Cross-linked polymer-ionic liquid composite materials

Article abstract of DOI:10.1021/ma021710n

A series of composites, some with permanent porosity, comprising polymers and ionic liquids have been prepared by in-situ polymerization. The characteristics of both composites and isolated polymers have been investigated. Slow mass and phase transfer characterize the interaction with a range of polymers (1-17) in the N,N′-dialkylimidazolium liquids, [emin]BF₄, [bmim]PF₆, or [omim]N(SO₂CF₃)₂. Free-radical homopolymerization of 1-vinyl-2-pyrrolidinone in [bmim]PF₆ or of 4-vinylpyridine in [omim]N-(SO₂CF₃)₂ gave viscous solutions from which polymer (M_w = 162 500 and 71 500 g mol^-1, respectively) could be isolated. Detectable ionic liquid residues were retained in the isolated polymers despite five reprecipitations from methanol. Copolymerization of 4-vinylpyridine (VP) with 5% divinylbenzene (DVB) or trimethylolpropane trimethacrylate (TRIM) in [omim]N(SO₂CF₃)₂, and homopolymerizations of the cross-linking monomes on their own, led with 90-100% monomer conversion to a series of gel-like composite materials, B-H, from which gross-linked polymers, B′-H′, could be isolated by Soxhlet extraction of the ionic liquid. Copolymers of VP with 5-30% DVB (E′, F′) showed a low degree of permanent porosity in the dry state. However, poly(DVB) (G′) and poly(TRIM) (H′) have bulk densities (<0.6 g cm^-3), intrusion volumes (>0.9 cm³ g^-1), BET surface area (70-320 m² g^-1), and morphology (from SEM studies) which demonstrate the porogenic character of the ionic liquids used. Comparisons with the related products I and J obtained in toluene reveal the sensitivity of these systems both to the properties of the porogen solvent and to the monomer used.

Full text of DOI:10.1021/ma021710n

Macromolecules 2003, 36, 4549-4556
4549
Cross-Linked Polymer-Ionic Liquid Composite Materials
P eter Sn ed d en ,†,‡ An d r ew I. Coop er ,‡ Keith Scott,§ a n d Neil Win ter ton *,†,‡
Leverhulme Centre for Innovative Catalysis and Department of Chemistry, University of Liverpool,
Liverpool, L69 7ZD, UK, and Department of Chemical and Process Engineering, Merz Court,
University of Newcastle, Newcastle-upon-Tyne, NE1 7RU, UK
Received November 25, 2002; Revised Manuscript Received April 9, 2003
ABSTRACT: A series of composites, some with permanent porosity, comprising polymers and ionic liquids
have been prepared by in-situ polymerization. The characteristics of both composites and isolated polymers
have been investigated. Slow mass and phase transfer characterize the interaction with a range of polymers
(1-17) in the N,N′-dialkylimidazolium ionic liquids, [emim]BF4, [bmim]PF6, or [omim]N(SO2CF3)2. Free-
radical homopolymerization of 1-vinyl-2-pyrrolidinone in [bmim]PF6 or of 4-vinylpyridine in [omim]N-
(SO2CF3)2 gave viscous solutions from which polymer (Mw ) 162 500 and 71 500 g mol-1, respectively)
could be isolated. Detectable ionic liquid residues were retained in the isolated polymers despite five
reprecipitations from methanol. Copolymerization of 4-vinylpyridine (VP) with >5% divinylbenzene (DVB)
or trimethylolpropane trimethacrylate (TRIM) in [omim]N(SO2CF3)2, and homopolymerizations of the
cross-linking monomers on their own, led with 90-100% monomer conversion to a series of gel-like
composite materials, B-H, from which cross-linked polymers, B′-H′, could be isolated by Soxhlet
extraction of the ionic liquid. Copolymers of VP with 5-30% DVB (E′, F ′) showed a low degree of permanent
porosity in the dry state. However, poly(DVB) (G′) and poly(TRIM) (H′) have bulk densities (<0.6 g cm-3),
intrusion volumes (>0.9 cm3 g-1), BET surface areas (70-320 m2 g-1), and morphology (from SEM studies)
which demonstrate the porogenic character of the ionic liquids used. Comparisons with the related products
I and J obtained in toluene reveal the sensitivity of these systems both to the properties of the porogen
solvent and to the monomer used.
In tr od u ction
to effect the ring-opening polymerization of ethylene
carbonate,3c condensation polymerizations leading to
polyamides and polyimides,3d,e metal-complex-catalyzed
oligomerization,4a-c and polymerization of alkenes4d,e
and phenylacetylene.4f Living, atom-transfer or conven-
tional radical homopolymerizations of methyl meth-
acrylate,5a-f 2-hydroxyethyl methacrylate,5e-g methyl,
butyl, hexyl, and dodecyl acrylate,5h styrene,5c,e,f and
acrylonitrile5f have been carried out in N,N′-dialkylimi-
dazolium salts of [PF6]-, [BF4]-, [N(SO2CF3)2]-, or
[F(HF)n]-. Alternating copolymers of styrene and CO6a,b
and block copolymers of styrene and methyl methacryl-
ate6c have also been prepared. Very recent papers
describe atom-transfer radical copolymerizations of N-
hexylmaleimide with styrene6d and of butyl acrylate
with methyl acrylate6e in similar media as well as the
reversible addition-fragmentation chain transfer po-
lymerization of styrene, methyl acrylate, and methyl
methacrylate.6f
Research into the preparation, structure, and proper-
ties of new composite and hybrid materials has been
driven by a range of possible technological applications
as well as by intrinsic interest. We are currently seeking
to develop novel materials for use in membrane reactors
in which ionic liquids are used as a medium in which
to effect a catalyzed chemical change1,2 and wish to
report on some associated investigations on a series of
polymer-ionic liquid composites.
