Insolubility by cross-linking

Sephadex A trade name for an insoluble hydrophilic substance prepared by cross-linking dextran, and used in gel filtration. It can also be linked to acidic or basic groups for ion exchange or to alkanes for the chromatography of lipophilic compounds. 

From a morphological point of view, block copolymer micelles consist of a more or less swollen core resulting from the aggregation of the insoluble blocks surrounded by a corona formed by the soluble blocks, as decribed in Sect. 2.3. Experimental techniques that allow the visualization of the different compartments of block copolymer micelles will be presented in Sect. 2.4. Other techniques allowing micellar MW determination will also be briefly discussed. Micellar dynamics and locking of micellar structures by cross-linking will be commented on in Sects. 2.5 and 2.6, respectively. 

In 2000, Benaglia and coworkers reported preparation of MeO-PEG supported quaternary ammonium salt (10) and examined the catalytic efficiency in a series of phase-transfer reactions (Fig. 5.3) [69]. The reactions occurred at lower temperatures and with shorter reaction times than with comparable insoluble 2% cross-linked polystyrene-supported quaternary ammonium salts, although yields varied with respect to classical solution phase quaternary ammonium salt catalyzed reactions. It was observed that yields dropped with a shorter linker, and that PEG alone was not responsible for the extent of phase-transfer catalysis. While the catalyst was recovered in good yield by precipitation, it contained an undetermined amount of sodium hydroxide, although the presence of this byproduct was found to have no effect on the recyclability of the catalyst... 

Antigens and their corresponding antibodies precipitate by cross-linking to form an insoluble network. Polysaccharides have multiple, repetitive immunodeterminants and virtually none have demonstrable tertiary structure in solution (except, perhaps, under viscous stress). The number of these immunodeterminant groupings on each macromolecule is large. In the case of dextran, for instance, there are several thousand of them (if the dextran has a molecular weight of several million), even if the determinant involves the hep-tasaccharide. There is, thus, ample opportunity to form a precipitating, crosslinked complex with divalent (or polyvalent) antibody molecules. 

Indium(III) alkoxides are readily prepared by the treatment of trihalide with NaOR.127 The chemistry of these compounds has been little studied, no doubt because they are insoluble materials in which the indium atom optimizes its coordination number by cross-linking via oxygen. The isopropoxides of aluminum and gallium react128 with In(OPr )3 to give In[M(OPr )4]3 (M = A1, Ga) which are volatile and monomeric in solution. The structure involves six-coordination at the smaller metal centre. 

As discussed earlier, hydrogels can be prepared by cross-linking water-soluble polymers. When immersed in water, such materials absorb water and swell, but they cannot dissolve because of the constraints imposed by the cross-linked structure. The volume expansion is limited by the degree of cross-linking. The minimum number of cross-links required to form an insoluble matrix is approximately 1.5 per chain, and this yields a system with the maximum expansion possible without separation of the chains into a true solution. Thus, a hydrogel may be more than 95% water, yet it has structure and, in that sense, has much in common with living soft tissues. 

Poly[fe(glyceryl)phosphazene] (3.88) is soluble in water, and it is hydrolyzed slowly to glycerol, which can be metabolized readily, and to phosphate and ammonia. A further advantage of this polymer is that the pendent hydroxyl groups are sites for the attachment of drug molecules. If a water-insoluble derivative is needed, this can be accomplished by the use of hydrophobic cosubstituents or by cross-linking of the chains. 

The use of insoluble, highly cross-linked anisotropic networks created by the polymerisation of photoreactive monomers, eliminates the problem of crystallisation, at least for organic materials, since polymer networks are macromole-cular structures incapable of crystallising, see Chapter 6. Furthermore, the fabrication of multilayer devices would be facilitated by the use of a cross-linked stable HTL next to the anode on the solid substrate surface, onto which subsequent layers can be deposited by vapour deposition. Multilayer OLEDs are intrinsically more stable than monolayer devices due to a better balance of charge-carriers and concentration of the charged species away from the electrodes. The synthesis and cross-linking of a suitable aromatic triarylamine derivative with a polymerisable oxetane group at each end of the molecule for use as a HTL has been reported recently, ... 

Other approaches for insolubilization have been proposed by Francotte more stable CSPs could be obtained by cross-linking polysaccharide derivatives photochemical I y initiated ] 163] or thermally initiated ] 164] (Tables 9.5 and 9.6) using a radical reaction. The new CSPs exhibited improved separations for many racemates, predominantly through the ability to use chloroform and other co-solvents. Further, compounds that were insoluble in the commonly used /)-heptane-2-propanol eluents could be easily separated into individual enantiomers, and for many enantioseparations run times can be reduced with chloroform, ethyl acetate or THF containing mobile phases. These improved CSPs are about to be commercialized. 

Preparation of active, insoluble enzymes by physical adsorption to matrices has already been discussed (see p. 362), and, in the case of polysaccharide matrices, most of the enzyme derivatives formed in this way depend upon ionic interaction between charged groups on the polysaccharide and enzyme (see Table II). The instability of these enzyme derivatives may be lessened by cross-linking the coupled enzyme molecules.A few enzymes have been insolubilized by inclusion in microcapsules of collodion, or by entrapment in a polysaccharide gel. - A partition eflFect, in which an enzyme is held in the aqueous phase of a cellulose column while the substrate flows through in an organic phase, has also been used. 

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