Chemistry Reference
In-Depth Information
:
ð
2
5
:
no
useful equivalents
total no
2
1
99
'
f
av
5
1
:
99
5
:
moles
At p
5
0.99,
2
2
2
pf
av
5
2
2
2
ð
0
:
99
Þð
1
:
99
Þ
5
X
n
5
67
injection molded or extruded into articles that are relatively unoriented compared
to fibers (cf. Section 1.8).
We can see from this simple example why step-growth polymerizations
require very high conversions of functional groups compared to syntheses of the
same linkages in conventional organic reactions.
Suppose now that the acid monomer for this polymerization consisted of
0.01 mol CH
3
(CH
2
)
4
COOH for every 0.99 mol of adi
pic
acid. The situation
would then be as shown in
Table 7.2
. It can be seen that X
n
would be only 67 at
p
0.99 compared to 100 if the
m
onomers were pure.
The same large decline in X
n
would result if there were 0.99 mol of one
bifunctional monomer for every mole of the coreactive monomer in this case.
It can be seen that it is relatively easy to carry out step-growth polymeriza-
tions inefficiently or entirely ineffectively if the requirements listed in
Section 7.3
are not fully appreciated.
As another practical example consider how the molecular weight
5
of
nylon-6,6 can be controlled to a target value, say, 50,000. The formula weight of
the repeating
unit
shown in structure 7-4 is 226 g
m
ol
2
1
, and the number of
repeating units DP
n
5
ð
M
n
Þ
221 in this case; X
n
then equals 442, by defi-
nition, since each repeating
u
nit in nylon-6,6 contains residues of two monomers.
Substitution of f
av
5
50
;
000
=
226
5
442 into
Eq. (7-19)
indicates that the target con-
version p for this number average molecular weight is 1
2 and X
n
5
0.9977. It is
not practical to try to control a factory-scale step-growth
p
olymerization to this
level of conversion. If p is 0.002 below this target value, X
n
would be only 237,
w
hile a p that is 0.002 higher than that calculated would produce a product with
X
n
5
1/442
2
5
4708
:
Table 7.2
Effect of Impurity on Development of Molecular Weight in Equilibrium
Step-Growth Polymerizations
Equivalents
a
Moles
Functionality
H
2
N
(CH
2
)
6
NH
2
1
2
2
a
a
HOOC
(
CH
2
)
COOH
0.99
2
1.98
a
a
a
a
CH
3
(
CH
2
)
4
COOH
0.01
1
0.01
a
a
a
a
2.0
3.99
a
Total amine equivalents
5
2 and total acid equivalents
5
1.99.
