Chemistry Reference
In-Depth Information
The concentration of these groups also determines the molecular weights of the final products
[
118
-
128
]. This type of equilibria also occurs in polymerizations initiated by amino acids or by salts
of carboxylic acids formed with primary and secondary amines. In the hydrolytic polymerizations of
caprolactam the above reactions involve only a few percent of the total lactam molecules present
[
144
,
145
]. The predominant propagation reaction is a step-growth addition of lactam molecules to
the end groups. It is acid catalyzed [
144
,
145
]:
O
NH
2
+
O
C
N
H
N
C
NH
2
H
The exact mechanism of this addition is uncertain. It was postulated that the addition steps are
through reactions of neutral lactam molecules with ammonium cations [
146
-
148
]. Others felt,
however, that the lactam molecules add to the undissociated salts [
149
].
Hydrolytic polymerizations are the smoothest of all three types of polymerization reactions
because the growing species are less activated than in either cationic or anionic polymerizations.
Many commercial processes utilize it in
-caprolactam polymerizations. Formation of irregular
structures, however, and even crosslinked material was detected. In addition, at elevated temperatures
deamination and decarboxylation of polycaprolactam can take place [
150
] Such reactions can result
in formations of ketones and secondary amine groups. The ketones, in turn, can react with amines and
form Schiff bases. This leads to branching and crosslinking.
In industrial preparations most of the water used to initiate the polymerizations is removed after
conversions reach 80-90% in order to attain high molecular weights. The final products contain
about 8% of caprolactam and about 2% of a cyclic oligomer [
150
]. These are removed by vacuum or
hot water extraction. The material is then dried under vacuum at 100-200
C to reduce moisture to
about 0.1%.
e
5.10 Polymerization of N-Carboxy-
a
-Amino Acid Anhydrides
The polymerizations of these anhydrides (or substituted oxazolidine-2,5-diones) can be carried out
with basic catalysts to yield polyamides:
H
RO
O
N
n
O
N
R
n
O
H
These polymerization reactions are important to biochemists because the products are poly(
a
-
amino acid)s and resemble the building blocks of naturally occurring polyamides.
When the polymerization is initiated with strong bases, the initiating step is hydrogen abstraction
from the anhydride by the base. This results in formation of
activated
species:
H
O
O
N
N
O
O
+
B
+
BH
R
R
