Water-soluble ionic liquids as novel stabilizers in suspension polymerization reactions: Engineering polymer beads

Article abstract of DOI:10.1002/chem.200600657

Aqueous solutions of ionic liquids have been used as novel and environmentally friendly reaction media to synthesize and "control" the size of different cross-linked polymer beads by suspension polymerization reactions. It was found that the investigated ionic liquids can act as novel stabilizing agents of the suspensions as a result of their surface-active properties. The results have demonstrated that the average size of polymer beads can be varied from the macro- to the nano-scale and their surface area can also be "adjusted" by this synthetic approach. Furthermore, the use of a combination of ionic liquids and water for the synthesis of polymers, the simple isolation of the products formed in this polymerization procedure, as well as the recycling of the continuous medium for further reactions open up possibilities for the development of "new and green" polymerization processes.

Full text of DOI:10.1002/chem.200600657

DOI: 10.1002/chem.200600657
Water-Soluble Ionic Liquids as Novel Stabilizers in Suspension
Polymerization Reactions: Engineering Polymer Beads
[a]
[a]
[b]
CarlosGuerrero-Sanchez, Tina Erdmenger, Petr Sereda, Daan Wouters,[a] and
ˇ
Ulrich S. Schubert*[a]
Abstract: Aqueous solutions of ionic
liquids have been used as novel and en-
vironmentally friendly reaction media
to synthesize and “control” the size of
different cross-linked polymer beads by
suspension polymerization reactions. It
was found that the investigated ionic
liquids can act as novel stabilizing
agents of the suspensions as a result of
their surface-active properties. The re-
sults have demonstrated that the aver-
age size of polymer beads can be
varied from the macro- to the nano-
scale and their surface area can also be
“adjusted” by this synthetic approach.
Furthermore, the use of a combination
of ionic liquids and water for the syn-
thesis of polymers, the simple isolation
of the products formed in this polymer-
ization procedure, as well as the recy-
cling of the continuous medium for fur-
ther reactions open up possibilities for
the development of “new and green”
polymerization processes.
Keywords: combinatorial chemis-
try · cross-linked polymer beads ·
green chemistry · ionic liquids · ion-
ic surfactants · suspension polymeri-
zation
Introduction
sion in which the nature of the suspended drops changes as
the polymerization reaction proceeds. Common suspension
polymerization reactions are carried out in stirred tank reac-
tors with a stabilizing agent (protective colloid) being added
to the aqueous phase to keep the suspended organic drops
from coalescing as they change from the liquid to the solid
state via a sticky phase. The size of the drops is determined
by the balance between the rates of drop break-up and coa-
lescence, which are influenced by several parameters such as
the density and viscosity of the continuous and dispersed
phases, interfacial tension, type and concentration of the
suspending agent, type of impeller, stirring speed, and tem-
perature. The continuous change in the properties of the dis-
persed phase, as a result of the ongoing polymerization reac-
tion, in addition to the aforementioned parameters, contrib-
ute to the size of the drops and to their distribution.[3] Of
major importance for the stability of suspensions are the
type and amount of suspending agent utilized owing to the
fact that it induces electrostatic charges on the surfaces of
suspended monomer–polymer particles which retard their
coalescence. Examples of stabilizing agents used in suspen-
sion polymerization are poly(vinyl alcohol), poly(vinylpyrro-
lidone), hydrophobically modified celluloses, phosphates,[1]
steric stabilizers such as hydrophilic and hydrophobic co-
polymers, soap solutions, water–agarose gels,[2a] and per-
fluoropolyethers for suspension polymerization reactions
carried out in supercritical carbon dioxide.[4] The selection
The heterogenization of polymerization processes is desira-
ble in some cases for an easy separation of the product. Het-
erogeneous polymerization processes, such as water-based
emulsions and suspensions, have been widely adopted for
the production of commodity materials[1] as well as special-
ized polymers for advanced applications.[2] Materials synthe-
sized by heterogeneous polymerization processes are regular
beads with sizes between 50 nm and 3000 mm, depending on
the characteristics of the procedure used (e.g., emulsion,
miniemulsion, precipitation, dispersion, soap-free emulsion,
seeded, or suspension polymerization). Heterogeneous poly-
merization can be defined as a reactive liquid–liquid disper-
[a] Ms. Sc. C. Guerrero-Sanchez, Dipl.-Chem. T. Erdmenger,
Dr. D. Wouters, Prof. Dr. U. S. Schubert
Laboratory of Macromolecular Chemistry and Nanoscience
Eindhoven University of Technology and Dutch Polymer Institute
(DPI)
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
Fax : (+31)40-247-4186
E-mail: u.s.schubert@tue.nl
ˇ
[b] Ms. Sc. P. Sereda
Department of Biomedical Engineering
BIOMIM - Image Analysis and Interpretation
Eindhoven University of Technology
Den Dolech 2, 5612 AZ Eindhoven (The Netherlands)
9036
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 9036 – 9045

FULL PAPER
of the suspending agent will mainly depend on the desired
particle size of the polymer beads, which is determined by
market requirements and/or applications (e.g., bone cement,
packing for chromatographic columns, expandable polystyr-
ene).[1,2] However, the prediction of particle size under spe-
cific polymerization conditions is still not fully understood.
Recently, the use of ionic liquids (ILs) as reaction media
for the synthesis of diverse polymers and composites has led
to a “new environmentally friendly” and novel synthetic ap-
proach with some kinetic advantages over the use of con-
ventional solvents.[5] Despite the developments in polymeri-
zation reactions in ILs, a clear understanding of how to opti-
mize the use of these novel reaction media in specific poly-
mer systems is still lacking, since many factors must be opti-
mized, including polymer isolation, IL recycling, and
toxicity. A better knowledge of all these factors is necessary
before industrial developments may be applied. Regarding
polymer isolation and IL recycling, an environmentally
friendly approach has recently been discussed in the litera-
ture.[6] This approach takes advantage of the poor solubility
of many polymers in most water-soluble ionic liquids
(WSILs), which facilitates polymer isolation and IL recy-
cling, avoiding the use of volatile organic compounds
(VOCs). The polymers obtained by this “new” sort of heter-
ogeneous polymerization process in WSILs resulted in
chalky solids, comparable to those reported in the literature
using a similar synthetic approach in different ILs.[7]
Scheme 1. Molecular structures of A) 1-butyl-3-methylimidazolium chlor-
ide ([C4MIM][Cl]), B) 1-decyl-3-methylimidazolium chloride ([C10MIM]
[Cl]), and C) 1-hexadecyl-3-methylimidazolium chloride ([C16MIM][Cl]).
