Synthesis of Cross-linked Ionic Poly(styrenes) and their Application as Catalysts for the Synthesis of Carbonates from CO2 and Epoxides

Article abstract of DOI:10.1002/cplu.201600461

A series of dicationic styrene-functionalized imidazolium-based salts, in which the two imidazolium rings are bridged by a functionalized spacer, are prepared. The salts are polymerized to afford cross-linked imidazolium-based ionic polystyrene materials,

Full text of DOI:10.1002/cplu.201600461

Accepted Article
Title: Synthesis of Cross-Linked Ionic Poly(Styrenes) and their Application as Cata-
lysts for the Synthesis of Carbonates from CO2 and Epoxides
Authors: Felix Daniel Bobbink; Antoine Philippe Van Muyden; Aswin Gopakumar;
Zhaofu Fei; Paul J. Dyson
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10.1002/cplu.201600461
Synthesis of Cross-Linked Ionic Poly(Styrenes) and their Application as Catalysts
for the Synthesis of Carbonates from CO2 and Epoxides
Felix D. Bobbink+, Antoine P. Van Muyden+, Aswin Gopakumar, Zhaofu Fei and Paul J. Dyson*
Institut des Sciences et Ingénieuries Chimique, École Polytechnique Fédérale de Lausanne (EPFL), 1015
Lausanne (Switzerland), Phone: +41 21 693 98 53, Email: paul.dyson@epfl.ch
+ These authors contributed equally to this work.
* Corresponding author.
Abstract
A series of di-cationic styrene-functionalized imidazolium-based salts in which the two
imidazolium rings are bridged by a functionalized spacer were prepared. The salts were
polymerized to afford cross-linked imidazolium-based ionic polystyrene materials which, due to
the presence of the functionalized spaces, should be highly active organocatalysts for the
cycloaddition of CO2 to epoxides to afford cyclic carbonates (CCE reaction). Consequently, the
catalytic activity of the polymers was evaluated in the CCE reaction. The most active catalyst
incorporates a diol functionality and is active at 80 °C and 4 bars at a loading of 5 mol%, which is
comparable to the most active organocatalysts. Moreover, high yields can be obtained under
atmospheric pressure when the temperature is increased to 120 °C. Under more forcing
conditions the catalyst is highly active at a loading one order of magnitude lower and highlights
the importance of benchmark conditions for the CCE reaction. Moreover, the polymer catalysts
are advantageous as they can be used at low catalyst loadings, the carbonate product is easily
isolated in pure form, and loss of activity of the recovered polymer catalyst is not observed
during reuse.
Keywords
Sustainable chemistry; CO2 chemistry; imidazolium salts; cycloaddition reaction; ionic polymers.
Introduction
1
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With carbon dioxide (CO2) levels increasing in the atmosphere, CO2 has become an increasingly
interesting C1 building block because of its abundance, low cost and low toxicity. In this context,
efforts to utilize CO2 as a building block, rather than to consider it a waste, continues to attract
considerable attention.[1,2] Although CO2 is cheap, abundant and non-toxic, incorporating CO2
into organic molecules remains challenging. Nevertheless, CO2 may be directly reduced to fuels
(CH4,[3,4] CH3OH,[5–7] and HCOOH[8]), used as a carbon source in the reductive functionalization of
amines,[9–12] and used to form heterocycles incorporating CO2 without altering its oxidation
state.[13] Moreover, CO2 has been incorporated in several types of polymers including
polycarbonates.[14–17] Recently, several examples of non-catalytic transformations employing CO2
have emerged that take place under mild conditions, emphasizing the potential of CO2 as C1
building block in chemistry.[18] A few of the reactions that use CO2 as a reagent have been
commercialized.[19] Notably, the cycloaddition of CO2 to epoxides to afford cyclic carbonates (the
CCE reaction) is currently performed on industrial scale (Scheme 1).[20] While this reaction is
atom-economical and efficient, it should be noted that epoxides are toxic, volatile and there are
safety concerns surrounding this reaction. In this aspect, several studies attempted to prepare
cyclic carbonates from the reaction of CO2 with more benign diols, instead of epoxides.[21,22]
Scheme 1. Generic scheme for the cycloaddition of CO2 to epoxides to afford cyclic carbonates,
the CCE reaction.
Despite the industrial success of this CO2 cycloaddition reaction, catalyst development is still
ongoing. A range of catalysts, both metallic, including alkali metals,[23] salen complexes,[24]
lanthanides[25] and others, as well as non-metallic catalysts such as organic salts based on
ammonium,[26,27] phosphonium[28,29] and imidazolium[30–32] systems or other organocatalysts
such as polyols[33] have been developed. Efforts to immobilize some of these catalysts on solid
supports including silica,[34,35] polyethylene glycol[36] and other materials[37][38] have also been
made.
