Chemistry Reference
In-Depth Information
OH
-H
2
O
NH
2
+
O
C
NH
2
NH
C
NH
2
+H
2
O
H
NH
CN
H
CNH
N
The above water elimination reaction results in formations of amidines. Acylamidinium ions can
also result from dehydration of the tetrahedral intermediates during the reactions of amino groups
with acyllactams. Such groups could also be present within the polymer molecules. The water that is
released in these reactions hydrolyzes the acyllactams, acylamidine salts, and lactam salts to yield
carboxylic acids [
114
].
In the cationic polymerization of lactams the ammonium and amidinium groups form N-terminal
chain ends. The C-terminal chain ends are in the form of carboxylic acid groups or alkylamide
residues. This is important, because the nature of the end groups and their reactivity determine the
steps that follow in the polymerizations. This means that the different types of cationic
polymerizations of lactams are the results of the different end groups that form during the initiation
steps. Formation of amidines increases with increasing acidity and concentration of the initiator and
with an increase in the temperature:
Cl
R
NH
C
O
R
NH
Cl
(CH
2
)
x
C
+
O
When strong acids or amine salts initiate the polymerizations, almost all amine salt groups become
converted to amidine salts shortly after the start of the initiation reaction [
108
]. Formation of
amidinium salts leads to a decrease in the reaction rate because they initiate polymerizations of
lactams less effectively than do ammonium salts [
125
,
126
]. Lewis acids act in a similar manner,
unless a co-reactant is present, like water. In that case, the Lewis acids are transformed into protonic
acids and the polymerizations proceed as if they were initiated by protonic acids [
114
].
N-substituted lactams can generally not be polymerized. Some exceptions, however, are known
when cationic mechanisms are employed [
122
] and when strong carboxylic or inorganic acids are
used as initiators. In such cases the anions of the initiating acids, like Cl
, react with the lactam
cations to yield amino acid chlorides [
114
]:
H
H
Cl
R
N
C
O
Cl
+
R
N
C
O
O
HCl
R
N
C
Cl
Only the more strained four-, eight- and nine-membered N-substituted lactams have so far been
shown to be capable of polymerizations [
113
]. The 2,2-dimethylquinuclidone is highly strained and
undergoes polymerizations at room temperature [
127
]. The propagation reaction of substituted
lactams can be illustrated as follows [
122
]:
