Abstract: A particulate interpenetrating network polymer that is composed of a polyolefin polymer and a vinyl aromatic polymer that includes residues of (meth)acrylic acid, is described. The particulate interpenetrating network polymer is formed by polymerization of a vinyl aromatic monomer composition substantially within the polyolefin polymer. The vinyl aromatic monomer composition includes vinyl aromatic monomer (e.g., styrene) and a comonomer that is composed at least in part of (meth)acrylic acid (e.g., (meth)acrylic acid and optionally butyl(meth)acrylate). The amount of (meth)acrylic acid present within the comonomer (and accordingly the vinyl aromatic polymer) is selected such that a molded article prepared from expanded particulate interpenetrating network polymer, has a volume shrinkage value of less than or equal to 5 percent, when subjected to a temperature of 100° C. for 24 hours.