March 3 - 5,1999, Karlsruhe, Germany - FZK

function of C0 2 content, of pressure, and of
temperature [4]. Figure 3 gives the ratio of kp-c M
values for polymerizations in C0 2 and in bulk as a
function of the relative initial monomer
concentration w M (referring to the pure monomer
density). Reaction pressures and temperatures were
as indicated in Fig. 3. For each system a reduction
of the propagation rate by 40 % is observed for the
highest experimental C0 2 content. A similar
reduction in kp has also been reported in the
literature [9], From the pressure and temperature
dependence of kp measured for high C0 2 contents
activation volumes and activation energies are
calculated that are identical with the corresponding
bulk data [10]. The observations are assigned to the
poor solvent quality of C0 2 which is associated with
more compact polymer coils as compared to the
situation in bulk and with a lower monomer
concentration in the vicinity of the free-radical site
[4]. The observed decrease in kp-c u (see Fig. 3) is
most probably due to a decrease in local monomer
concentration rather than to a reduction in kp. This
explanation is in accordance with results from PLP-
SEC experiments in C0 2, where low molecular
weight material is produced. Under such conditions
no influence on k p was observed [5, 11],
1.0 i
0.8
0.6
o BA, 200bar/ll°C
0.4 A BA, 1000bar/-l°C
• MMA, 1000 bar/30°C
0.2 2 I . i
1 , . i 1
i
• i '
i
• '—
1—
0.2 0.4 0.6 0.8 1.0
Figure 3. Ratio of kp-c M
values for homopoly­
merizations in C0 2 and in bulk of BA
and MMA. w M is the weight fraction of
initial monomer concentration relative to
the corresponding bulk value.
Termination rate coefficients for butyl acrylate and
styrene homopolymerizations in C0 2
Fig. 4 shows the conversion dependence of k t
for BA polymerizations in bulk and in 46 wt.% C0 2
at 40°C and 1000 bar. Over the entire conversion
range k t is higher in C0 2 solution. For
polymerizations in bulk, k t increases up to a
monomer conversion of about 10 %, followed by a
narrow plateau region. From 15 to 40 % of
monomer conversion k { decreases slightly. For
21
polymerizations in C0 2 the initial kt value is close
to the bulk value. In contrast to bulk
polymerizations, the increase in k { is steeper and
extends up to monomer conversions of about 20 %.
For higher conversion, within experimental
uncertainty a constant h is observed. The initial
increase of kt is supposed to be due to decreasing
solvent quality associated with the consumption of
monomer and with the increasing polymer
concentration. The reduction in solvent quality
results in more tightly coiled polymer molecules.
Polymerization in scC0 2 leads to a more
pronounced lowering of solvent quality and thus a
stronger influence on k t is observed. An
enhancement of k t originating from a decrease in
solvent quality has already been discussed in the
literature [e.g. 12, 13, 14].
46 wt.% C0 2
7 0-1 , 1 . 1 . 1 . 1 • '
/ , U
0 10 20 30 40 50
conversion / %
Figure 4. Conversion dependence of h for BA
polymerizations at 40°C and 1000 bar
[6].
The pressure dependence of kt has been
investigated for polymerizations in bulk and in
scC02 (42 wt.%) at 40°C. The activation volumes
are identical within experimental uncertainty.
Investigations into the temperature dependence of
kt, however, resulted in slightly, but distinctly
different activation energies. This difference is
another indication of the impact that solvent quality
has on the termination behavior.
Plotted in Figure 5 is the pressure dependence
of fct for thermally initiated styrene polymerizations
at 80°C. The triangles refer to polymerizations in
41 wt.% C0 2 and the dashed line represents bulk
data extrapolated from reference [15]. In the
presence of C02, kt values are by one order of
magnitude higher than in bulk. The activation
volume of kt, AJ% t) = 17 cm 3
-mol"\ is however
identical for both reaction media [11]. The observed
enhancement of k t is more pronounced in styrene
than in BA polymerizations, which is probably due
to the lower solvent quality that C0 2 offers for PS.