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Ionic liquids, which are defined as ion pairs with melting points below 100 °C, have been
thoroughly investigated as catalysts for the CCE reaction.[39] Examples of imidazolium salts
incorporating alcohols, diols, carboxylic acids, nitriles and other functional groups have been
used in the CCE reaction, as well as several polymeric and supported catalysts[40–42]. In particular,
it was found that incorporating H-donor groups in the catalyst result in higher activity, whereas
heterogenization of the catalyst usually leads to lower activities. Inspired by previous studies
demonstrating the importance of a H-donor group (alcohol or diol) near the imidazolium ring,
we developed a series of cross-linked, functionalized ionic poly(styrenes) (Figure 1) and
evaluated their activity in the CCE reaction.[43–45]
Figure 1. (a) Catalytically active imidazolium salt for the CCE reaction[31]. (b) Examples of
imidazolium-containing polymer catalysts[43,46]. (c) New cross-linked catalysts with a functional
group near the imidazolium ring, where R = alcohol, acid, diol, benzyl, dibenzyl or hexyl.
3
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Results and discussion
The synthetic route used to prepare the vinyl-functionalized di-imidazolium salt monomers
m1a/b to m6a/b and subsequent polymers p1a/b to p6a/b is depicted in Scheme 2. In the first
step the styryl-functionalized di-imidazolium salts are prepared from 4-vinylbenzylimidazole
(VBIm) (prepared according to ref 32, see experimental) by reaction with appropriate spacer
(organobromide or organochloride) precursors. For the more reactive organobromides, it was
possible to react the bromo-functionalized spacer directly with VBIm. The reaction was
1
monitored by H NMR spectroscopy and it was found that a slight excess of VBIm avoids any
contamination from the mono-quaternization product. The reaction was terminated when a
single peak corresponding to the C2-imidazolium ring proton was observed at ca. 9.4 ppm in the
1H NMR spectrum indicative of a single species in solution (see spectra in SI). All the reactions
with the organobromides went to completion, however, with organochlorides long reaction
times were required to obtain the di-substituted monomers in high yield. For spacers m4 and
m5 which contain hydroxyl groups (Scheme 2), the reaction did not proceed with the dichloride
starting materials. However, monomers m4a and m5a may be obtained via an alternative
synthetic route. The route involves reaction of the spacer with imidazole in presence of NaOH. In
a second step, the chloride imidazolium salt is obtained from the reaction of the di-imidazole
with 4-vinylbenzylchloride (see Scheme S1 for details). Under the conditions employed herein,
the C2 position of the imidazolium is not reactive, and no side-products with C2
functionalization were obtained.
The 1H NMR spectra of the monomers are as expected with the imidazolium-proton observed at
ca. 9.4 ppm. Note, the spectra of m1a (the only salt that was reported previously) is consistent
with the one reported in the literature.[47] In the electrospray ionization mass spectra of the
monomers peaks corresponding to the intact di-cation with one counter-anion are observed.
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Scheme 2. Synthesis of vinyl-functionalized di-imidazolium salts m1a/b to m6a/b and their
subsequent polymerization to form cross-linked polymers p1a/b to p6a/b.
i
All the monomers were readily polymerized in PrOH using 5 wt% AIBN as a radical initiator
(Scheme 2) yielding the polymer as a white powder. In contrast to the hygroscopic monomers,
i.e. m1a/b, m4a/b, m5a/b and m6a, all the polymers could be stored on the bench without
absorbing ambient moisture. The polymer with the hexyl spacer was reported previously and
evaluated as membrane, but with Tf2N as counter-anion.[48] In their work an alternative
synthetic route was used to prepare the monomer, i.e, they prepared the bridged di-imidazole
first, and reacted it with 4-vinylbenzyl chloride. Polymers p1a/b - p6a/b were characterized by IR
spectroscopy, thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) and scanning
electron microscopy (SEM). Notably, the polymers are insoluble in water, CH3CN, DMSO, DMF
and EtOAc. Comparison of the IR spectra of the monomers with the analogous polymers
demonstrates that the vinyl moiety (ca. 1620 cm-1 and 920 cm-1) has been polymerized.
Representative spectra are shown in Figure S1. BET measurements (Table S1) on the dried
polymer show that the polymers exhibit only modest porosities, and the spacer seems to have
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only a marginal effect on surface area. Compared to previously reported linear ionic
poly(styrenes) that exhibited no porosity,[46] the cross-linked polymers show small surface areas.
The SEM images of selected polymers give further insights on the overall morphological
structure in the solid-state (Figures 3 and S2). For example, polymer p1b with a hexyl-spacer
presents a smoother surface than p5b containing a diol group whereas the particle size of p5b
appears to be more homogeneous than that of p1b (see Figure S2 for selected polymers at 50
μm magnification).
Figure 3. SEM images of polymer p1b (top and middle) and p5b (bottom).
