Progress in Polymer Science 34 (2009) 1333 1347
Contents lists available at ScienceDirect
Progress in Polymer Science
journal homepage: www.elsevier.com/locate/ppolysci
Ionic liquids as solvents for polymerization processes Progress and
challenges
Przemysław Kubisa"
Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 A
ódz
, Poland
a r t i c l e i n f o a b s t r a c t
Article history:
Ionic liquids (ILs) are organic salts that are liquid at ambient temperatures. Ionic liquids have
Received 6 July 2009
emerged as a new class of solvents for practical applications due to their unique combina-
Received in revised form 27 August 2009
tion of low volatility, chemical stability, high conductivity, wide electrochemical window,
Accepted 4 September 2009
ability to dissolve organic and inorganic solutes and gases, and tunable solvent properties.
Available online 15 September 2009
In polymer science ionic liquids are used as solvents for polymerization processes and as
components of polymeric materials. In this review the advantages and limitations of appli-
Keywords:
cation of ionic liquids as solvents for polymerization processes are critically discussed, with
Ionic liquids
special emphasis on results published within last 5 years.
Polycondensation
© 2009 Elsevier Ltd. All rights reserved.
Radical polymerization
Ionic polymerization
Solvent properties
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1334
1.1. Ionic liquids and their properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1334
1.2. Application of ionic liquids as solvents for chemical reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335
2. Polycondensation processes in ionic liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335
2.1. Enzymatic polycondensations in ionic liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336
2.2. Polycondensation processes in ionic liquid under microwave irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1337
3. Radical polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1337
4. Ionic polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1339
4.1. Cationic polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1339
4.2. Anionic polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1340
5. From ionic liquids to supramolecular polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1340
6. Ionic liquids as solvents for cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1341
7. Miscellaneous application of ionic liquids in polymer chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1342
8. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1342
9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343
"
Tel.: [phone]; fax: [phone].
E-mail address: [e-mail].
0079-6700/$  see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.progpolymsci.2009.09.001
1334 P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347
1. Introduction ned as  the sum of all possible, non-specific interactions
between the solute ions and molecules and solvent mole-
1.1. Ionic liquids and their properties cules, excluding such interactions leading to definite chem-
ical alterations of the ions or molecules of the solute [5].
Ionic liquids (ILs) are organic salts that are liquid at Intuitively it can be expected that because ILs are
ambient temperatures, preferably at room temperature composed of positively and negatively charged ions, they
(RTIL  room temperature ionic liquids). Ionic liquids are should be highly polar. In order to qualify solvents as polar
composed of large organic cations and small inorganic or or non-polar the commonly used parameter is a dielectric
organic anions. Examples of typical cationic and anionic constant. ILs are electrolytes therefore direct measurement
components of ILs are shown in Fig. 1. of dielectric constant is not possible. Measurements of
It is estimated that the number of possible combinations polarity of ILs using solvatochromic and fluorescent probes
resulting in properties that are characteristic for ILs exceeds indicated that their polarity was close to polarity of lower
106 [1]. Thus ILs are very versatile class of solvents and their alcohols [6 8]. Application of dielectric spectroscopy in
properties can be easily tuned for specific application (so- megahertz/gigahertz regime with extrapolation to zero fre-
called task-specific ionic liquids) but at the same time it quency led, however, to static dielectric constant values
is difficult to discuss their properties in general because markedly lower (µ in the range of 10 15) than values found
some properties may differ considerably depending on the by spectroscopy with polarity-sensitive solvatochromic
structure of cation and anion. The most common group of dyes [9]. Those results indicate that ILs are solvents of only
ILs are imidazolium ILs and further discussion will mainly moderate polarity [10]. Dielectric relaxation spectroscopy
be related to this class of ILs. (DRS) studies over the range of temperatures and concen-
Although different authors use different abbrevia- trations confirmed the modest polarity of ionic liquids and
tions for ILs, most commonly abbreviations for cation provided insight into mechanism of dielectric relaxation
and anion structure are given in square brackets [11,12].
