Chemistry Reference
In-Depth Information
One of the reasons for the relatively lowmolecular weights of the products is the low reactivity of the
epoxide ring toward anionic propagation. Another reason is the tendency to chain transfer to monomers,
particularly in polymerizations of substituted ring structures, like, for instance, in propylene oxide:
O
O
+
Na
O
OH
+
CH
2
Na
The newly formed species rearranges rapidly:
O
very fast
CH
2
O
Na
CH
2
Na
Such transfer reactions are E-2 type eliminations. This was shown on tetramethylethylene oxide
that undergoes the reaction when treated with catalytic amounts of potassium
t
-butoxide [
5
]:
O
O
K
OH
KOC
4
H
9
C
4
H
9
OH
+
+
In propylene oxide polymerization, therefore, the E-2 type elimination reaction is in competition
with propagation:
O
O
O
propagation
O
+
transfer
O
OH
+
There are both allyl and propenyl ether end groups in the products, according to the infra-red
spectra [
5
]. This suggests that in addition to the E-2 type elimination, an intramolecular transfer takes
place by allylic hydrogen.
Raynaud et al. reported carrying out ring-opening polymerizations of ethylene oxide initiated by
heterocyclic carbene [
15
]. The reaction yields highmolecular weight polymers. It is illustrated as follows:
R
R
N
O
N
n-1
R
O
O
DMSO
50
o
C
N
+
O
O
N
n-1
N
R
R
N
R
