Living radical polymerizations (LRPs) in supercritical CO 2-swollen polymer substrates were investigated. Atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in supercritical CO 2-swollen polymer substrates produced interpenetration of PMMA chains in the mother polymer substrates. In addition, nitroxide-mediated polymerization (NMP) of styrene in supercritical CO2-swollen polymer substrates resulted in interpenetration of polystyrene (PS) chains in the mother polymer substrates. The chain extensions of the penetrated PMMA chains by ATRPs of methacrylates were also performed. Chain extension ATRPs were carried out by immersing PMMA penetrated polymer substrate in a methanol solution for ATRP of another methacrylate. The chain extension was also successful. This is important for the development of the science and technology of polymer blends produced via living radical polymerizations.