Chemistry Reference
In-Depth Information
The propagations take place by a similar process:
O
O
O
O
+
O
(CH
2
)
5
O
R
R
O
5
5
5
O
These steps repeat themselves until the chains are built up. Anionic polymerizations can yield
optically active polymers. This was observed in formations of poly(
a
-methyl,
a
-ethyl-
b
-
propiolactone) [
193
] that contains asymmetric carbon atoms.
5.8.3 Polymerization of Lactones by Coordination Mechanism
The mechanism of coordination polymerization was pictured by Yong, Malzner, and Pilato
[
90
] as being an intermediate between the above two modes of polymerization (a cationic and
anionic one):
Initiation
O
M
T
O
O
M
T
+
OM
T
R
(CH
2
)
5
(CH
2
)
5
O
R
O
R
5
where, M
T
means metal.
Propagation
(CH
2
)
5
O
O
O
O
O
OM
T
+
(CH
2
)
5
O
R
5
O
M
T
R
5
The above shown mechanism, however, is incorrect when caprolactone is polymerized with tin
compounds [
95
]. Yet, it appears to be correct for polymerizations of propiolactones with an ethylzinc
monoxide catalyst [
95
].
The bimetallic oxoalkoxides are useful catalysts for the polymerizations of
-caprolactone. The
general course of the reaction is quite similar to one for oxiranes. A typical coordination mechanism is
indicated fromkinetic and structural data [
97
]. The molecular weight increases with conversion and the
reaction exhibits a “living” character, because there is a linear relationship betweenDP and conversion.
When themonomer is all used up, addition of freshmonomer to the reactionmixture results in increases
in DP. By avoiding side reactions it is possible to achieve high molecular weights (up to 200,000) with
narrowmolecular weight distribution (
M
w
/
M
n
1.05) [
97
]. The reaction proceeds through insertion of
the lactone units in the Al-OR bonds. The acyl-oxygen bond cleaves and the chain binds through the
oxygen to the catalyst by forming an alkoxide link rather than a carboxylate one:
e
O
O
O
Al
OR
+
(CH
2
)
5
O
Al
OR
5
n
