Abstract: An expandable particulate interpenetrating network polymer composition that includes as an expansion agent pentafluorobutane and optionally a minor amount of heptafluoropropane is described. The expandable particulate interpenetrating network polymer, more particularly, includes a particulate interpenetrating network polymer that includes: (i) a polyolefin polymer present in an amount of from 10 percent by weight to 80 percent by weight; and (ii) a vinyl aromatic polymer present in an amount of from 20 percent by weight to 90 percent by weight, in each case based on total weight of the particulate interpenetrating network polymer. The expansion agent resides (or is impregnated) within the particulate interpenetrating network polymer. The pentafluorobutane may be 1,1,1,3,3-pentafluorobutane, and the heptafluoropropane may be 1,1,1,2,3,3,3-heptafluoropropane. In an embodiment, the expansion agent consists of 1,1,1,3,3-pentafluorobutane, and is substantially free of any other expansion agents.