The technical (as opposed to economic) characteristics
of composite materials will be determined by the
particular application envisaged and, in the case of
membranes for use in catalytic reactors, will include
coprocessability and compatibility of component materi-
als as well as mechanical and chemical robustness in
use.
In our approach we have investigated selected poly-
mer-ionic liquid interactions as a prelude to the design
of effective composite materials. These have included
studies of the solubility of a range of polymers in
selected ionic liquids, polymerizations and copolymer-
izations in ionic liquids, the characterization of targeted
polymer-ionic liquid composites, and the role of cross-
linking and gelation.
Polymerizations are among the earliest chemical
transformations to have been studied in ionic liquids,
with electrochemical polymerization of pyrrole first
described3a in N-alkylpyridinium chloroaluminate3a and,
more recently,3b in 1-ethyl-3-methylimidazolium tri-
fluoromethanesulfonate. Related media have been used
Interest in polymer electrolytes for possible use in new
battery, fuel cell, and other applications has led to the
polymerization of monomers or the functionalization of
polymers having some of the characteristics of ionic
liquids, including materials derived from vinyl and
imidazolium groups tethered to oligo(ethylene oxide),7a
from N-[(4-vinyl)benzyl]-N′-ethylimidazolium7b and N-vi-
nyl-N′-alkylimidazolium7c,d salts or from liquid salts
derived from 1-vinylimidazole and vinylsulfonic acid or
3-sulfopropyl acrylate.7e
Carlin et al. have reported a series of studies of gel-
polymer electrolytes derived from perfluorinated poly-
mer membranes and ionic liquids.2,8a-c High-tempera-
ture proton conducting membranes based on perfluorin-
ated ionomer membrane-ionic liquid composites have
also been described.8d
† Leverhulme Centre for Innovative Catalysis, University of
Liverpool.
‡ Department of Chemistry, University of Liverpool.
§ University of Newcastle.
* Author for correspondence: e-mail N.Winterton@liverpool.ac.uk.
10.1021/ma021710n CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/15/2003

4550 Snedden et al.
Macromolecules, Vol. 36, No. 12, 2003
Ta ble 1. P h a se Beh a vior a of Selected P olym er Solu tion s in Ion ic Liqu id s
IL:
[emim]BF4
11
[bmim]PF6
15
+ + + + +
o - - -
[omim]Tf2N
polymerb
weeks:
0
2
4
6
0
2
4
6
0
2
4
6
11
1, P(BuVP+Br--VP)
+ + + + +
o - - - -
- - - - -
+ + + + +
- - - - -
o - - - -
+ + + + +
+ + + + +
+ + + +
+
c
c
2, P(BuVP+Br--VP-styrene)
3, P(BuVP+Br--VP-BMA)
o
+ +
c
c
c
c
o - - - -
c
c
4, P(BuVP+Tf2N--VP)
c
c
c
c
c
c
o
c
c
o
c
c
c
-
+ + + +
+ + + +
+ + + +
+ + + +
+
+
+
+
-
+
-
+
+
-
-
-
-
-
5, P(MeVIm+Tf2N-)
c
6, P(VIm-VPyrr), 50% VPyrr
7, P(MeVIm+MeOSO3--VIm-VPyrr)
8, P(MeVIm+Cl--VIm-VPyrr)
9, P(MeVIm+MeOSO3--VIm-VPyrr-VCap)
10, P(acrylic acid), Mw 2 × 103 g mol-1
11, P(BMA), Mw 337 × 103 g mol-1
12, P(ethylene glycol), Mw 10 × 103 g mol-1
13, P(styrene), Mw 30 × 103 g mol-1
14, P(VP), Mw 160 × 103 g mol-1
15, P(VP-styrene), 10% styrene
16, P(VP-BMA), 10% BMA
+ c
-
c
c
+ + c
c
+
c
c
c
c
+ + o o
o
o
o
o
o
c
+ + + +
- - - -
+ + + +
+ + + +
- - - -
+ + + + +
+ + + + +
+ - - - -
- - - - -
- - - - -
- - - - -
- - - - -
+ + + + -
- - - - -
- - - - -
+ + + + +
- - - - -
- - - - -
- - - - -
- - - - -
+ + c - -
+
+
+
c
c
c
- -
- -
- -
17, P(VPyrr), Mw 10 × 103 g mol-1
Key: + (clear solution), c (cloudy), o (opaque), - (precipitate). 1-5 synthesized, 6-17 commercially available. Abbreviations: BMA
(butyl methacrylate), Tf2N- [(N(SO2CF3)2]-, VCap (1-vinylcaprolactam), VIm (1-vinylimidazole), VP (4-vinylpyridine), VPyrr (1-vinyl-2-
pyrrolidinone). Mw values for 1-9, 15, and 16 were not determined.
+ + + +
a
b
Resu lts a n d Discu ssion
liquids and completely insoluble in the third, for reasons
that are not immediately obvious.
P olym er Solu bilities in Ion ic Liqu id s. The solu-
bilities of a range of polymers were screened in a
selection of ionic liquids. The results obtained are shown
in Table 1. Polymers chosen for study included ho-
mopolymers of acrylic acid (10), butyl methacrylate (11),
ethylene oxide (12), styrene (13), 4-vinylpyridine (14),
and 1-vinyl-2-pyrrolidinone (17) and copolymers of
1-vinylimidazole (1-vinyl-2-pyrrolidinone, 6) and 4-vi-
nylpyridine (styrene, 15; butyl methacrylate, 16). Poly-
electrolytes chosen for study included partially quater-
nized 4-vinylpyridinium (1-4) and 1-vinylimidazolium
(7-9) copolymers and the 1-vinylimidazolium homopoly-
mer 5. Polymers 1-5 were synthesized,1 and polymers
6-17 were obtained from commercial sources. The three
ionic liquids chosen for study were [emim]BF4, [bmim]-
PF6, and [omim]Tf2N (Tf2N- ) [N(SO2CF3)2]-) to be
representative of typical cations and anions. From the
studies of the solvation characteristics of ionic liquids
reported so far,9a-i it seems reasonable to conclude that
the hydrophobicity of N,N′-alkylmethylimidazolium cat-
ions [rmim]+ (r ) e (ethyl), b (butyl); or o (octyl)) and
anions increase along the series emim < bmim < omim;
BF4 < PF6 < Tf2N. A trial experiment utilizing pow-
dered samples revealed the majority of polymers 1-17
to be insoluble in both [bmim]PF6 and [omim]Tf2N (5
wt % concentration). The apparent insolubility of the
polymers may have arisen from the relatively high vis-
cosity10a,b of ionic liquids, leading to slow dissolution.
