Chemistry Reference
In-Depth Information
Table 3.3
Effect of substituents on decomposition of
dibenzoylperoxide in benzene [
10
,
11
]
Substituent
K
i
10
3
log
K
1
/
K
s
i
+
s
0
p
0
-dimethoxy
p
,
7.06
0.447
0.536
p
-methoxyy
4.54
0.255
0.268
p
0
-dimethyl
p
,
3.68
0.164
0.340
p
0
-di-
p
,
t
-butyl
3.65
0.161
0.394
Parent compound
2.52
0.000
0.000
p
0
-dichloro
p
,
2.17
0.065
+0.454
m
0
-dichloro
m
,
1.58
0.203
+0.746
m
0
-dibromo
m
,
1.54
0.215
+0.782
p
0
-dicyano
p
,
1.22
0.314
+1.300
or both of the radicals may or may not expel a small molecule. For instance, benzoyl peroxide can and
often does decompose into a phenyl radical and carbon dioxide, as follows:
O
O
O
Δ
2
2
+
2CO
2
O
OO
The resultant phenyl radicals can combine and yield new and completely inactive species:
2
The above-described recombination reactions of free radicals are some of the causes of inefficiency
among initiators. The average time for recombination of free radicals inside a solvent cage and also the
time for their diffusion out of the cage is about 10
10
s[
7
]. In addition, the efficiency of the initiator is
affected by the monomer and by the solvent. It was shown that the viscosity of the medium is inversely
proportional to the initiator efficiency because the more viscous the solution, the greater the cage-
effect [
8
,
9
].
Numerous lists are available in the literature that give the decomposition temperatures or the half-
lives at certain elevated temperatures of many initiators [
6
]. Decompositions of peroxides may
proceed via concerted mechanisms [
10
,
11
] and the rates are structure dependent. This can be
illustrated on benzoyl peroxide. The benzoyl groups, the two halves of this molecule, are dipoles.
They are attached, yet they repel each other. Rupture of the peroxide link releases the electrostatic
repulsion between the two dipoles. Presence of electron donating groups in the
para
position
increases the repulsion, lowers the decomposition temperature, and increases the decomposition
rate. The opposite can be expected from electron attracting groups in the same position [
6
]. The
effect of substituents on the rate of spontaneous cleavage of dibenzoyl peroxide was expressed [
11
]in
terms of the Hammett equation, log (
. This is shown in Table
3.3
.
In addition, peroxides can cleave in two ways,
heterolytically
and
homolytically
. Heterolytic
cleavage of peroxides results in formation of ions,
K
/
K
O
)
¼ rs
R
0
R
þ
:
O-O-R
0
Ð
R-O-O-R
0
Ð
:
þ
R-O-O
but homolytic cleavage results in formation of radicals:
R-O-O-R
0
Ð
R-O
þ
O-R
0
