Chemistry Reference
In-Depth Information
What happens to the final radical is uncertain. High conversions were reported in polymerizations
of
n
-butyl vinyl ether in the presence of azobisisobutyronitrile (as a source of radicals) with
I
PF
6
at 50
C[
55
]. The above is an illustration of a thermal process [
12
]. Free radicals
can also be generated with the aid of UV light. This, for instance, was done with a UV light
decomposition of benzoin methyl ether in the presence of an oxidizing salt [
56
]:
ðp
- tolyl
Þ
2
O
O
h
ν
+
OCH
3
OCH
3
+
I
PF
6
+
2
PF
6
OCH
3
OCH
3
+
I
4.3.2.1 Charge Transfer Complexes in Ionic Initiations
Formations of copolymers by charge transfer mechanisms in free-radical polymerizations are
discussed in Chap.
3
. Reactions between donor and acceptor molecules, however, can also result in
some charge transfers that yield ion-radicals and subsequent polymerization by ionic mechanism:
+
A
D
[DA
D A
]
[D
A
DA]
ground state
excited state
The nonbonding form predominates in the ground state while the charge transfer predominates in
the excited state. The energy separation between the ground state and the first excited state is small. If
the ionization potential of the donor molecule is low and the acceptor molecule has a strong electron
affinity, transfer of an electron can occur to a significant extent even in the ground state. Mutual
oxidation-reduction takes place:
D
þ
A
þ
Ð
н
D
A
complex
Some vinyl compounds, i.e., vinyl ethers, can function as donor molecules because they possess a
low ionization potential. The acceptors can be neutral molecules like, quinones, anhydrides, nitrile
compounds, etc. They can also be ionic intermediates, such as metal ions, ionized acids, and carbon
cations. An interaction of an acceptor with a donor is followed by a subsequent collapse of the charge
transfer complex. This can results in formation of cation-radicals that are capable of initiating cationic
polymerizations [
57
]. The exact mechanism of the reaction of cation-radicals with olefins is still not
completely determined.
One example is a combination of an alkyl vinyl ether (donor) with vinylidene cyanide (acceptor)
that results in ionic polymerizations [
58
]. The reaction actually contains the ingredients of both
cationic and anionic type polymerizations [
59
]:
