Polymer resin beads

Two classes of micron-sized stationary phases have been encountered in this section silica particles and cross-linked polymer resin beads. Both materials are porous, with pore sizes ranging from approximately 50 to 4000 A for silica particles and from 50 to 1,000,000 A for divinylbenzene cross-linked polystyrene resins. In size-exclusion chromatography, also called molecular-exclusion or gel-permeation chromatography, separation is based on the solute s ability to enter into the pores of the column packing. Smaller solutes spend proportionally more time within the pores and, consequently, take longer to elute from the column. 

Resin bead polymer composition Either acrylic resins or, more generally, styrene (vinylbenzene, VB) are cross-linked with typically 4 to 20% divinylbenzene (DVB) in a copolymer network or matrix. 

The polymeric resin beads fill a need that arises from the instability of silica gel and its products to mobile phases of extreme pH (outside a pH range of about 4.0-7.0) and, consequently, are employed in most ion exchange separations. Organic moieties containing ionic groups can be bonded to silica and produce an effective ion exchange media, but the restrictions of pH on phase stability still apply. It follows that ion exchange bonded phases are less popular than the polymer bead alternatives. 

Hacroreticular resins are prepared by suspension polymerization of, for example, styrene-divinylbenzene copolymers in the presence of a substance which is a good solvent for the sononer but a poor swelling agent for the polymer [178-180]. Each resin bead is formed from many microbeads joined together during the polymerization process to create a network of holes and 7 channels. This results in greater mechanical stability,... 

Diffusion is not straightforward inside the resin phase, and this is due to the restrictive influences of the polymer network and because of the charge distribution connected with the fixed ions of the functional groups. The resin phase is consequently related to a porous solid. The effectual diffusivities of metal ions in the resin phase may differ but are largely less than those in the aqueous phase external to the resin phase. If Fick s law is applied to diffusion in a resin bead of radius, r, it may be represented as... 

The novel concept of synthesizing a molecule while attached to a swollen cross-linked resin bead was introduced and demonstrated by R. B. Merrifield with the solid-phase peptide synthesis method about 20 years ago (1,2). The procedure involves the covalent attachment of an amino-acid residue to the polymer bead followed by the addition of subsequent amino-acid units in a stepwise manner under conditions that do not disrupt the attachment to the support. At the completion of the assembly of the peptide, the product is cleaved from the resin and recovered. The macro-scopically insoluble support provides convenient containment of the desired product so that isolation and purification from soluble co-products in the synthesis can be achieved by simple... 

The most well-developed recent examples of catalysis concern catalysts for oxidation reactions these are essentially achiral or chiral metal-salen complexes. Taking into account a number of results suggesting the importance of a degree of mobility of the bound complex, Sherrington et al. synthesized a series of polymer-supported complexes in which [Mn(salen)Cl] units are immobilized in a pendant fashion by only one of the aromatic rings, to polystyrene or poly(methacrylate) resin beads of various morphology (Figure 6).78,79... 

Although the cross-linked pyrazoline polymer is insoluble in common organic solvents by virtue of the crosslinks present, it was discovered that films of this material could be cast onto a variety of substrates from a suspension in a swelling solvent such as benzene. The ability to cast films is presumably related to the substantial change in polymer resin morphology after the coupling reaction has proceeded. The initial resin is hard, spherical and white in color, while the reacted bead is powdery, irregular in appearance and yellow, the color of the pyrazoline monomer. 

Other chapters in this book will describe the developments of new polymer supports which move on and away from conventional resin beads. However, there is no magic polymer support which is optimal for all applications and the novel morphological variants of resin beads reviewed in this chapter offer new opportunities and require extensive evaluation in a range of applications. This work is already ongoing. 

A key element in the original Merriheld procedure of solid-phase synthesis is the solid support system. That system consists of two parts a resin head and a. linker, an organic compound used to join the hrst amino acid to the resin head. The resin beads used by Merriheld are small spherical objects made of cross-linked polystyrene. This material consists primarily of the polymer polystyrene whose linear molecules are linked to each other at various positions by the addition of divinylhenzene (CH2 = CHCgH jCH =CH2). The hnal cross-linked material is relatively rigid, with enough hexihility to permit... 

The linker in a support system has two functions first, to provide an anchor for the growing polymer being made during the synthesis, and second, to protect the functional group at one end of that polymer, the end attached to the resin. At the completion of the synthesis, the hnal product is released from the linker, which remains attached to the resin bead. 

The [Ru(bpy)3]2+ photosensitized reduction of methyl viologen (MV2+) proceeds rapidly in water-swollen iminodiacetic acid type chelate resin beads which adsorb both [Ru(bpy)3]2+ and MV2+ (RM resin). The reduction takes place with the aid of polymer-bound iminodiacetate as a donor 102). Photosensitized formation of hydrogen peroxide occurs in an aqueous solution containing RM resin and oxygen molecules (Fig. 8)102 The some reaction also occurs using a polystyrene-coated filter paper, onto which both [Ru(bpy)3]2 + and MV2+ were adsorbed 103). 

Polymer-bound methylcyclopentadienyltricarbonylmanganese, MeVCpMn(CO)3-co-R (MeVCpMn(CO)3 = methylvinylcyclopentadienyltricarbonylmanganese, R = styrene or vinylpyrrolidone), P-MeCpMn(CO)3, which is used as resin beads or membrane, reacts with acetylene to give stable acetylene complexes on irradiation with near ultraviolet- or visible light. U1,112,ll3)... 

Substrate-catalyst interaction is also essential for micellar catalysis, the principles of which have long been established and consistently described in detail [63-66]. The main feature of micellar catalysis is the ability of reacting species to concentrate inside micelles, which leads to a considerable acceleration of the reaction. The same principle may apply for polymer systems. An interesting way to concentrate the substrate inside polymer catalysts is the use of cross-linked amphiphilic polymer latexes [67-69]. Liu et al. [67] synthesized a histidine-containing resin which was active in hydrolysis of p-nitrophenyl acetate (NPA). The kinetics curve of NPA decomposition in the presence of the resin was of Michaelis-Menten type, indicating that the catalytic act was accompanied by sorption of the substrate. However, no discussion of the possible sorption mechanisms (i.e., sorption by the interfaces or by the core of the resin beads) was presented. 

---