Chemistry Reference
In-Depth Information
3.2 Reactions Leading to Formation of Initiating Free Radicals
Initiating free radical can come from many sources. Thermal decompositions of compounds with azo
and peroxy groups are common sources of such radicals. The radicals can also come from “redox”
reactions or through various light induced decompositions of various compounds. Ionizing radiation
can also be used to form initiating radicals.
3.2.1 Thermal Decomposition of Azo Compound and Peroxides
The azo compound and peroxides contain weak valence bonds in their structures. Heating causes
weak bonds in these compounds to cleave and to dissociate into free radicals as follows:
D
2R
þ
N
2
!
R-N=N-R
For many azo compounds such dissociations occur at convenient elevated temperatures. One
commonly used azo compound is
a
0
-azobisisobutyronitrile. An original synthesis of this compound
was reported to be as follows [
3
,
4
]:
a
,
O
OH
N
HCN
+
NH
Br
2
N
2
OH
N
H
2
N
N
N
+
N
N
N
NH
NH
2
The final products of decomposition of this compound are two cyanopropyl radicals and a
molecule of nitrogen:
Δ
N
N
N
N
+
N
N
N
N
N
N
2
+
As stated earlier, not all free radicals that form, however, initiate polymerizations. Some are lost to
side reactions. Thus, for instance, some free radicals that form can recombine inside or outside the
solvent cage, where the decompositions take place, to yield either tetramethylsuccinonitrile or a
ketenimine [
4
,
5
]:
N
N
N
2
N
N
