Chemistry Reference
In-Depth Information
The above can be shown on boron trifluoride initiation of the polymerization of isobutylene:
BF
3
+
H
2
O
BF
3
OH
H
BF
3
OH
+
H
BF
3
OH
In the same manner, a reaction of a Lewis acid with an alkyl halide and subsequent initiation can
be illustrated as follows:
+
MeX
n
RX
MeX
n+1
R
MeX
n+1
MeX
n+1
+
R
R
In the above shown examples the Lewis acid is actually the
co-initiator
and the water or the alkyl
halide are the
initiators
.
Numerous studies confirm the need by many Lewis acids for other molecules like, water or alkyl
halides to form initiating ions. For instance, pure TiCl
4
will not initiate the polymerization of
isobutylene [
16
,
17
]. This led many to believe that none of the Lewis acids are capable of initiating
cationic polymerizations of olefins by themselves [
18
]. Subsequent investigations, however,
demonstrated [
28
,
30
] that some strong Lewis acids, are capable of initiating such polymerizations.
Whether a Lewis acid is capable of initiating these polymerizations by itself was tested with the aid
of an early discovery. It has been known for some time that hindered bases like crowded pyridine
derivatives exhibit specificity toward reactions with protons [
19
]. Such bases might be used to
discriminate between two types of initiating mechanisms encountered with Lewis acids [
20
]. The
base will not interfere with direct electrophilic additions of the Lewis acids to the monomer. On the
other hand, it should prevent an initiation process by protons from taking place. If both pathways are
operative, then the pyridine derivatives can only quench the protonic initiation and will offer a means
of assessing the relative importance of each process.
Kemedy et al. investigated polymerizations initiated by Lewis acids-water in the presence of one
such “proton trap,” 2,6-di-
-butylpyridine [
21
]. The presence of this base markedly decreased
conversion from almost 100%, in some cases, to only a few percent. At the same time, there was a
marked increase in the molecular weight and a narrowing of the molecular weight distribution of the
products. This shows that the protonic reaction is by far the most important mode of initiation. It also
suggested to Kemedy et al., that not all the protonic initiations occur exclusively by free protons.
They might also proceed through concerted protonations. These would not be blocked by the “proton
traps” [
21
]:
t
H
H
H
H
H
O
O
O
X
Me
X
+
X
Me
X
X
Me
X
R
X
R
X
R
X
where, MeX
3
represents a Lewis acid. The di-
-butyl pyridine does not block the above reaction due to
steric compression. It also suggested that protonic initiations by water in the presence of Lewis acids
are probably unlikely when sufficient quantities of materials that act as “proton traps” are present.
t
