Chemistry Reference
In-Depth Information
A bimolecular initiating system, based in 2,2
0
-azobisisobutyronitrile was reported by Michl and
coworkers [
26
]. It consists of weakly solvated lithium in combination with the cyanopropyl radical (from
AIBN). The combination can initiate polymerizations of olefins. The reaction was illustrated as follows:
R
Li
+
R
R
CN
CN
Li
R
n
3.2.3 Boron and Metal Alkyl Initiators of Free-Radical Polymerizations
These initiators were originally reported a long time ago [
27
-
29
]. Oxygen plays an important role in
the reactions [
30
,
31
]. It reacts with the alkyl boride under mild conditions to form peroxides [
32
,
33
]:
R
R
+
R
B
O
O
R
B
O
O
R
R
Initiating radicals apparently come from reactions of these peroxides with other molecules of
boron alkyls [
34
,
35
]. One postulated reaction mechanism can be illustrated as follows [
35
]:
B
R
B
O
R
+
R
+
B
O
R
O
O
B
Another suggested reaction path is [
36
]:
R
2
B-O-O-R
þ
2R
0
3
B
!
2R
þ
R
0
2
B-O-BR
0
2
þ
R
0
2
B-O-R
Catalytic action of oxygen was observed with various organometallic compounds [
35
]. One
example is dialkylzinc [
37
] that probably forms an active peroxide [
38
]. The same is also true of
dialkylcadmium and of triethylaluminum [
38
]. Peroxide formation is believed to be an important step
in all these initiations. Initiating radicals, however, do not appear to be produced from mere
decompositions of these peroxides [
35
].
3.2.4 Photochemical Initiators
This subject is discussed in greater detail in Chap.
10
,
in the section on photo-cross-linking reactions
of coatings and films. A brief explanation is also offered here because such initiations are used, on a
limited scale, in a few conventional preparation of polymers.
