Chemistry Reference
In-Depth Information
reagents being isolated by filtration to give the pure
product in solution (Fig. 8.1) or bound to the
polymer (Fig. 8.2); no further extraction, chroma-
tography or extra distillation is generally required.
f
Concentration effects
S
0
S
1
High concentrations of catalysts or reagents can be
obtained inside the polymeric material; this concen-
tration may be beyond the common solubility. This
aspect is important in catalysis when a good diffu-
sion is provided. Conversely, with a very low degree
of functionalisation, intramolecular reactions are
favoured versus intermolecular reactions; this is an
alternative to the high dilution conditions that can
be achieved now in a restricted volume of reactants.
f
'
Fig.
8
.1
Polymer-supported reagents.
Safety of handling
Polymeric catalysts
Being insoluble and non-volatile, polymers can bind
ill-smelling or noxious reagents; once the reagent
has been grafted (with the usual care when handling
As, Br
2
, HF, S, Se or Sn products or their derivatives),
the polymer can be stored and used upon request
under less severe safety conditions.
Environmental requirements are fulfilled more
easily when wastes and co-reagents stay in a solid
and filtrable phase and when reduced amounts of
solvent are used in the final work-up.
Any kind of ion, organic or organometallic molecule
with a well-defined catalytic activity can be immo-
bilised ionically or covalently on polymer carriers
(Fig. 8.3); even enzymes have been entrapped into
polymeric matrices. This aspect is the common area
of both homogeneous and heterogenous catalysis; the
'immobilisation' of soluble catalytic species allows the
reuse or the recovery of expensive materials:
enzymes, precious metal derivatives, chiral species.
1.3 What is really possible and what is
still expected
Recycling
Depending on the relative price of the final product
versus that of the starting polymer beads, the poly-
meric carrier can be recycled or not. Immobilised
enzymes and organometallic derivatives of precious
metals are expected to be recycled as catalysts; ion-
exchanged resins usually are reloaded with fresh
salts.
In a different area, at the end of a multistep syn-
thesis of an expensive biologically active molecule,
the price of the starting polymer carrier often
becomes non-significant.
Easy separation
This is the first and most important feature: reactants
and products are separated by simple filtration,
affording a single product in one phase.
Solvent dependence
In usual reactions in solution, the solvent must dis-
solve all the components that are involved, but when
one reactant is immobilised the others can fit with
a larger choice of solvents. The polymer itself can
induce local solvation effects.
Workbench strategy
This approach allowed the bloom of multistep syn-
theses on a solid phase: once a starting molecule is
bound, a cascade of different reactions are performed
repeatedly with a series of reagents in solution;
Completion
Reactions can be made almost quantitative with
an excess of reagent, the remaining and the spent
