Abstract: The invention is directed to extruded foam based on thermoplastic polymers. The foam contains at least one styrene copolymer which contains copolymerized maleic anhydride units or copolymerized maleimide units, and optionally styrene-acrylonitrile copolymers (SAN), and thermoplastic polymers and at least one halogen-containing polymer as flame retardant, and at least one flame retardant synergist selected from antimony trioxide and dicumyl. The invention also relates to a process for producing the foam with introducing carbon dioxide as blowing agent, and acetone as coblowing agent; into the polymer melt in order to form a foamable melt, and extruding the foamable melt into a region of relatively low pressure with foaming to give the extruded foam. The invention also relates to the use of said foam as insulation material and as structural foam.