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TGA analysis of the dried polymers showed that the polymers are stable in the range 250 to 350
°C (Figures 3 and S3), with an average of 5 wt% loss associated with trace solvent evaporation.
The materials with hexyl and benzyl spacers are stable up until 350 °C, whereas the polymers
containing H-donor spacers show mass losses at around 250 °C, which is associated with slightly
lower stability of the spacer containing an alcohol, diol or acid compared to the more stable
hexyl, benzyl and dibenzyl spacers.
Figure 4. TGA curves for selected polymers p1b, p2b and p5b.
All the polymers were evaluated as catalysts in the cycloaddition of CO2 to epoxides using
styrene oxide (SO) to afford styrene carbonate (SC). Table 1 shows the yields of SC obtained
under standard conditions for the polymers in order to identify the most potent catalyst. The
anion is important for activity and in general bromide containing salts are more active than their
chloride counterparts[49]. However, for the ionic polymers with benzyl and dibenzyl spacers the
catalytic activity of the bromide and chloride salts are essentially the same (Table 1, entries 3-4
and 5-6). As expected, the nature of the spacer has a marked influence on the reaction: simple
spacers like hexyl, benzyl or dibenzyl lead to significantly less active catalysts than those
incorporating a H-donor, i.e. p4b, p5b and p6b (Table 1, entries 8-10-11). The most active
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catalyst is p5b which contains a diol spacer. Imidazolium salts modified with similar functional
groups to those described here have been evaluated as catalysts for the CCE reaction.[31,50]
However, for processing purposes, the polymer-based catalysts facilitate product separation. In
this context, an example of a diol-functionalized imidazolium cation grafted onto polystyrene has
also been evaluated in the CCE reaction.[51] A carboxylic acid functionalized imidazolium cation
has also been grafted onto silica,[34] and a hydroxyl-functionalized imidazolium cation grafted
onto a cross-linked polymer[52] to afford heterogeneous CCE catalysts.
Table 1: Evaluation of the polymers at catalysts for the synthesis of styrene carbonate.
Entry
Catalyst
Yield [%][a]
1
p1a
12
25
14
15
14
15
16
48
17
62
37
2
p1b
p2a
3
4
p2b
p3a
5
6
p3b
p4a
7
8
p4b
p5a
9
10
11
p5b
p6b
Conditions: catalyst (0.5 mol%), SO (1 g, 8.33 mmol), CO2 (2.5 MPa),
100 °C, 15 h, neat. [a] Determined by GC-FID using n-decane as
internal standard. All selectivities >99%.
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Using the most efficient catalyst, i.e p5b, the reaction conditions (pressure and temperature)
were further optimized with a catalyst loading of 0.5 mol% (Table 2). The diol-containing
polymer p5b afforded SC in near-quantitative yield at 130 °C under 2.5 MPa of CO2 and the SC
product was obtained by simply removing the catalyst by filtration (Table 2, entry 2). At a lower
temperature of 80 °C the catalyst is active and SC is obtained in 41 % yield is obtained after 15
hours reaction (Table 2, entry 3).
Table 2: Optimization of pressure and temperature employing p5b as the catalyst for the CCE
reaction.
Entry
P [MPa]
1
T [°C]
130
130
80
Yield [%][a]
1
2
3
4
62
99
41
62
2.5
2.5
2.5
100
Conditions: p5b (0.5 mol%), SO (1 g, 8.33 mmol), CO2, 100 °C, 15 h. [a] Determined by GC-FID
using n-decane as internal standard.
The activity of p5b is at least as good as other heterogeneous, supported catalysts which tend to
require a co-catalyst and employ much higher catalyst loadings.[53][54] Since a structurally-related
homogeneous imidazolium salt containing a hydroxyl functional group fully convert styrene
oxide at 80 °C and 4 bars of CO2 after 24 hours at 5 mol%. loading[31], we were interested to see
whether the catalyst presented herein would achieve high yields at a similar catalyst loading.
Table 3 compares the activity of the monomer and the corresponding polymer under milder
conditions than for m/p5b (note that for these experiments, 5 mol% cat. was used instead of 0.5
mol% used in Tables 1 and 2). Interestingly, full conversion is obtained under these conditions
after 24 hours, which is in contrast with the low yield of 41 % obtained at 80 °C and 2.5 MPa of
CO2 obtained with 0.5 mol% catalyst (Table 3, entries 1 and 2, and Table 2, entry 3). Moreover,
SC was obtained in 94 % yield under atmospheric CO2 pressure (balloon) at 120 °C (Table 3, entry
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5); at lower temperatures the yield is somewhat reduced (Table 3, entries 3-4). These results
demonstrate the importance of benchmark conditions for the CCE reaction, because direct
comparison between studies is difficult when more than one parameter differs.
Table 3. Catalytic activity of m5b and p5b at a high loading.
