Chemistry Reference
In-Depth Information
different kinetic parameters of each monomer,
including the cross-linking agent.
(2)
The functional monomer, which will be trans-
formed further into the required active group
(chloromethylstyrene, glycidyl methacrylate,
acrylonitrile, 4-vinyl pyridine, vinyl acetate; Fig.
8.5b).
(3)
The cross-linking agent—a bifunctional
monomer that gives insolubility to the
final material: divinylbenzene, ethylene glycol
dimethacrylate, trimethylolpropane trimetha-
crylate (Fig. 8.5c).
2.2 Required properties
The polymeric material that will be used as a carrier
must fulfil several requirements:
(1)
The polymer must be insoluble in a variety of
solvents used in the reaction; this is ensured by
connecting the individual chains (see below for
cross-linking).
(2)
The polymer must be chemically inert with
respect to the reactants and the product(s), espe-
cially with electrophilic and strong base com-
pounds.
(3)
The polymer must afford good diffusion of
solvent and reactants; this is provided by
swelling for gel-type polymers or by the high
porosity of macroporous polymers; swelling is
ensured when the monomeric units have a
similar polarity to that of the solvent. Accessibil-
ity is improved when the reactive function is put
away, by means of a 'spacer', from the polymer
chain.
(4)
The reactive functions must give complete, clean
and unambiguous reactions.
(5)
The polymer must possess a sufficient degree of
functionalisation to afford a good capacity of
functional groups.
(6)
The polymer must be thermally stable in the
usual range of temperature.
(7)
The polymer must be mechanically stable
enough under the stirring conditions; gel-type
resins are more resistant to attrition than macro-
porous resins but the damage can be reduced
when using the 'tea-bag' technique.
(8)
The polymer should be regenerated easily after
use by a simple procedure.
Cross-linking of polystyrene generally is performed
with technical divinylbenzene (DVB), which is a
mixture containing only about 50% of divinyl
benzene (one-third of the
meta
and two-thirds of the
para
isomer) and 50% of ethyl styrene, which
behaves like styrene; thus when a resin is said to be
1% cross-linked it means that 2% of technical DVB
was used in the copolymerisation procedure.
With acrylates and methacrylate-based mono-
mers, ethylene glycol dimethacrylate (EGDMA)
and trimethylolpropane trimethacrylate (TRIM) are
used as cross-linking agents; swelling properties can
be improved with a more flexible molecule such as
1,6-hexanediol diacrylate [23]. The preparation of
these copolymers is well documented in the litera-
ture [24-27].
Fortunately, several of these copolymers are avail-
able commercially; they are ready to use or can be
processed simply to get the required functional
groups. Some significant structures are given in Table
8.1. The suitable monomer CH
2
=CH—
f
n
, when
readily available, may not survive some anionic or
cationic polymerisation conditions, in which case it
is necessary to introduce protecting groups that are
removed afterwards [28].
According to the polymerisation process and the
amount of cross-linking agent introduced, two series
of polymeric beads can be obtained:
•
Gel type
: when usually less than 8% of cross-
linking agent is involved, the polymer network is
made of quite independent chains that are loosely
connected. In suitable solvents the polymer chains
become solvated as if they were free, so the mate-
rial swells up to the limit. This ensures a good dif-
fusion of reactants and products (Fig. 8.6a).
•
Macroporous type
: when the polymerisation is per-
formed with a very high content of cross-linking
agent, a network of tightly connected chains is
obtained. To ensure some diffusion in such a
2.3 Copolymerisation with usual monomers
The polymeric materials transformed into polymer-
supported reagents usually are obtained by radical
copolymerisation of a mixture of three vinylic
monomers:
(1)
The chain agent, which is the main physical con-
stituent of the carrier and plays the role of a
diluent along the chains (styrene, methyl-
methacrylate; Fig. 8.5a).
