Green Chemistry December 1999 G167 This journal is © The Royal Society of Chemistry 1999 F EAT U R E oncern over volatile organic solvent emissions and the generation of aqueous waste streams has prompted a number of chemists and chemical engineers to seek new, cleaner methods for polymer synthesis1 and polymer processing.2 The use of supercritical carbon dioxide (scCO2) has attracted particular attention in both of these areas for the following reasons: l CO2 is non-toxic, non-flammable, chemically inert, and inexpensive l Supercritical conditions are easily obtained: Tc (CO2) = 31.1°C; Pc, (CO2) = 73.8 bar (Figure 1) l The solvent may be removed by simple depressurisation l The density of the solvent can be ‘tuned’ by varying the pressure l Many polymers become highly swollen and plasticised in the presence of CO2 On the other hand, the use of supercritical fluids requires elevated pressures and relatively specialised equipment, and these considerations must be balanced carefully with the perceived advantages for a given application.However, there are many recent examples which suggest that the benefits of using CO2 as an alternative solvent might warrant the C G additional complexity associated with supercritical fluid technology, at least for certain applications.The aim of this article is to highlight some of these areas.3 Polymer synthesis Much of the pioneering work in the field of polymer synthesis using scCO2 has been carried out by Professor J. M. DeSimone and colleagues at the University of North Carolina at Chapel Hill (UNC), USA.4 In 1992, this group showed that it was possible to synthesise amorphous fluoropolymers in CO2 under relatively mild conditions by homogeneous solution polymerisation.5 Since the only other solvents for these polymers tend to be chlorofluorocarbons (CFCs), the use of CO2 represents a much cleaner route to materials of this type.However, with the exception of certain amorphous fluoropolymers and polysiloxanes, the vast majority of polymers show negligible solubility in CO2 under practicable conditions (<100 °C, <100 bar). Hence, a number of research groups have studied the synthesis of polymers by heterogeneous polymerisation in scCO2 (i.e., under conditions where the resulting polymer is not soluble in the supercritical solvent).In 1994, DeSimone demonstrated that it was possible to synthesise polymers such as poly(methyl methacrylate) (PMMA) in CO2 by dispersion polymerisation using specially designed, CO2-soluble surfactants.6 Since dispersion polymerisation is usually carried out in solvents such as hydrocarbons or C1–C5 alcohols, the use of CO2 has potential to reduce organic solvent usage.These techniques have recently been extended to the synthesis of a range of materials, including waterdispersible polymer powders7 and well-defined cross-linked microspheres (Figure 2).8 Water-dispersible powders are useful because they can be transported dry, thus saving on transport costs, while cross-linked microspheres are very important in applications such as chromatographic separations and polymer-supported synthesis. The use of scCO2 for the synthesis of porous polymers is an area of great interest, particularly since conventional processes tend to be solvent intensive and can generate materials containing organic solvent residues which may be difficult to remove.Carbon dioxide has allowed the ‘solvent-free’ preparation of polymeric materials with pore sizes spanning a very broad range, from microcellular foams down to macroporous resins (Figure 3) and mesoporous/microporous aerogels.9 In the UK, several academics are investigating aspects of the use of CO2 for polymer synthesis, including groups in Nottingham, Cambridge, and Liverpool.10 Polymer processing Carbon dioxide has been used in a wide range of polymer processing applications, the most established of which are polymer fractionation and extraction.11 Both of these techniques exploit the variable density which is associated with supercritical fluid solvents.More recently, there has been much interest n the use of CO2 for the infusion or ‘impregnation’ of molecules into Clean polymer synthesis and processing using scCO2 Andrew Cooper from Liverpool University in the UK describes how scCO2 (supercritical carbon dioxide) is proving to be a valuable green alternative to conventional solvents in polymer synthesis and processing C Figure 1 Schematic phase diagram for CO2 showing the supercritical region Figure 2 Cross-linked polymer microspheres synthesised in scCO2, average diameter = 410 nm, see ref. 8F EAT U R E C G G168 Green Chemistry December 1999 This journal is © The Royal Society of Chemistry 1999 polymeric materials.From an environmental perspective, a particularly exciting technique is disperse dyeing using scCO2.12 In this approach, the dye molecule has very low solubility in CO2 (e.g., mole fractions of 10-5–10-6) However, because the dye molecules partition preferentially into the polymer phase, significant quantities of dye can be loaded into the polymer in a relatively short period.13 In addition to using a clean solvent, very little dye is actually dissolved in the fluid at any given time, thus minimising dye loss and environmental burdens.Another