Entry
Cat
m5b
p5b
p5b
p5b
p5b
T [°C]
80
P [MPa]
0.4
Yield [%][a]
1
2
3
4
5
>99
>99
68
80
0.4
80
0.1
100
120
0.1
78
0.1
94
Conditions: cat (5 mol%), SO (0.1 g, 0.83 mmol), CO2, 24 h. [a] Determined by 1H NMR.
The scope of the CCE reaction was investigated with p5b using the optimized conditions with a
low catalyst loading (0.5 mol%, Table 2 entry 2), and it was found to selectively incorporate CO2
into a range of epoxides (Table 3, entries 1-4) in high yield, including cyclohexene oxide under
more forcing conditions (140 °C, 86 hours, Table 3, entry 5), as the bicyclic structure of
cyclohexene oxide leads to a much less reactive substrate.[55] Polymers p4b and p5b are
structurally related to a recently reported and highly active functionalized-imidazolium salt
catalyst,[31] which owe the high activity to the formation of favorable H-bonding interactions
with the substrate, see below.
Table 3. Investigation of the substrate scope of the CCE reaction using p5b as the catalyst under
optimized conditions.
Entry
Substrate
Product
Yield [%][a]
Selectivity
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[%]
1
98
> 99
2
3
94
99
> 99
> 99
4
5
98[b]
98[c]
> 99
> 99
Conditions: p5b (25 mg, 0.5% mol), epoxides (8.3 mmol), CO2 (2.5 MPa), 130 °C, 15 h, neat. [a] Isolated yield. [b]
Conditions: 48 h, 140 °C. [c] Conditions: 86 h, 140 °C.
The mechanism of the CCE reaction catalyzed by ion pairs has been proposed with the anion
initially opening the oxirane which, in turn, facilitates CO2 insertion. The catalytic cycle is
concluded with a ring-closing SN2 reaction also regenerating the catalyst. DFT calculations have
been used to confirm this mechanism for both ammonium and imidazolium salts.[26,56] Following
this mechanism, we hypothesize that the key interactions shown in the catalytic cycle (Scheme
3) result in the high activity of the diol containing polymer p5b. Since the rate-determining step
is usually the ring-opening of the epoxide (at least in the case of imidazolium salts),[56] the H-
bonding interactions between the diol and epoxide O-atom presumably help to reduce the
activation energy of this step. This is consistent with previous studies which show that catalysts
can be improved by incorporating a H-bonding functionalities.[40] While the polymers possess
two imidazolium centres, it is uncertain whether this has a catalytic advantage. However, this di-
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cationic structure makes the catalyst robust and insoluble, presumably due to the highly cross-
linked nature of the material. Note that with other catalysts such as polyphenols or aluminum
complexes, the mechanism can be different.[33,57]
Scheme 3. Tentative mechanism for the CCE reaction catalyst employing p5b as the catalyst.
Although diol containing ILs are generally hygroscopic, polymer p5b is not noticeably
hygroscopic and is bench-stable for months. The cross-linked nature of p5b provides a robust
and easily handled catalyst ideal for recycling and reuse. The catalyst was recycled three times
(Figure 5) without any loss of activity with epichlorohydrin as the starting material. After each
reaction the mixture was diluted in ethyl acetate and the product was recovered by filtration
1
without the need for any additional purification steps. The H NMR spectrum of the product
after the third recycling (see Figure S4) indicates that the product is pure and is not
contaminated by the catalyst and is free from unreacted starting material. It should be noted
that the catalyst turns from white to brown after several runs, although this has no influence on
catalytic activity.
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Figure 5. Recycling experiments using epichlorohydrin as a substrate under optimized
conditions: p5b (24.9 mg, 0.5 mol%), epichlorohydrin (0.76 g, 8.3 mmol), CO2 (2.5 MPa), 130 °C,
15 h.
Conclusions
Herein, we describe a series of cross-linked di-cationic ionic imidazolium polymers with different
functionalized spacers between the imidazolium rings. Cationic polymers resembling p1 have
been employed in membranes with TF2N- as the counter-anion, however, these materials as well
as the new polymers p2a/b to p6a/b were not used in catalysis. Consequently, we evaluated the
polymers as catalysts for the industrially important CCE reaction, as previous studies had shown
that imidazolium salts are efficient catalysts for this reaction, especially when H-donor
functional groups such as alcohols or diols are in proximity to the ring. The diol-functionalized
solid-phase organocatalyst reported here is highly active and operates under high CO2 pressures
or, at higher loadings, under ambient conditions so that autoclaves can be avoided. The most
active polymer catalyst of the series, i.e. p5b, the di-cationic polymer containing a diol-
functionalized spacer, can incorporate CO2 selectively in a range of epoxides including
cyclohexene oxide, and can be recycled without a marked decrease in activity.
