Chemistry Reference
In-Depth Information
R
0
T
0
T
1
Fig.
8
.2
Polymer-supported molecules
in multistep synthesis.
æ
S
0
ææÆ
æ
S
1
ææÆ
æ
S
1
-
T
0
ææÆ
æ
S
1
-
T
0
-
T
1
f
0
CH
2
CH
f
0
æ
cat
S
1
S
0
+
R
a
c
Fig.
8
.3
Polymer-supported catalytic species.
CH
2
CH
f
i
æ
f
i
b
the final elaborated molecule is released from the
support (not as simply as a ripe fruit from the
branch).
The allegated analogy relies on the following
sequence: molecules are brought on the workbench,
immobilised on the vice, carefully hand-processed
and finally released in a turn.
a
b
CH
2
CH
f
n
æ
f
n
Fig.
8
.4
General schemes of making functional polymers.
2 Making Functional Polymers
modifications are performed easily. It should be
recalled, however, that these modifications are not
introduced homogeneously inside the beads: the
outer sphere of the particles is more dense in func-
tional groups than the inner part beneath; this is not
important if it is a unique transformation (Fig. 8.4,
path
a
).
When using the final material for a given reaction,
the substrate need not diffuse further inside the
beads to be transformed; these 'superficial' reagents
allow fast reactions provided that there is enough
reagent. On the contrary, if a modification sequence
has been required (Fig. 8.4, path
c
) the transforma-
tions may not be completed deeper inside, giving P—
f
(
n
-
i
)
intermediates.
The copolymerisation method (Fig. 8.4, path
b
)
allows a possible purification of the monomer
CH
2
=CH—
f
i
at each step from CH
2
=CH—
f
0
to
CH
2
=CH—
f
n
and the final copolymerisation affords
a much more homogeneous material in terms of
spatial distribution of the functional groups, but a
significant part of the reactive functions buried in the
centre of the beads is less accessible to the substrate.
With this last approach, the polymerisation condi-
tions need to be experienced each time due to the
2.1 General schemes
Considering P—
f
n
as the required functional polymer,
there are two fundamental approaches: when a start-
ing polymeric material P—
f
0
is available, the native
function
f
0
is transformed into
f
n
by a reaction or a
(short) sequence of reactions (Fig. 8.4, path
a
); and
when a starting monomer containing
f
n
is available,
the polymer is obtained by copolymerisation with
that monomer CH
2
=CH—
f
n
or by modifying an
available monomer CH
2
=CH—
f
0
(Fig. 8.4, path
b
).
These are extreme situations. In fact, the monomer
may require a transformation from CH
2
=CH—
f
0
to
the intermediate monomer CH
2
=CH—
f
i
and then
copolymerisation gives the intermediate material P—
f
i
that is modified to get the required P—
f
n
(Fig. 8.4,
path
c
).
The choice depends on the availability of the ma-
terials, the stability and the reactivity of the relevant
monomers. The method preferred by most organic
chemists is to modify a ready-made polymer, espe-
cially when they are not familiar with polymerisa-
tion procedures.
The polymer in hand has a well-defined composi-
tion and morphology and the required chemical