To investigate the thermodynamic solubility of the
polymers, a second experiment was devised. Polymers
1-17, dissolved in molecular solvents (5 wt %), were
added to ionic liquids, and the volatile solvents were
removed. The phase behavior of the ionic liquid/polymer
solutions was monitored over the course of 11-15
weeks. As can be seen from Table 1, [omim]Tf2N
appeared to be the most effective solvent for a wide
range of polymers. The behavior of [emim]BF4 was
similar to that of [bmim]PF6. Polymers that showed
good initial solubility in all three ionic liquids included
the N-butylpyridinium polyelectrolyte, 1, and the hy-
drophilic polyether, PEG, 12. Highly hydrophobic poly-
(styrene), 13, was insoluble in all three ionic liquids.
On the other hand, the relatively hydrophobic poly(butyl
methacrylate), 11, was freely soluble in two of the ionic
Several of the polymer solutions exhibited slow phase
separation, suggesting that both polymer dissolution
and phase separation may be mass- or phase-transfer-
limited. Many initially homogeneous polymer-ionic
liquid solutions exhibited phase separation over periods
of days to weeks.
The solubility studies, undertaken to guide our pro-
gram of polymer synthesis and composite assembly,
have highlighted a rich range of behavior. It is evident
that the factors responsible for polymer solubility in
ionic liquids are complex and not readily predicted. The
detailed phase behavior of polymer-ionic liquid mix-
tures requires more in-depth investigation.
P olym er -Ion ic Liqu id Com p osites. The key goal
of this research program is to develop polymer-ionic
liquid composite materials that are suitable for use in
catalytic membrane reactors.11 In principle, there are
two main methods by which this can be achieved. First,
an appropriately porous material may be imbibed with
the ionic liquid. This material may be a commercially
available porous membrane,2,8a-c a chemically modified
membrane, or a polymeric material that is specifically
designed and synthesized for the particular application.
Second, one may produce a polymer-ionic liquid com-
posite in situ by reaction-induced phase separation (one
method among several12a-d of gelling of an ionic liquid
solution). Each approach has advantages and disadvan-
tages (not least associated with the point in the process
at which catalyst is introduced2,8a). Imbibing ionic
liquids into commercially available membranes is simple
but does not permit much “tuning” of the interactions
between the polymer and the ionic liquid phases (e.g.,
by modifying factors such as polymer wettability and
pore filling). In contrast, the direct formation of com-
posite materials by in situ reaction-induced phase
separation should allow fine control over the physical
and chemical nature of the composite materials. More-
over, this approach offers a route to “molded” composites
in a range of shapes and sizes for various applications
(e.g., membranes, monolithic column packings, filled
capillaries, etc.). A potential disadvantage of any direct
“gelation” route is that residual monomer may remain
in the composite after reaction. On the other hand, since

Macromolecules, Vol. 36, No. 12, 2003
Cross-Linked Polymer-Ionic Liquid Composites 4551
Ta ble 2. Lin ea r P olym er sa
polymer
monomer
solvent
selected characterization data
A′
VPyrr
[bmim]PF6
purified productb
Mw 162 500 g mol-1, Mn 61 300 g mol-1, PD 2.7c
composite material (B)
B′
C′
D′
VPyrr (+1 mol % DVBd)
VPyrr (+5 mol % DVBd)
VP
[bmim]PF6
[bmim]PF6
[omim]Tf2N
transparent tacky syrup; Tonset 326 °C, Tmax 475 °Ce
composite material (C)
opaque tacky syrup; Tonset 309 °C, Tmax 442 °Ce
purified productb
Mw 71 500 g mol-1, Mn 36 200 g mol-1, PD 2.0c
a
b
Polymerization conditions: monomer (1 vol), [omim]Tf2N (2.5 vol), AIBN, 80 °C, 4 h. After trituration and five reprecipitations.
c From GPC (calibrated with PMMA standards). DVB (55%; remainder composed of a mixture of 3- and 4-ethylvinylbenzene). e From
d
thermogravimetry in flowing air: Tonset, onset decomposition temperature; Tmax, maximal decomposition temperature.
Ta ble 3. Cr oss-Lin k ed P olym er -Ion ic Liqu id Com p osite Ma ter ia lsa
composite
appearance
composite
monomer
X-linker (mol %)
Tonset (°C)b
Tmax (°C)b
Fs (g mL-1)c (T/°C)
E
F
G
H
VP
VP
DVB
TRIM
5d
30d
55
gel
255
286
363
262
331, 432
355, 452, 491
469
1.291 (26.1)
1.301 (24.6)
1.282 (25.8)
1.327 (25.9)
waxy solid
chalky solid
waxy solid
100
309, 414, 494
a
b
Polymerization conditions: monomer (1 vol), [omim]Tf2N (2.5 vol), AIBN, 80 °C, 4 h. From thermogravimetry in flowing air: Tonset
,
onset decomposition temperature; Tmax, maximal decomposition temperature(s). c Skeletal density (from helium pycnometry). DVB (55%;
d
remainder composed of a mixture of 3- and 4-ethylvinylbenzene).
the ionic liquids chosen have essentially zero vapor
pressure, it should be possible to remove traces of most
monomers by evacuation at elevated temperatures.
whether the ionic liquid may undergo hydrogen atom
abstraction, a possibility raised recently by Muldoon and
co-workers.13b We would expect abstraction to occur to
a very limited extent, bearing in mind the electrophilic
character of [CMe2CN]•, a possible reactant, and the
relative unreactivity of imidazolium cations toward
hydrogen atom abstraction.13c While free-radical reac-
tions have been successfully investigated in N,N′-
dialkylimidazolium ionic liquids,13d,e our initial obser-
vations suggest that there may be a limit to their
usefulness as media in which to carry out free-radical
polymerizations.