as stabilizing agents in suspension polymerization reactions,
as well as their influence on the size and surface area of the
synthesized polymer particles. For this purpose, three
WSILs (1-alkyl-3-methylimidazolium chlorides, Scheme 1)
were investigated. Furthermore, it is thought that the non-
polar part of the ILs studied may act as a surfactant helping
to stabilize the suspended organic phase (Figure 1). For this
reason, the effect of variation of the length of the aliphatic
side-chain in the ILs on the stability of the suspensions is
also discussed. For the purposes of this work, the synthesis
of poly(ethylene glycol dimethacrylate–N-vinylimidazole)
cross-linked beads was chosen as the main suspension poly-
merization system to be studied (Scheme 2).[8]
ILs are defined as organic molten salts below 1008C,
therefore their use in suspension polymerization reactions
may be equivalent to the use of common stabilizing agents.
In other words, ILs may induce electrostatic charges on the
surfaces of monomer–polymer particles to keep them from
coalescence, as depicted in Figure 1. Based on this assump-
tion, this contribution describes in detail the use of WSILs
Results and Discussion
This work is a first attempt to elucidate the influence of ILs
used as reaction media and/or stabilizing agents in “suspen-
sion” polymerization reactions.[6,7] The use of WSILs is more
convenient since they facilitate the preparation of an aque-
Figure 1. Schematic representation of a water-soluble ionic liquid acting as a stabilizing agent and reaction medium in suspension polymerization reac-
tions.
Scheme 2. Reaction scheme for the synthesis of poly(ethylene glycol dimethacrylate–N-vinylimidazole) cross-linked polymer beads by free radical sus-
pension polymerization using water-soluble ionic liquids as stabilizing agents. AIBN=2,2’-azobisisobutyronitrile.
Chem. Eur. J. 2006, 12, 9036 – 9045
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9037

U. S. Schubert et al.
Table 1. Experiments performed in the laboratory-scale reactor (Fig-
ure 2A) to evaluate the potential of three different water-soluble ionic
liquids (Scheme 1) as stabilizing agents in the investigated suspension
polymerization reactions.
ous continuous reaction medium, the recovery of the IL for
further polymerization reactions, and the isolation of certain
polymers avoiding the use of VOCs.[6]
On the assumption that ILs may provide electrical charg-
es on the surfaces of suspended monomer–polymer particles
in order to keep them from coalescing during suspension
polymerization, in the first experiments carried out in this
research the concentration and the kind of IL (different ali-
phatic side-chain lengths, Scheme 1) in the aqueous phase of
the polymerization reactions were varied to evaluate their
effect on the stability of the suspensions and on the average
particle size of the resulting polymer beads. These experi-
ments were performed in the experimental set-up shown in
Figure 2A and the results are summarized in Table 1.
Expt
Ionic
liquid
Concentration of
ionic liquid in the
aqueous phase [wt%]
Average
particle
size
1
2
3
4
5
6
7
C
10
70
100
20
40
1
2.7 mm
1.6 mm
0.5 mm
1.7 mm
600 nm
1.9 mm
500 nm
5
the IL/water ratio, but the length of the aliphatic side-chain
of the WSILs (Scheme 1) also has an influence. By combin-
ing these effects (concentration and aliphatic side-chain
length) one can conclude that in order to obtain a specific
average particle size the concentration of the ILs in the
aqueous phase of the suspension has to decrease as the ali-
phatic side-chain length becomes longer. For example, the
average particle size remained between 1.6 and 1.9 mm
when the concentration of the WSIL was 70, 20, and even
1 wt% for 1-butyl-3-methylimidazolium chloride ([C4MIM]
[Cl]), 1-decyl-3-methylimidazolium chloride ([C10MIM][Cl]),
and 1-hexadecyl-3-methylimidazolium chloride ([C16MIM]
[Cl]), respectively (Expts 2, 4, and 6). When using the same
WSIL to “tune” the average size of the particle, its concen-
tration has to be increased in order to reduce the bead size
(i.e., Expts 1–3). Nevertheless, the most remarkable reduc-
tion in particle size (up to the nanometer scale) is mainly in-
fluenced by the aliphatic side-chain length of the WSIL. For
instance, and as shown in Table 1, nanometer-scale particles
could not be synthesized when [C4MIM][Cl] was used, even
at the highest possible concentration (pure IL) in the aque-
ous continuous medium of the suspension (Expt 3). On the
other hand, when [C10MIM][Cl] and [C16MIM][Cl] (Expts 5
and 7) were used as stabilizing agents, at a specific concen-
tration, beads with sizes in the nanometer range were ob-
tained, as revealed by dynamic light scattering (DLS).
To end up with gel-like reaction mixtures, suspended
waxy solids, and/or chalky solids similar those reported in
the literature[6,7] and as shown in Figure 3, a critical concen-
tration of a specific IL in the aqueous phase of the suspen-
sion has to be reached. For instance, for the investigated
WSILs these concentration were estimated to be around
100 wt% for [C4MIM][Cl], 40 wt% for [C10MIM][Cl], and
5% for [C16MIM][Cl] under the reaction conditions of the
experiments reported in Table 1. According to this finding,
it is thought that this critical concentration may establish
the limit of the smallest average particle size that it is possi-
ble to obtain using a specific WSIL as stabilizing agent
under certain experimental conditions (in this case under
the experimental conditions used in the laboratory-scale sus-
pension polymerization reactor shown in Figure 2A). When
the beads are small enough, they can be analyzed by DLS
or by atomic force microscopy (AFM). DLS provides infor-
Figure 2. Overview of the set-ups used to perform the suspension poly-
merization experiments. A) A laboratory-scale suspension polymerization
reactor (experiments summarized in Table 1). B) An automated parallel
synthesizer A-100 minipilot plant (experiments summarized in Table 2).
