Chemistry Reference
In-Depth Information
Repeated additions of the charged species and the resulting zwitterionic products lead to high
polymers:
M
N
M
E
M
N
M
E
n
The initial zwitterion that forms upon combination of a nucleophilic with an electrophilic
monomer is called a
genetic zwitterion
[
214
]. Intramolecular reactions can produce “macrocycles”:
M
E
M
N
n-1
M
N
M
E
M
N
M
E
n
The contribution of the cyclization reaction, however, is, apparently, small [
214
]. A reaction can
also take place between a free monomer and any zwitterion at one of the ionic sites:
M
N
M
N
M
E
M
N
M
E
n
M
N
M
E
M
N
M
E
n
M
N
M
E
M
N
M
E
M
E
n
Such reactions disturb the alternating arrangements of the units -M
N
-M
E
- in the products. The
reactivity of the monomers determines whether homopropagations occur as well. Alternating propa-
gation depends upon dipole-dipole interactions between M
N
and M
E
monomers in preference to ion-
dipole reactions between ion centers of zwitterions and monomers in homopropagations [
214
].
An example of an alternating copolymerization via zwitterion intermediates is a copolymerization
of 2-oxazoline with
-propiolactone. It takes place in a solution in a polar solvent like dimethyl-
formamide at room temperature over a period of a day to yield quantitative conversions [
215
]:
b
O
N
O
O
+
N
O
n
O
HO
A zwitterion that forms first is the key intermediate for the polymerization. The onium ring from 2-
oxazoline is opened by a nucleophilic attack of the carboxylate anion at carbon [
214
]:
O
O
N
O
N
O
2
O
O
O
N
H
O
O
In this reaction the number of copolymer molecules increases at first, then reaches a maximum and
finally decreases as the conversion becomes high [
214
-
226
]. When the concentration of both