For our first evaluation, we chose 1-vinyl-2-pyrroli-
dinone (VPyrr) and 4-vinylpyridine (VP) as monomers
for thermal free-radical polymerization studies in ionic
liquids. Poly(1-vinyl-2-pyrrolidinone) showed good initial
solubility in [bmim]PF6 and [omim]Tf2N, whereas con-
trasting behavior of poly(4-vinylpyridine) was seen. As
such, these ionic liquids were chosen to investigate
“good” and “poor” solvents/porogens for the formation
of linear and cross-linked VP and VPyrr polymers. The
results of these polymerizations are summarized in
Table 2. In the absence of cross-linker, the free-radical
polymerization of VPyrr in [bmim]PF6 led to a homo-
geneous viscous solution (A) of a linear polymer (A′)
with a weight-average molecular weight of 162 500 g
mol-1. Interestingly, despite repeated reprecipitations,
A series of more promising materials were obtained
when higher cross-linker concentrations were employed
(Table 3). The copolymerization of VP with 5 mol % DVB
in [omim]Tf2N afforded a transparent poly(4-vinylpyri-
dine)-[omim]Tf2N composite gel (E). An opaque gel was
formed when 30 mol % DVB was used (F ). Both of these
gels were completely insoluble in all organic solvents
tested. Soxhlet extraction (methanol, 72 h) afforded
brittle orange solids with gel fractions of 100%. Materi-
als were also produced by free-radical polymerization
of neat 55% DVB (G) and 100% trimethylolpropane
trimethacrylate (TRIM) (H). Both reactions gave rise
to opaque white solid polymer-ionic liquid composite
materials.
1
the H NMR spectrum of the free polymer revealed the
presence of residues believed to arise from [bmim]PF6
(∼4%). The copolymerization of VPyrr (40% v/v) in
[bmim]PF6 with 1 mol % DVB afforded a transparent,
tacky syrup (B). Increasing the cross-linker ratio to 5
mol % DVB led to opaque white tacky syrups (C). The
polymer isolated from B, B′, dissolved completely in
dichloromethane, suggesting that it might be a highly
branched copolymer rather than a cross-linked network.
The polymer isolated from C, C′, formed a viscous, milky
solution in dichloromethane, suggesting the presence of
at least some insoluble microgel particles. None of these
reactions yielded a gelled composite that might be
suitable for uses as a catalytic membrane.
An unusual feature of these composite materials is
that the liquid phase has essentially zero vapor pres-
sure: as such, a number of properties can be measured
for the composites that usually require “dry” samples.
For example, thermogravimetric analysis (Figure 1) was
carried out for the polymer-ionic liquid composite
materials, G and H, as well as for the isolated polymers,
G′ and H′. The results are interesting since it is possible
to check simultaneously for (i) decomposition of the
polymer, (ii) decomposition of the ionic liquid, and (iii)
loss of any volatiles associated with residual, unreacted
monomer. Figure 1a,b suggests that both polymer-IL
composites G and H are somewhat more thermally
stable than the respective polymers after extraction of
the ionic liquid (G′ and H′). The ionic liquid itself
(Figure 1c) is more thermally stable than either the
polymers or the composites. The fact that the DVB-
Polymerization of VP in [omim]Tf2N led to a homo-
geneous viscous solution (D) of a linear polymer (D′)
with a weight-average molecular weight of 71 500 g
mol-1. Repeated reprecipitation of D′ afforded poly(4-
1
vinylpyridine) as a white solid. Once again, H NMR
spectroscopy revealed the presence of residues from the
ionic liquid ([omim]Tf2N; ∼1%), highlighting the dif-
ficulty that may exist in removing all traces of these
nonvolatile solvents from polymeric materials. The
origin and nature of these retained materials are the
subject of a separate study,13a particularly to explore

4552 Snedden et al.
Macromolecules, Vol. 36, No. 12, 2003
Macr opor ou s Cr oss-Lin ked P olym er s Usin g Ion ic
Liqu id s a s P or ogen s. Removal of the ionic liquid from
the IL-polymer composites by Soxhlet extraction was
carried out in order to examine the porous properties
of the cross-linked materials. The pore structure was
analyzed by N2 sorption-desorption, by mercury intru-
sion porosimetry, and by helium pycnometry. The
results of this analysis are shown in Table 4. The two
materials synthesized using VP (4-vinylpyridine) as the
monomer (E′ and F ′) exhibited high bulk densities
(>0.75 g cm-3), low intrusion volumes (<0.3 g cm-3),
and low BET surface areas (<2 m2 g-1). This is indica-
tive of a low degree of permanent porosity in the dry
state, although the data suggest that some dry porosity
existed in the sample prepared at the higher cross-linker
ratio (F ′). Since both samples E′ and F ′ show marked
weight loss and pronounced shrinkage after extraction
of the ionic liquid, it is likely that the degree of cross-
linking in these materials14a,b is insufficiently high to
support the formation of permanent porosity and that
the polymers have some of the properties of gel-type
resins.15 The fact that the resins swell in the presence
of [omim]Tf2N and deswell after Soxhlet extraction is
consistent with the fact that the linear, non-cross-linked
polymer of VP is somewhat soluble in [omim]Tf2N (see
Table 1).
