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occurs.
structures of the epoxy resins and EMIM acetate are shown in
Fig. 1. Palladium acetate (Pd(OAc)2) and palladium acetylaceton-
ate (Pd(acac)2) were obtained from Aldrich. Ethyl cinnamate was
purchased from Alfa Aesar while cyclohexene and styrene were
obtained from Fluka. All other substrates were purchased from
Aldrich and hydrogen 5.0 was obtained from Basi. All chemicals
were used as received without purification.
properties [9–12]. As a result, numerous applications have been
reported, e.g., in the fields of alternative solvents [13–15], indus-
trial processes [16–18], separation techniques [10,19] and catalysis
[20–24]. A promising application of ionic liquids, particularly imid-
azolium salts, is their use as thermally latent initiators for the
polymerization of epoxy resins [25,26]. Polymerization reactions as
well as properties of the resulting polyethers can be largely influ-
enced by choice of appropriate cation–anion combinations. In that
way, resin formulations can be prepared that can be safely stored
and processed at room temperature and elevated temperatures
while polymerization can be started by increasing the tempera-
ture to a well-defined curing temperature. This temperature can be
adjusted by the ionic liquid type and its concentration in the resin
initiators and low amounts, typically in the range of a few percent,
lead to high crosslinking with epoxide conversion above 90%. Fur-
ther advantages are the good miscibility of imidazolium salts with
epoxy resins and high compatibility with other components in the
resin formulations [27–29].
Polymerization of the resin formulations was monitored by
differential scanning calorimetry (DSC) employing
a Mettler
Toledo DSC822e device. Typically, 3–10 mg resin samples were
employed. The onset temperatures of the exothermic polymer-
ization processes Tonset (tangent onset), peak maxima Tpeak and
polymerization enthalpies ꢀH were recorded. Glass transition tem-
peratures Tg of the polymerized materials were also determined by
DSC according to DIN 53765. Thermogravimetric analyses (TGA)
were carried out using a Mettler Toledo TGA/SDTA851e instrument
employing 2–6 mg samples. DSC as well as TGA measurements
were carried out under nitrogen atmosphere in the dynamic mode
and the heating rate was 10 ◦C/min.
Palladium loadings of the materials were determined by SEM-
EDX. SEM micrographs were recorded using a FESEM DSM 982
Gemini microscope from ZEISS equipped with a backscatter detec-
tor and coupled with an INCAPentaFET-x3 energy-dispersive X-ray
Recently, this strategy was employed for the preparation of cat-
alysts. A two-step procedure was described comprising epoxy resin
polymerization in the first step and modification of the result-
ing materials with a palladium complex in the second step [30].
attached to the polyether matrices by an impregnation procedure.
The resulting materials were employed as catalysts for the Heck
coupling of iodobenzene with methyl acrylate and the hydrogena-
tion of cinnamaldehyde [30,31].
Within this work, catalyst preparation by this approach was fur-
glycidylether of bisphenol A (DGEBA) and the trifunctional gly-
cidyl derivative of p-aminophenol (TGAP), a high-performance
resin with several applications in the aerospace sector, were used
[32–34]. It is shown that simple metal salts instead of metal com-
plexes can be employed and a convenient one-step procedure for
catalyst preparation is described. Palladium acetate (Pd(OAc)2) was
employed and compared to palladium acetylacetonate (Pd(acac)2).
Regarding the polymerization initiator, the ionic liquid 1-ethyl-3-
methylimidazolium acetate (EMIM acetate) was employed, which
is, among some related compounds, a highly efficient initiator for
the anionic polymerization of epoxy resins. The palladium-doped
polyether materials were tested as catalysts for the hydrogenation
of different alkenes under mild conditions. Metal leaching from
the catalysts to reaction solutions was investigated and catalyst
recycling experiments were carried out.
To investigate the redox state of Pd particles, surface sensi-
tive x-ray photoelectron spectroscopy (XPS) was applied. The dried
samples were prepared for XPS analysis by pressing them onto an
indium foil inside an anoxic glove box. The samples were conveyed
into the XP spectrometer, ULVAC-PHI model VersaProbe II, without
air contact by means of a vacuum transfer vessel. Monochromatic
Al K␣ X-rays (1486.6 eV) are used for excitation in conjunction with
a beam of low energy electrons concurrently with a beam of low
energy Ar+. The binding energy scale of the spectrometer was cali-
brated with the Cu 2p3/2 and Au 4f7/2 lines of pure and Ar+ sputter
cleaned metal foils [35]: the difference of binding energies between
both lines is adjusted by the electronics to coincide with the ref-
erence value of 848.66 eV within 0.1 eV. Standard deviation of
binding energies was within 0.2 eV. The C 1s line of adventitious
hydrocarbon at 284.8 eV was used for charge referencing. Atomic
concentrations are calculated from the areas of elemental lines
(after Shirley background subtraction) of survey spectra using sen-
sitivity factors of the elemental lines, asymmetry parameters, and
transmission function of the analyzer. Narrow scans of elemental
lines were recorded at 23.5 eV pass energy of the analyzer for deter-
mination of chemical shifts and spectral features. Data analysis was
performed using ULVAC-PHI MultiPak Version 9.5.
Powder X-ray diffraction (XRD) studies were performed on the
ground catalyst particles employing a PANalytical X’Pert Pro X-ray
diffractometer operating with Cu K␣ radiation.
Transmission electron microscopy (TEM, STEM-HAADF) was
performed with a FEI Tecnai F20 ST TEM (operating voltage 200 kV)
equipped with a field emission gun and EDAX EDS X-ray spectrom-
eter. For TEM analysis, the ground powder of the catalyst before and
after catalytic testing was suspended in isopropanol. A small drop
of each suspension was then deposited onto amorphous carbon-
coated 400 mesh copper grids and eventually air dried.
2. Experimental
2.1. Materials and methods
The diglycidyl ether of bisphenol A (DGEBA) was obtained
from LEUNA-Harze GmbH (Epilox A 18-00, epoxy equivalent
weight: ∼180 g/eq, viscosity at 25 ◦C: ∼9000 mPa s) and the trigly-
cidyl derivative of p-aminophenol (TGAP) was purchased from
Aldrich (N,N-diglycidyl-4-glycidyloxyaniline, epoxy equivalent
weight: ∼95 g/eq, viscosity at 25 ◦C: ∼600 mPa s). The imidaz-
olium salt 1-ethyl-3-methylimidazolium acetate (EMIM acetate,
EA) and the analogous compounds with thiocyanate, dicyanamide
and diethylphosphate anions were obtained from BASF SE. The
Palladium traces in the reaction solutions were quantified
by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES) employing a Varian Liberty 150 instrument. To reach
maximum accuracy, palladium concentration was enriched before
analysis by solvent evaporation and dissolving of the residues in
small amounts of concentrated HNO3.
Conversions and selectivities were analyzed by GC-FID using an
Agilent 6890 N gas chromatograph equipped with a J & W SCI-
ENTIFIC DB5 column (30 m × 0.25 mm, 0.25 m film thickness).
Please cite this article in press as: U. Arnold, et al., Ionic liquid-initiated polymerization of epoxides: A useful strategy for the preparation