The results shown in Table 1 reveal that the concentration
of WSILs has a large effect on the as-obtained average par-
ticle size of the synthesized polymer beads. In short, for the
three WSILs investigated, it was found that a low content of
IL in the aqueous continuous medium of the suspension
allows the synthesis of large polymer beads (with average
sizes in the millimeter range), whereas for a high IL content
the particle sizes decrease remarkably, down to the nanome-
ter range in some cases. Simultaneously, it can also be seen
in Table 1 that the average bead size is not only affected by
9038
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 9036 – 9045

Ionic Liquids as Stabilizers of Suspension Polymerizations
FULL PAPER
sion polymerization reactions is just feasible with specific
monomers (i.e. N-vinylimidazole, which has a similar chemi-
cal structure to the WSILs utilized and, therefore, may be
more compatible with the phase containing ILs in the sus-
pension), additional experiments were performed. These ex-
periments were carried out under the same reaction condi-
tions as those utilized in Experiments 1–3, but by using styr-
ene instead of N-vinylimidazole and divinylbenzene instead
of ethylene glycol dimethacrylate as co-monomers. In these
experiments, findings similar to those discussed above for N-
vinylimidazole-containing polymer beads were observed.
More specifically, a gel-like suspension was obtained when
pure [C4MIM][Cl] was used as the continuous medium of
the suspension, an average particle size of 1.5 mm when a
70 wt% aqueous solution of [C4MIM][Cl] was utilized in the
system, and polymer beads with an average size of 2.5 mm
in the case of a 10 wt% aqueous solution of [C4MIM][Cl].
Note that even though the polymer beads synthesized in a
high-content IL aqueous continuous medium were of a
small size, the reaction systems studied in this research
differ from precipitation polymerization reactions. In precip-
itation polymerization reactions, monomers form a continu-
ous phase with the reaction medium (solvent) but the poly-
mers do not and precipitate as small nanoparticles. In the
polymerization reactions investigated in this work, the or-
ganic phase (co-monomers and pore former mixture), in all
cases, is separated from the IL aqueous phase and therefore
they can be classified as suspension polymerization reac-
tions.[2c]
In addition to the fact that the investigated WSILs may
create electrical charges on the surfaces of the suspended
monomer–polymer particles in order to stabilize them, the
surface active properties of ILs, which have already been
discussed in the literature,[10] may also contribute to the sta-
bilization of the system. These studies mainly report on the
surface tension properties of some ILs and their aqueous
solutions, as well as their dependence on temperature.[10a–f]
Moreover, as an application of the surface active properties
of ILs, WSILs have been utilized as novel templates in the
preparation of highly ordered monolithic super-micro-
porous lamellar silica materials by the nanocasting techni-
que.[10g] From those studies, one can conclude that some ILs
exhibit surface active properties and aggregation behavior
in aqueous solutions. For this reason, surface tension meas-
urements of the aqueous solutions of the ILs given in
Scheme 1 were performed and the results are shown in
Figure 5. It was found that the critical concentration of ag-
gregation was 0.17 and 0.03 wt% for [C10MIM][Cl] and
[C16MIM][Cl], respectively. In the case of [C4MIM][Cl] it
was difficult to establish a value for the critical concentra-
tion of aggregation since the surface tension of the aqueous
solutions decreases continuously as the concentration of the
ionic liquid increases; this means a well-defined plateau was
not reached. The value of the critical concentration of ag-
gregation for [C16MIM][Cl] was the lowest of the three
WSILs; this was expected since it has the longest aliphatic
side-chain. However, the lowest surface tension value ob-
Figure 3. Impression of the gel-like reaction mixtures obtained after sus-
pension polymerization in a high ionic liquid content aqueous phase
(Expts 3, 5, and 7).
mation on the average diameter of the cross-linked polymer
beads in aqueous solution, when they are supposed to be in
a swollen state due to the presence of the solvent (water), as
reported in the literature for this kind of material.[8] AFM
measures the diameter of the beads in dry conditions (non-
swollen state).[9] For instance, Figure 4 shows an AFM
image utilized to determine the average size of the polymer
beads of Experiment 11 in dry conditions; a value of 100 nm
was obtained, whereas in the swollen state (DLS measure-
ments) the average size was 250 nm.
To investigate if the phenomena of “tuning” the size of
the beads by utilizing WSILs as stabilizing agents in suspen-
Figure 4. AFMimage of polymer beads formed in Experiment 11 in dry
conditions used to determine the average size of the beads.
Chem. Eur. J. 2006, 12, 9036 – 9045
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9039

U. S. Schubert et al.
ution). For instance, the interfacial tension between the or-
ganic phase used for the polymerization reaction and pure
water had a value of 4.5 mNmꢀ1, whereas the interface of
the organic mixture and pure [C4MIM][Cl] (Expt 3) had a
value of 1.8 mNmꢀ1 and for the interface of Experiment 1
the value was 3.9 mNmꢀ1. These results show that the inter-
facial tension indeed decreases as the content of IL in the
aqueous phase increases, suggesting the possibility of a
spontaneous emulsification process in the system allowing
the synthesis of small-sized polymer beads.
After considering the measurements obtained for both
surface tension (Figure 5) and particle size (Table 1), it is
clear that the aliphatic side-chain length and not only the
concentration of the WSILs plays an influential role in the
stabilization of the monomer–polymer particles in suspen-
sion polymerization reactions. According to Figure 5, the ex-
periments reported in Table 1 were performed above the
critical concentration of aggregation of ILs (at least in the
cases of [C10MIM][Cl] and [C16MIM][Cl]) and therefore in a
low surface tension aqueous phase. Similar to the case of
emulsion polymerization, the availability of hydrophobic ali-
phatic chains in the system promotes the creation of a great-
er interfacial area between the organic phase (monomer–
polymer particles) and the aqueous IL solutions. Therefore,
the synthesis of polymer beads with smaller average particle
size is possible.
This effect may be similar in common suspension poly-
merization reactions in which poly(vinyl alcohol) (PVA) and
other water-soluble polymers are used as protective colloids.
