Chemistry Reference
In-Depth Information
H
H
A
N
N
NN
A
NN
2
N
NH
2
+
H
2
The high degree of strain in the three-membered rings causes very rapid polymerizations.
A variety of cationic species act as efficient catalysts for such reactions. The propagating species
are iminium ions and the propagation steps result from nucleophilic attacks by the monomers
on the ions, as shown above. Branches form due to reactions of secondary amine groups with
the iminium centers. They can also result from attacks by the imine end groups of inactive
polymer chains on the iminium centers of the propagating species. As the reaction progresses, it
slows down
H
A
H
A
+
N
N
NN
NNH
2
N
NH
2
H
2
because the protons become equilibrated with various amines [
185
]. The polymer is also extensively
cyclized due to intramolecular nucleophilic attacks of primary and secondary amines on the iminium
group. The product contains cyclic oligomers and polymer molecules with large size rings.
The termination mechanism is still not fully explained. It is believed that it may take place by
proton abstractions from the iminium ions by the counterions, or by any nitrogen in the polymer
chains, or by the nitrogens of the monomer units. It was also suggested [
185
,
186
] that backbiting and
ring expansion terminate the reactions. Such ring expansions result in formations of relatively
unreactive piperazine end groups:
A
N
N
N
N
H
H
H
Substitution on the ethylene imine ring hinders polymerization [
185
]. The 2,3 and 1,2 substituted
aziridines fail to polymerize. Only low molecular weight linear and cyclic oligomers form from 1 and
2 substituted ethylene imines.
In polymerization of secondary cyclic amines, formation of the nonstrained ammonium salt is
actually a termination reaction. If the rate, therefore, of propagation, is not considerably higher than
the rate of termination, the formation of high molecular weight material will not be possible. Thus
ratio of
k
p
/
k
t
should, therefore be high. The rate of polymerization can be written as:
P
n
þ
R
p
¼
dm
=
d
t ¼ k
p
m
½
where, m is the concentration of the monomer and [P
n
+
] is the concentration of the growing chains.
Assuming that the termination is a first-order reaction, then, the rate of termination can be expressed as:
½
P
n
þ
=
t ¼ k
t
½
P
n
þ
R
t
¼
d
d
