Polymerization in SCFs

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Apart from pressure effects, which are not unique to SCFs, some other peculiar aspects of polymerization in SCFs are related to the physico-chemical properties of compounds under supercritical conditions. 

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. 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

The primary consideration is that polymer synthesis in SCFs are generally carried out at high pressure consequently, it is important to analyze the influence of this parameter on the polymerization process. The increase of pressure can affect a polymerization process by ... 

In traditional liquid solvents, the polymerization reaction rates are often limited by the local increase in viscosity during the process, as this lowers the mass transfer rate of the monomer to the reaction site. A lower viscosity and a higher diffusion coefficient in SCFs each contribute to overcome this limitation, however, allowing the polymerization rate to be significant up to high value of monomer conversion. 

High-pressure processes have been widely applied in the polymer industry. Near-critical and supercritical fluids (SCFs) are in particular used owing to their easily tunable density, which enhances the control of polymer solubility and their good separability from polymer material [1], SCF solvents (e.g. scC02) offer a potential advantage for separation process. The solubility of different polymeric material in SCFs can be systematically varied by changing operating conditions. Several... 

This application of SCFs, while highly touted, has received scant experimental attention (15). Investigations have been limited to Diels-Alder reactions (11,16), electrochemical reactions (17,18), polymerizations (19,20), and high temperature processes (21,22). Recent semiempirical treatments of SCF solvent properties (2 3,24) have provided a basis for interpreting solvent effects in SCFs. 

It should be noted that on an industrial scale, reactions or other processes in SCF media are not new. Many industrial reactions developed in the early part of the twentieth century are actually conducted under supercritical conditions of either their product or reagent including ammonia synthesis (BASF, 1913), methanol synthesis (BASF, 1923) and ethylene polymerization (ICI, 1937). 

The anomalies observed in the GPC traces of the early fractions implicate GTP mechanisms rather than SCF extraction efficiencies. Unlike anionic and free-radical polymerization mechanisms, GTP is catalyzed, and a silicon atom is central to the catalysis mechanism. It is plausible that the presence of nonfunctional PDMS may interfere with the normal rate of propagation (Hellstern, 1989) since silicon atoms in the PDMS chain may act as alternative catalyst sites. Hellstern (1989) compared the extent of MMA polymerization in the presence of nonfunctional PDMS with the extent of MMA-PDMS... 

Throughout this chapter, the examples of polymerizations in compressed CO2 have been primarily for chain growth polymerization processes. However, step-growth methods represent an area of new interest for SCFs. Initial experiments in this area include the synthesis of aromatic polyesters such as poly(ethylene terephthalate) (PET) in SCCO2 as illustrated in Scheme 4.5-9 [144]. An advan-... 

TABLE IX Polymerizations of Some Selected Vinyl Monomers in SCF... 

The study of supercritical fluid systems by Nuclear Magnetic Resonance (NMR) has been accomplished in two ways the high-pressure probe method and the high-pressure cell method. At this present stage, no paper on the study of polymerization reactions in SCF by NMR could be found. Nevertheless, other reactions have been studied in supercritical media and at extreme conditions [13], and these can be compared with polymerization reactions. 

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