Chemistry Reference
In-Depth Information
When weak carboxylic acids or acids of medium strength initiate lactam polymerizations at
anhydrous conditions, there is an induction period [
123
]. In addition, the rates of these reactions
are proportional to the
K
a
of the acids [
105
]. It appears that different reaction mechanisms are
involved, depending upon the acid strengths [
113
]. The nucleophiles are present in equilibrium:
p
O
RCOOH
+
O
C
H
R
C
O
+
O
C
NH
2
The acylation of the carboxylate anions is assumed to lead to formations of mixed anhydrides of
the acids with amino acids [
124
] and subsequent rearrangements:
O
O
O
C
+
O
O
C
NH
2
C
O
C
NH
2
O
O
C
NH
C
OH
When strong acids, however, initiate the polymerizations, the strongest nucleophiles present are
the lactam amide groups that undergo acylations. As a result, the acids are not incorporated into the
polymers.
The propagation steps in cationic polymerizations of lactams occur by transamidation reactions
between lactam rings and the ammonium groups formed during the steps of initiation. It is believed that
during the reaction proton transfers take place first from the amine salts to the lactams or to the acyllactams
to formcations. These in turn acylate the free amines that formwith the regeneration of ammoniumgroups:
NH
3
+
C
O
C
H
H
2
N
O
NH
2
+
H
O
O
+
NH
3
NH
2
C
NC
O
+
C
NC
O
O
H
H
O
O
+
NH
3
+
H
C
NC
N
O
C
NH
2
C
O
+
H
2
NC
NH
C
O
+
H
H
O
+
NH
3
H
2
NC
O
N
+
C
H
NH
3
The propagation step is very rapid when aminolysis takes place at the carbonyl group of the
activated acid derivative (like acyllactam or an acid chloride). It is slower, however, if it involves an
amide group of the monomer [
114
]. As is typical of many carbonyl reactions, acylations are followed
by eliminations [
125
]:
