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Green synthesis of polymers using supercritical carbon dioxide
Colin D. Wood a, Andrew I. Cooper b,*, Joseph M. DeSimone a,c,*
a Department of Chemistry, University of North Carolina at Chapel Hill, NC 27599, USAb Donnan and Robert Robinson Laboratories, Department of Chemistry, University of Liverpool, Liverpool, L69 3BX, UK
c Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695, USA
Abstract
Supercritical carbon dioxide is the most extensively studied supercritical fluid (SCF) medium for polymerization reactions and
organic transformations. This can be attributed to a list of advantages ranging from solvent properties to practical environmental
as well as economic considerations. Aside from these gains, CO2 finds particularly advantageous application in the synthesis and
processing of fluoropolymers and porous materials and as such, this will be the subject of this review. The overall aim is to highlight
areas where the unique properties of SCF solvents can be exploited to generate polymers that would be difficult or inconvenient to
obtain by other routes.
2005 Elsevier Ltd. All rights reserved.
Keywords: Supercritical; Fluoropolymer; Porous polymers; High-throughput
1. Introduction
Supercritical carbon dioxide has emerged as the most
extensively studied environmentally benign medium for
organic transformations and polymerization reactions.
This stems from a list of advantages ranging from sol-
vent properties to practical environmental and economic
considerations. Increasing environmental regulation has
prompted a tremendous effort to develop and apply
environmentally benign alternative media or solvent free
processes to a variety of commercially significant, sol-
vent-intensive processes [1]. Carbon dioxide in the liquid
and supercritical state is a potential alternative for manyof the processes experiencing ever increasing regulation,
owing to its non-toxic, non-flammable nature. Addition-
ally, its ubiquity makes it inexpensive and readily avail-
able. Several excellent reviews are available that discuss
the utilization of carbon dioxide for many solvent- and
waste-intensive industries [25]. Aside from the gains
provided by CO2 as a reaction medium in general, itfinds particularly advantageous application in the syn-
thesis and processing of fluoropolymers and porous
materials [611] as such, this will be the subject of this
review. In particular the aim is to highlight areas where
the unique properties of supercritical fluid (SCF) sol-
vents can be exploited to generate polymers that would
be difficult or inconvenient to obtain by other routes.
The general properties of SCFs in relation to chemical
synthesis [8] and extraction [2] have been reviewed previ-
ously and will not be a focus herein.
2. Fluoroolefin polymerization in CO2
2.1. Introduction
Fluoropolymers are typically synthesized in aqueous
polymerization systems (both emulsion and suspension),
non-aqueous systems (Freon-113), or in hybrid Freon-
113/aqueous hybrid systems [12]. Such processes require
the use of large quantities of water, CFCs (for non-aqueous
1359-0286/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cossms.2005.02.001
* Corresponding authors.
E-mail address: aicooper@liverpool.ac.uk (A.I. Cooper).
Current Opinion in Solid State and Materials Science 8 (2004) 325331
mailto:aicooper@liverpool.ac.ukmailto:aicooper@liverpool.ac.uk7/28/2019 [13]desimone
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polymerizations), and fluorinated surfactants for emul-
sion polymerization. Many of the fluorinated surfac-
tants typically employed in aqueous emulsion and
suspension polymerizations are currently under scrutiny
due to bioaccumulation and environmental persistence
[13,14]. These issues have collectively resulted in an
increasingly urgent impetus for transition from the con-ventional fluoropolymer synthesis platforms to alterna-
tives that meet the requirements of emerging public
and regulatory demands. Most commercially available
fluoropolymers are prepared from a relatively small
group of olefins including tetrafluoroethylene (TFE),
chlorotrifluoroethylene (CTFE), vinylidene fluoride
(VF2), hexafluoropropylene (HFP), ethylene, and per-
fluoroalkyl vinyl ethers (PAVEs). Many of these mono-
mers are flammable and some are explosive. For
example, TFE is flammable when mixed with air and
has a high propensity for explosion during expansion
to a gas from its liquid phase under pressure. Further,
TFE is highly explosive as a gas at elevated tempera-