In these cases PVA supplies covalently linked carbon chains
to the interface of the immiscible phases, avoiding coales-
cence of the monomer–polymer particles formed. However,
one of the disadvantages of using water-soluble polymers as
stabilizing agents in suspension polymerizations is the fact
that these materials are difficult to dissolve in water at room
temperature. Added to this is the limited understanding of
how the average size of the polymer beads is affected by the
use of conventional active surface modifiers. To investigate
whether the use of conventional ionic surfactants in suspen-
sion polymerization reactions has the same effects as those
obtained with the investigated WSILs, experiments were
performed under similar reaction conditions to those report-
ed in Table 1. The main difference in these experiments is
the use of sodium dodecyl sulfate (SDS) (a well-known
ionic surfactant) as the stabilizing agent instead of WSILs.
Four different concentrations of SDS (0.1, 0.33, 1, and
5 wt%), one below and three above the critical micelle con-
centration value (0.23 wt%),[11] were used for these experi-
ments. The average sizes of the polymer beads obtained
from these experiments were around 0.8 mm in all cases.
Therefore, the use of SDS does not allow the average size of
the beads to be “tuned” as the WSILs of Scheme 1 do (at
least for the reaction systems investigated).
Figure 5. Surface tension measurements of the aqueous solutions of the
ionic liquids of Scheme 1. A) [C4MIM][Cl], B) [C10MIM][Cl], and C)
[C16MIM][Cl].
tained, once the critical concentration of aggregation was
reached, was 27 mNmꢀ1 for [C10MIM][Cl] in comparison to
38 mNmꢀ1 for [C16MIM][Cl]. In the case of [C4MIM][Cl]
the lowest possible surface tension was 46 mNmꢀ1 (pure
IL).
An important aspect that may help us to understand the
role of the WSILs in “tuning” the size of the synthesized
polymer beads is the interfacial tension between the organic
(co-monomers) and the IL aqueous phases. In polymeriza-
tion reactions in which spontaneous emulsification occurs
(e.g., microemulsion),[5c] the interfacial tension between the
monomer and the aqueous continuous medium phase ap-
proaches zero due to the presence of surfactants that allow
the synthesis of polymer beads in the nanometer range. In
this context, interfacial tensions in some of the experiments
reported in Table 1 were qualitatively estimated using the
Du Nouy ring method (by considering the value of the inter-
facial tension recorded by the tensiometer at the maximum
value of the force exerted on the ring as it passes through
the interface of the organic mixture and the IL aqueous sol-
Furthermore, in the cases in which beads with sizes in the
millimeter scale were synthesized, the polymer can be easily
isolated from the reaction medium by simple decanting and/
or filtration and therefore the IL aqueous solutions can be
9040
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 9036 – 9045

Ionic Liquids as Stabilizers of Suspension Polymerizations
FULL PAPER
reused to carry out new polymerization reactions. This ap-
proach was investigated by reusing the IL aqueous solution
of Experiment 6 to perform more polymerization cycles
under the same reaction conditions. In this case, after three
polymerization cycles the average size of the beads (2 mm)
was comparable to that of the first reaction cycle (1.9 mm).
These results suggest that the IL aqueous solutions can be
recovered and reused several times to synthesize new mate-
rials with comparable properties.
temperature) on the average particle size of polymer beads
synthesized by suspension polymerization have been report-
ed in the literature.[3]
It is known that the agitation speed has an important
effect on particle size in suspension polymerization reac-
tions.[3] From the experiments performed in this work using
the two different experimental set-ups shown in Figure 2, it
was also found that the type of agitation influences the par-
ticle size and stability of the suspensions. For instance, in the
experiments reported in Table 1 (magnetic agitation was ap-
plied to the laboratory-scale reactor shown in Figure 2A),
larger average particle sizes were obtained in comparison to
those achieved in the experiments reported in Table 2 (note
that the experiments reported in Table 1 were aimed at in-
vestigating the WSILs of Scheme 1 as stabilizing agents in
suspension polymerization reactions under fixed reaction
conditions). On the other hand, in the experiments reported
in Table 2 (mechanical agitation was used in the experimen-
tal set-up shown in Figure 2B), polymer beads with sizes in
the nanometer range could be obtained by using a low con-
centration of stabilizing agent ([C4MIM][Cl]). This was the
case for Experiment 11, which showed an average particle
size of 250 nm in solution, as revealed by DLS, and around
100 nm, as determined by AFMin dry conditions (Figure 4).
It has been reported that polymers containing imidazole
groups are able to bind different transition metals to their
surface, finding applications in the removal of transition-
metal ions from waste streams[8,13] and as supports for palla-
dium-catalyzed reactions.[14] An important property of these
polymeric materials is their surface area, since this factor
determines to a large extent the amount of chemical species
that can be loaded. Other polymer architectures with large
surface areas and an intrinsic porosity have also found appli-
cations as replacements of inorganic materials such as zeo-
lites and/or activated carbon used as heterogeneous catalytic
supports[15] and more recently
The simultaneous effects of other reaction parameters on
the stability of the suspensions and on the average size of
polymer beads were briefly investigated by performing addi-
tional polymerization reactions in an automated parallel
synthesizer[12] (Figure 2B). The reaction conditions and the
results of these experiments are summarized in Table 2. Ac-
cording to these results, qualitatively, it was found that the
content of IL in the aqueous phase of the suspension (x1),
acting as a stabilizing agent, is inversely proportional to the
average particle size, as already discussed above. The reac-
tion temperature (x2) has an effect similar to x1; higher reac-
tion temperatures lead to particles with a smaller average
size. The presence of N-vinylimidazole in the polymer beads
(x3) also showed effects similar to x1 and x2; the higher the
content of N-vinylimidazole, the smaller the average particle
size. Finally, the content of toluene (x4) in the formulations
is thought to have almost no influence on the stability of the
suspensions and/or on the size of the particles, but mainly
on the formation of pores in the beads. For example, when
toluene was not included in the formulation, the particles
obtained were translucent. On the other hand, when toluene
was used, the particles obtained were white and opaque (the
presence of pores may disperse the light through the beads
turning them opaque). Similar effects of some of the reac-
tion parameters investigated in the experiments reported in
Table 2 (concentration of the stabilizing agent and reaction
in hydrogen storage.[16] Even
though the incorporation of a
pore former (toluene in this
work) is essential to promote
the synthesis of polymer beads
with large surface areas,[8] it
has been found in this study
Table 2. Experimental design performed in the automated parallel synthesizer (Figure 2B) for the synthesis of
cross-linked beads by suspension polymerization using a water-soluble ionic liquid as stabilizing agent.[a],[b]
Expt
x1
x2
x3
x4
Remarks
Average particle size
8
9
ꢀ
+
ꢀ
ꢀ
ꢀ
+
ꢀ
+
ꢀ
ꢀ
+
+
+
+
ꢀ
+
ꢀ
ꢀ
+
ꢀ
ꢀ
+
ꢀ
ꢀ
+
+
ꢀ
+
+
ꢀ
+
+
ꢀ
ꢀ
ꢀ
+
ꢀ
ꢀ
+
ꢀ
+
ꢀ
+
+
ꢀ
+
+
+
ꢀ
ꢀ
ꢀ
ꢀ
+
ꢀ
+
+
ꢀ
+
ꢀ
ꢀ
+
+
+
+
collapsed translucent beads
collapsed translucent beads
stable suspension, translucent beads
white stable suspension
large (3 mm)
medium (1.5 mm)
medium (1.4 mm)
nanometer scale (250 nm)
large (2.8 mm)
small (0.65 mm)
small (0.8 mm)
nanometer scale (600 nm)
nanometer scale (525 nm)
small (0.7 mm)
nanometer scale (405 nm)
n.a.