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Experimental
All starting materials were purchased from commercial providers and used as received. Reaction
solvents were of absolute purity (according to Acros® brand). The solvents used for washing of
the catalysts and for carbonate extraction were of technical grade. All the monomers and
polymers were prepared under a N2 atmosphere using standard Schlenk techniques. 4-
Vinylbenzylimidazole was prepared using a literature procedure.[32] GC-MS were carried out on a
Gas Chromatograph Agilent 7890B equipped with a Agilent 7000C MS triple quad detector and a
capillary column from Agilent (l x d x f: 30 m x 0.25 mm x 0.25 μm) using N2 as carrier gas. 1H
and 13C NMR spectra were recorded on a Bruker 400 MHz instrument and electrospray
ionization mass spectra were obtained on a LTQ-Orbitrap Elite instrument (Thermofisher)
operated in positive ionization mode using a literature method.[58] Elemental analysis was
determined on a Flash 2000 Organic Elemental Analyzer. IR spectra were recorded on a Perkin-
Elmer FT-IR 2000 instrument. TGA was carried out on a Perkin-Elmer TGA 4000. BET experiments
were obtained on a Quantachrome Autosorb-IQ/MP-XR with N2 as gas for analyte. SEM pictures
were obtained on a FEI XLF30- FEG with Schottky field emission gun operated between 1 - 30 kv.
Typical procedure for ionic monomer synthesis, m1a-b to m6a-b:
4-Vinylbenzylimidazole (4.2 mmol), the appropriate spacer (2 mmol) and CH3CN (10 mL) were
loaded into a 25 mL two-neck flask. The mixture was heated to 80 °C for 12 – 168 h depending
on the spacer. The reaction was monitored by 1H NMR spectroscopy and when the reaction was
complete (i.e, all the mono-quaternization product disappeared), the reaction mixture was
cooled down to r.t and transferred to a 100 mL round bottom flask. The product was precipitated
with EtOAc (50 mL) and sonicated for 10 min, after which the white solid was left to settle. The
liquid phase was carefully removed, and the washing procedure was repeated twice with EtOAc
(2 x 50 mL), and twice with 50 mL Et2O. Then, the white solid product was dried under vacuum
overnight.
3,3'-(hexane-1,6-diyl)bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) chloride was isolated as a white
hygroscopic powder. Reaction time: 12 h. Yield 98 %. 1H NMR (400 MHz, DMSO-d6) δ 9.46 (s, 2H), 7.84 (s,
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4H), 7.53 (d, 4H), 7.42 (d, J = 7.8 Hz, 4H), 6.75 (dd, J = 17.6, 11.0 Hz, 2H), 5.88 (d, J = 17.7 Hz, 2H), 5.43 (s,
4H), 5.31 (d, J = 10.9 Hz, 2H), 4.17 (t, J = 7.2 Hz, 4H), 1.79 (t, J = 7.2 Hz, 5H), 1.25 (tt, J = 8.1, 3.4 Hz, 4H).
13C NMR (101 MHz, DMSO-d6) δ 138.1, 136.7, 136.38 , 134.77 , 129.16 , 127.13 , 123.00, 115.78 , 52.14 ,
49.27 , 29.46 , 25.33. Yield: 99%. Anal. Calc. for C30H36Cl2N4: C 68.82, H 6.93, N 10.7; Found: C 67.82, H
7.56, N 10.79. HRMs Calc. for C21H27N4.2+: 335.2225; Found: 335.2254.
3,3'-(hexane-1,6-diyl)bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) bromide was isolated as a white solid.
Reaction time: 12 h. Yield: 98 %. 1H NMR (400 MHz, Methanol-d4) δ 9.26 – 9.13 (m, 2H), 7.76 – 7.63 (m,
4H), 7.59 – 7.50 (m, 4H), 7.50 – 7.40 (m, 4H), 6.85 – 6.69 (m, 2H), 5.84 (dd, J = 17.7, 0.9 Hz, 2H), 5.44 (d, J
= 4.8 Hz, 4H), 5.31 (dd, J = 11.0, 0.9 Hz, 2H), 4.27 (q, J = 7.4 Hz, 5H), 4.12 (q, J = 7.1 Hz, 1H), 3.46 (td, J =
13
6.7, 5.2 Hz, 6H), 2.00 – 1.84 (m, 5H), 1.51 – 1.37 (m, 5H), 1.26 (t, J = 7.2 Hz, 2H). C NMR (101 MHz,
MeOD) δ 138.73, 135.84, 133.15, 128.58, 126.70, 122.68, 122.43, 114.12, 60.11, 52.52, 49.40, 29.36,
25.12, 13.04. Anal. Calc. for C30H36Br2N4: C 58.83, H 5.93, N 9.15; Found: C 54.20, H 5.88, N 8.64.
