Water-soluble polymers cross-linked

Because poly(vinylpyrrolidone) is a water-soluble polymer, cross-linked copolymers of this material imbibe water and swell. This fact has been applied to the possible preparation of contact lenses which are soft and flexible on contact with water or aqueous fluids not only by grafting appropriate monomers to preformed PVP but also by direct copolymerization of N-vinylpyrrolidone with other comonomers. For example, a cross-linked hydrogel was prepared by terpolymerizing A-vinylpyrrolidone, methyl acrylate, and tetraethylene glycol dimethacrylate in the presence of AIBN [44]. 

It is necessary to discuss another chemical feature related to water-soluble polymers cross-cross-linking — the component that separates viscous systems from gel systems. Viscous systems flow, and it follows, therefore, that they do not possess the tensile properties of muscles. High-viscosity systems have structural integrity, gels provide the necessary combination of tensile strength and elongation or stretch. 

Alkanolamine Chelates. Alkanolamine chelates, which are prepared by reaction of tetraalkyl titanates with one or more alkanolamines, are used primarily in cross-linking water-soluble polymers (qv) (see Alkanolamines) (104). The products are used in thixotropic paint emulsion paints, in hydrauhc fracturing and drilling of oil and gas wells, and in many other fields. The stmcture of... 

Cross-linked versions of water-soluble polymers swollen in aqueous media are broadly referred to as hydrogels (52) and have a growing commercial utility in such apphcations as oxygen-permeable soft contact lenses (qv) (53) (Table 4) and controUed-release pharmaceutical dmg deflvery devices (54). Cross-linked PVP and selected copolymers fit this definition and are of interest because of the following stmcture/performance characteristics ... 

High-performance size exclusion chromatography is used for the characterization of copolymers, as well as for biopolymers (3). The packings for analyses of water-soluble polymers mainly consist of 5- to 10-/Am particles derived from deactivated silica or hydrophilic polymeric supports. For the investigation of organosoluble polymers, cross-linked polystyrene beads are still the column packing of choice. 

In previous works [18-20,23,102] water-soluble polymers such as polyacrylamide (pAM), polysodium acrylate (pAA Na), poly(acrylamide-sodium acrylate) (pAM-AA Na), poly(acrylamide-diallyethylamine-hy-drochloride) (pAM-DAEA-HCl), and poly(acrylamide-sodium acrylate-diallyethylamine-hydrochloride) (pAM-AANa-DAEA-HCl) were used in the recovery of cations and some radioactive isotopes from aqueous solutions. It was found that the floe is formed between the added polymer and ions of the solution in the flocculation process with the formation of a crosslinked structure. The formed cross-linked structure is characterized by [103-105] ... 

The core of reversed micelles can be transformed to a highly viscous domain (nanogel) by entrapping appropriate species, such as viscous solvents and hydrophilic macromolecules, or by performing in situ appropriate polymerization reactions or intramolecular cross-linking of water-soluble polymer chains [232-234]. 

The use of ionizing radiation to induce cross-linking is another important technique for producing hydrogels from linear water-soluble polymers. When such polymers are bombarded by ionizing radiation, either in the bulk state or in solu-... 

Croscarmellose sodium is derived by internally cross-linking the ether, sodium car-boxymethylcellulose. This water-soluble polymer is composed of repeating units of... 

The amount and the rate of water solubility or swelling can be decreased by cross-linking and the substitution of nonpolar units for polar units in the polymer. Most water-soluble polymers possess both hydrophobic and hydrophilic moieties. This combination of hydrophobic and hydrophilic moieties affects the shape of the polymer chains in solution. Thus many water-soluble polymers in aqueous solution exist as random or partially helical chains which are partially extended to allow fuller hydrogen bonding with the water molecules. 

In summary, we described a system in which a backbone containing a highly polar group contains a degree of cross-linking. Put another way, the system is a three-dimensional network of water-soluble polymer and cross-linking that serves as the basis for all hydrogels, natural or synthetic. 

Cross-linked versions of water-soluble polymers swollen in aqueous media are broadly referred to as hydrogels and have a growing commercial utility in such applications as oxygen-permeable soft contact lenses and... 

Membrane design and fabrication requires more optimization than the synthesis of the right type of polymer. For example, those phosphazene polymers that contained the highest ratios of methylamino groups were too brittle to be used as membranes (because of the high glass-transition temperatures) and too soluble in aqueous media. However, the polymers could be made insoluble in water by radiation cross-linking as shown in reaction (54). 

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. 

N-(Hydroxymethyl)acrylamide (HMA, Tokyo Kasei Co.) was recrystallized from ethyl acetate. N,N-Dimethylacrylamide (DMA, Tokyo Kasei Co.) and styrene (St, Kashima Kagaku Yakuhin Co.) were distilled at 54°C/3.5 mmHg and 40°C/14.5 mmHg, respectively. In some copolymerizations cross-linking reagents were added to reduce the formation of water-soluble polymer. N -Methylenebisacrylamide (MBA, Nakarai Chemicals Co.) and N-allylacrylamide (AAA, Polysciences, Inc.) were used as received. Divinylbenzene (DVB, Tokyo Kasei Co.) was treated with 10 % sodium hydroxide and dried. Two kinds of initiators were used Potassium persulfate (KPS, Taisei Kagaku Co.) was recrystallized from water and azobis(isopropyl-... 

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