nanometer scale (650 nm)
nanometer scale (400 nm)
small (0.75 mm)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
that the incorporation of
a
collapsed white beads
WSIL ([C4MIM][Cl]) into the
aqueous continuous medium of
the suspension also has an im-
portant effect on the creation
of surface area in the particles,
as shown in Table 3. The re-
sults given in Table 3 show the
surface area values (as deter-
mined by nitrogen gas adsorp-
tion) for different polymer
beads synthesized in the exper-
imental set-up shown in Fig-
ure 2A and under reaction
stable suspension, translucent beads
stable suspension, white beads
cloudy stable suspension
white stable suspension
stable suspension, white beads
yellow stable suspension
yellow swollen gel-like
white stable suspension
white stable suspension
stable suspension, white beads
white stable suspension
small (0.35 mm)
[a] Mechanical stirring speed was 400 rpm. [b] Four reaction parameters were analyzed as follows: x1 =content
of ionic liquid [C4MIM][Cl] in the aqueous phase (ꢀ=20 wt%, +=70 wt%), x2 =reaction temperature (ꢀ=
758C, +=908C), x3 =content of N-vinylimidazole in the polymer beads (ꢀ=50 mol%, +=87.5 mol%), and
x4 =content of porogenic agent (toluene) in the organic phase (ꢀ=0% vol., +=30% vol.).
Chem. Eur. J. 2006, 12, 9036 – 9045
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9041

U. S. Schubert et al.
Table 3. Surface area properties, as determined by the nitrogen gas adsorption technique, of cross-linked poly-
mer beads.[a]
Table 3 also reveal a strong in-
fluence of the content of
[C4MIM][Cl] on the formation
of surface area. First, when
both [C4MIM][Cl] and pore
former (toluene) are included
in the synthetic formulation of
the beads (Expts. 24–28) and
the amount of toluene remains
constant, the creation of sur-
face area is strongly related to
the content of IL in the aque-
ous continuous medium of the
suspension and therefore to
Expt Content of [C4MIM][Cl]
Average
Langmuir
surface
Porosity
Pore
shape
BET average pore
diameter [nm]
in the aqueous phase [wt%] particle
size [mm] area [m2 gꢀ1
]
24
25
26
27
28
29
30
100
70
50
30
10
0.5
1.6
1.9
2.4
2.7
2.7
1.3
57.6
191.9
208.8
268.8
303.8
158.8
1.7
nonporous
mesopores ink bottle 3.8
mesopores ink bottle 3.7
mesopores slit
mesopores slit
mesopores cylindrical 7.7
nonporous
–
–
4.4
4.6
0 (908C)
0 (908C, no toluene)
–
–
[a] The cross-linked polymer beads were synthesized under similar reaction conditions to those used in experi-
ments reported in Table 1.
conditions similar to those used in the experiments reported
in Table 1. The pores have been classified according to the
shape of the adsorption isotherms[17] obtained for each
sample and according to the IUPAC classification. Figure 6
shows nitrogen adsorption/desorption isotherms for some of
the experiments reported in Table 3. According to the shape
of the isotherms, the materials can be classified with regard
to pore shape.[17] The results in Table 3 show that beads with
a “large” average particle size (>1.5 mm) can be classified
as good porous materials. More important, the results in
the size of the beads obtained, as explained before. These
results reveal that a low content of WSIL in the reaction
medium yields particles with large sizes and surface areas
whilst a high content of WSIL promotes the synthesis of
beads with small sizes and surface areas. Second, the inclu-
sion of a pore former has been found to be essential to
create surface area in the beads, as revealed by additional
experiments in which toluene was not used, and even in
presence of WSILs, nonporous materials were obtained. In
general, the absence of WSILs or stabilizing agents in the
reaction system leads to unstable systems or coalescence of
the beads in suspension polymerization reactions. However,
to investigate the pure effect of toluene as a pore former
and in the absence of any stabilizers, Experiments 29 and 30
(Table 3) were performed. These suspension polymerization
reactions were carried out at 908C owing to the fact that at
lower reaction temperatures the suspension may be unstable
and show coalescence. From these results, it is again con-
firmed that the absence of toluene yields nonporous materi-
als with a small surface area (Expt 30). In the case of Ex-
periment 29, beads with a relatively large surface area were
formed as expected with the inclusion of toluene. However,
when comparing results of Experiments 28 and 29, it is clear
that the combination of pore former and a relatively small
amount of WSILs (10 wt%) in the reaction mixture pro-
motes a more efficient formation of surface area (around
twice as large) for beads with similar average particle sizes
(2.7 mm). The morphology of some of the synthesized poly-
mer beads was also investigated by scanning electron micro-
scopy (SEM) to gain an insight into the formation of pores.
Images of the morphologies obtained are shown in Figure 7,
which shows the presence of pores in the beads. Moreover,
from the results in Table 3 and the shape of the adsorption/
desorption isotherms of Figure 6 it can also be observed that
the shape of the pores formed in the beads can be modified
by varying the concentration of the WSIL during synthesis.
