Combinatorial Chemistry Review

Resins for Solid Phase Synthesis

In solid phase support synthesis, the solid support is generally based on a polystyrene resin. The most commonly used resin supports for SPS include spherical beads of lightly cross linked gel type polystyrene (12% divinylbenzene) and poly(styrene-oxyethylene) graft copolymers which are functionalised to allow attachment of linkers and substrate molecules. Each of these materials has advantages and disadvantages depending on the particular application.

Crosslinked Polystyrene

Lightly cross-linked gel type polystyrene (GPS) (Figure) has been most widely used due to its common availability and inexpensive cost. GPS beads which are functionalised with chloromethyl-, aminomethyl-, and a variety of linkers are commercially available from a variety of sources. A prominent characteristic of GPS beads is their ability to absorb large relative volumes of certain organic solvents (swelling). This swelling causes a phase change of the bead from a solid to a solvent-swollen gel, and therefore, the reactive sites are accessed by diffusion of reactants through a solvent-swollen gel network. In solvents, which swell the polymer well, the gel network consists of mostly solvent with only a small fraction of the total mass being polymer backbone. This allows relatively rapid diffusional access of reagents to reactive sites within the swollen bead. In solvents, which do not swell the polymer, the cross-linked network does not expand and the diffusion of reagents into the interior of the bead is impeded.

cross-linked gel polystyrene (GPS)

GPS has good swelling characteristics in solvents of low to medium polarity ranging from aliphatic hydrocarbons to dichloromethane. Polar, protic solvents, such as alcohols and water, do not swell GPS resins, and accessibility to all reaction sites may be compromised. Hence GPS supports are most suitable for chemistry performed in solvents of low to medium polarity.