Chemistry Reference
In-Depth Information
Table 3.2
Decomposition rates of some azonitrile initiators
a
Compound
K
d
b
(s
1
)
T(
C)
Solvent
2,2
0
-azobisisobutyronitirle
10
5
Benzene
78.0
8
2,2
0
-azobis-2-ethylpropionitrile
10
3
Nitrobenzene
100.0
1.1
2,2
0
-azobis-2-cyclopropylpropionitrile
10
5
Toluene
50.0
8.2
1,1
0
-azobiscyclohexanenitrile
10
6
Toluene
80.0
6.5
2,2
0
-azobis-2-cyclohexylpropionitrile
10
6
Toluene
80.0
8.3
1,1
0
-azobiscyclooctanenitrile
10
4
Toluene
45.0
1.5
a
From ref. [
13
], and from other sources in the literature
b
e
ðE
a
=RTÞ
K
c
¼ A
Examples of other, fairly efficient, azo initiators include the following:
1.
N
-Nitrosoacylanilides,
O
N
N
OR
Δ
NR
Δ
N
O
O
O
N
2
+
+
OR
2. Bromobenzenediazohydroxide,
N
Br
OH
Δ
Br
N
N
2
+
NO
+
3. Triphenylazobenzene,
Δ
+
+
N
N
2
N
3
3
The triphenylmethyl radical shown above is resonance stabilized and unable to initiate
polymerizations. The phenyl radical, on the other hand is a hot radical. It initiates polymerizations
readily. Decomposition rates of some azonitrile initiators are listed in Table
3.2
. There are also many
peroxides available for initiating free-radical polymerizations. These can be organic and inorganic
compounds. There are, however, many more organic peroxides available commercially than are the
inorganic ones. The organic ones include dialkyl and diaryl peroxides, alkyl and aryl hydroperoxides,
diacyl peroxides, peroxy esters, and peracids. Hydrogen peroxide is the simplest inorganic peroxide.
Syntheses, structures, and chemistry of various peroxides were described thoroughly in the
literature [
5
]. Here will only be mentioned some properties of peroxides and their performance as
they pertain to initiations of polymerizations. Decompositions of peroxides, such as the azo
compounds, are also temperature dependent [
6
]. This means that the rates increase with temperature.
The rates are also influenced by the surrounding medium, such as the solvents that imprison or “cage”
the produced pairs of free radicals. Before undergoing a net translational diffusion out of the cage, one