From these findings one can conclude that, at least for the
investigated reaction system, the combination of a pore
former and the use of a WSIL as a stabilizing agent in sus-
pension polymerization reactions may allow the surface area
of polymer beads to be “tuned” during their synthesis.
Figure 6. Shape of the nitrogen adsorption/desorption isotherms at 77 K
of some of the experiments reported in Table 3. The materials can be
classified according to the shape of the adsorption isotherms. A) Expt 24,
nonporous beads, B) Expt 26, mesoporous beads with ink-bottle-like
pores, C) Expt 27, mesoporous beads with slit-shape pores, and
D) Expt 29, mesoporous beads with cylindrical pores.
9042
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 9036 – 9045

Ionic Liquids as Stabilizers of Suspension Polymerizations
FULL PAPER
Figure 7. Morphology of some of the synthesized polymer beads as investigated by SEM. Images of polymer beads A) and B) obtained from Expt 28 and
C) obtained from Expt 25.
washed several times with ethyl acetate before drying under vacuum at
408C, and [C4MIM][Cl] was just dried under vacuum at 408C since the
yield of this reaction was close to 100%. The H NMR spectra of the syn-
Conclusion
1
thesized WSILs confirmed their chemical structure (Scheme 1) and the
purity of the materials. The long aliphatic chain WSIL, [C16MIM][Cl],
was obtained as a white powder at room temperature. Even though its
This contribution shows, for the first time, that WSILs have
potential as stabilizing agents in the synthesis of polymers
by suspension polymerization as a result of their surface
active properties. This approach allows “tuning” of the aver-
age particle size (from the macro- to the nanoscale) of the
synthesized polymer beads by adjusting the concentration
and the aliphatic side-chain length of the investigated ILs in
the aqueous phase of the suspension. Furthermore, the sur-
face area of the beads can also be “adjusted” with a combi-
nation of a pore former and a WSIL used as a stabilizing
agent during their synthesis. Moreover, the IL aqueous solu-
tions used could be recovered and reused to carry out fur-
ther reactions in a completely “efficient” synthetic ap-
proach.
melting point was found to be 418C, as revealed by DSC measurements,
it can still be classified as a room-temperature IL (melting point below
1008C). The melting point of [C10MIM][Cl], found by DSC, was around
48C. Although [C4MIM][Cl] was obtained as a liquid substance at room
temperature from the microwave-assisted synthesis, after some days it
started to crystallize; its melting point was determined to be around
408C. The rest of the experiments in this work involving [C4MIM][Cl]
were performed using the liquid substance before crystallization (as ob-
tained from the synthesis and purification procedures).
Synthesis of cross-linked polymer beads by suspension polymerization in
IL aqueousoslutions : Typical suspension polymerization experiments
were performed using a similar formulation to that reported in the litera-
ture for the synthesis of poly(ethylene glycol dimethacrylate–N-vinylimi-
dazole) cross-linked beads.[8] The main difference between this latter syn-
thetic approach and the one used here lies in the preparation of the
aqueous phase. In order to study the stability of the suspensions using
WSILs as stabilizing agents, the aqueous phases were prepared by dis-
solving predetermined amounts of the corresponding IL [instead of pol-
y(vinyl alcohol)][8] in purified water (demineralized). The organic phase
was formulated (on the basis of 50 g of aqueous phase) with ethylene
glycol dimethacrylate (6 mL, 32 mmol), toluene (4 mL, 38 mmol) (pore
former), N-vinylimidazole (3 mL, 33 mmol), and 2,2’-azobisisobutyroni-
trile (100 mg, 0.6 mmol). The aqueous phase was placed in the corre-
sponding experimental set-up (as described below) at the desired reac-
tion temperature. Subsequently, the organic phase was dispersed into the
aqueous medium by vigorous stirring. This moment was considered to be
the beginning of the polymerization process, which was conducted for
0.5 h to obtain almost 100% yield of polymer beads based on the initial
amount of monomer (as determined by gravimetry). The beads obtained
were separated from the aqueous phase by filtration, washed with water
and ethanol to remove unreacted monomer and pore former (toluene),
dried, and stored at room temperature.
Experimental Section
Materials: 1-Methylimidazole and N-vinylimidazole (Aldrich) were distil-
led under reduced pressure and stored at room temperature prior to use.
Ethylene glycol dimethacrylate (Aldrich) was purified by passing it
through active alumina (Merck) and was stored at 48C prior to use. All
other materials were used as received.
WSILs by microwave-assisted synthesis: WSILs were synthesized by mi-
crowave heating in a similar way to those methods reported in the litera-
ture,[18] by reacting 1-methylimidazole with a molar excess (1.1 mol) of
the corresponding chloroalkane (1-chlorobutane, 1-chlorodecane, or 1-
chlorohexadecane; Aldrich). The reactions were performed in sealed re-
action vials (10 mL) specially designed for the single-mode microwave
system Emrys Liberator (Biotage, formerly Personal Chemistry). These
vials were filled with the reagents, sealed, and degassed with argon for
2 min. The reaction volume for every batch was 5 mL (per vial). The re-
agents were mixed during the microwave-assisted reaction by using a
magnetic stirring system included in the microwave apparatus. Optimal
reaction conditions for the synthesis of these ionic liquids in the afore-
mentioned microwave platform were found to be 1708C and a reaction
time of 6–7 min under a high absorption reaction mode (150 Watts). At
the end of the reaction time the vials were decapped and any unreacted
material (upper organic phase) was decanted. Note that under the reac-
tion conditions described, the lower organic phases in the reaction vials
(WSILs) were almost colorless liquids in the case of [C4MIM][Cl] and
[C10MIM][Cl], and a white solid in the case of [C16MIM][Cl]. The synthe-
sized WSILs (Scheme 1) were further purified to remove traces of un-
reacted starting materials as follows: [C16MIM][Cl] was recrystallized
from acetone and dried under vacuum at 408C, [C10MIM][Cl] was
To determine the stability of the suspension polymerization reactions in
the presence of WSILs, two different experimental set-ups were em-
ployed. A typical laboratory-scale reactor (Figure 2A) consisting of a
round-bottomed flask (50 mL) placed in an oil bath (808C) and stirred
(400 rpm) with a cylindrical PTFE-coated magnetic stirring bar. The ex-
perimental scale for this laboratory set-up was 25 mL based on the aque-
ous phase. This set-up was utilized to evaluate the potential of WSILs as
stabilizing agents in the suspension polymerization system described and
therefore the rest of the reaction conditions remained unmodified (under
the previously mentioned conditions). The reaction conditions utilized in
the suspension polymerization experiments performed with the experi-
mental set-up of Figure 2A are summarized in Table 1.
