Chemistry Reference
In-Depth Information
Propagation
O
H
H
O
O
H
R
O
NSn
N
(C
4
H
9
)
3
CH
3
O
+
O
N
Sn
R
CH
3
O
NO
(C
4
H
9
)
3
R
O
O
R
O
H
O
H
R
-CO
2
N
Sn
N
(C
4
H
9
)
3
CH
3
O
R
O
H
-carboxyanhydride polymerizations are initiated by secondary amines with small
substituents, the amines act as nucleophiles, similarly to primary amines [
157
]. Secondary amines
with bulky substituents, however, produce only
When
N
-carboxyanhydride anions. The same is true of
tertiary amines. These anions in turn initiate polymerizations that proceed by the “active monomer
mechanism.”
Messman and coworkers did a mechanistic study of
N
-amino acid carboxy anhydride polymeriza-
tion [
158
] They polymerization in high vacuum with polymerization at atmospheric The conclusion
of their work was that poly(
a
-benzyl-
L
-tyrosine) prepared in vacuum yields a polymer by normal
amine mechanism with minimum termination. By contrast when the reaction was not carried out at
high vacuum, there were several termination products.
O
5.11 Metathesis Polymerization of Alicyclics
Ring-opening polymerizations of alicyclics by Ziegler-Natta type catalysts resulted from general
studies of olefin metathesis [
158
-
160
]. These interesting reactions can be accomplished with the aid
of many catalysts. The best results, however, are obtained with complex catalysts based on tungsten
or molybdenum halides. One such very good catalyst forms when tungsten hexachloride is combined
in right proportions with ethylaluminum dichloride and ethanol.
Several reaction mechanisms were proposed to explain the course of olefin metathesis. Most of the
evidence supports a carbene mechanism involving metal complexes, originally suggested by Harrison
and Chauvin [
160
-
163
]. A typical metathesis reaction of olefins can be illustrated as follows:
R"
R"
R'
R'
WCl
6
C
2
H
5
AlCl
2
+
C
2
H
5
OH
+
+
+
R'
R"
R'
R"
R'
R"
W
W
+
R'
R"
R'
R"
R'
R"
