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Chem. Rev. 1999, 99, 543−563 543 Polymerizations in Supercritical Carbon Dioxide Jonathan L. Kendall,† Dorian A. Canelas,† Jennifer L. Young, and Joseph M. DeSimone* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290 Received June 30, 1998 (Revised Manuscript Received November 23, 1998) Contents I. Introduction 543 and kinetics. For example, conditions that give a low viscosity supercritical fluid result in diminished solvent cage effects in free-radical initiator decom- positions.4 When carbon dioxide is used as the supercritical solvent, additional advantages can be realized. The chemical industry has become increasingly aware of environmental concerns over the use of volatile organic solvents and chlorofluorocarbons in the manu- facture and processing of commercial polymer prod- ucts. The use of water alleviates these problems somewhat, but still results in large amounts of hazardous aqueous waste that require treatment. As a result of these environmental concerns, supercriti- cal CO2 represents a more environmentally friendly alternative to traditional solvents. CO2 is naturally occurring and abundant: it exists in natural reser- voirs of high purity located throughout the world. In addition, it is generated in large quantities as a byproduct in ammonia, hydrogen, and ethanol plants and in electrical power generation stations that burn fossil fuels.5 CO2 has an easily accessible critical point with a Tc of 31.1 °C and a Pc of 73.8 bar.6 Because it is an ambient gas, CO2 can be easily recycled after use as a solvent to avoid any contribution to green- house effects. Finally, it is inexpensive, nonflam- mable, and nontoxic, making it an attractive solvent for large-scale synthesis. There are several important issues, such as drying, solubility, and polymer plasticization, that are in- volved when supercritical CO2 is used as a polymer- ization solvent. Because CO2 is an ambient gas, the polymers can be isolated from the reaction media by simple depressurization, resulting in a dry polymer product. This feature eliminates energy-intensive drying procedures required in polymer manufactur- ing to remove solvent and represents potential cost and energy savings for CO2-based systems. Solubility plays a very important role in the synthesis of polymers in supercritical CO2. While CO2 is a good solvent for most nonpolar and some polar molecules of low molar mass,7 it is a poor solvent for most high molar mass polymers under mild condi- tions (<100 °C, <350 bar). For example, poly(methyl acrylate) requires 2000 bar CO2 and 100 °C for a 105 g/mol polymer to dissolve in CO2.8 Such high pres- sures are not practical and are too costly for their widespread implementation in polymer manufactur- ing. The only polymers that show good solubility in CO2 under mild conditions are amorphous fluoropoly- mers and silicones.1,4,9-12 546 A. Free-Radical Polymerizations 546 1. Homogeneous Polymerizations 547 2. Precipitation Polymerizations 547 3. Dispersion and Emulsion Polymerizations 548 II. Chain-Growth Polymerizations in CO2 4. Polymer Blend Synthesis B. Cationic Polymerizations C. Transition Metal-Catalyzed Polymerizations 556 D. Thermal Ring-Opening Polymerization 557 III. Step-Growth Polymerizations 557 A. Melt-Phase Condensation Polymerizations 557 B. Sol−Gel Polymerizations 559 C. Oxidative Coupling 559 IV. Conclusions 561 V. References 561 I. Introduction In the search for new polymerization solvents, scientists have turned to supercritical fluids. Super- critical carbon dioxide possesses many properties that have allowed it to emerge as the most extensively studied supercritical fluid for polymerization reac- tions. The work presented in this review shows that supercritical CO2 is a viable and promising alterna- tive to traditional solvents used in polymer synthesis. Much of this promise results from its fluid properties, effects on polymers, and environmental advantages. Supercritical fluids have the best of two worlds: they can have gaslike diffusivities (which have important implications for reaction kinetics) while having liquidlike densities that allow for solvation of many compounds. They exhibit changes in solvent density with small changes in temperature or pres- sure without altering solvent composition.1 Because of these advantages, chemists have explored reaction mechanisms and solvent cage effects in supercritical fluids.2,3 Changing solvent quality also has substan- tial effects on reaction workup: it can affect the separation of the polymer from starting materials and catalysts and the polymer molecular weight fractionation. In addition, the low viscosity of super- critical fluids and their ability to plasticize glassy polymers have implications on polymer processing † Present address: Lord Corp., Thomas Lord Research Center, P.O. Box 8012, Cary, NC 27512-8012. 10.1021/cr9700336 CCC: $35.00 Published on Web 01/22/1999 553 553 © 1999 American Chemical SocietyPDF Image | Polymerizations in Supercritical Carbon Dioxide
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