Supercritical fluid polymerization

Canelas, D.A. Desimone, J.M. Polymerizations in liquid and supercritical carbon dioxide. Adv. Polym. Sci. 1997, 133 (Metal Complex Catalysts, Supercritical Fluid Polymerization, Supramolecu-lar Architecture), 103-140. 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

Runaway reaction or polymerization—e.g., vinyl chloride monomer (Kim-E and Reid, The Rapid Depressurization of Hot, High Pressure Liquids or Supercritic Fluids, chap. 3, in M. E. Paulaitis et al., eds.. Chemical engineering at Supercritical Fluid Conditions, Ann Arbor Science, 1983, pp. 81-100)... 

Figure 3.10 Optimisation strategy for supercritical fluid extraction of polymeric samples. After Lou et al. [145]. Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, A Division of Preston Industries, Inc.

For the analysis of organic additives in polymeric materials, in most cases, prior extraction will be necessary. Depending on the nature of the additive, many different approaches are employed. These include soxhlet extraction with organic solvent or aqueous media, total sample dissolution followed by selective precipitation of the polymer leaving the additive in solution, assisted extraction using pressurised systems, ultrasonic agitation and the use of supercritical fluids. In trace analysis, solid phase extraction (SPME) from solution or solvent partition may be required to increase the analyte concentration. 

In 1994, we reported the dispersion polymerization of MM A in supercritical C02 [103]. This work represents the first successful dispersion polymerization of a lipophilic monomer in a supercritical fluid continuous phase. In these experiments, we took advantage of the amphiphilic nature of the homopolymer PFOA to effect the polymerization of MMA to high conversions (>90%) and high degrees of polymerization (> 3000) in supercritical C02. These polymerizations were conducted in C02 at 65 °C and 207 bar, and AIBN or a fluorinated derivative of AIBN were employed as the initiators. The results from the AIBN initiated polymerizations are shown in Table 3. The spherical polymer particles which resulted from these dispersion polymerizations were isolated by simply venting the C02 from the reaction mixture. Scanning electron microscopy showed that the product consisted of spheres in the pm size range with a narrow particle size distribution (see Fig. 7). In contrast, reactions which were performed in the absence of PFOA resulted in relatively low conversion and molar masses. Moreover, the polymer which resulted from these precipitation... 

The advantages of supercritical fluid chromatography for polymer separations have been illustrated in the literature for many years. A recent example is the separation of long-chain polyprenols using SFC with matrix-assisted laser-desorption ionization TOF mass spectrometry [10]. The generic name for 1,4-polyprenyl alcohols is polyprenol these compounds generally have smaller polymerization chains of less... 

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. 

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