Poly (styrene-oxyethylene) graft copolymers, first reported by Bayer and Rapp,
are another class of widely used supports for organic synthesis. As with the polyacrylamide
resins, in order to produce a polar reaction milieu that is closer to the solvents generally used
by solution synthetic chemists, grafted polymer beads have been prepared. The most pre-eminent of
these is TentaGel resin which consists of polyethylene glycol attached to cross-linked polystyrene
through an ether link, and combines the benefits of the soluble polyethylene glycol support with the
insolubility and handling characteristics of the polystyrene bead. The resin was originally prepared
by the polymerisation of ethylene oxide on cross-linked polystyrene already derives with tetraethylene
glycol to give polyethylene glycol chains. (Figure) Poly (styrene-oxyethylene) graft copolymers beads
display relatively uniform swelling in a variety of solvents from medium to high polarity ranging from
toluene to water. The polymers are produced by grafting ethylene oxide from the polystyrene backbone
creating long flexible chains that terminate with a reactive site spatially separated from the more
rigid polystyrene backbone.
Some disadvantages of Poly(styrene-oxyethylene) graft copolymers supports are:
- Relatively low functional group loading compared with GPS; the potential for the PEG chains to complex Lewis acids;
- The potential instability of PEG;
- The presence of linear PEG impurities found in the small molecule products after cleavage from the resin;
- The tendency for resins to become sticky and difficult to handle as the synthesis progress.
