Chemistry Reference
In-Depth Information
H
H
2
N
NH
2
+
HOC
O
C
O
OH
H
2
N
N
C
C
O
OH
+
H
2
O
O
(12-11)
lies much farther to the right than that in reaction (12-1).
This difference is reflected in the synthesis of nylon-6,6, where the initial step
is the production of a salt (12-1) that is recrystallized from methanol to ensure
exact equivalence of reactants:
H
3
N
O
NH
3
H
2
N
NH
2
+
HOOC
CH
2
COOH
CH
2
CH
2
2
4
6
6
C
C
O
(12-12)
4
O
O
12-1
An aqueous slurry of the salt is heated with acetic acid end-group stabilizer
(Section 7.4.2) and the reaction is completed at 270
280
C and atmosphere pressure:
H
H
H
3
N
NH
3
CH
2
6
+
C
O
O
H
2
O
N
CH
2
N
CH
2
6
4
x
O
C
CH
2
C
O
4
O
O
(12-13)
The equilibrium in this case allows formation of polyamide linkages even in the pres-
ence of high concentrations of water. This is not possible with polyesters, and practical
processes for the production of both polymer types differ fundamentally for this reason.
The need to drive the polymerizations to completion is common to all step-growth
reactions that are carried out under conditions in which polymerization
depolymeriza-
tion equilibria are significant (Section 7.4.2). This is accomplished in general by
removal of a volatile product such as water or an alcohol. The rate of polymerization is
often limited by the rate of transfer of such condensation products into the vapor state.
A complete kinetic description of the process must then involve both the chemical reac-
tion rate and the rate of mass transfer. The latter depends on the details of reactor design
and stirring and therefore so does the rate of polymer production
[1]
.
12.3
Chain-Growth Polymerizations
The major commercial examples of chain-growth polymerizations involve reac-
tions across C
C double bonds to produce polymers with all-carbon backbones.
The enthalpies of polymerization of such monomers are of the order of 60
Q
85 kJ/
