Chemistry Reference
In-Depth Information
100
100%
80%
60%
40%
80
10%
60
40
20
0
Time
FIGURE 8.3
Conversion-time plots for the polymerization of methyl methacrylate in benzene at 50
C.
The labeled curves are for the indicated monomer concentrations
[17]
.
methacrylate in benzene at 50
C
[17]
. At monomer concentrations less than
about 40 wt% in this case, the rate is approximately as anticipated from the
standard kinetic scheme described in this chapter.
R
p
decreases gradually as
the reaction proceeds and the concentrations of monomer and initiator are
depleted.
An acceleration is observed at higher monomer concentrations, however.
This is due to a decrease of
k
t
at higher conversions in these polymerizations.
When the polymer concentration becomes high enough, the macroradicals
will become entangled with segments of other polymer chains. As a result,
the rate of diffusion of the polymeric radicals and the frequency of their
mutual
encounters will decrease. The
rate of
termination is
reduced
accordingly.
The rate of propagation is affected much less than the rate of termination. The
propagation reaction involves the reaction of a large radical with a small mono-
mer whose diffusion rate is not changed significantly, whereas the termination
process involves two macroradicals whose ends have reduced mobility, because
motion of their centers of mass has become restrained. The net result in this case
is an increase in the effective
k
p
=
k
1
=
2
ratio in
Eq. (8-29)
and an increase in the
t
rate of polymerization.
Because vinyl polymerizations are exothermic, the increased polymer produc-
tion associated with the autoacceleration effect can cause a temperature rise and
faster initiator decomposition. Runaway reactions or explosions may result if the
heat of reaction is not removed efficiently.
The polymer concentration at which autocatalytic effects are significant varies
from system to system. When a polymer is insoluble in its own monomer, radicals
can be occluded in the precipitated polymer-rich phase where they are prevented
