In this thesis, the feasibility of scCO2 as both a processing medium and a polymerization medium for preparation of polymer/clay nanocomposites has been explored, with the first part discussing a CO2-mediated intercalation of poly(ethylene oxide) (PEO) in clay. It has been shown that CO2 can act as a plasticizer to promote intercalation similar to that achieved in polymer melts. Intercalation kinetics in both melt intercalation and CO2-mediated intercalation were studied by Differential Scanning Calorimetry (DSC). Data and results towards both intercalation kinetic and thermal behavior of PEO were discussed. In the second part, we explored the feasibility of scCO2 as a polymerization medium for in-situ polymerization of vinyl monomers and exfoliation of clay. By using a CO2- philic fluorinated surfactant (10F-clay) to modify clay, partially exfoliated poly(methyl methacrylate) (PMMA)/clay nanocomposites were synthesized in high yields via a pseudo-dispersion polymerization of MMA in scCO2. It was found that 10F-clay was an effective stabilizer (as compared to conventional hydrocarbon surfactant modified clay) for PMMA polymerization in CO2. A stabilization mechanism was proposed, wherein FT-IR studies indicated hydrogen bond formation between MMA and clay. Thermal and mechanical properties of the PMMA nanocomposites were also studied. Pseudo-dispersion polymerization was also conducted on polystyrene to study the effect of clay on non-hydrogen-bonding polymers. By using a poly(dimethylsiloxane) (PDMS) surfactant to modify clay, PMMA and polystyrene/clay nanocomposites were synthesized and compared in this study. The effects of the PDMS-clay concentration on polymer conversion, molecular weight, and morphology were investigated. The distributions of clay in both polymers were compared, and two different interaction mechanisms were proposed. The effects of clay distribution on both thermal properties and mechanical properties of the polymers have also been discussed.