New method for synthesis of methacrylate-type polymerizable ionic liquids

Article abstract of DOI:10.1080/00397911.2012.745159

A new method for the synthesis of polymerizable ionic liquids bearing a methacrylate moiety was developed with the aim to avoid premature polymerization of synthesized compounds. Spacer length between the imidazolium cation and the polymerizable functional group varied from 2 to 10 carbon atoms. Different 1-(n-hydroxyalkyl)-3-methylimidazolium bromides and 1-[n-(methacryloyloxy)- alkyl]-3-methylimidazolium bromides were obtained with very good yields (more than 90%).

Full text of DOI:10.1080/00397911.2012.745159

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New Method for Synthesis of
Methacrylate-Type Polymerizable Ionic
Liquids
a b
a b
b
Kaija Põhako-Esko
, Triinu Taaber
, Kristjan Saal , Rünno
b
b
a
Lõhmus , Ilmar Kink & Uno Mäeorg
a
University of Tartu, Institute of Chemistry , Tartu , Estonia
University of Tartu, Institute of Physics and Estonian
b
Nanotechnology Competence Center , Tartu , Estonia
Accepted author version posted online: 31 May 2013.Published
online: 25 Jul 2013.
To cite this article: Kaija Phako-Esko , Triinu Taaber , Kristjan Saal , Rnno Lhmus , Ilmar Kink & Uno
Meorg (2013) New Method for Synthesis of Methacrylate-Type Polymerizable Ionic Liquids, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
43:21, 2846-2852, DOI: 10.1080/00397911.2012.745159
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1
Synthetic Communications , 43: 2846–2852, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.745159
NEW METHOD FOR SYNTHESIS OF METHACRYLATE-
TYPE POLYMERIZABLE IONIC LIQUIDS
1
,2
1,2
Kaija P o˜ hako-Esko, Triinu Taaber, Kristjan Saal,
2
2
R u¨ nno L o˜ hmus, Ilmar Kink, and Uno M a¨ eorg
University of Tartu, Institute of Chemistry, Tartu, Estonia
University of Tartu, Institute of Physics and Estonian Nanotechnology
Competence Center, Tartu, Estonia
2
1
1
2
GRAPHICAL ABSTRACT
Abstract A new method for the synthesis of polymerizable ionic liquids bearing a methacry-
late moiety was developed with the aim to avoid premature polymerization of synthesized
compounds. Spacer length between the imidazolium cation and the polymerizable functional
group varied from 2 to 10 carbon atoms. Different 1-(n-hydroxyalkyl)-3-methylimidazo-
lium bromides and 1-[n-(methacryloyloxy)-alkyl]-3-methylimidazolium bromides were
obtained with very good yields (more than 90%).
[Supplementary materials are available for this article. Go to the publisher’s online
1
edition of Synthetic Communications for the following free supplemental resource: Full
experimental and spectral details.]
Keywords Imidazolium salts; ionic liquids; methacrylate; polymerizable ionic liquids
INTRODUCTION
Ionic liquids (ILs) are a diverse group of salts that are liquid at ambient tem-
perature. The formation of a stable crystal lattice is prevented by poorly coordinated
ions. Cation is generally an organic structure with low symmetry, and the anion can
be either organic or inorganic. ILs are the focus of intense research because of their
[
1,2]
extraordinary chemical and physical properties.
Recently, polymeric forms of ILs (PILs) have attracted attention. Typical
characteristics of ILs combined with mechanical durability of polymers make PILs
a key material for development of a wide range of emerging technologies, for
Received October 16, 2012.
Address correspondence to Kaija P o˜ hako-Esko, University of Tartu, Institute of Chemistry, 14a
Ravila St., 50411, Tartu, Estonia. E-mail: kaija.pohako@ut.ee
2
846