A second experimental set-up consisting of an automated parallel synthe-
sizer[12] (A-100 minipilot plant, Chemspeed Technologies) (Figure 2B)
Chem. Eur. J. 2006, 12, 9036 – 9045
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9043

U. S. Schubert et al.
was used to perform a qualitative study involving the variation of more
reaction conditions simultaneously. In this way, the influence of reaction
temperature and the content of one of the utilized WSILs ([C4MIM]
[Cl]), co-monomers, and pore former on the stability of the suspension
polymerization reactions could be qualitatively investigated. The advant-
age of performing experiments in parallel (using the same type of reac-
tors, the same stock solutions of reagents, etc.) is that the reaction condi-
tions can be varied systematically and that the results are, therefore,
easily comparable. The main characteristic of this experimental set-up is
the mechanical agitation that can be afforded to the reaction systems,
which is, next to the stabilizing agent, one of the most important varia-
bles influencing the stability of monomer–polymer drops in suspension
polymerization reactions. An anchor-type impeller and an agitation
speed of 400 rpm were found to be suitable for performing the required
suspension polymerization experiments. The experimental scale for this
set-up was 50 mL based on the aqueous phase. The reaction conditions of
the suspension polymerization experiments carried out in this synthesizer
are summarized in Table 2.
the image processing toolbox incorporated into the MATLAB software
to obtain the average particle size and particle-size distribution (see the
results given in Table 1 and Table 2). Figure 8 shows an overview of this
characterization technique for two selected experiments reported in
Table 1 and Table 2.
Some of the experiments reported in Table 1 and Table 2 yielded parti-
cles with average sizes in the nanometer range (Expts 5, 7, 11, 15, 16, 18,
20, and 21). In these cases, the materials obtained had a similar appear-
ance to those synthesized in other pure ionic liquids (gel, waxy solid,
and/or chalky solid compounds).[6,7] In all the experiments described, the
composites were dispersed in water (~0.1 gLꢀ1) under vigorous stirring
until stable emulsion-like dispersions were obtained. The resulting disper-
sions were analyzed by DLS; the average particle sizes are reported in
Table 1 and Table 2. DLS experiments were performed at 258C at 908 on
a Malvern 4700 DLS Particle Size Analyzer apparatus equipped with a
488 nm laser. The values of the refractive index and viscosity of water
used were 1.331 and 0.89 cPoise (1 cPoise’0.001 Kgmꢀ1 sꢀ1), respectively.
Imaging of one selected sample (Expt 11) in dry conditions was per-
formed by AFM[9], as shown in Figure 4. For this measurement, a sample
from Experiment 11 used during the DLS measurements was additionally
diluted with water (100-fold) and spin-cast (to obtain near monolayer
coverage) onto a clean silicon wafer substrate. Imaging was performed in
intermittent contact mode with a Multimode SPM (Digital Instruments,
Characterization techniques: 1H NMR spectra of the synthesized WSILs
were recorded at room temperature on a Varian Gemini 300 spectrome-
ter using deuteriated dimethyl sulfoxide (Cambridge Isotopes Laborato-
ries).
The melting points of the synthesized WSILs were determined by DSC
on a Netzsch DSC 204 F1 instrument calibrated with indium. Multiple
heating and cooling scans (20, 40, and 108Cminꢀ1 scan rates) were gener-
ated over a temperature range of ꢀ100 to 2508C using nitrogen as the
purge gas. The melting point temperatures were noted at the maxima of
the heat capacity peaks during the heating scans.
Santa Barbara, CA) using NSC36-type tips (~1 Nmꢀ1
, Mikromasch,
Spain). The heights of the beads were determined from histograms after
zero-order leveling of the images.[9]
Surface and interfacial tension parameters were measured with a Krüss
K10T tensiometer with a Wilhemy plate and by the Du Nouy ring
method, respectively. From the surface tension measurements the critical
micellar concentrations were determined.
Surface area, pore size, and pore-size distribution of the synthesized poly-
mer beads were determined using nitrogen adsorption (TriStar 3000 ap-
paratus of Micromeritics) at 77 K. Samples were dried under vacuum at
408C for three days just prior to measurement.
Particle sizes and particle-size distributions were determined using digital
image analysis techniques.[19] Images of large- and medium-sized particles
(>1 mm) (for the particle sizes see Table 1 and Table 2) were obtained
with a conventional digital camera. Images of small-sized particles (<
1 mm) (for the particle sizes see Table 2) were obtained with an optical
microscope (Optem, zoom 125) equipped with a digital camera (Sony
Exwave HAD). In both cases, the images obtained were analyzed using
The surface morphology and porosity of the polymer beads was exam-
ined using scanning electron microscopy (SEM) (JEOL JSM-840 A). The
samples were initially dried under vacuum at 408C for three days before
being analyzed. Dried beads were mounted on a SEMsample holder,
sputter coated with gold for 2 min, and scanned at the desired magnifica-
tion.
Figure 8. Overview of the determination of the average particle size and particle-size distribution of the synthesized cross-linked polymer beads by a digi-
tal image analysis technique. A) Digital images of large-sized particles (Expt 1) and their respective particle-size distribution, as obtained after the corre-
sponding statistical analysis. B) Digital images of small-sized particles (Expt 22) obtained with an optical microscope.
9044
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 9036 – 9045

Ionic Liquids as Stabilizers of Suspension Polymerizations
FULL PAPER
[8] A. Kara, L. Uzun, N. Besirli, A. Denizli, J. Hazard. Mater. B 2004,
106, 93–99.
Acknowledgements
[9] C. Guerrero-Sanchez, D. Wouters, C. A. Fustin, J. F. Gohy, B. G. G.