tures. In the presence of oxygen it will undergo autopo-
lymerization, a process sufficiently exothermic to ignite
an explosion. Aqueous suspension and dispersion poly-
merizations of copolymers of TFE with a variety of
comonomers including hexafluoropropylene (HFP)
and various perfluoroalkyl vinyl ethers (PAVEs) typi-
cally exhibit high levels of carboxylic acid end groups
[7,15]. The presence of acid end groups often proves del-
eterious to the intended properties and function of the
polymer not to mention the introduction of complica-
tions into certain post polymerization, melt-processing
steps. This problem is particularly prevalent in copoly-mers including PAVE comonomers (perfluoroalkyl vinyl
ethers) which are an essential component in the control
of crystallinity in TFE-based fluoroplastics. In order to
prevent decomposition, discoloration and emissions of
hydrogen fluoride, polymers containing high levels of
acid end groups may require high-temperature hydroly-
sis and fluorination finishing steps. A propagating poly-
meric radical derived from a terminal PAVE monomer
unit has one of two possible reaction pathways (Scheme
1). First, and most obvious, it can cross propagate to
monomer continuing the polymerization reaction or itcan undergo b-scission, resulting in an acid fluoride ter-
minated polymer and a perfluoroalkyl radical capable of
initiating further polymerization which is essentially a
chain-transfer-to-monomer step [7,15]. The use of CFCs
circumvents these problems; however, CFCs have fallen
under exceedingly strict regulation due to environmental
concerns and as a result, are no longer viable to be used
as large scale reaction media.
2.2. TFE-based materials
Tetrafluoroethylene (TFE)-based copolymers have
become premium high performance materials for a
broad range of applications that demand chemical and
thermal resistance, and melt processing capability [16].
As mentioned earlier, caution must be taken when
TFE is used as a monomer due to potential explosions.
It has been demonstrated that CO2/TFE mixtures are far
less susceptible to ignition as TFE forms a pseudo
azeotrope with CO2 [17]. This makes handling and deliv-
ering monomer much safer. A range of TFE-based
fluoropolymers have been successfully synthesized in
CO2. These include FEP, PFA, ETFE, TFE/vinyl ace-
tate polymers, Nafion-type materials, and Teflon
AF-type materials (Fig. 1) the details have recently beenreviewed elsewhere [11]. Further, it is worth noting that
fluoropolymers synthesized in sc-CO2 exhibit signifi-
cantly diminished levels of acid end groups leading to
very high molecular weight materials [15]. Indeed, in
such instances, it is even necessary to add chain transfer
agents to control molecular weight and maintain melt-
processability of the product [18]. IR analysis has con-
firmed that fluoropolymers synthesized under these
conditions exhibit acid end group levels an order of
magnitude below (03 end groups per 106 carbon atoms)
those which are synthesized in conventional organic and
aqueous reaction systems [14,18]. The most likely expla-
nation for this is that the tremendous plasticizing
capability that CO2 provides for efficient transport of
monomer into the polymer phase, maintaining high
effective concentrations of monomer in the vicinity of
the active chain ends, thus favoring the bimolecular
propagation step over the unimolecular b-scission pro-
cess (Scheme 1). The other explanation may be the lower
temperatures that one can use with CO2 which may
favor cross propagation over b-scission. Additionally,
it has been demonstrated that dense CO2 is exceptionally
inert to free radical chemistry in general [19]. This dem-
onstrates the broad applicability of liquid and supercrit-
CF2 C
F
O
CF2
CF2
CF3
CF2 C
F
O
CF2
CF2
CF3
CF2 C
F
F
CF2 C
F
O
CF2
CF2
CF3
CF2 C
O
F
F3C CF2 CF2
TFE
CrossPropagation
+
-scission
Scheme 1. Reaction sequence for b-scission in fluoroolefin polymer-
ization [7,15].
326 C.D. Wood et al. / Current Opinion in Solid State and Materials Science 8 (2004) 325331
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ical CO2 in the synthesis and processing of fluorinated
polymers which has led to the commercialization of a
CO2-based approach in the manufacturing of certain
grades of polymers based on tetrafluoroethylene (Tef-
lonTM). DuPont has recently invested an initial $40 mil-
lion in the construction of a TeflonTM FEP production
facility in North Carolina. The process employs CO2as the continuous phase instead of 1,1,2-trichloro-
1,2,2-trifluoroethane or water and surfactant [1,2023].
