Supercritical carbon dioxide (scCO 2 ) is a unique solvent with a wide range of interesting properties. This review focuses upon recent advances in the use of scCO 2 in materials synthesis and materials processing. In particular, we consider the advances made in three major areas. First the design and application of new surfactants for use in scCO 2 , which enable the production of metal nanoparticles, porous polymers and polymers of high molecular weight with excellent morphology. Second the development of new polymer processing and polymer blend technologies in scCO 2 , which enable the synthesis of some very complex polymer composites and blends. Finally, the application of scCO 2 in the preparation of novel biomedical materials, for example biodegradable polymer particles and scaffolds. The examples described here highlight that scCO 2 allows facile synthesis and processing of materials, leading to new products with properties that would otherwise be very difficult to achieve.

New stabilizers for the dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide (scCO2) were prepared and studied in terms of their anchor group architecture. The same perfluoropolyether (PFPE) chain was used in each case as the CO2-philic portion of the stabilizer and four different PMMA-philic headgroups were investigated as anchoring units:  an alcohol, an acetate group, a methacrylate unit, and a PMMA block. When compared to the stabilizing ability of PFPE−alcohol, incorporation of an anchor group as small as an acetate group, or a reactive group such as a methacrylate unit, was found to have a dramatic effect upon the dispersion polymerization of MMA in scCO2. Their incorporation led to a significant increase in PMMA yield and molecular weight and an improvement of the morphology of the polymer product. A method for the synthesis of PFPE-b-PMMA diblock copolymers is reported via atom transfer radical polymerization (ATRP) from a PFPE−bromoester macroinitiator in a fluorinated solvent (pentafluorobutane). This method allows the controlled synthesis of PFPE-b-PMMA diblock copolymers with well-defined architecture. These copolymers were found to be effective stabilizers in scCO2, leading to excellent PMMA yield with high molecular weight and a fine morphology. The effect of PFPE and PMMA block length on the copolymer stabilizing ability was also studied to probe the influence of the stabilizer anchor-soluble balance (ASB). In addition, the phase behavior of each stabilizer in CO2 or a mixture of CO2/MMA was studied to elucidate the effect of stabilizer structure on CO2-philicity and stabilizing ability.

Poly(ether carbonate)s (PEC) and poly(ether ester)s (PEE) were synthesized by step growth polymerization, and the reaction conditions were optimized. The solubility of the polymers in CO 2 was then evaluated using two methods: (i) observation of the cloud point and (ii) gravimetric extraction. Our results suggest that it is difficult to give an exact description of the solubility profiles for these polymers using cloud points alone, particularly in cases where there is a broad distribution of molecular weights. We suggest that the cloud point data are more easily interpreted in combination with a quantitative gravimetric extraction method that we have developed. It was found that the CO 2 solubility of the PEC and PEE samples was strongly affected by both the polymer molecular weight and the chemical structure.

The batch homopolymerization of vinylidene fluoride (VDF) in supercritical carbon dioxide (scCO2) was studied using diethyl peroxydicarbonate (DEPDC) as a free radical initiator. Experiments were carried out to investigate the effects of monomer concentration, initiator concentration, and agitation. It was shown that poly(dimethylsiloxane) monomethacrylate (PDMS-ma) acts effectively as a stabilizer influencing both the morphology and the molecular weight of the poly(vinylidene fluoride) (PVDF) product. Scanning electron microscopy (SEM) and laser scattering particle size analysis were employed to characterize polymer particle morphology and particle size distribution. Thermal analysis of the products by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) is also reported. 19F nuclear magnetic resonance (NMR) was performed to characterize the microstructure of the PVDF polymer chain.