In contrast, the two polymers synthesized from DVB
(G′) and from TRIM (H′) have relatively low bulk
densities (<0.6 g cm-3), higher intrusion volumes (>0.9
cm3 g-1), and much higher BET surface areas (70-370
m2 g-1). Interestingly, polymers G′ and H′, both of which
were prepared using [omim]Tf2N as the porogen, were
quite different in comparison with control polymers (I
and J ) synthesized using toluene as the porogen. In the
case of the DVB polymer, G′, the average pore size of
the material was much greater than the equivalent
polymer synthesized using toluene (I) (see Figure 2a).
F igu r e 1. Thermogravimetry traces (in flowing air) for: (a)
poly(divinylbenzene)s (55 mol %) before (G, bold line) and after
(G′, faint line) removal of porogen, [omim]Tf2N; (b) poly-
(trimethylolpropane trimethacrylate)s (100 mol %) before (H,
bold line) and after (H′, faint line) removal of porogen, [omim]-
Tf2N; (c) [omim]Tf2N.
As a result, the surface area for sample G′ (73.2 m2 g-1
)
was found to be much lower than for sample I (321 m2
g-1).16 In contrast, the poly(TRIM) sample, H′, exhibits
properties that are very similar to the equivalent
material synthesized using toluene as the porogen (J ).
Both samples have high surface areas (∼370 m2 g-1
)
and similar average pore sizes (0.040 and 0.029 µm,
respectively). Figure 2b shows that the pore size dis-
tributions as measured for the two samples by mercury
intrusion porosimetry are very similar. Sample H′, or
similar materials, may be useful for a range of practical
applications (e.g., membrane catalysis) because the pore
size distribution is relatively broad and incorporates
large pores (>100 nm) (to facilitate mass transport) that
based composite G shows a greater mass loss in the
temperature range 20-200 °C is consistent with the fact
that the yield in this reaction (90%) was lower than that
observed for the methacrylate composite, H (yield ∼
100%). The methacrylate composite, H, shows good
thermal stability and very little mass loss up to at least
250 °C, despite the fact that the material has not been
purified or treated in any way after polymerization.
Ta ble 4. Cr oss-Lin k ed P olym er sa
c
cross-linker
(mol %)
yield
(%)b
Fs (g mL-1
(T/°C)
)
Fb
SABET
Vtot
Atot
Dmv
P
d
e
d
d
monomer
solvent
(g mL-1
)
(m2 g-1
)
(mL g-1
)
(m2 g-1
)
(µm)d
(%)d
E′
F ′
G′
I
K
H′
J
VP
VP
5f
30f
55
55
55
100
100
100
[omim]Tf2N
[omim]Tf2N
[omim]Tf2N
toluene
chloroform
[omim]Tf2N
toluene
∼100
1.278 (24.6)
1.141 (26.5)
1.126 (24.7)
1.080 (25.4)
1.075 (27.9)
1.264 (26.7)
1.245 (26.7)
1.219 (26.5)
1.050
0.768
0.242
0.788
0.741
0.559
0.679
0.744
1.6
1.7
73.2
321
4.5
365
374
2.5
0.13g
0.26g
2.62
0.17
0.26
0.89
0.59
0.29
<10g
<10g
63
14
19
50
40
21
89
DVB
DVB
DVB
TRIM
TRIM
TRIM
90
67
59
53.6
0.57
0.056
0.033
0.040
0.029
90.2
19.1h
66.3
∼100
170h
97
140h
62.3
L
chloroform
97
a
b
Polymerization conditions: monomer (1 vol), [omim]Tf2N (2.5 vol), AIBN, 80 °C, 4 h. After Soxhlet-extraction (refluxing MeOH, 72
h). c Skeletal density (from helium pycnometry). From mercury intrusion porosimetry: Fb, bulk density (at 1 psia); Vtot, total intrusion
d
volume; Atot, total pore area; Dmv, median pore diameter by volume; P, porosity. e BET surface area. f DVB (55%; remainder composed of
g
a mixture of 3- and 4-ethylvinylbenzene). Most apparent porosity can be ascribed to interparticulate filling at low mercury pressure.
BET surface area suggests significant proportion of pores <7 nm that are not detected by mercury intrusion porosimetry.
h

Macromolecules, Vol. 36, No. 12, 2003
Cross-Linked Polymer-Ionic Liquid Composites 4553
The differences between these two pairs of samples
may arise from the sensitivity of these systems to the
porogen solvent quality, which influences the mecha-
nism of phase separation, nucleation, aggregation, and
pore formation.15 Similar effects have been documented
for conventional organic solvent porogens. For example,
Peters et al. observed a dramatic porogen effect on the
structure of porous copolymer monoliths synthesized
from ethylene glycol dimethacrylate (EGDMA), butyl
methacrylate, and 2-acrylamido-2-methyl-1-propane-
sulfonic acid (AMPS).17a A ternary porogen mixture was
used consisting of 10 wt % water and 90 wt % of a
mixture of 1-propanol and 1,4-butanediol. The average
pore size in the monoliths was tuned by controlling the
ratio of the two alcohols. Somewhat surprisingly, the
porous structure was found to be extremely sensitive to
the porogenic solvent composition: for example, when
the proportion of propanol was increased from 55 to 63
wt %, the average pore size decreased from ∼5000 to
just 150 nm.17a It was suggested that this could be
attributed to the degree of solvation for the AMPS
moieties, since the effect was more pronounced at higher
AMPS incorporations. In a similar way, the average
pore size in EGDMA/glycidyl methacrylate copolymer
monoliths was controlled by using a cyclohexanol/
dodecanol porogen mixture and varying the ratio of the
two alcohols.17b Xie et al. studied a homologous series
of alcohols (methanol to dodecanol) mixed with dimethyl
sulfoxide as porogens for the preparation of poly-
(acrylamide-co-methylenebis(acrylamide)) monoliths and
found that the average pore size increased significantly
with increasing chain length of the porogenic alcohol.17c
Moreover, we have shown that the pore structure in
permanently porous cross-linked poly(methacrylate)
monoliths using supercritical CO2 as the porogen is
highly sensitive to changes in the CO2 density.18 Each
of these studies illustrates the sensitivity of the struc-
ture of highly cross-linked, permanently porous poly-
acrylates and polymethacrylates to the precise nature
of the porogenic diluent. That the DVB system interacts
quite differently with [omim]Tf2N and toluene, while the
TRIM system gives rise to quite similar materials, is
further illustrative of this phenomenon.