(without charges) thus [bmim][PF6] denotes 1-butyl- In a recent mini-review entitled  Ionic Liquids: fact and
3-methylimidazolium hexafluorophosphate, [emim][Cl] fiction there is the following statement:  all that can be
denotes 1-ethyl-3-methylimidazolium chloride, etc. unequivocally stated is that ionic liquids are not as polar as
Room temperature ionic liquids (sometimes abbrevi- often assumed and many more studies are required to gain
ated as RTILs) have emerged as a new class of solvents for a better understanding of their microscopic properties [1].
practical applications due to their unique combination of Another factor that is seldom considered by synthetic
low volatility, chemical stability, high conductivity, wide chemists using ILs as solvents is a competition between
electrochemical window, ability to dissolve organic and two kinds of interactions that are possible in bulk ILs, i.e.
inorganic solutes and gases, and tunable solvent proper- electrostatic interactions of fragments where charges are
ties. Those properties of ILs are frequently taken as granted localized and collective short-range interactions involv-
without proper understanding of possible limitations. ing non-polar parts of the side chains (e.g. alkyl groups
Ionic liquids are indeed non-volatile in that sense that at in alkylimidazolium cations) [13]. Therefore, ILs do not
near ambient temperatures their vapor pressure is negligi- behave like simple molecular solvents and form spatially
ble. It has been shown recently, however, that at least some heterogeneous domains. Due to electrostatic interactions
ionic liquids can be distilled [2,3]. Several ionic liquids can and extended hydrogen-bond systems in liquid state ILs are
ć%
be vaporized under high vacuum at 200 300 C and then highly structured [14 17] thus, as recently was pointed out,
recondensed. Convincing proofs that volatilization involves there is some analogy between structure of ILs and three-
ionic species (without dissociative proton or alkyl transfer) dimensional hydrogen-bonded network of water [18]. Such
have been presented [2,4]. Vapor pressure of ILs remains, behavior of ILs as a pre-organized medium can modify the
however, negligible at near ambient conditions thus for all molecular reactivity by formation of inclusion complexes
practical purposes they may be considered as non-volatile between reactive species and ILs [19].
solvents. ILs are generally considered as chemically stable sol-
Also the earlier opinion that ILs are highly polar solvents vents. This is essentially true but again chemical stability
has more recently been questioned. The concept of polarity cannot be taken as granted. Proton in C(2)-position of imi-
is not easily defined. In IUPAC document polarity is defi- dazolium cation is acidic and deprotonation leading to
carbene is possible under basic conditions [20]. This phe-
nomenon was exploited in polymer chemistry by applying
imidazolium ILs as precatalysts for carbene initiated poly-
merizations [21,22].
Acidity of C(2)-protons may also lead to unex-
pected side-reactions of imidazolium ILs. Thus, when
paraformaldehyde was dissolved in imidazolium IL slow
insertion of formaldehyde occurred leading to imida-
zolium cation containing  CH2OH group as substituent
at 2-position [23]. In attempted anionic polymerization
of methyl methacrylate (MMA) in imidazolium IL, chain
transfer to ionic liquid involving substitution of N-alkyl
Fig. 1. Typical cationic and anionic components of ionic liquids.
group by growing poly-MMA chain was observed [24].
P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347 1335
These examples show that although in most cases when There is a growing number of papers published every
ILs were used as solvents for polymerization processes they year describing application of ionic liquids as solvents for
acted as chemically inert reaction media, the possibility of polymerization processes. It is, however, not always clear
side-reactions cannot be a priori excluded because ILs are what authors expect to achieve by replacing typical organic
not entirely chemically stable under any condition. solvents with ionic liquids. In spite of improved meth-
In many papers dealing with application of ionic liquids ods of synthesis and commercial availability of various
as solvents authors stress the  green chemistry aspect. ionic liquids [35] they are still more expensive than typical
This is sometimes overemphasized in reports when ILs are organic solvents. Therefore, application of ILs as solvents
used as solvents for polymerization but volatile organic for polymerization processes were justified only if due to
solvents have to be used for polymer isolation and purifi- their specific properties some special effects, difficult to
cation. What is more important, one cannot claim that ILs achieve in more conventional solvents, could be expected.
