Supercritical fluids free radical reactions

Much of the research discussed in this chapter has demonstrated that sc C02 is a viable alternative solvent for free-radical reactions. Moreover, several of these studies demonstrate that the unique properties of a supercritical fluid, specifically the ability to change important solvent properties such as viscosity by varying pressure (and temperature), can be exploited to manipulate reaction yield and selectivity. Solvent viscosity is particularly important for reactions that are diffusion-controlled or those for which cage-effects are important. 

Ganapathy, S. Carlier, C. Randolph, T. W. O Brien, J. A. Influence of Local Structural Correlations on Free-Radical Reactions in Supercritical Fluids A Hierarchical Approach, Ind. Eng. Chem. Res. 1996, 35, 19-27. 

With the growth of PTC, various new technologies have been developed where PTC has been combined with other methods of rate enhancement. In some cases, rate enhancements much greater than the sum of the individual effects are observed. Primary systems studied involving the use of PTC with other rate enhancement techniques include the use of metal co-catalysts, sonochemistry, microwaves, electrochemistry, microphases, photochemistry, PTC in single electron transfer (SET) reactions and free radical reactions, and PTC reactions carried out in a supercritical fluid. Applications involving the use of a co-catalyst include co-catalysis by surfactants (Dolling, 1986), alcohols and other weak acids in hydroxide transfer reactions (Dehmlow et al., 1985,1988), use of iodide (traditionally considered a catalyst poison, Hwu et... 

Professor DeSimone s group at North Carolina has focused on carrying out polymerization reactions in supercritical fluids (7representative example is shown in Figure 4(b), where a fluorinated monomer which is soluble in CO2 is polymerized via free radical reactions. Control of molecular weight distribution has been achieved and the rates are comparable to other synthesis methods. As a direct result, there is considerable commercial interest in using supercritical CO2 as solvent replacement for fluorinated polymer synthesis. 

Supercritical fluid (SCF) solvents are unique in that their densities can be varied continuously from gas-like to liquid-like values simply by varying the thermodynamic conditions. Because many of a fluid s solvating properties are strongly dependent on the fluid density, such large changes in density can have dramatic effects on solute reactivity [1,2]. For example, at low pressures supercritical water supports homolytic, free radical reactions, whereas at higher pressures, heterolytic, ionic reactions dominate [3,4]. Thus, thermodynamic control of SCF solvent densities promises to enable us to control reaction outcome and selectively produce desired products. 

S Ganapathy, C Carlier, TW Randolph, JA O Brien. Influence of local structural correlations of free-radical reactions in supercritical fluids a hierarchical approach. Ind Eng Chem Res 35 19, 1996. 

TW Randolph, C Carlier. Free-radical reactions in supercritical ethane a probe of supercritical fluid structure. J Phys Chem 96 5146, 1992. 

Supercritical fluids have also been used purely as the solvent for polymerization reactions. Supercritical fluids have many advantages over other solvents for both the synthesis and processing of materials (see Chapter 6), and there are a number of factors that make scCCH a desirable solvent for carrying out polymerization reactions. As well as being cheap, nontoxic and nonflammable, separation of the solvent from the product is achieved simply by depressurization. This eliminates the energy-intensive drying steps that are normally required after the reaction. Carbon dioxide is also chemically relatively inert and hence can be used for a wide variety of reactions. For example, CO2 is inert towards free radicals and this can be important in polymerization reactions since there is then no chain transfer to the solvent. This means that solvent incorporation into the polymer does not take place, giving a purer material. 

Rate of complex formation between chiral alcohols and DBTA monohydrate in hexane suspension is quite slow (see Figure 1) and numerous separation steps are necessarry for isolation of the alcohol isomers (filtration of the diastereoisomeric complex then concentration of the solution, decomposition of the complex, separation of the resolving agent and the enantiomer, distillation of the product). To avoid these problems, alternative methods have been developed for complex forming resolution of secondary alcohols. In a very first example of solid phase one pot resolution [40] the number of separation steps was decreased radically. Another novel method [41] let us to increase the rate of complex forming reaction in melt. Finally, first examples of the application of supercritical fluids for enantiomer separation from a mixture of diastereoisomeric complexes and free enantiomers [42, 43] are discussed in this subchapter. 

Water has been shown to be an effective solvent in some chemical reactions such as free radical bromination. Supercritical fluids such as liquified carbon dioxide are already commonly used in coffee decaffeination and hops extraction. However, supercritical carbon dioxide can also be used as a replacement for organic solvents in polymerization reactions and surfactant production. Future work may involve solventless or neat reactions such as molten-state reactions, dry grind reactions, plasma-supported reactions, or solid materials-based reactions that use clay or zeolites as carriers. 

Radical polymerization can be carried out under homogenous as well as heterogenous conditions. This difference is classified based on whether the initial mixture and/or final product are homogenous or heterogenous. Some homogenous mixtures become heterogenous as polymerization proceeds as a result of insolubility of the resulting polymer in the reaction media. There are many other specialized processes that are used to synthesize materials via free-radical polymerization. These include interfacial polymerization, gas phase reactions ( popcorn polymerization ), as well as the use of specialized media like supercritical fluids. Current research efforts include the study of such... 

The advantage of conducting the precipitation polymerization in supercritical fluids is the ease with which the unreacted monomer can be recovered from the reaction medium and the ease of recovering the produced polymer from the solvent. Free-radical polymerization in SCF hydrocarbon solvents makes use of the relationship between solvent power and SCF density to alter the threshold of precipitation of the polymer chains and also to minimize the swelling of the precipitate. This process produces polymers with controlled molecular weight with a narrow molecular weight distribution. 

Reaction schemes exploiting supercritical fluid diffusivities. The dif-fusivity of a dilute solute in a supercritical fluid, somewhat removed from the critical point, is typically an order of magnitude greater than in liquid solvents at comparable temperatures. Thus, radical initiators under supercritical fluid conditions are able to escape more readily from solvent cages, and the rate coefficient for the initiation process is markedly increased. Processes propagated by free radicals, such as polymerisation, are rate enhanced for this reason, as are enzymatic reactions. 

JO Metzger. Thermal organic reactions at supercritical fluid conditions Functionalization of alkanes by free radical additions to unsaturated compounds. In G Brunner, M Perrut, eds. Proceedings of the 3rd International Symposium on Supercritical Fluids, Strasbourg, France, Vol. 3, 1994, pp 99-102. 

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