Chemistry Reference
In-Depth Information
5.5.1 The Initiation Reaction
The initiations result from coordination of the cation catalysts with the oxygen of the monomers to
form oxonium ions [
48
,
49
]. This weakens the oxygen-carbon bonds and leads to ring openings after
reactions with a second molecule of the monomer. New oxonium ions are generated in the process:
R
3
O BF
4
+
O
R
O
+
R
2
O
BF
4
O
R
O
+
O
R
O
BF
4
BF
4
[(C
2
H
5
O)
3
O]
+
BF4
,
[(C
2
H
5
O)
3
O]
+
SbCl
6
,
Some active oxonium salts are [
48
-
50
]:
[(C
2
H
5
O)
3
O]
+
FeCl
4
, and [(C
2
H
5
O)
3
O]
+
AlCl
4
Examples of carbon cations that can initiate polymerizations of tetrahydrofuran, as well as some
other cyclic ethers are:
O
(ClO
4
)
HClO
4
+
+
O
OH
O
O
O
(ClO
4
)
+
AgCl
AgClO
4
+
Cl
O
O
SnCl
4
+
SnCl
5
Cl
Cl
+
FeCl
3
CH
2
FeCl
4
O
R
O
R
S
+
BF
3
S
BF
3
O
RO
O
RO
SbCl
5
+
C
SbCl
6
CCl
3
3
The initiation mechanisms, however, by many carbon cations as, for instance, by triphenylmethyl
cations, are not straightforward. Initially, hydride ions are abstracted from the monomers to form
triphenylmethanes [
51
-
53
]. Simultaneously, acids are released from the counterions. The acids