as a class conform to 12 Principles of Green Chemistry The  green aspect of ILs is often overestimated because
as defined by US Environmental Protection Agency [25] at present it is difficult to imagine application of ILs as
that require among other such properties as biodegrad- solvents in large scale industrial processes.
ability and low toxicity. These points were addressed in What are the properties of ILs that may be of interest for
a paper under somewhat provocative title:  Are ionic liq- synthetic polymer chemists?
uids green solvents? presented at 2002 ACS Symposium Good thermal stability and non-volatility may offer
on Ionic Liquids [26]. Unquestionable advantage of ILs is, some advantage for the processes which require removal
however, their recyclability thus ILs (frequently containing of by-products at relatively high temperatures such as
dissolved catalyst) can be repeatedly used in subsequent polycondensation processes. Another property of ILs is
reaction cycles [27]. their ability to dissolve several inorganic or organometal-
lic compounds that are used as catalysts in polymerization
1.2. Application of ionic liquids as solvents for chemical processes. This offers at least two advantages. Polymer-
reactions izations in ILs may be conducted under homogeneous
conditions otherwise difficult to achieve (an example is a
Ionic liquids are intriguing solvents and their potential solubility of ATRP catalysts in ILs). Other advantage is a pos-
to replace organic solvents in different areas of chemistry sibility of recycling and reusing solutions of catalyst in ILs,
has been firmly established [28]. The growth of number of especially when expensive catalysts (e.g. those based on
publications and patents devoted to ionic liquids in recent noble metals) are used.
10 years is steeper than simply exponential. Although Polarity of ionic liquids is often cited as a property that
application of ILs in polymer chemistry not always elim- justify application of ILs, especially as solvents for ionic
inates the need for volatile organic solvents (quite often polymerization. On the basis of recent reports, as discussed
resulting polymers have to be separated from ILs using later in the text, it seems, however, that in spite of their
organic solvents) the set of properties displayed by ILs ionic nature ILs are only moderately polar solvents and
may be advantageous for certain specific applications [29]. their estimated dielectric constant values are lower than
In order to take benefit of rather unusual properties of these of e.g. nitromethane or dimethylformamide. On the
those liquids it is, however, necessary to really understand other hand the high charge density in ILs, existence of
advantages and limitations of using ILs as solvents for poly- hydrophobic (longer alkyl chains) and hydrophilic (ionic
merization processes. groups) domains in bulk ILs, ability of certain ILs (e.g. 1-
The use of ionic liquids in polymer science is not lim- alkyl-3-methylimidazolium chloride) to break hydrogen
ited to their application as solvents. Ionic liquids are used as bonds and possibility of specific interactions of cationic or
additives to polymers (plasticizers, components of polymer anionic components with growing species or monomers
electrolytes, porogenic agents). More recently properties may lead to effects that do not appear in solution in
of polymers containing chemically bound ionic liquid moi- typical organic solvents. Investigations of polymerization
ety (polymeric ionic liquids) are studied and possibilities processes in ILs may therefore provide useful information
of their applications are being explored. These subjects concerning polymerization mechanism.
have been discussed in recently published reviews entitled: In the subsequent sections the possible advantages and
 Macromolecules in ionic liquids: Progress, challenges, and limitations of application of ionic liquids as solvents for
opportunities [30] and  Advanced applications of ionic liq- polycondensation, radical polymerization and ionic poly-
uids in polymer science [31]. In these reviews application merization will be discussed. Resent results concerning
of ionic liquids as solvents for polymerization processes is enzymatic polymerization, and microwave-assisted poly-
only briefly discussed. Review papers dealing specifically merization will be presented separately. Electrochemical
with the application of ionic liquids as polymerization sol- polymerization leading to conducting polymers in ILs will
vents were published before year 2005 [32 34]. Since then not be discussed because it has been covered in recent
not only several papers appeared concerning this subject reviews [30,31].
