Thermoplastic foam and process for the preparation thereof

An improved extruded closed-cell resinous foam, the foam being prepared from an ethylenic addition polymer consisting essentially of mer units having the general formula ##STR4## wherein A is at least one tetravalent organic radical having a first pair of valences or vicinal carbon atoms attached to the carbonyl groups and a second pair of valences or vicinal carbon atoms forming part of the polymer chain and arising by addition polymerization of a C.dbd.C ethylenic double bond in the monomeric form of A, i.e. 2 to 30 weight percent based on the weight of the polymer an ethylenically unsaturated dicarboxylic acid cyclic anhydride, B is the polymerized mer unit corresponding to at least one ethylenically unsaturated addition polymerizable monomer, and x and y are numbers such that the ratio thereof represents the average proportion on a mole basis of the respective mer units in the polymer and the sum thereof represents the average degree of polymerization in the polymer by heat plastifying polymer in the presence of about 0.5 to 20 parts by weight per 100 parts by weight of polymer of a blowing agent selected from the group consisting of ammonium carbonate, ammonium bicarbonate, ammonium oxalate and mixtures thereof, the foam containing maleimide groups as determined by infrared spectroscopy.