Styrene supercritical water

Lilac, W.D. Lee, S. Kinetics and mechanisms of styrene monomer recovery from waste polystyrene by supercritical water partial oxidation. Adv. Envi. Res. 2001, 6, 9-16. 

Zhang, R., Sato, O., Zhao, E et al. (2(K)4) Heck coupling reaction of iodobenzene and styrene using supercritical water in the absence of a catalyst. Chem. Eur. J., 10,1501-6. 

Styrene ion-exchange resins, which are used in the nuclear industry [29]. It is argued that the destruction of pulp mill sludges from paper manufacture by supercritical water oxidation is now an economic proposition [12]. These sludges contain dioxins and furans and therefore have high land-fill costs. 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

Even superheated (to 260 °C) or supercritical (to 400 °C) water was employed in the Heck reaction with several catalyst precursors and aryl halides with styrene. However, all conversions show large amounts of side products and the yields were in the 5-30% range, indicating radical intermediates and by-products from decomposition of the arene starting material [15]. 

Especially in case of hydrophobic monomers, solubility becomes a limiting factor for the efficiency of enzyme-initiated polymerizations. Biocatalyst solubilization in organic media may be a viable solution as proposed recently by Li and coworkers [29]. Polar organic solvents can be used to some extend to increase the monomer solubility in the reaction medium [12,30]. However, the delicate balance between enhanced monomer solubility on the one hand and decreased enzyme stability in the presence of water soluble organic solvents on the other hand [27] has to be considered. Emulsion polymerizations have been reported for highly unpolar substrates such as styrenes [14, 21, 31] or use of unconventional reaction media such as supercritical C02 [19]. 

The potential of microemulsions for organometaUic-catalyzed hydrogenations in water/scC02 biphasic systems has been assessed using the rhodium-catalyzed hydrogenation of styrene as a common test reaction [Eq. (7)] [31]. The water-soluble Wilkinson complex [RhCl(TPPTS)3] was applied as catalyst precursor together with anionic perfluoropolyether carboxylates, cationic Lodyne A, or nonionic poly-(butene oxide)-b-poly(ethylene oxide) surfactants. The interfacial tension is small in the presence of the supercritical fluid and small amounts of surfactant (0.1-2.0 wt.%) suffice to form stable microemulsions. The droplet diameter of the microemulsions varied between 0.5 and 15 pm and a surface area of up to 10 m was obtained. 

In a classical heterogeneous dispersion polymerization, the continuous phase is organic in nature, althou in some instances water has been used as a component of the continuous phase to increase polarity. Research investigations have focused on the composition of the dispersion m um, reaction kinetics, the structure and influence of the stabilizer polymer, particle size, molecular weight and molecular weight distribution (4, S). Dispersions of poly(methyl methacrylate) (8) and poly(styrene) (9) are widely studied and among Ae best characterized systems. Recently, dispersion polymerizations conducted in supercritical carbon dioxide have also been reported (9-12). 

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