Chemistry Reference
In-Depth Information
The overall rates of polymerizations depend on the concentrations of acyllactams and diacylamine
groups as well as on the lactam anions. The latter result form dissociations of the lactam salts,
depending upon the nature of the metal:
Me
NC
O
Me
+
NC
O
where Me means metal The alkali metals can be rated in the following order with respect to rates of
initiations and propagations [
113
,
132
]:
Li
<
Na
<
K
<
Ca
or cocatalysts, such as acyl halides, anhydrides, or isocyanates, can
result in elimination of the induction period. These additives insure formations of stabilized
adducts:
Additions of
activators
O
O
O
CN
R
C
+
O
CN
C
R
+
Cl
Cl
O
O
O
O
CN
C
R
+
N
C
O
O
CN
CN
C
R
O
O
CN
O
CN
H
C
+
R
N
C
N
R
O
H
The structures of the activators can determine the rates of addition to the first lactam anion [
113
].
If, for instance, the acyl group is large, as in pivaloylcaprolactam, the decrease in the rate can be
merely due to steric hindrance [
113
,
129
]. On the other hand, substituents like the benzoyl group
increase the rates of additions to the first lactam anions [
113
,
129
]. In addition, the structures of the
activators can also affect the course of the polymerization. This is because they become incorporated
at the end of the polymeric molecules and may influence the basicity during the polymerization
reactions.
Polymerizations in the presence of acylating agents are often called
polymerization. If
the acylating agents are absent from the reaction mixture the reactions may be called
activated
nonactivated
.
Sometimes the terms
are used instead.
Several reaction mechanisms were offered to explain the mechanism of anionic ring-opening
polymerizations of lactams. One mechanism is based on nucleophilic attacks by the lactam anions at
the cyclic carbonyl groups of N-acylated lactams. This leads to formations of intermediate symmet-
rical mesomeric anions that rearrange with openings of the rings [
133
,
134
]:
assisted
and
nonassisted
O
O
O
O
O
O
NC
R
C
+
NC
R
C
NC
NC
O
O
O
R
C
NC
NC
