Divinylbenzene polymeric resins

The polarity values of binary acetonitrile/water and methanol/water mobile phases used in RPLC were measured and compared with methylene selectivity (acH2) for both traditional siliceous bonded phases and for a polystyrene-divinylbenzene resin reversed-phase material [82], The variation in methylene selectivity for both was found to correlate best with percent organic solvent in methanol/water mixtures, whereas the polarity value provided the best correlation in acetonitrile/water mixtures. The polymeric resin column was found to provide higher methylene selectivity than the siliceous-bonded phase at all concentrations of organic solvent. 

Reversed-phase chromatography employs a nonpolar stationary phase and a polar aqueous-organic mobile phase. The stationary phase may be a nonpolar ligand, such as an alkyl hydrocarbon, bonded to a support matrix such as microparticulate silica, or it may be a microparticulate polymeric resin such as cross-linked polystyrene-divinylbenzene. The mobile phase is typically a binary mixture of a weak solvent, such as water or an aqueous buffer, and a strong solvent such as acetonitrile or a short-chain alcohol. Retention is modulated by changing the relative proportion of the weak and strong solvents. Additives may be incorporated into the mobile phase to modulate chromatographic selectivity, to suppress undesirable interactions of the analyte with the matrix, or to promote analyte solubility or stability. 

In the latter half of the 1990s, porous, highly cross-linked polystyrene divinylbenzene (PS-DVB) resins with smaller, spherical particle sizes more suitable for SPE uses became available (Figure 2.23). The new generation of apolar polymeric resins is produced in more purified form, reducing the level of impurities extracted from the sorbent. Polymeric resins are discussed in more detail by Huck and Bonn [69], Fritz [73], Thurman and Mills [75], and Pesek and Matyska [87],... 

Various types of sorbents used for SPE can be grouped (Table 2.6) according to the primary mechanism by which the sorbent and the analyte interact [32,72]. Reversed-phase bonded silica sorbents having alkyl groups such as octadecyl (Qg, C18), octyl (C8, C8), or ethyl (C2, C2) covalently bonded to the silica gel backbone or cyclohexyl (CH) or phenyl groups and sorbents composed of polymeric resins such as polystyrene-divinylbenzene... 

Ion exchange is widely used for the selective separa-tion/removal of metal ions from an aqueous medium either for environmental clean-up, for the removal of undesirable components, or for metal recovery. Other species that have been treated include nitrate, ammonia, and silicate. This process is normally based on the use of polymeric resins (typically styrene/divinylbenzene polymer backbones) that have chemically active groups attached to them. Sulfonic or carboxylate groups are regularly used for the removal of cations, and quaternary ammonium groups are used for the removal of anions. 

In addition to true ion exchange, other interactions can take place between the sample solutes and the resin. Adsorption is one of the commonest of these interactions. For example, the benzoate anion appears to be adsorbed somewhat by the poly-styrene-divinylbenzene polymeric matrix of organic ion exchangers. This may be due to an attraction of the k electrons of the aromatic polymer for the benzoate. Benzoic acid, which exists mostly in the molecular form, is absorbed to a much greater degree than benzoate salts. 

N-oxide salts (HBTU and TBTU, respectively) [39], or from l-hydroxy-7-azabenzotriazole (HOAt) such as N-[(dimethylamino)-lH-l,2,3-triazolo[4,5-fe] pyridino-l-y]methylene]-N-methy]methanaminium tetrafluoroborate N-oxide (HATU) [40], are well established reagents. They are especially devoted to peptide coupling reactions due to their efficiency and the low degree of undesirable race-mization produced in the final peptide compared to the use of classical carbodi-imide-coupling methods. Therefore, as the polystyrene-supported HOBt is an often used polymeric reagent (Section 7.6.3) [41], its transformation in a supported HOBt and tetramethylurea-derived aminium salt analog to HBTU and TBTU resulted directly. Thus, the reaction of polystyrene-2% divinylbenzene copolymer resin P-HOBt (20) with tetramethylchloroformamidinium tetrafluoroborate (21) (4 equivalents) in the presence of triethylamine gave polymeric N-[(lH-benzotriazol-l-yl)(dimethylamino)methylene]-N-methylmethanaminium tetrafluoroborate N-oxide (P-TBTU, 22) (Scheme 7.6) [42],... 

The insoluble polymeric resin is synthesised by the suspension-radical copolymerisation of styrene with divinylbenzene (DVB) (see Figure 4.3). The polymerisation results in a three-dimensional column framework which is porous in character and can thus allow diffusion of ions to take place through it. By conducting the polymerisation in an aqueous medium beads of definite size can be produced. The extent of cross-linkage and hence pore size can be varied by altering the proportion of DVB. 

Fmoc-aminomethyl-3,5-dimethoxyphen-oxy)valeric acid PEG, polyethylene glycol PEG-PS, polyethylene glycol-cross-linked polystyrene graft copolymer PS, copoly-(styrene-1% divinylbenzene) polymeric support , resin SPPS, solid-phase peptide synthesis tBu, tert.-butyl TEA, trifluoro-acetic acid TFE, trifluoroethanol THF, te-trahydrofuran TLC, thin-layer chromatography. Amino acid symbols denote the L-con-figuration. All solvent ratios and percentages are v/v unless stated otherwise. 

There are two major reasons for modification of pore surfaces in some polymeric resins (1) hydrophiiization of hydrophobic surfaces and (2) an increase in loading capacity and kinetics of ion-exchange resins. The former is typical of styrene-divinylbenzene copolymers that are too hydrophobic to be used directly for the separation of biopolymers in some modes. In this case, hydrophilic polymers such as dextran, poly(oxyethylene), poly(ethylene imine), and poly-(vinyl alcohol) are adsorbed and cross-linked on [35,43] or covalently linked to the pore surface to form a thin biopolymer friendly barrier on the hydrophobic surface. Regnier [44] was one of the first to develop a covalently attached hydrophilic coating that substantially decreased the nonspecific irreversible adsorption of proteins. 

The organic and aqueous phases are prepared in separate tanks before transferring to the reaction ketde. In the manufacture of a styrenic copolymer, predeterrnined amounts of styrene (1) and divinylbenzene (2) are mixed together in the organic phase tank. Styrene is the principal constituent, and is usually about 90—95 wt % of the formulation. The other 5—10% is DVB. It is required to link chains of linear polystyrene together as polymerization proceeds. DVB is referred to as a cross-linker. Without it, functionalized polystyrene would be much too soluble to perform as an ion-exchange resin. Ethylene—methacrylate [97-90-5] and to a lesser degree trivinylbenzene [1322-23-2] are occasionally used as substitutes for DVB. 

The point at which two polymeric chains are joined together by a cross-linker such as divinylbenzene, or sites where tertiary hydrogens are located in the stmcture, are other locations for oxidative attack. In both cation- and anion-exchange resins, oxidative attack results in the removal of cross-linking. 

Divinylbenzene. This is a specialty monomer used primarily to make cross-linked polystyrene resins. Pure divinylbenzene (DVB) monomer is highly reactive polymericaHy and is impractical to produce and store. Commercial DVB monomer (76—79) is generally manufactured and suppHed as mixtures of m- and -divinylbenzenes and ethylvinylbenzenes. DVB products are designated by commercial grades in accordance with the divinylbenzene content. Physical properties of DVB-22 and DVB-55 are shown in Table 10. Typical analyses of DVB-22 and DVB-55 are shown in Table 11. Divinylbenzene [1321 -74-0] is readily polymerized to give britde insoluble polymers even at ambient temperatures. The product is heavily inhibited with TBC and sulfur to minimize polymerization and oxidation. 

Divinylbenzene copolymers with styrene are produced extensively as supports for the active sites of ion-exchange resins and in biochemical synthesis. About 1—10 wt % divinylbenzene is used, depending on the required rigidity of the cross-linked gel, and the polymerization is carried out as a suspension of the monomer-phase droplets in water, usually as a batch process. Several studies have been reported on the reaction kinetics (200,201). 

A macroporous polystyrene-divinylbenzene copolymer is produced by a suspension polymerization of a mixture of monomers in the presence of water as a precipitant. This is substantially immiscible with the monomer mixture but is solubilized with a monomer mixture by micelle-forming mechanisms in the presence of the surfactant sodium bis(2-ethylhexylsulfosuccinate) (22). The porosity of percentage void volume of macroporous resin particles is related to percentage weight of the composite (50% precipitant, 50% solvent) in the monomer mixture. 

Recently, new approaches of sorbent construction for reversed-phase chromatography have been developed. Silicas modified with hydrocarbon chains have been investigated the most and broadly utilized for these aims. Silica-based materials possess sufficient stability only in the pH 2-8 range. Polymeric HPLC sorbents remove these limitations. Tweeten et al. [108] demonstrated the application of stroongly crosslinked styrene-divinylbenzene resins for reversed-phase chromatography of peptides. 

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