Chemistry Reference
In-Depth Information
radical [
155
]. Hydroquinone is often employed as an inhibitor; it requires, however, oxygen for activity
[
156
,
157
]:
H
X
H
X
O
0
2
+
O
H
X
O
H
X
O
O
+
HO
OH
OH
+ HO
O
Oxygen, however, can also act as a comonomer in a styrene polymerization:
O
+
O
2
O
O
O
It causes marked retardation, however, in the polymerizations of methyl methacrylate [
158
]. The
same is true of many other free-radical polymerizations.
The ability of phenols to inhibit free-radical polymerizations appears to increase with the number
of hydroxyl groups on the molecules [
157
]. The locations of these hydroxyl groups on the benzene
rings in relationships to each other is important. For instance, catechol is a more efficient inhibitor
than is resorcinol [
158
].
Aromatic nitro compounds can act as strong retarders. Their effect is proportional to the quantity
of the nitro groups per molecule [
160
,
161
].
O
X
N
X
O
O
2
N
+
X
O
N
O
Figure
3.1
illustrates the effect of inhibitors and retarders on free-radical polymerization [
162
].
The equation that relates rate data to inhibited polymerizations is
R
P
2
k
t
=k
P
2
2
2
½
M
þR
P
½
Z
k
z
=k
P
½
M
R
i
¼
0
