Inverse temperature dependence polymer solubilities

In 1993 Bergbreiter prepared two soluble polymer-supported phosphines that exhibited an inverse temperature-dependent solubility in water [52]. Although PEG-supported phosphine undergoes a phase-separation from water at 95-100 °C, the PEO-poly(propylene oxide)-PEO supported catalyst was superior as it is soluble at low temperatures and phase-separates at a more practical 40-50 °C. Treatment of a diphenylphosphinoethyl-terminated PEO-PPO-PEO triblock copolymer... 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acryhc acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubihty in water and retain this property after functionahzation with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by coohng the solution to 0 °C [92]. Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

Materials that typify thermoresponsive behavior are polyethylene—poly (ethylene glycol) copolymers that are used to functionalize the surfaces of polyethylene films (smart surfaces) (20). When the copolymer is immersed in water, the poly(ethylene glycol) functionalities at the surfaces have solvation behavior similar to poly(ethylene glycol) itself. The ability to design a smart surface in these cases is based on the observed behavior of inverse temperature-dependent solubility of poly(alkene oxide)s in water. The behavior is used to produce surface-modified polymers that reversibly change their hydrophilicity and solvation with changes in temperatures. Similar behaviors have been observed as a function of changes in pH (21—24). 

Bergbreiter " synthesized smart phosphine ligands 26 and 27 (Figure 6) from an ethylene oxide-propylene oxide-ethylene oxide triblock co-polymer that possesses a property termed inverse temperature-dependent solubility. This means that at low temperature, the phosphines and their rhodium complexes are soluble in water through the formation of hydrogen bonds between oxygen atoms and water molecules, but, on heating above the critical-temperature cloud... 

The inverse temperature-dependent solubility in aqueous media of polymer-bound palladium(0)-phosphine catalysts, based on the water-soluble polymer poly(Wisopropyl)acrylamide (PNIPAM) 28, was also used to recycle and reuse these catalysts in nucleophilic allylic substitutions (Equation (8)) and cross-coupling reactions between aryl iodides and terminal alkynes (Equation (9)). The catalyst was highly active in both reactions, and it was recycled 10 times with an average yield of 93% in the allylic nucleophilic substitution by precipitation with hexane. ... 

Thermal precipitation by cooling is the scheme chemists normally use in recrystallizations and is the normal behavior of small molecules. Macromolecules are different in that they can often be phase separated from solution by heating [ 119,120]. Thermal precipitation by heating is a process that produces a solid polymer without addition of anything other than heat. It is the inverse of the process used with the polyethylene oligomers discussed above. This inverse temperature-dependent solubility of macromolecules is a phenomenon that is most simply ascribed to the unfavorable entropy of solvation of a macro-... 

Wong and coworkers have investigated the enzymatic oligosaccharide synthesis on a thermoresponsive polymer support (Scheme 16.33). The copolymers of 7V-/-propyla-crylamide (NIPAm) and functionalized monomers are thermoresponsive and exhibit inverse temperature-dependent solubility in water they are soluble in cold water but become... 

A variant on this theme is to attach a transition-metal complex of a smart polymer, the solubility of which can be dramatically influenced by a change in a physical parameter, e.g., temperature [23] (cf. Sections 4.6 and 4.7). Catalyst recovery can be achieved by simply lowering or raising the temperature. For example, block copolymers of ethylene oxide and propene oxide show an inverse dependence of solubility on temperature in water [24]. Karakhanov et al. [25] prepared water-soluble polymeric ligands comprising bipyridyl (bipy) or acetylacetonate (acac) moieties covalently attached to poly(ethylene glycol)s (PEGs) or ethylene oxide/propene oxide block copolymers 9 and 10. 

Another method to prepare membranes utilizes the thermally induced phase inversion (TIP) process. TIP refers to a process whereby the polymer is dissolved in a solvent in which the solubility of the polymer in the solvent is temperature dependent. 

Eq. VI - 74 is the basic equation for liquid transport and it is the same as that for gas transpon (see eq. VI - 46) and vapour transpoit. However, due to the (high) interaction between organic liquids and polymer the permeability coefficient Pj is dependent on composition and temperature. Both solubility and diffusivity are concentration and temperature dependent as have been discussed in chapter V Eq. VI - 74 illustrates the important parameters involved, the permeabilitj coefficient is a membrane- or material-based parameter. Other parameters of interest are the effective membrane thickness I and the partial pressure difference Apj. The permeation rate is inversely proportional to the membrane thickness and proponional to the partial pressure difference across the membrane. In general eq. VI - 74 can be w ritten as... 

In order to explain the experimental phase diagrams of water-soluble polymers, a number of semiempirical approaches that assume the concentrational dependence of the Flory-Huggins solubility parameter were developed. " " Hie two-state models, which involve equilibtium coexistence of two interconvertible (solvophilic and solvophobic) states of the monomer units, as well as the n-duster model, which assumes temperature-dependent inversion of the higher order virial coeffident, allow to rationalize the apparent concentrational dependence of the solubility parameter in aqueous solutions of water-soluble polymers. 

Figures 12.1.22 and 12.1.23 explain technical principles behind formation of efficient and selective membrane. Figure 12.1.22 shows a micrograph of hollow PEI fiber produced from N-methyl-2-pyrrolidone, NMP, which has thin surface layer and uniform pores and Figure 12.1.23 shows the same fiber obtained from a solution in dimethylformamide, DMF, which has a thick surface layer and less uniform pores. The effect depends on the interaction of polar and non-polar components. The compatibility of components was estimated based on their Hansen s solubility parameter difference. The compatibility increases as the solubility parameter difference decreases. Adjusting temperature is another method of control because the Hansen s solubility parameter decreases as the temperature increases. A procedure was developed to determine precipitation values by titration with non-solvent to a cloud point. Use of this procedure aids in selecting a suitable non-solvent for a given polymer/solvent system. Figure 12.1.24 shows the results from this method. Successfid in membrane production by either non-solvent inversion or thermally-induced phase separation requires careful analysis of the compatibilities between polymer and solvent, polymer and non-solvent, and solvent and non-solvent. Also the processing regime, which includes temperature control, removal of volatile components, uniformity of solvent replacement must be carefully controlled.

The permeability provides a quantitative measure of the easiness with which a certain species penetrates the membrane. The permeability depends on the nature of the gas, on the polymeric matrix, on temperature, and on the pressure values, Pl and Ph, on both sides of the membrane. For simple gases the solubility increases as the gas diameter increases, since the gas becomes easier to condense. The gas diffusivity through a membrane is inversely proportional to its molecular size, because large molecules interact more with the polymer chains. 

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