Process and intermediate for preparing an intrinsically foamed thermoplastic polymer comprising a vinyl monomer and maleic acid and/or maleic anhydride, in which process a polymer composition comprising:a) a core polymer based ona.1) 12-50 moles % maleic acid and/or maleic anhydridea.2) a vinyl monomer, anda.3) optionally a third monomer, where the sum total of a1+a2+a3 is 100 moles % and the weight average molecular weight of the core polymer is 50000-500000, andb) optionally a nucleating agent, is introduced into equipment suited for making foam, in which process the core polymer, and optionally the nucleating agent, are brought to a temperature which is high enough to release CO.sub.2 from the core polymer, and in which process the polymer melt is expanded to form a polymer foam with the desired density. The core polymer is preferably a polymer of styrene and maleic anhydride.