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

these mixtures, at a given temperature, a wide
range of fluid densities are obtainable at different
pressures. This was used in a novel way to conduct
polymerization by matching fluid mixture density
to the density of polystyrene. This is referred to as
levitation polymerization. The concept is to hinder
precipitation of the polymer when the molecular
weight increases to a level where the polymer is no
longer soluble in the medium. This helps in
producing even higher molecular weights. Indeed
experiments conducted at 51 and 73 °C, show a
maximum in the polymer molecular weight if
polymerizations were carried out at pressures that
would give a fluid density close to 1.05, the
density of polystsyrene. For example, at 51 °C and
21 MPa, fluid density is 1.05 and the molecular
weight of the polymer is 175,000.
As pointed out in the previous section, molecular
weight maximum with pressure has been noted in
some other systems, however the fluid mixture
density information for those systems have not
been reported to test if density plays a key role in
those system also. The notion of solubility
maximum has not been evaluated for the
polymerization in the SF 6+ C0 2 mixtures.
The notion of levitation by density matching offers
the possibility for a psuedo-dispersion
polymerization process that is free of added
stabilizers (such as surfactants) used in
conventional dispersion polymerizations.
Heterogeneous Dispersion and Emulsion
Polymerization. In dispersion polymerization, the
monomer and the initiator are soluble in the
continuous solvent phase, the polymer phase
separates but is stabilized as a colloid with
stabilizer additives. Polymerization proceeds to
high degrees of polymerization and the end
product is recovered as spherical particles. In
emulsion polymerization, the initiator is
preferentially dissolved in the continuous phase
and not the monomer phase, and the monomer
does not have high solubility in the continuous
phase.
For dispersion polymerization in carbon dioxide,
special stabilizer compounds have been developed.
These molecules have C0 2 - philic segments that
show high solubility in C0 2 and CO rphobic
anchoring segments that are relatively insoluble in
carbon dioxide. They are either homopolymers
such as poly (FAO) which has an acrylic-like
anchor and fluorinated C0 2-philic side chain, or
9
block copolymers of polystyrene and poly (FAO),
or block copolymers of polystyrene and
Poly(dimethylsiloxane) where the FOA or siloxane
blocks function as the C0 2-philic segments [3, 27].
Another group of stabilizers are the comblike graft
copolymers, such as poly(niethylmethacrylate-cohydroxyethylmcthacryalte)-graft-poly(perfluoropropylene
oxide) [28].
These polymerizations have been successfully
carried out in carbon dioxide with high
polymerization rates to produce polymers of high
molecular weight that are recovered as 1-3 micron
spherical particles. For example, polymerization at
65 °C and 205 bar, with PFAO as stabilizer and
using AIBN as initiator, polymethyl methacrylate
of Mn =200,000 to 315,000 have been produced
[27]. With a comb-like stabilizer at 65 °C and 380
bar polymethyl methacrylates in the molecular
range 100,000 to 355,000 have been produced
[28].
A recent study [29] demonstrates the dependence
of dispersion polymerizations on solvent quality
by conducting poly(FOA)-stabilized dispersion
polymerization of methyl methacrylate in
supercritical carbon dioxide in the presence of
different amounts of added helium. It is shown that
in the presence of helium, higher molecular weight
polymer, higher yields, smaller particles sizes and
a narrower particle size distributions are obtained.
At 65 °C and 344 bar, polymer produced in pure
carbon dioxide has a molecular weight of 204,000,
and a particle size and particle size distribution of
1.93 microns and 1.29. In the presence of 10 mole
% helium, molecular weight becomes 365,000,
particle size reduces to 1.64 microns with a
particle size polydispersity of 1.06. It is argued
that in the presence of He, solvent quality is
reduced, and as a result, critical chain length of
PMMA in the continuous phase is reduced, and the
polymer chain nucleates more effectively. When
more nuclei are stabilized, a reduction in particle
size is observed. Higher conversions and higher
molecular weights in C0 2 + He mixtures are
attributed to lower degree of swelling which in
terns leads to higher viscosity and gel effect.
Ionic Polymerizations
Ionic polymerizations are either cationic where
polymerization proceeds by adding monomers to a
terminal carbocation, or anionic where monomers
add to a negatively charged terminal carbon.