3,3'-(1,4-phenylenebis(methylene))bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) chloride was isolated as a
white powder. Reaction time: 12 h. Yield 96 %. 1H NMR (400 MHz, DMSO-d6) δ 9.53 (s, 2H), 7.85 (h, J = 2.1
Hz, 4H), 7.53 (d, J = 8.0 Hz, 4H), 7.50 (d, J = 6.4 Hz, 4H), 7.42 (d, J = 8.0 Hz, 4H), 6.75 (dd, J = 17.7, 11.0 Hz,
2H), 5.88 (d, J = 17.7 Hz, 2H), 5.44 (d, J = 7.5 Hz, 8H), 5.32 (d, J = 11.0 Hz, 2H). 13C NMR (101 MHz, DMSO-
d6) δ 138.1, 136.8, 136.4, 135.8, 134.6, 129.5, 129.3, 127.2, 123.4, 115.8, 52.3. Anal. Calc. for C32H32Cl2N4:
C 70.71, H 5.93, N 10.31; Found: C 70.50, H 6.11, N 10.42. HMRs Calc. for C23H23N4.2+: 355.1912; Found:
355.1939.
3,3'-(1,4-phenylenebis(methylene))bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) bromide was isolated as a
white hygroscopic powder. Reaction time: 12 h. Yield 97 %. 1H NMR (400 MHz, DMSO-d6) δ 9.44 (s, 1H),
7.90 – 7.80 (m, 2H), 7.54 (d, J = 8.0 Hz, 4H), 7.48 (s, 2H), 7.42 (d, J = 8.1 Hz, 2H), 6.76 (dd, J = 17.7, 10.9
Hz, 1H), 5.89 (d, J = 17.7 Hz, 1H), 5.44 (d, J = 8.1 Hz, 4H), 5.32 (d, J = 10.9 Hz, 1H). 13C NMR (101 MHz,
DMSO-d6) δ 138.1, 136.8, 136.4, 135.8, 134.6, 129.5, 129.3, 127.2, 123.4, 115.8, 52.2. Anal. Calc. for
C32H32Br2N4: C 60.77, H 5.1, M 8.86; Found: C 59.07, H 5.32, N 8.83. HMRs Calc. for C23H23N4.2+: 355.1912;
Found: 355.1947.
3,3'-([1,1'-biphenyl]-4,4'-diylbis(methylene))bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) chloride
was
1
isolated as a white powder. Reaction time: 12 h. Yield: 95 %. H NMR (400 MHz, DMSO-d6) δ 9.62 – 9.49
(m, 1H), 8.06 (s, 2H), 7.89 (dq, J = 10.7, 2.0 Hz, 2H), 7.80 – 7.65 (m, 3H), 7.54 (d, J = 7.8 Hz, 5H), 7.44 (dd, J
= 6.5, 4.4 Hz, 2H), 6.76 (ddd, J = 17.7, 10.9, 3.3 Hz, 1H), 5.89 (dd, J = 17.8, 3.3 Hz, 1H), 5.48 (dd, J = 18.6,
13
3.6 Hz, 5H), 5.32 (dd, J = 10.9, 3.5 Hz, 1H). C NMR (101 MHz, MeOD) δ 171.55, 138.74, 135.85, 128.57,
128.41, 126.71, 126.69, 123.27, 122.48, 114.12, 60.11, 52.63, 19.44, 13.05. Anal calc. for C38H38Cl2N4: C
.+
73.42, H 6.16, N 9.01; Found: C 71.45, H 6.31, N 8.58. HMRs Calc. for C19H18N2 : 274.1465; Found
274.1470.
3,3'-([1,1'-biphenyl]-4,4'-diylbis(methylene))bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) bromide was
1
isolated as a white powder. Reaction time: 12 h. Yield: 98 %. H NMR (400 MHz, DMSO-d6) δ 9.49 (s, 2H),
7.87 (dt, J = 9.7, 1.9 Hz, 4H), 7.82 – 7.71 (m, 5H), 7.59 – 7.50 (m, 9H), 7.43 (d, J = 8.2 Hz, 4H), 6.75 (dd, J =
17.7, 11.0 Hz, 2H), 5.88 (dd, J = 17.7, 0.9 Hz, 2H), 5.47 (d, J = 19.4 Hz, 9H), 5.32 (dd, J = 10.9, 0.9 Hz, 2H).
13C NMR (101 MHz, DMSO) δ 138.1, 136.8, 136.4, 134.7, 134.6, 129.5, 129.3, 127.8, 127.2, 123.4, 115.8,
52.3, 52.2. Anal calc. for C38H38Br2N4: C 64.42, H 5.12, N 7.91; Found: C 63.35, H 5.15, N 7.71.
