Functionalized polystyrenes are available as linear noncrosslinked and as cross-linked polymers. The latter, generally referred to as PS resins, are among the most commonly used solid supports for solid-phase organic synthesis (SPOS), solid-phase-supported peptide (SPPS) synthesis and nucleotide synthesis. Nowadays, numerous types of polystyrene resins are commercially available and a nonex-haustive list of the most frequently used resins is given in Table 3.1. They are available in different sizes, with different loading capacities (amount of functional-ization) and degrees of crosslinking.
Macroporous and microporous polystyrene resins are typically prepared by suspension polymerization. The basis of this process is the dispersion of an organic phase (consisting of a monomer, a radical initiator, a crosslinking agent and potentially a comonomer) into an aqueous phase [13, 14]. The size of the initial droplets is adjusted by emulsifying the organic phase under stirring in the presence of a polymeric surfactant, which governs the final size and the final size distribution of the beads after polymerization. The different bead sizes are then separated by a multiple sieving process. This is why the size of the beads is usually given in mesh (number of sieve holes per inch); however, for most chemists the mm size of the final bead is more relevant for handling (Fig. 3.2). Resin beads used for combinatorial synthesis are spherical particles typically in the range 50-500 mm which can be easily handled (weighting, filtering and drying).
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