Chemistry Reference
In-Depth Information
An alternative procedure would involve feeding the
po
lymerization reactor
with a deliberate imbalance of diacid and diamine to limit M
n
to 50,000 even if p
were effectively forced close to unity. This is not attractive in this particular case
because hexamethylene diamine and adipic acid react to form a zwitterion salt
(7-5) that can be recrystallized from methanol and used to provide a pure feed-
stock with a stoichiometric balance of coreactive functional groups.
+
+
H
3
NCH
2
NH
3
6
O
C
CH
2
CO
4
-
O
O
-
7-5
The most general solution to the problem involves adding a monofunctional
reactant such as acetic acid to the reaction mixture to control the product molecu-
lar weight. The composition of the polymerization recipe is calculated as follows.
From
Eq. (7-19)
with X
n
5
442 and p
5
1, f
av
5
2
2
2/442
5
1.9955. (Note: Do
not round off to f
av
5
2.0.) If we keep the adipic acid and ethylene diamine in
stoichiometric balance and add y mol acetic acid for every mole of diacid, the ini-
tial reaction mixture is shown in the accompanying tabulation. There are (2
1
y)
acid groups for every two amine groups, so only two of these acid groups can
react. From
Eq. (7-16)
f
av
5
ð
2
2
Þ=ð
2
Y
Þ
5
1
:
9955
3
1
The solution is then y
0.00445 mol acetic acid for each mole of adipic acid.
5
Moles
f
Equivalents
Adipic acid
1.0
2
2
Hexamethylene diamine
1.0
2
2
Acetic acid
y
1
y
(As a check on the results of this calculation we can make use of the fact
that the number of molecules, N, left in the system at an extent of conversion p
is equal to the original number of moles of monomer minus the number of link-
ages formed. From a mass balance, the number of linkages that have been
formed is p times the number of moles of deficient groups and equals 2p
2
5
in the present example. Then N
2
y
2
0.0045. As before, the number
5
1
2
5
of structural units is N
0
5
the initial number of moles of monomer
2
y
5
1
445. This is a close enough check. The
difference between 445 and the target value of 442 reflects the effects of rounding-
off decimals.)
2.0045. X
n
5
N
0
/ N
2.0045/0.0045
5
5
5