F igu r e 2. Pore size distributions for: (a) Soxhlet-extracted
poly(divinylbenzene)s (55 mol %); polymerizations in [omim]-
Tf2N (G′, bold line) and toluene (I, faint line); (b) Soxhlet-
extracted poly(trimethylolpropane trimethacrylate)s; poly-
merization in [omim]Tf2N (H′, bold line) and toluene (J , faint
line).
are interconnected with much smaller pores for in-
creased surface area.
The morphology of the porous materials was inves-
tigated by electron microscopy (Figures 3 and 4). Sample
G′ exhibited a much coarser structure (Figure 3a) than
that observed for the equivalent polymer synthesized
using toluene as the porogen (I, Figure 3b). In contrast,
the two poly(methacrylate) samples, H′ and J , showed
very similar morphologies.
Con clu sion s
The solubility of polymers in ionic liquids is charac-
terized most notably by mass- and phase-transfer
F igu r e 3. Scanning electron micrographs (×15 000) of: (a) Soxhlet-extracted poly(divinylbenzene)s (55 mol %); polymerization
in (i) [omim]Tf2N (G′) and (ii) toluene (I); (b) Soxhlet-extracted poly(trimethylolpropane trimethacrylate)s; polymerization in (i)
[omim]Tf2N (H′) and (ii) toluene (J ).

4554 Snedden et al.
Macromolecules, Vol. 36, No. 12, 2003
F igu r e 4. Scanning electron micrograph (×15 000) of Soxhlet-extracted poly(trimethylolpropane trimethacrylate)s; polymerization
in (a) [omim]Tf2N (H′) and (b) toluene (K).
quaternization reaction of polymers 14-16 with butyl bromide
in chloroform.1 Polymer 4 was prepared by the ion-exchange
reaction of aqueous solutions of polymer 1 and lithium bis-
(trifluoromethylsulfonyl)amide.1 Polymer 5 was prepared by
the persulfate-initiated free-radical polymerization of 1-meth-
yl-3-vinylimidazolium bis(trifluoromethylsulfonyl)amide in wa-
ter.1 Divinylbenzene (DVB; tech., 55%, mixture of isomers),
trimethylolpropane trimethacrylate (TRIM; tech.), 4-vinylpyr-
idine (VP; 95%), and 1-vinyl-2-pyrrolidinone (VPyrr; 99%) were
obtained from Aldrich and used as supplied. R,R′-Azoisobuty-
ronitrile (AIBN) was purchased from Fisher and recrystallized
(methanol) prior to use. 1-Butyl-3-methylimidazolium hexaflu-
orophosphate ([bmim]PF6; g99%) and 1-ethyl-3-methylimida-
zolium tetrafluoroborate ([emim]BF4; g98%) were obtained
from Solvent Innovation. 1-Methyl-3-octylimidazolium bis-
(trifluoromethylsulfonyl)amide ([omim] Tf2N) was prepared by
the ion-exchange reaction of aqueous solutions of 1-methyl-3-
octylimidazolium chloride and lithium bis(trifluoromethylsul-
fonyl)amide.19
processes. Few general relationships between polymer
solubility and polymer functionality or the nature of the
ionic liquid are evident. The phase behavior of polymer-
ionic liquid mixtures requires more in-depth investiga-
tion.
Free-radical-initiated homopolymerizations in N,N′-
dialkylimidazolium ionic liquids yield polymers which
retain 1-4% of the salt residues (determined from 1H
NMR measurements), despite multiple reprecipitations
from solvents known to dissolve them. This conclusion,
if shown to be general, may limit the usefulness of ionic
liquids as media for free-radical polymerizations.
Ionic liquids are porogenic solvents for the efficient
polymerization of the cross-linking monomers divinyl-
benzene and trimethylolpropane trimethacrylate. Little
unreacted monomer is detected in the high-gel-fraction
ionic liquid-polymer composite by thermogravimetric
analysis (made possible because of the involatility of the
porogenic solvent). Copolymers of 4-vinylpyridine with
5-30% divinylbenzene have limited permanent poros-
ity.
Pore-size distribution of poly(divinylbenzene) using
an ionic liquid as porogen is significantly different from
that observed when toluene is used; that for poly-
(trimethylolpropane trimethacrylate) is similar whether
an ionic liquid or toluene is the porogen.
Direct gelation in ionic liquids has led to ionic liquid-
polymer composite resin materials suitable for evalua-
tion in catalytic membranes.
Acetone (Prolabo, AnR grade), chloroform (Prolabo, AnR
grade), cyclohexane (BDH, GPR grade), dichloromethane
(BDH, GPR grade), ethyl acetate (BDH, GPR grade), methanol
(BDH, AnR grade), and toluene (BDH, AnR grade) were used
as supplied.
