Chemistry Reference
In-Depth Information
The polymerization rate depends upon the concentration of the amine and the monomer. The
degree of polymerization is often but not always equal to the ratio of the monomer to the amine
[
154
]. It means that the reaction may be similar to but not identical to a living type polymeriza-
tion. In addition, the molecular weight distribution curve may be broadened or bimodal. This may
be due to some chemical termination reactions. These can be intramolecular reactions of the
terminal amine group with some functional group in the side chain and lead to formation of
hydantoic acid end groups [
154
]. It may also be due to physical termination from precipitation of
the product.
Dialkylzinc initiated polymerizations apparently take place by a different mechanism. The first
step is pictured by Makino, Inoue, and Tsuruta as a hydrogen abstraction by dialkylzinc from NH
[
155
]. This is similar to the reaction with a base shown earlier. The second stage of initiation,
however, is a reaction between two molecules of the activated carboxyanhydrides, and formation of
zinc carbamate [
155
]:
Zn
O
R
N
N
R
N
-R'H
2
O
+
R'
2
Zn
R
O
O
O
O
OO
O
R
N
N
R
Zn
O
O
O
O
O
O
-carboxyanhydride
to form a mixed anhydride. The mixed anhydride then changes into an amide group with elimination
of carbon dioxide [
155
]:
The propagation is a carbonyl addition of the zinc carbamate to the activated
N
N
N
R
R
Zn
O
O
O
O
O
O
O
Zn activated N-carbpxyanhydride
N
R
N
O
Zn activated N-caroxyanhydride
O
O
O
O
where, the activated
N
-carboxyanhydride portion is [
155
]:
R
R
CH
N
CH
N
C
O
O
C
O
C
C
O
O
O