The use of carbon dioxide results in a surfactant free
system with no issues of chain-transfer-to-solvent. This
process not only avoids many of the environmental
and safety concerns but also provides a product with
superior properties. This is the first commercial example
of fluoropolymer resins made using supercritical carbon
dioxide as the solvent.
Teflon AF is an amorphous copolymer of tetraflu-
oroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-
difluoro-1,3-dioxole (PDD). It combines the properties
of amorphous plastics, such as optical transparency
and solubility in organic solvents, with those of perfluo-
rinated polymers, including high thermal stability, excel-
lent chemical stability and low surface energy. Moreover
Teflon AF exhibits the lowest dielectric constant (1.90
for Teflon AF 2400) and the lowest refractive index
(1.29 for Teflon AF 2400) known for any solid organic
polymer [12]. As such, Teflon AF is well-suited for use
as an optical material. Teflon AF-based materials have
also been synthesized in supercritical carbon dioxide
(see Fig. 1) [24]. A range of copolymers with various
compositions and different molecular weights were pre-
pared in yields as high as 74% and their properties were
compared to commercially available Teflon AF. The
glass transition temperatures for the materials ranged
from 67 to 334 C. The synthesis of these copolymers
in CO2 has several key advantages compared to conven-
tional polymerization techniques. The low reaction tem-
perature and the use of a perfluorinated initiator resultsin copolymers with properties which are comparable to
those of the analogous commercial product. However,
an additional fluorination step is not necessary and the
product is isolated directly from the reactor without
contamination from solvents or surfactants. Moreover,
the synthesis uses TFE/CO2 mixtures instead of pure
TFE which has inherent safety advantages [17].
2.3. VF2-based materials
VF2 has been successfully synthesized in CO2 in a
continuous precipitation process using a continuous
stirred tank reactor [25]. In such a process monomer,
solvent (CO2), and initiator are continuously added to
the reactor. The advantage of a continuous process over
a batch process is that the monomer and the CO2 can be
removed and recycled, leading to a high rate of polymer-
ization. The authors later reported that, although this
was a precipitation polymerization, a model based on
the kinetics for a homogeneous polymerization fit the
experimental data, confirming the reaction order of 0.5
and 1 for the initiator and monomer respectively [26].
Further, the effect of VF2 flow rate, residence time
and temperature were investigated in this system [27].
Fluorinated Ethylene Propylene Resin
(FEP)
Perfluoroalkoxy Resin
(PFA)
Ethylene Tetrafluoroethylene Resin
(ETFE)
Nafion
CF2 CF2 CH2 CH2
CF2 CF2 CF2 CF
ORf
CF2 CF2 CF2 CF
CF3
CF2 CF2 CF2 CF
OCF2CF(CF3)OCF2CF2SO2F
CFCF
O O
CF3F3C
CF2CF2
n
nn
n n
Teflon
AF
CF2 CF2 CH2 CH
n
O
O
CH3
Tetrafluoroethylene-co-vinyl acetate
Fig. 1. Tetrafluoroethylene-based fluoropolymers synthesized in carbon dioxide.
C.D. Wood et al. / Current Opinion in Solid State and Materials Science 8 (2004) 325331 327
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It was found that monomer feed concentration had a
pronounced effect upon the modality of the molecular
weight distribution. At a high monomer concentration,
the resulting polymer exhibited a bimodal molecular
weight distribution. The authors suggested that at high
monomer concentrations the kinetics may no longer ad-
here to the homogeneous kinetic model proposed earlier[26]. Several other parameters were investigated such as
chain branching and agitation but they were unable to
explain the bimodality of the polymer.
