Polymer-supported ultrafiltration

Owing to some advantages, in particular low energy requirements and fast reaction kinetics, polymer-supported ultrafiltration is gaining in importance as a pro-... 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

In contrast to solid-phase Suzuki couphng, very low amounts of the Pd-catalyst (0.2 mol%) were sufficient and high conversions (87-99%) to biaryls (65) were obtained to yield relatively pure products (>90%, GC/MS, NMR) after ultrafiltration. In some cases most of the polymer supported boronic compound precipitated during the reaction and therefore no further purification was required. Nonetheless, quantitative removal of catalyst traces was not yet possible with either work-up protocol. 

Kragl 13) pioneered the use of membranes to recycle dendritic catalysts. Initially, he used soluble polymeric catalysts in a CFMR for the enantioselective addition of Et2Zn to benzaldehyde. The ligand a,a-diphenyl-(L)-prolinol was coupled to a copolymer prepared from 2-hydroxyethyl methyl acrylate and octadecyl methyl acrylate (molecular weight 96,000 Da). The polymer was retained with a retention factor > 0.998 when a polyaramide ultrafiltration membrane (Hoechst Nadir UF PA20) was used. The enantioselectivity obtained with the polymer-supported catalyst was lower than that obtained with the monomeric ligand (80% ee vs 97% ee), but the activity of the catalyst was similar to that of the monomeric catalyst. This result is in contrast to observations with catalysts in which the ligand was coupled to an insoluble support, which led to a 20% reduction of the catalytic activity. 

The ultrafiltration method was found to be very effective for the removal of the excess reagent. An enzymatic method was employed in this case for the splitting of the peptide from the polymer support. 

Possible alternatives to cross-linked polymer supports are soluble and colloidal polymers. They would require large scale ultrafiltration for industrial use. Although ultrafiltration is not yet economical for desalination of seawater, it might be for a separation of a more expensive product. One example is the catalytic partial hydrogenation of soybean oil (361 with soluble polymer-bound transition metal complexes. Solid inorganic supports such as silica gel and alumina are usually not subject to these physical attrition and filtration problems. 

These methods do not find wide, general applications in monitoring of solid-phase synthesis. Some of the polymer-mediated syntheses have been carried out using a soluble polymer support. Nucleotides show characteristic uv absorption, and the attachment of a nucleotide residue to a soluble polymer is easily followed by separating it by ultrafiltration or precipitation. The polymer-bound nucleotide is then redissolved and the concentration of the bound nucleotide determined by uv absorption (Hayatsu and Khorana, 1967). 

After cleavage of the amino protecting group, the next amino acid of the sequence was coupled dir tly to the support. For coupling DCC was used and the ultrafiltration technique for removal of excess reagents from the polymer support. 

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

Membranes used for the pressure-driven separation processes, microfiltration, ultrafiltration and reverse osmosis, as well as those used for dialysis, are most commonly made of polymeric materials 11. Initially most such membranes were cellulosic in nature. These are now being replaced by polyamide, polysulphone, polycarbonate and a number of other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation. This process has four main steps (a) the polymer is dissolved in a solvent to 10-30 per cent by mass, (b) the resulting solution is cast on a suitable support as a film of thickness, approximately 100 11 m, (c) the film is quenched by immersion in a non-solvent bath, typically... 

Membrane extraction offers attractive alternatives to conventional solvent extraction through the use of dialysis or ultrafiltration procedures (41). The choice of the right membrane depends on a number of parameters such as tlie degree of retention of the analyte, flow rate, some environmental characteristics, and tlie analyte recovery. Many early methods used flat, supported membranes, but recent membrane technology has focused on the use of hollow fibers (42-45). Although most membranes are made of inert polymers, undesired adsorption of analytes onto the membrane surface may be observed, especially in dilute solutions and when certain buffer systems are applied. 

E. Ruckenstein, L. Liang, Pervaporation of ethanol-water mixtures through poly(vinyl alcohol)-poly(acrylamide) interpenetrating polymer network membranes unsupported and supported on polyether-sulfone ultrafiltration membranes a comparison, J. Membr. Sci. 110... 

One promising approach that may help to establish APS as the fraction of HMW DOM that assembles into particulate matter is to compare the chemical composition of polymer gel assemblies with HMW DOM. In one such experiment, HMW DOM recovered by ultrafiltration of seawater or spent culture media was redissolved in seawater and agitated by bubbling to produce particles which collect at the top of a bubble tower (Gogou and Repeta, unpublished). Particles formed by bubbling have the same neutral sugars in approximately the same proportions as APS. NMR data likewise show particles to be rich in carbohydrate and have the same major resonances as APS, although the relative amount of major biochemicals differs between the two samples. This and other similar approaches further support the hypothesis that APS is the reactive fraction of... 

Most of the earlier studies on the immobilization of enzymes were directed towards the attachment of the enzymes to water-insoluble polymeric supports such as cellulose dextran derivatives, polyacrylamide and porous glass Diffusion problems and steric hindrance are two main factors affecting the application of such supports. The introduction of soluble polymers for immobilization purposes overcomes these difficulties to a greater extent. These soluble enzyme derivatives were synthesized in order to increase the effective molecular size of parent en mes this would rmit the use of ultrafiltration without any los of the enzyme. O NeiD etal. immobilized the enzyme chymotrypsin on soluble dextran for... 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

Coating of ultrafiltration/microfiltration membrane supports such as polyvinylidene fluoride (PVDF) or polysulfone (PSF) with solutions of polymers such as poly(ether-hlocfc-amide) [51]. 

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