least one polyisocyanate component with at least one polycarboxylic acid having at least 3 COOH groups per molecule or its anhydride, (B) 5 to 50 wt % of at least one polyol component B, (C) 1 to 59 wt % of at least one polycarboxylic acid component C, and (D) 0.01 to 3 wt % of at least one Lewis base component D, (E) 0 to 5 wt % of at least one foam stabilizer component E, wherein the reaction takes place with release of carbon dioxide.