but also our understanding of properties of ionic liquids
has advanced considerably. The aim of the present review 2. Polycondensation processes in ionic liquids
is therefore to critically review the progress in the field
of application of ionic liquids as solvents for polymeriza- Polycondensation is typically conducted at relatively
tion processes on the basis of recent literature, with main high temperature thus non-volatile and thermally stable
emphasis on possible advantages and limitations. ILs seem to be suitable solvents for polycondensa-
1336 P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347
Fig. 2. Polycondensation of aromatic dianhydrides with aromatic diamines.
Fig. 3. Polycondensation of glycolic acid.
tion processes. Research in this area has mainly been of -caprolactone (CL) in ILs with PGA in the presence of
directed towards synthesis of polyamides, polyimides and Ti(OBu)4 catalyst with subsequent transesterification led
polyesters. to random copolymers [46].
In the early studies on polycondensation of di- or tetra- More recently two step procedure (involving post-
carboxylic acid chlorides or anhydrides with diamines, polycondensation in IL) was applied for polycondensation
limitations due to the limited solubility of some aromatic of sebacic, adipic and succinic acid with aliphatic diols.
substrates in ILs were indicated [36,37]. On the other hand Aliphatic polyesters with Mw up to 6 × 104 were obtained
in some studies of polycondesation catalytic effect of ILs and again it was noted that solubility of polyesters in
was observed. Thus when dicarboxylic acids (less toxic IL was a limiting factor. Solubility depends on structure
although less reactive than corresponding chlorides or of IL (nature of cation and anion) and correlation was
anhydrides) were used in direct polycondensation with found between the miscibility of aliphatic polyester/ionic
diamines, relatively high molecular weight polyimides liquid system and the extent to which their solubility
were obtained in the absence of any added catalysts and parameters matched [47]. Thus, in reports dealing with
it was concluded that ILs act not only as solvents but also synthesis of polyesters by polycondesation in ILs, difficul-
as catalysts [38,39]. In polycondensation of aromatic dian- ties in obtaining high molecular weight polymers due to
hydrides with aromatic diamines, shown schematically in limited solubility of monomers and/or polymers are clearly
Fig. 2, solubility of starting materials could be improved indicated.
by addition of imidazolium type zwitterion which led to Catalytic effect of ionic liquids was observed in other
higher molecular weights of resulting polyimides [40]. type of polycondensation process namely in polycondensa-
Ionic liquids were also used as solvents for the synthesis tion of phenol and formaldehyde [48]. This approach may
of various optically active polyamides [41,42]. Polyimide lead to preparation of transparent ion conductive phenol
networks have been obtained by polycondensation of resin-ionic liquids hybrid films in which ionic liquids are
pyromellitic anhydride with aromatic di- and tri-amines. dispersed in phenol resin matrix at the nanometer level.
Due to the compatibility between branched polyimide Enzyme catalyzed polycondensation processes in ILs
network and ionic liquids, products that formed self- have also been studied as discussed in separate section.
supporting gels even at low content of polyimide (6 wt%) Presented results show that application of ILs as sol-
were formed. Gels showed good thermal stability and sta- vents for polycondensation processes offers only limited
ble ion conduction in a wide temperature range [43]. Ionic advantage. In some systems additional catalysts can be
liquids were also employed for interfacial polymerization avoided because ILs catalyze polycondensation reaction.