METHACRYLATE-TYPE IONIC LIQUIDS
2847
[
3–4]
example, electrochemical devices, such as batteries and fuel cells.
[
Another major
6]
area of PIL research is the use of the material as gas absorber. The extraordinary
solvation properties inherent to ILs make PILs attractive for the preparation of com-
[7]
[8,9]
posite materials such as ‘‘bucky plastic’’ or PIL-cellulose composites.
[
10]
PILs can be prepared either from polymerizable cations or anions. To date,
research has been focused more on the introduction of polymerizable groups (acry-
late, methacrylate, or vinyl) to the IL cation. Several papers have been published
addressing electrical and mechanical properties of PILs and their potential applica-
tions. However, at the same time the synthesis of PIL monomers has received rela-
tively little attention. The general approach for the preparation of acrylate- or
methacrylate-containing PIL monomers is accomplished in a two-step synthesis.
In the first step haloalcoholis treated with (meth) acryloyl chloride, resulting in
formation of the corresponding ester. In the second step N-alkylimidazole is
[
11,12]
quaternized with ester from the previous step.
as the counteranion and are introduced by anion exchange.
A variety of anions can serve
[
2]
Contrary to the conventional synthetic route, we decided to introduce the
methacrylate function into the IL structure in the last step (Scheme 1). This helps
to avoid the premature polymerization of synthesized PIL monomers, which is the
[
11,12]
main drawback of the published method.
Previously the idea has been noted
[13]
in the literature, but to the best of our knowledge it has not been studied further.
With this work we aim to investigate its scope and limitations. Alkyl chain lengths
between imidazolium cation and polymerizable moiety were also varied because it
Scheme 1. Synthesis of PIL monomers.

˜
K. POHAKO-ESKO ET AL.
2848
has been shown earlier that increasing the spacer length leads to greater electrical
[
14]
conductivity of the polymerized material.
mediates of the synthesis, 1-(n-hydroxyalkyl)-3-methylimidazolium salts, have been
It is worthwhile to mention that inter-
[15]
applied in tribology and in organic synthesis because of their catalytic activity
[
in a coupling reaction to obtain cyclic carbonates.
16,17]
DISCUSSION
In the current study different PIL monomers containing methacrylate groups
as polymerizable moieties were synthesized (3a–e). x-Bromo alcohols with alkyl
chain of different lengths (1a–d) were used for quaternization of N-methylimidazole.
The obtained 1-(n-hydroxyalkyl)-3-methylimidazolium salts (2a–d) were acylated with
methacryloyl chloride for introducing the polymerizable functional group. The poly-
merizable ionic liquid monomers (3a–e) were obtained with good yields. The structures
of compounds 2a–d and 3a–e were confirmed by spectral techniques [Infrared (IR),
1
13
H and C NMR]. Compounds 2c, 2d, 3c, and 3d are new, and their molecular
structures were additionally confirmed by high-resolution mass spectroscopy.
For synthesis of x-bromo alcohols, a well-known method starting from corre-
[18,19]
sponding a,x-diols was used.
The reaction resulted in a mixture from the
desired product, unreacted diol, and corresponding dibromide. To separate the pro-
duct, a liquid–liquid extraction method, innovative for the particular synthesis, was
developed.
The following step, quaternization of N-methylimidazole, occurs smoothly at a
reasonable temperature and results in different 1-(n-hydroxyalkyl)-3-methylimidazo-
lium bromides (2a–d) with quantitative yields (Scheme 1, Table 1).The reaction can
be carried out without any solvent, as the reagents are liquids and mutually miscible.
The reaction time depends on the particular x-bromo alcohol. The reactivity of alkyl
halides in substitution reactions generally decreases with increasing alkyl chain
[
2]
length. In the case of 2-bromoethanol it is about 24 h but with other applied x-
bromo alcohols a reaction time up to 48 h is necessary to complete the reaction.
The reaction can be accelerated with moderate heating but the temperature should
ꢀ
not exceed 40–50 C to avoid darkening the product, supposedly caused by
[
20]
chromophores formed in side reactions.
-(n-hydroxyalkyl)-3-methylimidazolium bromides depends on the length of the
alkyl chain (Table 1).
As the last step of the synthesis, the polymerizable moiety was introduced to
The physical state of obtained
1
the IL cation (Scheme 1, Table 2). 1-(n-Hydroxyalkyl)-3-methylimidazolium bro-
mides obtained in the previous step were acylated with methacryloyl chloride. The
Table 1. Yields of 1-(n-hydroxyalkyl)-3-methylimidazolium bromides 2a–d
Compound
Yield (%)
Physical state
2
2
2
2
a
b
c
99
99
89
94
Solid
Viscous liquid
Viscous liquid
Solid
d