Lohmeijer, U. S. Schubert, Macromolecules 2005, 38, 10185–10191.
[10] a) J. Bowers, C. P. Butts, P. J. Martin, M. C. Vergara-Gutierrez,
Langmuir 2004, 20, 2191–2198; b) G. Law, P. R. Watson, Langmuir
2001, 17, 6138–6141; c) J. Sung, Y. Jeon, D. Kim, T. Iwahashi, T.
Iimori, K. Seki, Y. Ouchi, Chem. Phys. Lett. 2005, 406, 495–500;
d) M. Gonzale-Melchor, F. Bresme, J. Alejandre, J. Chem. Phys.
2005, 122, 104710/1–104710/8; e) A. Bagno, C. Butts, C. Chiappe, F.
D’Amico, J. C. D. Lord, D. Pieraccini, F. Rastrelli, Org. Biomol.
Chem. 2005, 3, 1624–1630; f) S. L. Zang, Q. G. Zhang, M. Huang, B.
Wang, J. Z. Yang, Fluid Phase Equilib. 2005, 230, 192–196; g) Y.
Zhou, M. Antonietti, Chem. Mater. 2004, 16, 544–550.
This research forms part of the research program of the Dutch Polymer
Institute (DPI), project No. 401. The authors thank the Fonds der Chemi-
schen Industrie for financial support and appreciate the collaboration of
Chemspeed Technologies, Solvent Innovation, and Merck during the real-
ization of this work. The authors also thank Peter J. L. Lipman and Moni-
que A. Kirkels for their assistance during the gas adsorption measure-
ments. The authors appreciate the cooperation of the group of Prof. Dr.
Bert G. de With (Laboratory of Materials and Interface Chemistry of the
Eindhoven University of Technology), especially Srdjan Kisin and Nick
Lousberg, for their help with scanning electron microscopy, and Dr. Mar-
shall Ming for his assistance during the surface tension measurements.
[11] F. H. Quina, P. M. Nassar, J. B. S. Bonilha, B. L. Bales, J. Chem.
Phys. 1995, 102-103, 17028–17031.
[12] C. Guerrero-Sanchez, R. M. Paulus, M. W. M. Fijten, M. J. de la M-
ar, R. Hoogenboom, U. S. Schubert, Appl. Surf. Sci. 2006, 252,
2555–2561.
[1] a) Modern styrenic polymers (Eds.: J. Scheirs, D. Priddy), Wiley,
West Sussex, 2003, and references therein; b) T. Bilgic, M. Karali,
O. T. Savasci, Angew. Makromol. Chem. 1993, 213, 33–42.
[2] a) P. Giusti, L Lazzeri, G. Pizzirani, G. Polacco, C. Rizzo, J. Mater.
Sci. Mater. Electron. 1994, 5, 858–861; b) Y. Hatate, Y. Uemura, K.
Ijichi, Y. Kato, T. Hano, Y. Baba, Y. Kawano, J. Chem. Eng. Jpn.
1995, 28, 656–659; c) H. Kawaguchi, Prog. Polym. Sci. 2000, 25,
1171–1210; d) D. Hoffmann, K. Landfester, M. Antonietti, Magne-
tohydrodynamics 2001, 37, 217–221; e) J. H. Yeum, Q. Sun, Y.
Deng, Macromol. Mater. Eng. 2005, 290, 78–84.
[3] F. Jahanzad, S. Sajjadi, B. W. Brooks, Ind. Eng. Chem. Res. 2005, 44,
4112–4119.
[4] H. M. Woods, C. Nouvel, P. Licence, D. J. Irvine, S. M. Howdle,
Macromolecules 2005, 38, 3271–3282.
[5] a) P. Kubisa, Prog. Polym. Sci. 2004, 29, 3–12, and references there-
in; b) Ionic liquids in polymer systems: solvents, additives, and novel
applications (Eds.: C. S. Brazel, R. D. Rogers), ACS Symp. Ser. 913,
American Chemical Society, Washington DC, 2005, and references
therein; c) F. Yan, J. Texter, Chem. Commun. 2006, 2696–2698;
d) C. Guerrero-Sanchez, R. Hoogenboom, U. S. Schubert, Chem.
Commun. 2006, DOI: 10.1039/b608364a.
[13] N. Pekel, O. Güven, Polym. Bull. 2004, 60, 307–314.
ˇ
[14] C. Guerrero-Sanchez, C. Abeln, P. Sereda, S. Kisin, B. G. de With,
U. S. Schubert, unpublished results.
[15] a) A. Corma, Chem. Rev. 1997, 97, 2373–2419; b) I. F. J. Vankele-
com, Chem. Rev. 2002, 102, 3779–3810.
[16] a) P. M. Budd, B. S. Ghanem, S. Makhseed, N. B. McKeown, K. J.
Msayib, C. E. Tattershall, Chem. Commun. 2004, 230–231; b) N. B.
McKeown, B. Ghanem, K. J. Msayib, P. M. Budd, C. E. Tattershall,
K. Mahmood, S. Tan, D. Book, H. W. Langmi, A. Walton, Angew.
Chem. 2006, 118, 1836–1839; Angew. Chem. Int. Ed. 2006, 45, 1804–
1807.
[17] S. J. Gregg, K. S. W. Sing, Adsorption, surface area and porosity, 2nd
ed., Academic Press, London, 1982.
[18] M. Deetlefs, K. R. Seddon, Green Chem. 2003, 5, 181–186.
[19] T. Allen, Particle size measurement. Vol. 1: Powder sampling and
particle size measurement, 5th ed., Chapman & Hall, Padstow (UK),
1997.
[6] C. Guerrero-Sanchez, F. Wiesbrock, U. S. Schubert, ACS Symp. Ser.
2005, 913, 37–49.
[7] a) P. Snedden, A. I. Cooper, K. Scott, N. Winterton, Macromolecules
2003, 36, 4549–4556; b) P. Snedden, A. I. Cooper, Y. Z. Khimyak,
K. Scott, N. Winterton, ACS Symp. Ser. 2005, 913, 133–147.
Received: May 10, 2006
Published online: September 8, 2006
Chem. Eur. J. 2006, 12, 9036 – 9045
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9045