Chemistry Reference
In-Depth Information
This reasoning predicts that a reactivity ratio or an
r
1
r
2
product greater than
unity will decrease with increasing temperature and vice versa. The tendency for
random polymerization will increase and the tendency for monomer alternation
will decrease with increasing reaction temperature, so long as the same copoly-
merization mechanism predominates over the experimental temperature range.
These predictions are essentially confirmed by experience. Most free-radical
reactivity ratios are measured by convention at temperatures near 60
C, and the
effect of changes in conditions in the range 0
90
C is usually assumed to be
negligible, compared to the experimental difficulties in detecting the effects of
slight variations in
r
1
or
r
2
.
9.12.2
Pressure
In transition-state theory, the temperature dependency of a rate constant
k
on pres-
sure
P
can be expressed as
T
52
Δ
V
RT
@
ln
k
@
(9-77)
P
V
represents the volume change that occurs when
the transition state is formed from the reactants. The pressure dependence of a
reactivity ratio is then of the form
where the activation volume
Δ
V
ii
2
Δ
V
ij
RT
@
ln
r
i
=@
P
5
Δ
(9-78)
2
and reflects the differences between activation volumes for reaction of radical M
i
with monomer M
i
or M
j
. This difference is usually not large, and the variation of
reactivity ratios with pressure appears to be very small in the systems that have
been studied.
9.12.3
Medium
The copolymer composition equation is written in terms of monomer concentra-
tions at the locus of reaction. The same reactivity ratios should apply in principle
whether the polymerization is carried out in bulk, solution, suspension, or emul-
sion systems. In general, the only concentration values available to the experi-
menter are the overall bulk figures. Deviations of copolymer composition can be
expected, therefore, if the concentrations at the polymerization sites differ from
these figures. This can occur in emulsion systems, for example, if the monomers
differ appreciably in aqueous solubility and diffusion rates.
The occurrence of a homogeneous reaction system is also implicit in the deri-
vation of the copolymer composition equation. Some polymers, like poly(vinyli-
dene chloride), are insoluble in their own monomer and are not highly swollen by
monomer. In emulsion copolymerizations of such reactants the relative concentra-
tions of the comonomers in the polymerizing particles will be influenced by the
