Chemistry Reference
In-Depth Information
9.4.3.1 Reactions of Polystyrene
Photo chlorination
of polystyrene involves replacement of hydrogens at the
a
and
b
positions
[
175
,
176
]. It was believed in the past that the chlorine atoms react preferentially at the
-position
until the chlorine content of the product reaches 20% by weight. After that, it was thought that the
chlorines are introduced into the other position. Later, however, this was contradicted [
177
]. In fact,
when polystyrene is photo-chlorinated in carbon tetrachloride at low temperatures, like 13
C, it is
substituted equally at both positions. At higher temperatures, like 78
C, substitutions at the
a
b
-position
actually predominate [
177
].
Chlorinations of poly(
-methyl styrene) are somewhat more selective for the pendant methyl
groups and result in di- and tri-substitutions at the
p
-methyl position. Only small amounts of chlorine
are introduced into the polymer backbones [
178
]. Substitutions at the backbones, however, are
possible with the use of SO
2
Cl
2
as the chlorinating agent. In this case, half of the chlorines still
replace the methyl hydrogens, but the other half replace hydrogens on the backbone.
Free radical additions of maleic anhydride to polystyrene backbones can be carried out with the
help of either peroxides or ultraviolet light [
164
]. Approximately 2% of the anhydride can be
introduced. If, however, the additions are carried out on
a
-brominated polystyrene, the anhydride
content of the polymer can be raised to 15% [
164
]:
p
O
O
O
O
O
O
hν
+
Note
: the extra hydrogen shown above on the maleic anhydride moiety of the product presumably
comes from chain transferring.
-bromosuccinimide and benzoyl peroxide in CCl
4
at room
temperature can achieve a 61% conversion in 4 h. Considerable degradation, however, accompanies
this reaction [
180
].
Bromination
of polystyrene with
N
9.4.3.2 Chloromethylation Reactions
Chloromethylation reactions of the aromatic rings
of polystyrene and styrene copolymers are being
carried out extensively. Chlorodimethyl ether (a carcinogen) is a good solvent for these polymers. It is,
therefore, commonly employed as the reagent [
181
,
182
]. Laboratory preparations can be carried out in
mixtures of carbon disulfide and ether, using zinc chloride as the catalyst. A 9-h reaction at room
temperature yields 10% substitution [
183
]. The chloromethylation process [
184
] occurs in two steps.
Benzyl methyl ether forms as an intermediate. Cross-linking reaction between the aromatic nuclei and
the formed CH
2
Cl group occurs as side reaction. There are strong indications that the chloromethylation
takes place only at one position on the ring [
185
]. The same is true of bromomethylation [
185
].
Stannic chloride is a very effective catalyst for this Friedel-Craft reaction [
186
]. Iodomethylation
can also be carried out in the same manner with similar results [
179
]. When the reactions are carried
out on cross-linked styrene copolymers with chlorodimethyl ether and stannic chloride catalyst, they
are accompanied by strong morphological changes [
187
]. If these reactions are carried out with low
levels of chloromethylating agents or catalysts, they occur more or less homogeneously. Larger levels
