Hyperbranched polymeric ionic liquids with onion-like topology as transporters and compartmentalized systems

Article abstract of DOI:10.1002/anie.201205130

A new family of hyperbranched polymeric ionic liquids ("hyperILs" ) with onion-like topology and facile polarity design were tailored as transporters and compartmentalized systems. Applications include transport and dispersion of water-soluble dyes and functionalized graphene nanosheets from aqueous phase into nonpolar fluids, including polymer melts.

Full text of DOI:10.1002/anie.201205130

Angewandte
Chemie
DOI: 10.1002/anie.201205130
Compartmentalized Ionic Systems
Hyperbranched Polymeric Ionic Liquids with Onion-like Topology as
Transporters and Compartmentalized Systems**
Fabian Schꢀler, Benjamin Kerscher, Fabian Beckert, Ralf Thomann, and Rolf Mꢀlhaupt*
Among the rapidly expanding families of ionic liquids (ILs),[1]
amphiphilic polymer electrolytes, containing IL moieties
either in their backbone or side chains, offer attractive
opportunities for designing novel nano- and mesophase
materials, including unique compartmentalized systems. Poly-
meric ionic liquids (polyILs) combine attractive IL properties,
such as negligible vapor pressure, low flammability, and ionic
conductivity, with properties typical for polymers, particularly
durability, low toxicity, as well as their potential for facile
construction of highly multifunctional systems, combined with
easy polymer processing. Dispersions, films, and moldings are
fabricated without encountering leakage problems associated
with low-molecular-weight ILs.[2] Envisioned applications
range from electrolytes to solvents, sorbents, liquid separation
media, catalyst scaffolds, dispersing agents, and compartmen-
talized fluids that can be used as nano- and microreactors for
particle preparation and as transporters for shuttling a great
variety of functional materials. An important research
objective is to establish novel versatile synthetic routes,
enabling easy scale-up and tuning of molecular architectures
for polarity design and facile tailoring of multifunctional
systems without the need for tedious multi-step syntheses and
extensive purification. Moreover, to qualify as polymer
additives, polyILs must be extremely robust and able to
withstand the high shear forces and temperatures well above
2008C during polymer melt compounding.
mers[4] and of ionic liquid-crystalline polymers[5] has been
exploited to create polyIL nanostructures for applications in
electronics. The integration of 3D-ordered macropores in
polyILs has led to a new generation of smart materials as
tunable photonic crystals, electro-optical switches, and func-
tional surfaces.[6] PolyILs are known as very effective phase-
transfer media that enable transport of carbon nanotubes
(CNT) from aqueous into non-aqueous phase.[7] New polyIL/
CNT nanohybrid materials were reported to be electroactive
nanomaterials.[8] There is substantially less information avail-
able for highly branched polyILs as cascade macromolecules
with micelle-like molecular architectures, which are very
attractive on account of their considerably lower viscosity,
shear- and pH-independent stable conformation, and multi-
functionality compared to linear polyILs. Sophisticated
dendritic polyILs were prepared by means of multistep
syntheses[9] and assembled to form cylindrical nano-objects.[10]
In view of the potential industrial applications, the prepara-
tion of hyperbranched polyILs (“hyperILs”) is a much more
viable synthetic route to highly branched polyILs. A prom-
inent example is the commercially available hyperbranched
polyethyleneimine, prepared by cationic aziridine polymeri-
zation, which has been functionalized in various ways to
produce a great variety of multifunctional nanoparticles
targeting different applications, for example, microbio-
cides.[11] Another facile route to hyperILs exploits ionene
chemistry, for example, poly(N-alkylation) of lutidine deriv-
atives.[12] Although such hyperbranched ionenes could be
used as the core for micelle-like hyperILs, it is more attractive
to create the inverse structures with IL moieties as the shell.
