Chemistry Reference
In-Depth Information
[Note that
Eq. (9-30)
cannot be used when
r
1
5
1.0,
r
2
5
0.5. In that case
make calculations with
r
1
5
0.95 or 1.05. This will make very little difference to
the calculated values. Alternatively, one can use both suggested
r
1
values and
interpolate, if absolutely necessary.]
Since
F
1
is the ratio of the number of moles of M
1
converted divided by the
total number of moles of M
1
and M
2
polymerized in the same interval,
m
1
2
f
1
2
m
1
f
1
ð
1
p
Þ
2
F
1
5
(9-31)
m
2
Þ
5
m
1
2
m
2
2
ð
m
1
Þ
1
ð
p
Equation (9-30)
gives the feed composition after a fraction
p
of the initial
monomer mixture has been reacted. The composition of the copolymer made
during this interval of reaction is given by
Eq. (9-31)
. For given
r
1
,
r
2
, and
f
1
;
one assumes an
f
1
and estimates
p
from
Eq. (9-30)
. (It is necessary to remember
that
f
1
will decrease if M
1
is the more reactive monomer, and vice versa.) The
corresponding value of
F
1
is obtained from
Eq. (9-31)
with the
p
figure calculated as described. The
F
1
values for a series of stepwise
f
1
levels
are calculated by repeating this sequence with
f
1
in one step becoming
f
1
in the
next estimate. The cumulative average copolymer composition can also be calcu-
lated in a straightforward manner by entering
Eq. (9-31)
with the cumulative
value of
p
and the initial value of
f
1
:
(As a check, the average value of
F
1
at
1 must equal
f
1
:
p
)
Figure 9.3
records the changes of monomer feed and copolymer compositions
with conversion in the case of glycidyl methacrylate and styrene. This copolymer-
ization would produce an essentially styrenic polymer that is cross-linkable
through the pendant epoxy groups of the methacrylate residues. The last 10% of
copolymer formed is practically pure polystyrene. In the styrene-butadiene copol-
ymerization depicted in
Fig. 9.4
, the product composition is almost constant for
the first 70% of the reaction where this polymerization would normally be halted
anyway (Section 9.2.3).
The variation of copolymer composition during the course of a batch poly-
merization can be reduced by conducting the reaction as a so-called semibatch
process. This is a starved feed operation in which part of the charge is fed to
the reaction vessel and polymerization is started. The remainder of the mono-
mer feed is pumped in continuously or intermittently at a rate sufficient to
keep the copolymerization going at the desired rate. (In effect this could be at
a rate sufficient to keep the temperature of the reaction mixture contents in a
small range established by experience; these are exothermic reactions.) The
degree of conversion of the comonomer mixture
in the reactor
is always high
and relatively invariant, and the value of
F
1
in
Eq. (9-31)
remains sensibly con-
stant since
p
is almost
5
the same for reaction of most of the total monomer
charge.