exciting processing technique is the Vedoc Advanced Materials Process or ‘VAMP®’, which was recently commercialised by Ferro Corporation (Cleveland, OH).14 This process utilises the fact that CO2 is a good plasticising agent (i.e., it can cause polymers to soften and flow, even at low temperatures).The method has been applied successfully to low-temperature processing of a range of composite materials, particularly polymer–pigment formulations for use as powder coatings. Other important methods for polymer particle formation include rapid expansion from supercritical solutions (RESS) and a range of antisolvent precipitation techniques.15 Supercritical CO2 has also been exploited for the solvent-free application of protective fluorinated coatings16 and for dry-cleaning,17 the latter of which has already shown real commercial promise.Finally, carbon dioxide has been viewed as a potential solvent for microlithography.At present, the semiconductor industry produces millions of gallons of organic and aqueous waste effluent every year, all of which requires treatment. Supercritical solvents offer the possibility of simpler separations and recycling, and new processes are under evaluation which use CO2 as the solvent, both in the spin-coating stage and also in the development step.18 Conclusions Carbon dioxide has great potential as an alternative solvent for polymer synthesis and processing.The drive to use CO2 is especially strong in the case of processes which use volatile organic solvents. However, there may also be cases where CO2 is a viable substitute in aqueous processes, particularly if separations are simplified by the use of a supercritical solvent.A major breakthrough in the acceptance of this technology would be the generation of novel polymeric materials that are difficult or even impossible to obtain without the use of supercritical fluids. Whilst this may be a challenging goal, the rapid growth of this area over the last few years suggests that we will see future developments in the use of CO2 for the synthesis and processing of progressively more advanced materials.References 1 J. L. Kendall, D. A. Canelas, J. L. Young and J. M. DeSimone, Chem. Rev., 1999, 99, 543. 2 (a) B. Bungert, G. Sadowski and W. Arlt, Ind. Eng. Chem. Res., 1998, 37, 3208; (b) C. A. Eckert, B. L. Knutson and P. G. Debenedetti, Nature, 1996, 383, 313. 3 This short article is based on a fulllength review which will appear in the Journal of Materials Chemistry: A.I. Cooper, J. Mater. Chem., 2000, 10, in press. 4 http://www.unc.edu/depts/chemistry/ faculty/jmd/jmdhmpg.html 5 J. M. DeSimone, Z. Guan and C. S. Elsbernd, Science, 1992, 257, 945. 6 J. M. DeSimone, E. E. Maury, Y. Z. Menceloglu, J. B. McClain, T. J. Romack and J. R. Combes, Science, 1994, 265, 356. 7 M. Z. Yates, G. Li, J. J. Shim, S. Maniar, K. P. Johnston, K. T. Lim and S. Webber, Macromolecules, 1999, 32, 1108. 8 A. I. Cooper, W. P. Hems and A. B. Holmes, Macromolecules, 1999, 32, 2156. 9 (a) K. L. Parks and E. J. Beckman, Polym. Eng. Sci., 1996, 36, 2417; (b) A. I. Cooper and A. B. Holmes, Adv. Mater., 1999, 11, 2170; (c) D. A. Loy, E. M. Russick, S. A. Yamanaka, B. M. Baugher and K.J. Shea, Chem. Mater., 1997, 9, 2264. 10 http://www.nottingham.ac.uk/ ~pczsp/clnthome.html http://www.ch.cam.ac.uk/ CUCL/MLPS/ http://www.liv.ac.uk/Chemistry/Staff/ coopera.html 11 M. A. McHugh and V. J. Krukonis, Supercritical Fluid Extraction, 2nd ed., Butterworth-Heinemann, Stoneham, MA, 1994. 12 S. G. Kazarian, N. H. Brantley and C. A. Eckert, Chemtech, 1999, 29, 36. 13 B. L. West, S. G. Kazarian, M. F. Vincent, N. H. Brantley and C. A. Eckert, J. Appl. Polym. Sci., 1998, 69, 911. 14 F. S. Mandel, Proc. 5th ISASF Meeting on Supercritical Fluids, Chemistry and Materials: Nice, France, 1998, T1, pp 69. 15 (a) P. G. Debenedetti, J. W. Tom, X. Kwauk and S.-D. Yeo, Fluid Phase Equilib., 1993, 82, 311; (b) E. Reverchon, J. Supercrit. Fluids, 1999, 15, 1. 16 F. E. Henon, M. Camaiti, A. L. C. Burke, R. G. Carbonell, J. M. DeSimone and F. Piacenti, J. Supercrit. Fluids, 1999, 15, 173. 17 http://www.micell.com 18 (a) C. K. Ober, A. H. Gabor, P. GallagherWetmore and R. D. Allen, Adv. Mater., 1997, 9, 1039; (b) J. M. DeSimone, Proc. Polymer ’98 Meeting, Brighton, UK, 9-11 September 1998, pp. 5. Figure 3 Internal structure of a porous polymer monolith synthesised using scCO2, average pore size = 7.8 mm, see ref. 9(b) Biography Andrew Cooper obtained his PhD at the University of Nottingham, working with Professor Martyn Poliakoff in the area of organometallic chemistry. He then spent two years (1995–1997) as an 1851 Research Fellow at the University of North Carolina at Chapel Hill, USA, working with Professor Joseph DeSimone on the development of novel dendritic surfactants for extractions using CO2 as the solvent (Cooper et al., Nature, 1997, 389, 368). From 1997 to 1999, he held a Ramsay Memorial Research Fellowship at the Melville Laboratory for Polymer Synthesis in Cambridge, UK, working with Professor Andrew Holmes on polymer synthesis using supercritical CO2 (Cooper et al., Macromolecules, 1999, 32, 2156). He was appointed as a Royal Society University Research Fellow at the University of Liverpool, UK, in January 1999.