Macroporous Resins

Macroporous resins are generally highly (>5%) crosslinked polystyrene micro-beads [12]. The term ''macroporous'' refers to their inner skeleton, which is made up of a permanent porous structure even in the dry state (see Scheme 3.1c). Historically, functionalized macroporous resins have mainly been used for ion exchange. Nowadays, many new applications, especially in the field of polymer-supported reagents [26, 27], have been developed (see also Section end).

Macroporous resins are prepared by suspension polymerization of monomers such as styrene, vinyl pyridine, acrylamide, or glycidyl methacrylate with a porogen agent (Scheme 3.1) [28, 29]. Thus, a mixture of monomer with potentially a como-nomer and a crosslinking agent are copolymerized after dispersion in an aqueous medium in the presence of the porogen, which, remains within the beads during the polymerization and acts as a template for the formation of the permanent internal porous structure of the final resin. Porogen agents can be of different natures (e.g. solvents, noncrosslinked polymers). After completion of the polymerization the porogens are removed by processes dependent on their characteristics and a hard opaque bead with a rough surface remains. The opacity of the macro-porous resins, compared with the glassy appearance of the microporous beads, is due to their heterogeneous structure, which is made up of highly crosslinked polymeric microdomains and pores that are devoid of polymer [20, 30].

Scheme 3.1. Synthesis and structure of interconnected pores. (1) Porogen and network macroporous resins: (a) polymer network start to phase separate; (2) porogen phase forming; (b) porogen phase acts as pore removed to yield pores (hatched area, template; (c) dry macroporous resin with large crosslinked polymer; dots, porogen phase).

Scheme 3.1. Synthesis and structure of interconnected pores. (1) Porogen and network macroporous resins: (a) polymer network start to phase separate; (2) porogen phase forming; (b) porogen phase acts as pore removed to yield pores (hatched area, template; (c) dry macroporous resin with large crosslinked polymer; dots, porogen phase).

As mentioned above the generation of pores can take place in two ways: noncrosslinked polymers as well as organic solvents [28, 31]. For instance, if linear

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