2.4. Fluoroalkyl acrylate polymerizations in CO2
One of the more difficult issues associated with the
use of CO2 as a polymerization medium has been the
limited solubility of most high molecular weight poly-
mers in CO2. As such, much of the work reported for
polymerizations in CO2 are typically heterogeneous pre-
cipitation, dispersion or emulsion processes as discussed
above. However, it has been demonstrated that oligo-
meric perfluoropolyethers and oligomeric poly(chlo-
rotrifluoroethylene) oils are soluble in liquid CO2[2]. Additionally, it has been reported that highly fluori-
nated polyacrylates of high molecular weight (>250,000)
exhibit exceptional solubility in supercritical CO2. Not
surprisingly, since this discovery, it has been demon-
strated that high molecular weight, amorphous fluoro-
polymers can be synthesized homogeneously in CO2utilizing free radical initiators [21,2833]. Most amor-
phous fluoropolymers exhibit solvent resistance to
common organic solvents and therefore conventional
approaches to synthesis and processing have dependedon chlorofluorocarbon (CFC) platforms. Owing to the
previously mentioned environmental concerns with the
use of CFC refrigerants and solvents, CFCs are no
longer acceptable for large-scale commercial use. As
an effective reaction medium for homogeneous polymer-
izations under mild conditions, CO2, again, is an
increasingly attractive, inexpensive and harmless plat-
form option for the synthesis of highly fluorinated
monomers (as domestic and international regulation of
CFCs becomes ever more restrictive in the coming
years). Various fluorinated acrylate monomers, such as
1,1-dihydroperfluorooctyl acrylate (FOA) have been
successfully polymerized in CO2. It is also noted that
reaction conditions for these polymerizations are mild
in terms of temperature and pressure and not entirely
unlike those encountered in conventional solvent based
processes [21]. Further, studies have shown CO2 to be
inert to radical chemistry, eliminating chain transfer to
solvent as a side reaction [8]. Additionally the low vis-
cosity of sc-CO2 removes issues of Tromsdorf effect
and autoacceleration [8]. Therefore, as a reaction med-
ium, CO2 is ideally suited to free radical polymeriza-
tions. In all cases, the polymerizations remained
homogeneous throughout the reaction, illustrating the
high solubility of the polymers in supercritical CO2.
The solution properties of poly(FOA) in CO2 have also
been investigated using small-angle neutron scattering
(SANS) and it was shown that CO2 is an excellent sol-
vent for poly(FOA) [34].
2.5. Photooxidation of fluoroolefins in liquid CO2
Another class of fluoropolymer known to be readily
soluble in CO2 are perfluoropolyethers (PFPEs) [2]. As
a unique class of fluoropolymers, PFPE polymers and
copolymers have been established as high performance
materials, exhibiting low surface energies and low
moduli, as well as excellent thermal and chemical stabil-
ities. PFPEs are primarily found in high performance
lubricant applications such as for magnetic data storage
media and as heat exchanger fluids. Recently, DeSimone
et al. developed a novel solvent resistant photocurable
PFPE that can be used for the fabrication of microflu-
idic devices [35] and as materials for high resolution soft
lithography to replicate sub-100 nm sized features with
no indications of limits to going to even smaller in size
[36]. Moreover, the material has several specific advan-
tages over traditional glass and PDMS-based materials
and broadens the utility of both processes. In addition,
the process exemplifies a solvent-free approach.
One of the main industrial processes for the produc-
tion of PFPEs is photooxidation of fluoroolefins [37].
Currently, only TFE and HFP are used commercially
in this process. Typically, HFP is photooxidized in bulk
owing to its very low reactivity while TFE requires an
inert diluent in order to prevent homopolymerizationof the olefin. DeSimone and coworkers reported recently
the photooxidation of various concentrations of HFP
in CO2 in concert with parallel reactions carried out in
bulk HFP and in perfluorocyclobutane for purposes of
comparison [38]. Based on the data collected in this
study, it was demonstrated that there is a strong depen-
dence of molecular weight and composition on HFP
concentration (i.e., lower HFP concentrations gave
lower peroxide content). Additionally, viscosity effects
in liquid CO2 appeared to significantly reduce the
amount of peroxidic linkages in the final product.
2.6. Porous materials and supercritical fluids
Porous materials are used in a wide range of appli-
cations, including catalysis, chemical separations, and
tissue engineering [9]. However, the synthesis of these
materials is often solvent intensive. Supercritical carbon
dioxide as an alternative solvent for the production of
functional porous materials can circumvent this issue
as well as affording a number of specific physical, chem-
ical, and toxicological advantages listed below. For
example, in order to dry porous materials often requires
energy intensive steps whereas; the transient dry nat-
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ure of CO2 circumvents these issues. Pore collapse can
occur in certain materials when removing conventional
liquid solvents; this can be avoided using SCFs because
they do not possess a liquidvapour interface. Porous
structures are important in biomedical applications
(e.g., tissue engineering) where the low toxicity of CO2
offers specific advantages in terms of minimizing theuse of toxic organic solvents. In addition, the wetting
properties and low viscosity of CO2 offers specific bene-
fits in terms of surface modification.
