Chemistry Reference
In-Depth Information
All initiators can be divided into two groups. To the first one belong strong bases capable of
forming lactam anions by removing the amide proton. This starts the anionic polymerization reaction.
To the second one belong active hydrogen compounds capable of protonating the amide bond and
thereby affecting cationic polymerization [
114
].
Side reactions are common in lactam polymerizations. Their nature and extent depends upon the
concentration and character of the initiators, the temperatures of the reactions, and the structures of
the lactams. When cationic polymerizations of lactams are initiated by strong acids, strongly basic
amidine groups can be produced. These groups bind the strong acids, inactive the growth centers, and
decrease the rate of polymerization. Use of strong bases to initiate polymerizations of lactams
possessing at least one
a
-hydrogen also result in side reaction. Compounds form that decrease the
basicity of lactams and polyamides and slow the polymerizations. Also, side reactions give rise to
irregular structures, namely branching.
The ring-opening polymerization reactions depend upon thermodynamic and kinetic factors, and
on the total molecular strain energies of the particular ring structures. Six-membered
-valerolactam
is the most stable ring structure and most difficult to polymerize. Also, presence of substituents
increases the stability of the rings and decreases the ability to polymerize.
d
5.9.1 Cationic Polymerization of Lactams
The catalysts for cationic polymerization can be strong anhydrous acids, Lewis acids [
115
], salts of
primary and secondary amines, carboxylic acids, and salts of amines with carboxylic acids that split
off water at elevated temperatures [
114
]. The initiators react by coordinating with and forming rapid
pre equilibrium lactam cations. These cations are the reactive species in the polymerizations.
Initiations of this type are also possible with weakly acidic compound, but such compounds are not
able to transfer protons to the lactam. They are capable, however, of forming hydrogen bonds with the
lactams. The high reactivity of the lactam cations may be attributed to the decreased electron density
at the carbonyl carbon atoms. This makes them more subject to nucleophilic attacks [
114
].
Protonations of the amides occur at the oxygens [
116
], but small fractions of N-protonated amides
are also presumed to exist in tautomeric equilibrium. To simplify the illustrations, all lactams will be
shown in this section as:
C
O
N
H
So, while the above structure commonly represents propiolactam, in this section it can mean any
lactam, like a caprolactam, valerolactam, etc. Thus, the equilibrium can be shown as follows:
C
OH
N
H
H
H
O
CN
H
N
C
H
O
C
H
NH
2
