Chemistry Reference
In-Depth Information
There are potentially four active sites per trinuclear catalytic molecule. The number of actual sites,
however, depends upon the aggregation of the oxoalkoxides. Two different types of OR groups exist,
depending upon the bridging in the aggregates. Only one is active in the polymerization. This results
in a catalytic star-shaped entity. The fact that the dissociated catalysts generate four growing chains
per each Al
2
(CH
2
)
5
CO
2
(OR)
4
molecule [
97
] tends to confirm this.
The commercially available aluminum triisopropoxide was reported to be a very effective initiator
for the “living” ring-opening polymerizations of
-caprolactone, lactides, glacolide, and cyclic
anhydrides [
98
]. Based on kinetic and structural data, the ring-opening polymerization is believed to
take place by a coordination-insertion mechanism. While the molecules of aluminum triisopropoxide
are coordinatively associated in toluene, in the presence of lactones single isolated monomeric species
form and are believed to remain unassociated during the propagation reaction [
98
].
Actually, ring-opening polymerizations of
e
-caprolactone were achieved by various catalysts.
Only a few, however, initiate “living” polymerizations. Among these are the aluminum alkoxides
described above, bimetallic
e
-alkoxides [
99
], porphynatoaluminum [
100
], mono(cyclopentadienyl)
titanium complexes [
101
], and rare earth alkoxides [
102
,
103
]. Examples of rare earth alkoxides are
Ln, Nd, Y, or Nd isopropoxy diethyl acetoacetates and (C
5
H
5
)
2
LnOR and [C
5
(CH
3
)
5
]
2
LnCH
3
(donor)
complexes. It was suggested that the steric effect of bulky groups of these catalysts is to suppress an
interfering transesterification reaction by screening linear polymeric chains from the active centers
during the reactions and yield “living” polymerizations [
104
]. These catalysts also are useful in
formation of various block copolymers of lactones with other monomers [
104
,
105
]. Among other
lactones that were polymerized with the help of such rare earth catalysts are lactide [
106
-
108
],
d
m
-valerolactone [
109
],
b
-propiolactone [
109
], and
b
-butyrolactone [
107
].
-butylphenoxy)
yttrium and 2-propanol is first order with respect to the monomer and initiator [
105
]. This led to the
conclusion that the reaction proceeds via a three-step mechanism that can be illustrated as follows [
105
]:
Polymerization of
e
-caprolactone with a catalyst system consisting of tris(2,6-di-
tert
O
3 ROH
+
Y
O
O
OR
Y
+
3HO
RO
OR
O
O
Y(O
OR)
3
Y(OR)
3
+
3
O
O
O
3x
H
+
Y(O
OR)
3
O
OR
O
3H
O
x