2.7. Free-radical polymerization using sc-CO2 as a
pressure-adjustable porogen
Recently, Cooper et al. demonstrated the formation
of permanently porous crosslinked poly(acrylate) and
poly(methacrylates) monoliths using sc-CO2 as the por-
ogenic solvent [39,40]. Materials of this type [41,42] are
useful in applications such as high-performance liquid
chromatography, high-performance membrane chroma-
tography, capillary electrochromatography, microflui-
dics, [43] and high-throughput bioreactors [44]. In this
process, no organic solvents are used in either the syn-
thesis or purification. It is possible to synthesize the
monoliths in a variety of containment vessels (Fig. 2),
including chromatography columns and narrow-bore
capillary tubing. Moreover, the variable density associ-
ated with SCF solvents was exploited in order to
fine-tune the polymer morphology. Fig. 3 shows the
variation in BrunnauerEmmetTeller (BET) surface
area for a series of crosslinked monoliths synthesized
using sc-CO2 as the porogen over a range of reactionpressures [45]. As can be seen the BET surface area
and the average pore size in these materials could be
continuously varied over a broad range simply by vary-
ing the SCF solvent density. It is interesting to note the
minimum in area (and a maximum in the average pore
diameter, not shown) was observed at a reaction pres-
sure of around 180 bar. This can be rationalized by
considering the variation in solvent quality as a function
of CO2 density and the resulting influence on the mech-
anism of nucleation, phase separation, aggregation,
monomer partitioning, and pore formation [46]. This
approach was also extended to synthesize well-defined
porous, cross-linked beads by suspension polymeriza-
tion, again without using any organic solvents [47].
The surface area of the of the beads could be tuned over
a broad range (5500 m2/g) simply by varying the CO2density.
2.8. Templating of supercritical fluid emulsions
Emulsion templating is useful for the synthesis ofhighly porous inorganic, [4851] and organic materials
[5254]. In principle, it is possible to access a wide range
of porous hydrophilic materials by reaction-induced
phase separation of concentrated oil-in-water (O/W)
emulsions. A significant drawback to this process is that
large quantities of a water-immiscible oil or organic
solvent are required as the internal phase (usually
>80 vol.%). In addition, it is often difficult to remove
this oil phase after the reaction. Inspired by studies on
SCF emulsion stability and formation [55], we have
developed methods for templating high internal phase
CO2-in-water (C/W) emulsions (HIPE) to generate
highly porous materials in the absence of any organic
solventsonly water and CO2 are used [56]. If the emul-
sions are stable one can generate low-density materials
(0.1 g/cm3) with relatively large pore volumes (up to
6 cm3/g) from water-soluble vinyl monomers such as
acrylamide and hydroxyethyl acrylate. Fig. 4 shows a
crosslinked polyacrylamide material synthesized from
a high internal phase C/W emulsion, as characterized
by SEM and confocal microscopy (scale = 230 lm
230 lm). Comparison of the two images illustrates quite
clearly how the porous structure shown in the SEM
image is templated from the C/W emulsion (as
Fig. 2. Polymer monolith, synthesized using sc-CO2 as the porogen as
isolated from the reaction vessel. The monolith adopts the shape of the
reactor. Reproduced with permission; copyright 2003, American
Chemical Society [45].
50
100
150
200
250
300
350
100 150 200 2 300 350 4 45
Pressure (bar)
250 300 400 450
Surfac
eArea(m2/g)
Fig. 3. Surface area (as measured by N2 adsorptiondesorption using
the BET method) as a function of the CO2 pressure employed during
the polymerization. Reproduced with permission; copyright 2003,
American Chemical Society [45].
C.D. Wood et al. / Current Opinion in Solid State and Materials Science 8 (2004) 325331 329
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represented by the confocal microscopy image of the
pores). In general, the confocal image gives a better mea-
sure of the CO2 emulsion droplet size and size distribu-
tion immediately before gelation of the aqueous phase.