Ch a r a cter iza tion . Gel permeation chromatography (GPC)
was used to measure molecular weights and molecular weight
distributions with respect to poly(methyl methacrylate) stan-
dards. GPC experiments were carried out at 80 °C in DMF
(with 0.01 M LiBr to suppress interactions with column
packing) using a Waters 150CV instrument fitted with a
differential refractometer detector (columns: PLgel 2 × mixed
bed-B, 30 cm, 10 µm; flow rate ) 1.0 mL min-1). 1H NMR
spectra were recorded on a Bruker 400 MHz spectrometer;
chemical shifts (ppm) are reported relative to Me4Si. Onset
decomposition temperatures (Tonset) and maximal decomposi-
tion temperatures (Tmax) were determined by thermogravim-
etry in flowing air (heating rate ) 20 °C min-1) on a Perkin-
Elmer TGA 7 thermogravimetric analyzer. Total intrusion
volumes (Vtot), bulk densities (Fb), total pore areas (Atot), median
pore diameters (by volume, Dmv), and porosities (P) were
measured using a Micromeritics AutoPore IV 9500 mercury
intrusion porosimeter. Samples were subjected to a pressure
cycle starting at approximately 0.5 psia, increasing to 60 000
psia in predefined steps to give pore size/pore volume informa-
tion. Skeletal densities (Fs) were measured using a Micromer-
itics AccuPyc 1330 helium pycnometer. BET surface areas
(SABET) were evaluated on a Micromeritics ASAP 2010 surface
area and pore size analyzer; samples were degassed for 2 h at
110 °C prior to analysis. Scanning electron microscopy was
performed on a Hitachi S-2460N scanning electron microscope.
The samples were sputter-coated with approximately 10 nm
gold before analysis.
Exp er im en ta l Section
Ma ter ia ls. Poly(1-vinylimidazole-co-1-vinyl-2-pyrrolidino-
ne), 6 (Luvitec VPI K72W; comonomer content ) 50%), was
kindly donated by BASF. Poly(1-methyl-3-vinylimidazolium
methyl sulfate-co-1-vinylimidazole-co-1-vinyl-2-pyrrolidinone),
7 (Luviquat MS370), poly(1-methyl-3-vinylimidazolium chloride-
co-1-vinylimidazole-co-1-vinyl-2-pyrrolidinone), 8 (Luviquat
HM552), and poly(1-methyl-3-vinylimidazolium methyl sulfate-
co-1-vinylimidazole-co-1-vinyl-2-pyrrolidinone-co-N-vinylcapro-
lactam), 9 (Luviquat HOLD), were obtained from Fluka.
Poly(acrylic acid), 10 (Mw ) 2000 g mol-1), poly(butyl meth-
acrylate), 11 (Mw ) 337 000 g mol-1), poly(ethylene glycol), 12
(Mw ) 10 000 g mol-1), poly(4-vinylpyridine), 14 (Mw ) 160 000
g mol-1), poly(4-vinylpyridine-co-styrene), 15 (styrene content
) 10%), poly(4-vinylpyridine-co-butyl methacrylate), 16 (butyl
methacrylate content ) 10%), and poly(1-vinyl-2-pyrrolidi-
none), 17 (Mw ) 10 000 g mol-1), were obtained from Aldrich.
Poly(styrene), 13 (Mw ) 30 000 g mol-1), was obtained from
Polysciences. Polymers 1-3 were prepared by the respective

Macromolecules, Vol. 36, No. 12, 2003
Cross-Linked Polymer-Ionic Liquid Composites 4555
Gen er a l P r oced u r e for P olym er Solu bility Scr een in g
Exp er im en ts. Polymers 1-17 (25 mg) were individually
dissolved in solvent (0.5 mL: acetone 4, 5; MeOH 1-3, 6-10;
CH2Cl2 11-17), and the resultant polymer solutions (5 wt %)
were injected (glass syringe) into glass vials (2 mL capacity,
fitted with silicone-PTFE seals) containing ionic liquid (0.5
mL; [emim]BF4, [bmim]PF6, or [omim]Tf2N); the resultant
solutions were briefly agitated. A syringe needle was inserted
into each sample vial lid seal (to provide an air bleed), and
the volatiles were removed in vacuo (oil pump) at 50 °C for 3
days. Phase changes in the ionic liquid/polymer solutions were
monitored over the course of 11 ([emim]BF4, [omim]Tf2N) or
15 weeks ([bmim]PF6).
Gen er a l P r oced u r es for P olym er iza tion s a n d P olym er
P u r ifica tion . A degassed (three freeze-pump-thaw cycles)
solution of monomer (VP, VPyrr, DVB, or TRIM; 29% v/v),
DVB (0, 1, 5, or 30 mol %), AIBN (1 mol %), and solvent
([bmim]PF6, [omim]Tf2N, CHCl3, or toluene) in a Schlenk tube
was stirred (magnetic follower) under vacuum at 80 °C for 4
h. Linear polymers were purified by repeated reprecipitation.
Cross-linked polymers were purified by Soxhlet extraction
(refluxing MeOH, 72 h). Purified polymers were dried in vacuo
(50 °C) to constant weight.
P olym er iza tion of VP yr r a n d DVB (0, 1, a n d 5 m ol %).
All polymerization reactions were carried out in [bmim]PF6.
Poly(VPyrr): A (triturated three times with EtOAc and then
reprecipitated five times from CH2Cl2-EtOAc), white solid, Mw
) 162 500 g mol-1, Mn ) 61 000 g mol-1, Mw/Mn ) 2.7 (data
averaged over two runs). δH (CDCl3, 400 MHz): 1.30-2.50 [m,
6H, CH2CH2C(O) + CHCH2], 3.20 (m, 2H, NCH2), and 3.50-
4.00 (m, 1H, CHCH2). Attempts to prepare DVB-cross-linked
poly(VPyrr)-[bmim]PF6 composite materials afforded tacky
syrups (B, C); solubility tests suggested the products to have
no (B′), or very little (C′), cross-linking: B (1 mol % DVB),
transparent yellow tacky syrup (completely soluble in CH2-
Cl2), Tonset 326 °C, Tmax 475 °C; C (5 mol % DVB), opaque white
tacky syrup (produced milky solution in CH2Cl2), Tonset 309 °C,
Tmax 442 °C.
solid (59% conversion), Tonset 331 °C, Tmax 469 °C, skeletal
density ) 1.075 ( 0.014 g mL-1 (27.9 °C), bulk density ) 0.741
g mL-1, SABET ) 4.5 ( 0.1 m2 g-1, total intrusion volume )
0.26 mL g-1, total pore area ) 66.3 m2 g-1, median pore volume
) 0.033 µm, porosity ) 19%.