METHACRYLATE-TYPE IONIC LIQUIDS
2849
Table 2. Yields of 1-[n-(methacryloyloxy)-alkyl]-3-methylimidazolium
bromides 3a–d
Compound
Yield (%)
3
3
3
3
a
b
c
95
97
93
92
d
reactions were carried out at room temperature to avoid polymerization of the
synthesized products. To force the reaction run completely, excess reagent was used.
Washing with dichloromethane was effective for removing any residue of hydroxyl
functionalized ionic liquid from the final product.
To conclude, a new method for synthesis of PIL monomerswith methacrylate
as the polymerizable functional group was developed. Unlike conventional methods,
the new method does not suffer from premature polymerization of the product—a
drawback that would significantly limit its practical applications. As a result, differ-
ent 1-[n-(methacryloyloxy)-alkyl]-3-methylimidazolium bromides were obtained with
very good yields (more than 90%).
EXPERIMENTAL
Materials
N-methylimidazole (Aldrich), methacryloyl chloride (Fluka), 2,6-di-tert-butyl-
-methylphenol (BHT; Fluka), HBr (40% aqueous solution), and all solvents except
4
acetonitrile (ACN) were used as received. ACN (Rathburn) was distilled over CaH2.
2-Bromoethanol (Fluka) was distilled prior to use, collecting a fraction with boiling
ꢀ
point 149–150 C at atmospheric pressure.
Characterization
1
13
H and C NMR spectra were recorded at ambient temperature on a Bruker
Avance II 200 spectrometer, using dimethylsulfoxide (d -DMSO) or CDCl as a sol-
6
3
1
13
vent. The H NMR spectra were measured at 200 MHz and the C NMR spectra
were determined at 50 MHz. The chemical shifts were internally referenced by the
residual solvent signals relative to tetramethylsilane. The IR spectra were obtained
on a Perkin-Elmer Spectrum BXII FT-IR instrument. Gas chromatography–mass
Spectrometry (GC-MS) was performed on Agilent Technologies 7890A GC System
MSD 5975C (column HP-5 ms 30 m ꢁ 0.25 mm). The spectra of high-resolution mass
spectrometry (HRMS) were measured on a Thermo Electron LTQ Orbitrap ESI
spectrometer, and the sample was dissolved in ACN.
[
18]
General Procedure for Synthesis of x-Bromo Alcohols (1b–d
)
In a typical experiment a,x-diol (0.33 mol) was mixed with 1.2 eq of concen-
trated HBr (40% aqueous solution) and toluene. The reaction mixture was refluxed