By attaching IL groups to a flexible non-ionic highly branched
polymer core, the functional groups in the shell are rendered
more accessible. Softening temperatures and viscosities are
markedly lowered, approaching those of ILs. The first
example of this type of hyperIL was prepared by esterification
of polyglycidol with w-bromoacyl chlorides and alkylation of
tertiary amines. The resulting hyperILs containing pyridinium
or imidazolium groups in the shell were used as scaffolds for
recyclable catalysts.[13] Similar hyperILs were prepared by
tosylation of polyglycidol and substitution of the tosyl groups
by 1-methylimidazole.[14]
The recent focus of polyIL development has shifted
towards controlled nanostructure formation. A great variety
of nanostructured polyelectrolytes, mesoporous polymers,
and nanoparticles have been prepared from IL monomers.
Highly ordered polyIL nanoparticles with a tunable multi-
lamellar or unilamellar vesicular inner structure, resembling
liposomes, were formed by precipitation polymerization from
water.[3] Moreover, the self-assembly of ionic block copoly-
[*] Dr. F. Schꢀler, B. Kerscher, F. Beckert, Dr. R. Thomann,
Prof. Dr. R. Mꢀlhaupt
Freiburg Materials Research Center and Institute for Macromolec-
ular Chemistry of the University of Freiburg
Stefan-Meier-Strasse 31, 79104 Freiburg (Germany)
E-mail: rolfmuelhaupt@web.de
Homepage: http://www.fmf.uni-freiburg.de
Although it would be feasible to render such hyperILs
amphiphilic by creating a hydrophobic outer shell, most of
these hyperILs, including polyethyleneimines, have limited
thermal and thermoxidative stability. This prompted us to
design a novel hyperIL family with an onion-like topology
containing hyperbranched poly(3-ethyl-3-hydroxymethyloxe-
tane) (PEHO) as the core, an inner shell of covalently linked
imidazolium cations with variable counteranions, and an
outer shell of n-alkyl chains with variable chain length. In
[**] The authors gratefully acknowledge financial support by the
Deutsche Forschungsgemeinschaft (DFG), SFB 428, during the very
early phase of the project, and by the Freiburg Materials Research
Center (FMF). Research on functionalized graphene dispersions
was supported by the Federal Ministry of Education and Research
(BMBF) as part of the “FUNgraphen” project (Fkz: 03X0111A).
Supporting information for this article, including experimental
details and analytical data, is available on the WWW under http://
dx.doi.org/10.1002/anie.201205130.
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Table 1: HyperILs with different alkyl chains and anions.
contrast to polyglycidol, which contains a mixture of primary
and less-reactive secondary hydroxy groups, PEHO contains
only primary hydroxy groups. Moreover, PEHO does not
contain any hydrogen atoms in b position and consists
exclusively of sterically shielded 1,3-diether groups. This is
of particular interest for improving both the thermal and
chemical stabilities in harsh environments, for example, for
battery electrolytes, reaction media, and polymer melt
dispersion processes.
As illustrated in Scheme 1, ring-opening cationic poly-
merization of 3-ethyl-3-hydroxymethyl oxetane, following
procedures reported by Bednarek et al.,[15] produces hyper-
branched polyoxetanes with number-average molar masses of
1800–2400 gmolÀ1, polydispersities of 1.5–2, and degrees of
branching of 50% (detailed information is given in the
Supporting Information). Subsequently, the hydroxy groups
are tosylated and used to alkylate 1-(n-alkyl)imidazoles to
produce hyperImOTs with methyl, butyl, dodecyl, and
octadecyl N-substituted imidazolium cations and tosylate
Anion[a]
Tg[b]
[8C]
Tm
[b]
CnhyperImX
Alkyl substituent
[8C]
C1hyperImOTs
C1hyperImOTf
C1hyperImBF4
C1hyperImPF6
C1hyperImNTf2
C1hyperImDBS
C4hyperImOTs
C4hyperImOTf
C4hyperImBF4
C4hyperImPF6
C4hyperImNTf2
C12hyperImOTs
C18hyperImOTs
methyl
methyl
methyl
methyl
methyl
methyl
n-butyl
n-butyl
n-butyl
n-butyl
n-butyl
n-dodecyl
n-octadecyl
OTs
OTf
BF4
25
14
16
41
À2
18
18
À3
0
19
À12
31
–
–
–
–
–
–
–
–
–
–
–
–
–
37
PF6
NTf2
DBS
OTs
OTf
BF4
PF6
NTf2
OTs
OTs
[a] OTs=tosylate, OTf=triflate, NTf2 =bis(trifluoromethylsulfonyl)-
imide, DBS=4-dodecylbenzenesulfonate. [b] Tg =glass transition tem-
perature, Tm =melting temperature.
