The invention relates to a process for the production of plastics containing amide groups with elimination of CO.sub.2 by reaction of polyfunctional isocyanates, carboxylic acids and, optionally, alcohols in the presence of tertiary amines, more particularly heteroaromatic amines. The heteroaromatic amines preferably contain another heteroatom and/or substituents having +I and/or +M effects. The reaction preferably starts at room temperature. Fine-cell foams having an acid value of less than 40 and a density of at most 150 g/l can be produced in short reaction times at room temperature without any need for additional blowing agents.