(at hexane/IL interface) leading to polyureas with macrop- More interesting seems to be an application of ILs as sol-
orous structure. It was concluded that surface interactions vents for polycondensations leading to networks that form
between IL and polyurea were responsible for observed hybrid gels with ILs embedded and finely dispersed in poly-
porous structure [44]. mer matrix, which may lead to interesting ion-conducting
Other group of processes that has been studied in IL materials.
solutions involves synthesis of polyesters by polyconden-
sation (cf. Fig. 3). Direct polycondensation of glycolic acid
2.1. Enzymatic polycondensations in ionic liquids
in ILs gave only oligomeric products. Postpolycondensa-
tion of a preformed oligomer in IL led to polyesters with
Ionic liquids (ILs) have been investigated as an inter-
moderate molecular weights (DPn up to 45). Higher molec- esting alternative to organic solvents for enzymatic
ular weights could not be achieved due to precipitation of
conversion of small molecules [49 51]. An enzyme deacti-
polymers [45].
vation, frequently observed in many polar organic solvents
Poly(glycolic acid) oligomers (PGA) were further used
like methanol or DMF, is typically diminished in common
for the synthesis of PGA/CL copolymers. Polymerization
ILs.
P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347 1337
Introduction of enzymes for polymer synthesis in
organic solvents has led to increased research efforts in this
field [52,53] and ionic liquids have been tested as possi-
ble replacement of organic solvents in polymer synthesis.
In several reports synthesis of polyesters by polyconden-
sation of hydroxyacids or ring-opening polymerization
Fig. 4. Schematic representation of PSt-b-MMA block copolymer forma-
of lactones was described [54 58] and in some cases
tion.
enhanced enzyme activities (with additional microwave
irradiation) was observed. Activity of enzyme (lipase) in IL
solution was analyzed in terms of anion s H-bond basicity, decrease in termination rate was related to increasing vis-
enzyme dissolution, anion ionic association ability, cation cosity of the medium. Taken together these effects explain
hydrophobicity, and substrate ground-state stabilization or higher overall rates and higher molecular weights observed
hydrophobic interactions [59]. for polymerization of MMA in ionic liquids [68]. Those
conclusions were confirmed later by results of the investi-
2.2. Polycondensation processes in ionic liquid under gation by the same method of propagation rate coefficients
microwave irradiation (kp) of methyl methacrylate (MMA) and glycidyl methacry-
late (GMA) radical polymerizations in four different ionic
Application of ILs as media for microwave-assisted liquids [69,70].
reactions offers several advantages [60]. Typical organic Extending of the range of investigated ILs is important
solvents are frequently flammable and volatile, which is because it has been pointed out that the course of radi-
a safety hazard for high-temperature and closed-vessel cal polymerization depends on the structure of IL, i.e. the
applications using microwaves. In contrast, ILs have high length of the alkyl substituent in cation and the nature of
boiling-points, low vapor pressures and high thermal anion [71].
stabilities. In addition, typical ILs have moderately high An interesting observation related to application of
dielectric constants (in the range of 10 15), and relatively ILs as solvents for radical polymerization was that block
low heat capacities (in the range of 1 2 J/g K) [59]. This copolymer of styrene and methyl methacrylate could be
combination allows ILs to absorb microwaves efficiently. formed efficiently in conventional radical polymerization
Owing to these advantages, ILs have been investigated [72]. This was explained by poor solubility of polystyrene
as solvents in a number of microwave-mediated reac- in IL used ([bmim][PF6]) and precipitation of polymer
tions. Accelerating effect of ILs (as compared to common hindering diffusion and increasing life-time of propagat-
organic solvents) was observed for microwave-assisted ing radicals. Upon addition of second monomer, which
polymerization of oxazoline [61,62] and -caprolactone was miscible with the polymer dispersion, polymeriza-
[63]. Microwave heating in ionic liquids was used also tion continued and block copolymer was formed, as shown
for polycondensation reactions leading to polyamides [64] schematically in Fig. 4.