IL
1-methyl-3-octadecylimidazolium
tosylate
(C1C18ImOTs) exhibit very poor solubility in toluene.
Second, the outer shell was rendered hydrophobic by
exchanging the tosylate anion with hydrophobic anions,
such as 4-dodecylbenzenesulfonate (DBS) to form
C1hyperImDBS, which is the only methyl-substituted
C1hyperIm that dissolves in chloroform. Variation of
both the n-alkyl chain length and the type of counter-
anion afford excellent control of polarity, which is
readily matched with that of fluids and even polymers.
A detailed overview of the influence of n-alkyl sub-
stitution and the anion of the hyperIL on the solubility
in water and various organic solvents is given in the
Supporting Information.
As is apparent from Table 1, the majority of the
hyperIL tosylates are amorphous, with glass transition
temperatures at around room temperature. Only
C18hyperImOTs crystallizes at 378C, owing to crystal-
lization of the octadecyl groups. This greatly facilitates
handling of such solid materials in polymer processing.
As a function of the substitution patterns, it is possible
to lower the glass transition temperatures well below
room temperature. According to the thermogravimetric
analysis, no weight loss occurs upon heating to 3008C.
Consequently, all hyperImOTs are remarkably robust under
polymer processing conditions and qualify for the envisioned
application as a polymer additive.
The novel organophilic hyperIL tosylates (C18hyper-
ImOTs), which contain an outer nonpolar octadecyl shell
and an inner imidazolium shell, are highly effective phase-
transfer agents and transporters. This is illustrated for the
transport of the water-soluble food colorant Brilliant Blue
FCF (E133), a blue triphenylmethane dye, into the organic
phase of chloroform and polypropylene (PP). As illustrated in
Figure 1, E133 is transferred completely from the aqueous
phase to the chloroform phase immediately after adding
C18hyperImOTs. Moreover, the dye can be solubilized and
loaded in C18hyperIL. This dye-loaded C18hyperIL was
Scheme 1. Routes to nanostructured hyperILs with onion-like topology and
facile polarity design. Ts=p-toluenesulfonyl (tosyl).
counteranions. As nearly quantitative conversion is achieved
in both polymer analogous reactions, this method produces
hyperILs with degrees of modification that generally lie well
above 90%. By means of very effective anion exchange
reactions, the tosylate anions can be exchanged for a variety
of other anions, which are listed in Table 1.
Two strategies have been established for rendering water-
soluble N-methyl-substituted C1hyperImOTs soluble in
organic solvents. First, the n-alkyl chain length was increased
to enhance the hydrophobicity of hyperImOTs, as reflected
by the very low water solubility and excellent toluene
solubility for octadecyl substitution (C18hyperImOTs). It
should be noted that common polyelectrolytes, low-molec-
ular-weight ionic surfactants,[16] and even the corresponding
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For average particle sizes of C18hyperImOTs phases ranging
from 120 to 160 nm, the color is formed exclusively by light
absorption, whereas at much larger particle sizes in the
micrometer range for C4hyperImOTs, scattering of light
impairs light transmission. The C18hyperImOTs-analogous
low-molecular-weight IL C1C18ImOTs, which was used for
comparison, affords a very inhomogeneous dispersion of
E133 in PP. More detailed information is given in the
Supporting Information.
