Gel Preparation by Covalent Bonding

Polyurethane [6], polyurea, and epoxy resin [7] are obtained by polyaddition polymerization. Formation of crosslinks depends on reaction conditions. It is complex because, in many cases, both the combination of various addition reactions and condensation contribute, and the number of functional groups and reactivities change. 

Formaldehyde also reacts with aromatic hydrocarbons or amines at various ratios and forms relatively small molecular weight reaction intermediates [6]. These are multifunctional and easily form crosslinks by heating with appropriate hardeners. Phenolic resin, urea resin, and melamine resin are typical examples and 3D networks are formed by addition condensation reactions that repeats the addition and condensation 

Condensalion polymerizalion. condensalion addition polymerization, addition condensation polymerization. addMon polymeri ion 

Helix-coii transition, partial crystaliization by hydrogen bonding, interpolymer complex 

Aggregation of hydrophobic group, poiymer micelle formation 

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The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

A polymer gel is a network of flexible cross-linked chains. Structures of this type can be obtained by chemical or physical processes. Some gels are cross-linked chemically by covalent bonds (chemical gel), whereas other gels are cross-linked physically by weak forces, such as hydrogen bonds, van der Waals forces, or hydrophobic and ionic interactions (physical gel). Physical gelation processes are usually reversible and are called sol-gel transitions. The final gel structures and properties are sensitive to the preparation meth-... 

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