3,3'-(2-carboxypropane-1,3-diyl)bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) bromide was isolated as a
1
yellowish hygroscopic powder. Reaction time: 24 h. Yield 85 %. H NMR (400 MHz, Methanol-d4) δ 9.16
15
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10.1002/cplu.201600461
(dt, J = 74.5, 1.6 Hz, 2H), 7.79 (t, J = 1.8 Hz, 1H), 7.66 (dq, J = 5.1, 1.9 Hz, 2H), 7.52 (dt, J = 8.5, 1.9 Hz, 4H),
7.44 – 7.35 (m, 4H), 6.86 – 6.69 (m, 2H), 5.85 (dt, J = 17.6, 0.9 Hz, 2H), 5.46 (d, J = 3.6 Hz, 3H), 5.31 (dd, J
13
= 10.9, 0.8 Hz, 2H), 4.12 (q, J = 7.1 Hz, 2H), 1.23 (dt, J = 24.3, 7.1 Hz, 4H). C NMR (101 MHz, MeOD) δ
171.6, 138.7, 135.9, 128.6, 128.4, 126.7, 126.7, 123.3, 122.5, 114.1, 60.1, 52.6, 19.4, 13.1. Anal. Calcd for
+
C28H32Br2N4O2: C 54.56, H 5.23, N 9.09; Found: C 54.24, H 5.47, N 9.09. HRMs Calc. for C28H29N4O2 :
453.2285; Found 453.2288.
3,3'-(2-hydroxypropane-1,3-diyl)bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) bromide was isolated as a
white hygroscopic powder. Reaction time: 168 h. Yield 93 %. 1H NMR (400 MHz, DMSO-d6) δ 9.33 (s, 1H),
7.83 (d, J = 23.1 Hz, 2H), 7.53 (d, J = 50.0 Hz, 4H), 7.43 (d, J = 8.1 Hz, 4H), 6.75 (dd, J = 17.6, 11.0 Hz, 2H),
5.98 (d, J = 5.6 Hz, 1H), 5.88 (d, J = 17.7 Hz, 2H), 5.47 (s, 4H), 5.31 (d, J = 10.9 Hz, 2H), 4.45 (dd, J = 13.3,
2.4 Hz, 2H), 4.31 – 4.10 (m, 2H). 13C NMR (101 MHz, DMSO-d6) δ 138.0, 137.3, 136.4, 134.7, 129.2, 127.1,
123.9, 122.8, 115.8, 67.9, 65.4, 52.6, 52.1. Anal. Calcd for C27H30Br2N4O: C 55.31, H5.16, N 9.55; Found: C
54.65, H 5.58, N 9.26. HRMs Calc. for C27H30BrN4O+: 505.1603; Found 505.1593.
3,3'-(2,3-dihydroxybutane-1,4-diyl)bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) bromide was isolated as a
white hygroscopic powder. Reaction time: 168 h. Yield 78 %. 1H NMR (400 MHz, DMSO-d6) δ 9.27 (s, 3H),
7.79 (d, J = 24.2 Hz, 6H), 7.54 (d, J = 8.1 Hz, 6H), 7.41 (d, J = 8.1 Hz, 6H), 6.76 (dd, J = 17.6, 11.0 Hz, 3H),
5.89 (d, J = 17.7 Hz, 3H), 5.57 (d, J = 6.6 Hz, 3H), 5.45 (s, 5H), 5.32 (d, J = 10.6 Hz, 3H), 4.38 – 4.28 (m, 3H),
4.25 – 4.14 (m, 3H), 3.84 – 3.79 (m, 3H). 13C NMR (101 MHz, DMSO-d6) δ 138.0, 137.2, 136.4, 134.7,
129.1, 127.1, 124.0, 115.8, 70.3, 52.1. Anal. Calc. for C28H32Br2N4O2: C 54.56, H 5.23, N 9.09; Found: C
52.60, H 5.15, 8.82. HMRs Calc. for C28H32N4O2Br+: 535.1708, 537.1693; Found 535.1705, 537.1689.
Alternative procedure for the synthesis of ionic monomers m4a and m5a
Imidazole (1.64 g, 24 mmol, 3 eq) and NaOH (0.96 g, 24 mmol, 3 eq) and CH3CN (10 mL) were
loaded into a 25 mL two-neck flask. To the suspension the appropriate organohalide (8 mmol9
was added dropwise. The mixture was heated to 60 °C for 15 h. After reaction, the mixture was
cooled to r.t. and diluted in DCM (ca. 20 mL), and the resulting white solid was collected. An
aqueous work-up was not possible as the product is soluble in water. The solid was washed with
EtOH to solubilize the target compound. The insoluble solid was discarded, and the EtOH
evaporated to afford the di-imidazole product. In a second step, the di-imidazole compound
(1.34 g, 7 mmol, 1 eq) and CH3CN (10 mL) were loaded in a 25 mL two-neck flask. Then, 4-
vinylbenzyl chloride (2.48 g, 14.7 mmol, 2.1 eq) was added dropwise to the solution. The
mixture was heated to 80 °C for 15 h. After reaction, the mixture was cooled to r.t. and
transferred to a 250 mL flask. The imidazolium salt was purified by precipitation in EtOAc and
diethyl ether using to the procedure described above.