C18hyperImOTs was also found to be a highly effective
dispersing agent for many other (nano) materials, including
graphenes. Functionalized graphene (FG), prepared by
thermal reduction of graphite oxide, is readily dispersed in
polar solvents by means of high-pressure homogenization,
and forms mainly single FG nanosheets.[17] In the presence of
C18hyperImOTs, stable FG dispersions are formed in non-
polar media, such as toluene, whereas dispersions of FG in
pure toluene undergo rapid sedimentation. In fact, as can be
seen in Figure 3, when C18hyperImOTs (10:1 by mass with
respect to FG) is added to toluene, the graphene nanosheets
Figure 1. C18hyperImOTs as phase-transfer agent and transporter for
the water-soluble dye E133 in chloroform/water (left; b) and PP (right;
d). E133 without C18hyperImOTs in water/chloroform (a) and PP (c) is
shown for comparison.
added to PP during melt extrusion at 2008C. As shown in
Figure 1, even a low content of 0.11 wt% E133 and 0.50 wt%
C18hyperImOTs affords very effective brilliant blue colora-
tion of PP. Without C18hyperImOTs, the hydrophilic dye is
poorly dispersed in the nonpolar PP matrix. The dye transport
using C18hyperIL does not occur by host–guest “molecular
bottle”-type transportation, as proposed for dendritic amphi-
philes. Clearly, the dye is too large to fit inside the
C18hyperIL. Close inspection of the morphology of
C18hyperImOTs/PP blends by means of scanning electron
microscopy (SEM) reveals the presence of uniformly distrib-
uted C18hyperIL nanodroplets (Figure 2). The average nano-
droplet size increases with increasing C18hyperImOTs con-
tent from 121 nm at 0.25 wt% to 163 nm at 2.0 wt% without
affecting the shape and uniform distribution. In contrast, the
less compatible N-butyl-substituted C4hyperImOTs affords
a significantly larger diameter of 489 nm at 0.25 wt% and
increases into the micrometer range with 1453 nm at 2.0 wt%,
accompanied by broadening of the particle size distribution.
Figure 3. Left: FG in water/toluene after ultrasonification in the
a) absence and b) presence of C18hyperImOTs. Right: TEM image of
a melt compounded PS nanocomposite containing 2.5 wt% FG and
2.5 wt% C18hyperImOTs.
are transferred from water into the organic phase. The ability
of C18hyperImOTs to stabilize FG in organic media can be
utilized to disperse graphene nanosheets in polystyrene (PS).
When PS is dissolved in C18hyperIL-stabilized FG disper-
sions in toluene, melt-processable PS/graphene nanocompo-
sites are formed. At 2.5 wt% graphene content, the addition
of the same amount of C18hyperImOTs results in a 20-fold
increase in the electrical conductivity with respect to the
composite containing 2.5 wt% graphene and no hyperIL,
namely from 3.6 ꢀ 10À5 ScmÀ1 to 7.7 ꢀ 10À4 ScmÀ1. As shown
in Figure 3, very effective FG nanosheet dispersion and
formation of conducting networks is achieved in the presence
of C18hyperImOTs. Presumably, C18hyperImOTs assembles
at the graphene surface and changes its conformation in an
octopus-like fashion to produce ultrathin C18hyperIL coat-
ings on FG nanosheets, thus accounting for very effective
steric and electrostatic stabilization without preventing per-
colation.
In conclusion, a novel facile synthetic route has been
developed to produce new families of hyperbranched poly-
Figure 2. SEM image of PP melt-compounded with 2.0 wt% C18hyper-
ImOTs.
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a core of hyperbranched poly(1,3-diether), a polar inner
imidazolium cation shell, and an nonpolar outer n-alkyl shell.
High thermal stability up to 3008C and extraordinary robust-
ness enable these hyperILs to be used in reaction media,
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All experimental details, including analytical data, are
given in the Supporting Information.
Received: June 30, 2012
Revised: August 31, 2012
Published online: November 4, 2012
Keywords: graphene · hyperbranched polymer · ionic liquids ·
nanocomposites · transporters
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