Chemistry Reference
In-Depth Information
respectively, resulting in a wide range of functionalized polymers. For various modification reactions
of polystyrene, see Chap.
9
.
Two types of cross-linked polystyrene are often favored. One is a polymer that is cross-linked by
only 1-2% of divinyl benzene. The material, though fairly strong mechanically, swells and expands
significantly in volume when dispersed in proper solvents. It is called
micro porous
. A copolymer
with up to 20% divinyl benzene is the second type. It is prepared in the presence of large quantities of
diluents to retain the products in expanded form. As a result, the structures are
macro porous
or
macro reticular
. The advantages of micro porous over macro porous structures are faster reactions,
less fragility, and easier handling. Although, macro reticular supports are less often used, they have
the advantage of being useful in almost any solvent.
As a variation of the process, Fr´chet and coworkers introduced
reactive filtration
[
6
]. In place of
beads, they used discs of cross-linked polystyrene that were cut from a rod of the material. On the
surface of these discs, they grafted 2,2-dimethylazlactone. These discs were then used in a filtration to
efficiently scavenge excess amines from a reaction mixture. Subsequently, Frechet and coworkers
also used such discs in a flow through acylation reaction [
7
].
To improve the access to the functional groups, Lee and coworkers developed a process of placing
most of the functional groups on the outer surface of the polystyrene beads [
7
]. This was done by
surface aminomethylation of preformed cross-linked commercially available polystyrene beads. Such
bead usually range in size from 100 to 400 mesh (in.) in diameter and can be functionalized by surface
reactions or by surface grafts. This yielded materials with a lower number of functional groups, but
with the majority of them being accessible. Lee and coworkers then utilized the beads in a solid
peptide synthesis [
7
].
Lee and coworkers also prepared beads with
-heterocyclic carbene ligands located on the outer
shell. In this preparation, they used beads formed in a suspension polymerization [
8
,
9
].
Fr
´
chet and coworkers [
7
] reported a variation of the strategy by attaching functional groups to
cross-linked polystyrene in the interior of a soluble star polymer matrix. In this process, the catalytic
groups are core-confined through the use of low molecular weight macroinitiators that form the
surface of the final polymer. Presumably, this allows using simultaneous incompatible reagents, like
acids and bases, because they are physically isolated from one another.
Lu and Toy [
10
] illustrated a similar approach: by showing how core-functionalized star polymers
are prepared to form sulfonic acid-functionalized core material:
N
N
1. styrene
divinylbenzen
125
o
C
2. K
OH
3. H
+
O
Core functionalized
star polymer
n
S
OO
O
There is a drawback, however, of using the polymer described above, because many functional
groups end up imbedded inside the resin and access to them requires the type of solvents that can
thoroughly swell the resin. Many polar solvents, however, fail to swell cross-linked polystyrene
adequately, yet may be required for specific reactions. That led to modifications, such as, the use
