Abstract: A polymer composition that includes a first polyolefin polymer and an interpenetrating network polymer. The interpenetrating network polymer includes a second polyolefin polymer present in an amount of from 10 percent by weight to 80 percent by weight, based on total weight of the interpenetrating network polymer, and a vinyl aromatic polymer present in an amount of from 20 percent by weight to 90 percent by weight, based on total weight of the interpenetrating network polymer. As initially provided in the polymer composition, the interpenetrating network polymer is substantially free of crosslinking. The polymer composition itself is at least partially crosslinked. An expandable polymer composition is provided that includes the polymer composition and an expansion agent, which can be expanded to form an expanded polymer composition that can have a density of from 16 to 400 Kg/m3.