and poly(urea urethanes) [65]. Although indeed ILs, due Indeed it should be remembered that only some poly-
to their properties, are suitable solvents for microwave- mers are soluble in typical ILs thus solubility factors may
assisted chemical processes, application of this approach play important role. The role of polymer solubility in ILs
for polymer synthesis is still limited. Certain advantages has been discussed in a recent review [73]. Alternative
have been indicated, but until now only slight improve- explanation was forwarded recently, involving assumption
ment of reactions conditions (more efficient heating, higher that a  protected radical mechanism was in operation.
rates) has been achieved. The observed effect was explained by radical  protection
in IL as a part of the process of monomer separating into
3. Radical polymerization extremely small domains in the IL leading to significant
partitioning of radicals into IL domain. The existence of
In 2002 the first report appeared indicating that rate  protected radicals leads to formation of block copolymer
constants of propagation and termination in radical poly- when other monomer (MMA) is added to a system after
merization may be significantly affected by ILs. Kinetics polymerization of the first monomer (St) [74]. No indica-
of radical polymerization of methyl methacrylate (MMA) tion of  trapped radical effect was observed, however, in
in [bmim][PF6] was studied by Pulse Laser Polymeriza- RAFT polymerization of styrene in pyridynium ionic liquids
tion (PLP) technique. The kp of MMA increased steadily in which polystyrene was soluble [75].
as the concentration of IL increased. At 50 vol.% of IL kp Effect of enhancement of polymerization rate was
was approximately twice that of bulk MMA [66]. It was also observed for photoinitiated polymerization of poly
argued that increase of kp value was due to lowering of (ethylene glycol) mono- and di-methacrylates [76]. Mea-
activation energy for propagation. Further investigation of surements of viscosity revealed an interesting pheno-
this system revealed that also the rate of termination was menon viscosity of monomer/IL mixture for some ILs was
affected by IL [67]. The rate of termination decreased by higher than simple additive combination of components.
an order of magnitude as IL concentration was increased It has been postulated that this viscosity synergism is
to 60 vol.%. The enhancement of propagation rate was important for observed kinetic effects [77]. Influence of the
attributed to increasing polarity of the medium allow- viscosity on the propagation and the termination reaction
ing greater contribution from charge-transfer structures as well as the molecular weight distribution in MMA poly-
and lowering thus the energy of transition state while the merization was also observed by other researchers [78].
1338 P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347
Fig. 5. Schematic representation of activation deactivation processes in
ATRP.
Fig. 7. Schematic representation of reactions involved in RAFT polymer-
ization.
An interesting feature of ILs as solvents for ATRP is that
in some systems organic ligand (typically amine) may be
Fig. 6. Disproportionation of Cu(0) species.
avoided. This was shown for polymerization of MMA with
iron-based catalysts in phosphonium type ILs [92] or poly-
In the last few years several papers appeared confirm- merization of acrylonitrile in imidazolium ILs [93]. Similar
ing that when conventional radical polymerizations are effects as for ATRP in ILs have been observed also for reverse
conducted in ionic liquid media the rates and molecu- ATRP [94 96].
lar weights are higher than for polymerization in bulk or An interesting application of ILs for growing polymer
organic solvents [68,79 83]. Observation, that in radical brushes was reported recently [97]. Thus, small droplets
polymerization in ILs rate constants of propagation are of IL containing ATRP catalyst were placed on a surface
higher and rate constants of termination are lower than of silicon wafer and methyl methacrylate was introduced
in polymerization in bulk or typical organic solvents, has into the droplet. Polymer brushes were formed only at the
a special significance for processes of controlled radical area covered by IL droplets. Thus, IL droplets were used as
polymerization, especially atom transfer radical polymer- microreactors in which polymerization proceeded in con-
ization (ATRP) shown schematically in Fig. 5 [84,85]. fined geometry.