Initially we used low molecular weight (Mw 550 g/
mol) perfluoropolyether ammonium carboxylate surfac-
tants to stabilize the C/W emulsions [56] but there are
some practical disadvantages of using these surfactants
in this particular process such as cost and the surfactant
is non-degradable. It was subsequently shown that it is
possible to use inexpensive hydrocarbon surfactants to
stabilize C/W emulsions and that these emulsions can
also be templated to yield low-density porous materials
[57]. In this study it was shown that all of the problems
associated with the initial approach could be overcome
and it was possible to synthesize C/W emulsion-tem-
plated polymers at relatively modest pressures (60
70 bar) and low temperatures (20 C) using inexpensive
and readily available hydrocarbon surfactants. More-
over, we demonstrated that this technique can in princi-ple be extended to the synthesis of emulsion-templated
HEA and HEMA hydrogels that may be useful, for
example, in biomedical applications [5860].
2.9. High throughput solubility measurements in CO2
As discussed CO2 is an excellent environmentally
benign alternative solvent for the synthesis of fluoro-
polymers and porous materials. Moreover, a range of
environmental as well as practical advantages can be
realized from using CO2, which has resulted in the com-
mercial synthesis of certain grades of Teflon by DuPont.
However, the discovery of inexpensive CO2-soluble sur-
factants, ligands and phase transfer catalysts would
greatly increase the utility of CO2. As mentioned the
only polymers with appreciable solubility in CO2 are
amorphous fluoropolymers and polysiloxanes [61]. Inex-
pensive poly(ether carbonate) copolymers have been re-
ported to be soluble in CO2 under moderate conditions
[62,63] and could function as building blocks for inex-
pensive surfactants, but numerous practical difficulties
remain. Recently, Cooper et al. reported a new method
which allows for the rapid parallel solubility measure-
ments for libraries of materials in supercritical fluids
[64]. The technique was used to evaluate the solubilityof a mixed library of 100 synthetic polymers including
polyesters, polycarbonates, and vinyl polymers. This
method is at least 50 times faster than other techniques
in terms of the rate of useful information that is ob-
tained and has broad utility in the discovery of novel
SCF-soluble ligands, catalysts, biomolecules, dyes, or
pharmaceuticals for a wide range of materials
applications.
3. Conclusions
In general CO2 is an attractive solvent alternative for
the synthesis of polymers because it is environmentally
friendly, non-toxic, non-flammable, inexpensive, and
readily available in high-purity from a number of
sources. Product isolation is straightforward because
CO2 is a gas under ambient conditions, removing the
need for energy intensive drying steps. Moreover, addi-
tional advantages are realized in the manufacture of flu-
oropolymers such as increased safety in handling
explosive monomers and enhanced polymer properties
in many cases. As demonstrated CO2 is not only an ade-
quate alternative, but in many cases a superior one. This
Fig. 4. Emulsion-templated crosslinked polyacrylamide materials
synthesized by polymerization of a high-internal phase CO2-in-water
emulsion (C/W HIPE): (a) SEM image of sectioned material and (b)
confocal image of same material, obtained by filling the pore structure
with a solution of fluorescent dye. As such (a) shows the walls of the
material while (b) show the holes formed by templating the sc-CO2emulsion droplets. Both images = 230lm 230 lm. Ratio of CO2/
aqueous phase = 80:20 v/v.d Pore volume = 3.9 cm3/g. Average pore
diameter = 3.9 lm. Adapted from Butler et al. [56].
330 C.D. Wood et al. / Current Opinion in Solid State and Materials Science 8 (2004) 325331
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is demonstrated no more poignantly that DuPonts re-
cent $275 million investment in its CO2 based TeflonTM
production process. In addition, CO2 affords a number
of specific advantages that can be derived from the use
of SCFs for the synthesis and modification of porous
materials. There are a number of specific chemical
advantages that are realized by using CO2 for the syn-thesis of a range of materials but moreover, they offer
the potential of reducing organic solvent usage in the
production of materials.
Acknowledgments
We thank the NSF STC (grant number 537494), EPA
(grant number 535325), ONR (grant number 535747),
EPSRC. A.I.C. acknowledges the support of the Royal
Society for the provision of a Research Fellowship.
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