P olym er iza t ion of TR IM (100 m ol %). Polymerization
reactions were carried out in [omim]Tf2N (H), toluene (J ), or
chloroform (L). Poly(TRIM)-[omim]Tf2N composite material:
H, pale flesh-colored waxy solid, Tonset 262 °C, Tmax 309, 414,
and 494 °C, skeletal density ) 1.327 ( 0.004 g mL-1 (25.9 °C).
Poly(TRIM)s: H′ (Soxhlet extraction of H), white solid (∼100%
conversion), Tonset 263 °C, Tmax 275, 392, and 448 °C, skeletal
density ) 1.264 ( 0.024 g mL-1 (26.7 °C), bulk density ) 0.559
g mL-1, SABET ) 365 ( 2 m2 g-1, total intrusion volume ) 0.89
mL g-1, total pore area ) 170 m2 g-1, median pore volume )
0.040 µm, porosity ) 50%; J , white solid (97% conversion),
Tonset 260 °C, Tmax 272, 397, and 475 °C, skeletal density )
1.245 ( 0.023 g mL-1 (26.7 °C), bulk density ) 0.679 g mL-1
,
SABET ) 374 ( 1 m2 g-1, total intrusion volume ) 0.59 mL
g-1, total pore area ) 140 m2 g-1, median pore volume ) 0.029
µm, porosity ) 40%; L, white solid (97% conversion), Tonset 280
°C, Tmax 354 and 500 °C, skeletal density ) 1.219 ( 0.016 g
mL-1 (26.5 °C), bulk density ) 0.744 g mL-1, SABET ) 2.5 (
0.2 m2 g-1, total intrusion volume ) 0.29 mL g-1, total pore
area ) 62.3 m2 g-1, median pore volume ) 90.2 µm, porosity
) 21%.
Ack n ow led gm en t. The EPSRC is thanked for fi-
nancial support (through Grants GR/NO5765 and GR/
R15597). A.I.C. acknowledges the support of the Royal
Society through the provision of a Research Fellowship.
We thank Dr. Steve Holding of RAPRA Technology for
GPC data and Dr. Peter Beahan (Department of Engi-
neering, University of Liverpool) for SEM images.
Refer en ces a n d Notes
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P olym er iza tion of VP a n d DVB (0, 5, a n d 30 m ol %).
All polymerization reactions were carried out in [omim]Tf2N.
Poly(VP): D′ (triturated three times with EtOAc and then
reprecipitated five times from CH2Cl2-EtOAc), white solid, Mw
) 71 000 g mol-1, Mn ) 36 000 g mol-1, Mw/Mn ) 2.0 (data
averaged over two runs). δH (CDCl3, 400 MHz): 1.44 (m,
backbone; sh at 1.55), 1.84 (m, backbone; sh at 1.90), 6.38 (m,
2H, ArH), and 8.32 (m, 2H, ArH). DVB-cross-linked poly(VP)-
[omim]Tf2N composite materials: E (5 mol % DVB), transpar-
ent orange gel, Tonset 255 °C, Tmax 331 and 432 °C, skeletal
density ) 1.291 ( 0.006 g mL-1 (26.1 °C); F (30 mol % DVB),
pale flesh-colored waxy solid, Tonset 286 °C, Tmax 355, 452, and
491 °C, skeletal density ) 1.301 ( 0.007 g mL-1 (24.6 °C).
DVB-cross-linked poly(VP)s: E′ (Soxhlet extraction of E),
orange solid (∼100% conversion), Tonset 169 °C, Tmax 388 °C,
skeletal density ) 1.278 ( 0.036 g mL-1 (24.6 °C), bulk density
) 1.050 g mL-1, SABET ) 1.6 ( 0.2 m2 g-1, total intrusion
volume ) 0.13 mL g-1, porosity ) <10%; F ′ (Soxhlet extraction
of F ), flesh-colored solid (89% conversion), Tonset 320 °C, Tmax
417 °C, skeletal density ) 1.141 ( 0.002 g mL-1 (26.5 °C), bulk
density ) 0.768 g mL-1, SABET ) 1.7 ( 0.1 m2 g-1, total
intrusion volume ) 0.26 mL g-1, porosity ) <10%.
P olym er iza tion of DVB (55 m ol %). Polymerization
reactions were carried out in [omim]Tf2N (G), toluene (I), or
chloroform (K). Poly(DVB)-[omim]Tf2N composite material:
G, white chalky solid, Tonset 363 °C, Tmax 469 °C, skeletal
density ) 1.282 ( 0.001 g mL-1 (25.8 °C). Poly(DVB)s: G′
(Soxhlet extraction of G), white solid (90% conversion), Tonset
312 °C, Tmax 386 and 568 °C, skeletal density ) 1.126 ( 0.016
g mL-1 (24.7 °C), bulk density ) 0.242 g mL-1, SABET ) 73.2
( 0.7 m2 g-1, total intrusion volume ) 2.62 mL g-1, total pore
area ) 53.6 m2 g-1, median pore volume ) 0.57 µm, porosity
) 63%; I, white solid (67% conversion), Tonset 325 °C, Tmax 408
and 454 °C, skeletal density ) 1.080 ( 0.009 g mL-1 (25.4 °C),
bulk density ) 0.788 g mL-1, SABET ) 321 ( 1 m2 g-1, total
intrusion volume ) 0.17 mL g-1, total pore area ) 19.1 m2
g-1, median pore volume ) 0.056 µm, porosity ) 14%; K, white
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