16
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3,3'-(2-hydroxypropane-1,3-diyl)bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) chloride was isolated as a
white hygroscopic powder. Yield 50%. 1H NMR (400 MHz, DMSO-d6) δ 9.51 – 9.30 (m, 2H), 7.84 (s, 4H),
7.65 (s, 2H), 7.60 – 7.48 (m, 6H), 7.42 (t, J = 7.0 Hz, 5H), 6.75 (dd, J = 17.6, 11.0 Hz, 3H), 5.88 (d, J = 17.7,
1.3 Hz, 3H), 5.45 (d, 4H), 5.31 (d, J = 10.9 Hz, 3H), 4.43 (t, J = 12.2, 11.2 Hz, 2H), 4.30 – 4.13 (m, 2H).13C
NMR (101 MHz, DMSO) δ 138.1, 137.3, 136.4, 134.7, 129.2, 128.8, 127.1, 127.0, 123.9, 122.8, 115.8,
68.6, 67.9, 65.4, 52.5, 52.1, 36.4, 15.7. Anal. Calcd for C27H30Cl2N4O: C 65.19, H 6.08, N 11.26; Found: C
59.24, H 6.25, N 11.83.
3,3'-(2,3-dihydroxybutane-1,4-diyl)bis(1-(4-vinylbenzyl)-1H-imidazol-3-ium) chloride was isolated as a
white powder. Yield 58%. 1H NMR (400 MHz, DMSO-d6) δ 9.27 (s, 2H), 7.79 (d, J = 24.3, 1.8 Hz, 4H), 7.54
(d, 4H), 7.41 (d, 4H), 6.76 (dd, J = 17.7, 11.0 Hz, 2H), 5.88 (d, J = 17.7, 1.0 Hz, 2H), 5.57 (d, J = 6.6 Hz, 2H),
5.44 (s, 2H), 5.32 (d, 2H), 4.33 (d, J = 13.8, 3.0 Hz, 2H), 4.25 – 4.12 (m, 2H). 13C NMR (101 MHz, DMSO-d6)
δ 138.0, 136.4, 134.7, 129.3, 129.2, 127.2, 127.1, 123.9, 123.3, 115.8, 70.3, 52.1.
Typical polymerization procedure:
The monomer (0.2 g), azobisisobutyronitrile (AIBN) (5 wt%) and isopropanol (40 ml) were placed
into a 100 ml two neck flask equipped with a condenser. The mixture was heated to 90 °C for 15
h. After reaction, the reaction mixture was cooled down to r.t., and the white precipitate was
transferred to a 250 ml round bottom flask. 100 ml of diethyl ether was added. The precipitate
was sonicated for 15 minutes. This procedure was repeated three times and the white product
was dried under vacuum overnight.
Typical catalytic experiment (at high pressure):
The epoxide (8.3 mmol) and the catalyst (0.5 mol%) are loaded into a 45 ml stainless steel
autoclave. CO2 is added to the desired pressure. The autoclave was purged 3 times with CO2 to
ensure that no air remained in the system. The autoclave was heated in an oil bath set to the
appropriate temperature. To quench the reaction, the autoclave is cooled down in an ice cold
water bath and slowly de-pressurized. 5 mL of EtOAc were added to dilute the reaction mixture.
After decane was added as internal standard, the yields were determined by GC-MS. For the
scope, all the carbonate products were obtained in pure form after either a column-
chromatography or by simple filtration of the solid catalyst.
Typical catalytic experiment (at atmospheric pressure):
17
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The catalyst (5 mol%) and the epoxide (0.83 mmol) were loaded in a 10 mL two-neck flask
equipped with a stirring bar. The flask was connected to a Schlenk line and a CO2 balloon. After
3 vacuum-CO2 cycles, the flask was brought to the desired temperature in an oil bath for the
appropriate time. After reaction, the mixture was cooled to RT. CDCl3 (1 mL) was added to the
flask, as the product was in solid phase, to extract the epoxide/carbonate mixture. The yield was
determined by 1H NMR (D1 = 3 sec).
Typical recycling experiment:
The recycling experiment was performed with epichlorohydrin as a starting material. After
reaction, 5 mL ethyl acetate was added to the mixture, and the liquid phase was carefully
removed with a pipette. This washing was done 3 times. The catalyst was then dried under
vacuum and reused in the next catalytic experiment.
Acknowledgement
The EPFL, Swiss National Science Foundation and the CTI Swiss Competence Center of Energy
Research (SCCER) on Heat and Electricity Storage is thanked for funding.
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Table of contents :
Cross-Linked Ionic (Poly)Styrenes: Highly cross-linked imidazolium-based polymers were
prepared and used as catalysts for the cycloaddition of CO2 with epoxides for carbonate
production. The catalysts operates efficiency using CO2 at atmospheric pressure and therefore
doesn’t require the use of autoclaves.
22
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