Chemistry Reference
In-Depth Information
O
O
Fe
Fe
O
O
Fe
Fe
Fe
Fe
O
O
O
O
R
O
R
R
O
Fe
Fe
O
O
R
The forces of interaction between the iron atoms and the various oxygen atoms as shown above
assure a
cis
opening of the epoxide ring. The mechanism of the reaction of the ferric alkoxide is an
S
N
2 type. There is, therefore, increased restriction on the conformation of the monomer unit as it
approaches the reaction center [
22
].
Many other coordinated anionic catalysts that are
are also much
more reactive in the presence of water or alcohols. The function of these co reactants is to modify the
catalyst itself. For instance, diethylzinc combined with water in a ratio of 1:1 yields a very reactive
species. The exact nature of the catalyst is still not fully established, however, the reaction product is
pictured as follows [
23
,
24
]:
metal alkoxides
or
metal alkyls
C
2
H
5
Zn
O
Z
n
C
2
H
5
Several reaction mechanisms were proposed. One suggested pathway for propylene oxide poly-
merization pictures an initial coordination of the monomer with a cationically active center [
25
]:
C
2
H
5
δ
O
O
C
2
H
5
C
2
H
5
O
C
2
H
5
Zn
Zn
+
Zn
Zn
δ
O
The propagation is preceded by an intramolecular rearrangement:
δ
δ
C
2
H
5
C
2
H
5
O
C
2
H
5
O
C
2
H
5
Zn
Zn
Zn
Z
n
δ
O
δ
O