˜
K. POHAKO-ESKO ET AL.
2850
in Dean–Stark equipment until water separation was completed. While the reaction
mixture cooled down to room temperature, two layers were formed. The layers were
separated. Toluene was removed by evaporation and the product was separated
from the evaporation residue using the following extraction: 100 mL of mixture of
ethylene glycol and methanol (1:1) were added to the evaporation residue and
extracted with 3 ꢁ 50 mL of hexane, which removed the dibromide side product.
Methanol was removed by evaporation and 100 mL of toluene was added to ethylene
glycol. Ethylene glycol was washed out with water, and a solution of x-bromo alco-
hol in toluene was obtained. After drying the solution with MgSO and removing
4
toluene by evaporation, x-bromo alcohol was obtained with the purity of more than
98% according to GC-MS. The yield varied between 40 and 60% depending on the
starting material.
General Procedure for Synthesis of 1-(n-Hydroxyalkyl)-3-
methylimidazolium Bromides (2a–d
[21,22]
)
n-Bromoalcohol (0.1 mol) was dropwise added to 1 eq N-methylimidazole at
room temperature under an inert atmosphere. The reaction mixture was stirred
for 24–48 h. Depending on the alkyl chain length of n-bromoalcohol used, a white
solid or colorless viscous liquid was formed with quantitative yield. The obtained
product was vacuumed (<1 mm Hg) at room temperature for 12 h.
[21]
Compound 2a: 1-(2-Hydroxyethyl)-3-methylimidazolium bromide
1
H NMR (200 MHz, DMSO): d 9.10 (s, 1H, -N-CH-N-), 7.74 (s, 1H, -N-
CH¼CH-N-), 7.70 (s, 1H, -N-CH=CH-N-), 5.52 (s, 1H, -OH), 4.23 (t, 2H,
J ¼ 5.2 Hz, -N-CH -), 3.87 (s, 3H, -N-CH ), 3.72 (t, 2H, J ¼ 5.2 Hz, -CH -OH).
2
3
2
1
3
C NMR (50 MHz, DMSO): d 137.21, 123.80, 123.11, 59.79, 52.08, 36.19. IR
cm ): 3239 (O-H), 3057 (C-Harom-), 2928 (C-H), 2856 (C-H), 1571 (C=C), 1427
C ¼ N-), 1231 (C-N), 1168 (C-N), 1069 (C-O).
ꢂ1
þ
(
(
General Procedure for Synthesis of 1-[n-(Methacryloyloxy)-alkyl]-3-
methylimidazolium Bromide (3a–d
[23,24]
)
1-(n-Hydroxyalkyl)-3-methylimidazolium bromide (0.1 mol) was suspended in
50 mL of acetonitrile and 5 mg of BHT was added. The obtained suspension was
1
ꢀ
stirred in an argon atmosphere and cooled on an ice bath to 0 C. Methacryloyl
chloride (2 eq) diluted with 20 mL of acetonitrile was added dropwise. The reaction
mixture was allowed to warm to room temperature and stirred 48 h in an argon
atmosphere. Acetonitrile was removed by evaporation. After that the residue was
extracted with 3 ꢁ 25 mL of diethyl ether to remove the unreacted methacryloyl
chloride. After extraction, a new amount of BHT was added. The obtained viscous
liquid was vacuumed (<1 mm Hg) 12 h on an ice bath. The product was obtained as
yellowish oil with yield 93–97%.

METHACRYLATE-TYPE IONIC LIQUIDS
2851
Compound 3a: 1-[2-(Methacryloyloxy)-alkyl]-3-methylimidazolium
[
bromide
24]
1
H NMR (200 MHz, DMSO): d 9.39 (s, 1H, -N-CH-N-), 7.89 (s, 1H,
-
1
N-CH¼CH-N-), 7.81 (s, 1H, -N-CH=CH-N-), 6.06 (s, 1H, CH ¼ C-), 5.73 (s,
2
H, CH ¼ C-), 4.60 (t, 2H, J ¼ 5.4 Hz, -N-CH -), 4.46 (t, 2H, J ¼ 5.2 Hz, -CH -
3 2 3
2
2
2
1
3
O-), 3.91 (s, 3H, -N-CH ), 1.86 (s, 3H, CH ¼ C-CH ). C NMR (50 MHz, DMSO)
ꢂ
d 166.53, 137.57, 135.70, 127.09, 124.12, 123.22, 63.18, 48.44, 36.38, 18.34. IR (cm
1
): 3024 (C-H
-), 2961 (C-H), 2949 (C-H), 1718 (C=O), 1635 (C=CH ), 1575
C=C), 1452 (C ¼ N-), 1317, 1295 (C-O), 1161 (C-O, C-N overlapped) 945 (C=H ).
arom
þ
2
(
2
SUPPORTING INFORMATION
1
13
Full experimental detail and H and C NMR spectra can be found via the
‘Supplementary Content’’ section of this article’s Web page.
‘
ACKNOWLEDGMENTS
The authors acknowledge the Estonian Science Foundation (Grant Nos. 8428
and 8794), Estonian Ministry of Education and Research (Targeted Project No.
SF0180058s07), the European Regional Development Fund (Centre of Excellence
‘
3
‘Mesosystems: Theory and Applications’’, TK114; and ‘‘TRIBOFILM’’
.2.1101.12-0028), and the graduate school ‘‘Functional Materials and Technolo-
gies’’ receiving funding from the European Social Fund under Project
.2.0401.09-0079 in Estonia. The authors acknowledge Nord Forsk (Excellent
1
Nordic Chemistry) for support.
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