Controlled radical polymerization is not living because Lower (meth)acrylates are frequently used as
termination is not eliminated. Any factor leading to the monomers for polymerizations in ILs because both
increase of kp/kt ratio would therefore widen the window monomers and polymers are readily soluble in most ILs.
of polymerization conditions within which polymeriza- The limitations due to the polymer solubility were clearly
tion can be controlled. This is one of the advantages pointed out in the first report on reversible addition-
of conducting controlled radical polymerization in ILs. fragmentation chain transfer (RAFT) polymerization
Another advantage is solubility of several transition metal (shown schematically in Fig. 7) in imidazolium ionic
complexes that are used as ATRP catalysts (e.g. Cu salts liquids.
in conjunction with amine ligands) in ILs [86]. Thus in While homogeneous RAFT polymerizations of methyl
many systems polymerization proceeds under homoge- methacrylate (MMA) and methyl acrylate (MA) were fully
neous conditions and catalyst can be easily separated from controlled leading to polymers with Mn close to calcu-
polymer [87]. In several cases solution of catalyst in IL was lated values and low dispersity, polymerization of styrene
recovered and reused [29,88,89]. stopped at limited conversion due to polymer precipitation
Ionic liquids, among other polar solvents, have been [98 100]. Recently, however, successful RAFT polymer-
found to favor disproportionation of Cu(I)X species ization of styrene in pyridinium ILs under homogeneous
(X = halogen) into Cu(0) and Cu(II)X2 species (as shown in conditions was reported [75]. Analysis of the course of RAFT
Fig. 6) in the presence of different N-containing ligands polymerization in different ILs led to conclusion that it
(catalytic systems frequently used in ATRP). depends on both nature of RAFT agent employed and type
Cu(II) species provide the reversible deactivation of rad- of IL [101]. The effect of IL structure was related to solubility
icals into alkyl halide species while Cu(0) promotes the of monomer and polymer in IL and it was shown that many
activation of active species. It has been postulated that of those phase partitioning effects may be overcome by
this process proceeds by the outer-sphere single electron using IL-tethered RAFT agent (IL functionalized with RAFT
transfer process with low activation energy. Therefore, agent by click chemistry, as shown in Fig. 8) [101].
activation and deactivation steps are very fast and Nitroxide mediated radical polymerization of methyl
bimolecular termination is negligible. Very fast controlled acrylate (MA) in imidazolium ILs was successfully con-
ć%
polymerization leading to very high molecular weight ducted although relatively high temperature (140 C) was
polymers (Mn up to 1.5 × 106) was achieved for a range of required. At these conditions significant contribution of
monomers containing electron-withdrawing groups such spontaneous initiation was noted [102,103].
as acrylates, methacrylates, and vinyl chloride, initiated ILs have been used also as solvents for radical copoly-
with alkyl halides, sulfonyl halides, and N-halides [90]. merization processes. It has been shown that in IL solution,
For polymerization of MMA initiated with arenesulfonyl statistical copolymers from methacrylates of strongly dif-
chlorides in [bmim][PF6] catalyzed by Cu2O/2,2 -bipyridine ferent polarities and solubilities are formed. The relative
system a strong accelerating effect of IL was observed reactivity of monomers and thus the composition of
[91]. Nearly complete conversion of MMA to polymer with copolymers depended on the structure of imidazolium ILs
DPn = [monomer]0/[initiator]0 and Mw/Mn <" 1.1 could be [104,105].
obtained at room temperature within a few hours. These Reactivity ratios in copolymerization in ILs may also be
results indicate that although polymerization is fast, the modified by changing the mechanism of copolymerization
good control is still maintained. No mechanistic interpre- from radical to charge-transfer (CT) mechanism. Thus for
tation of the accelerating effect of IL in the studied system styrene methyl methacrylate pair reactivity of styrene in
was presented until now. CT copolymerization was enhanced as compared to radical
P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347 1339
Fig. 8. Synthesis of IL-tethered RAFT agent.
Fig. 9. Cationic polymerization of styrene initiated by organoborate acids (HBOB).
polymerization [106]. The effect of ILs on CT homopolymer- as TiCl4 or BCl3 are typically used) initiates polymeriza-
ization of methyl methacrylate and styrene have also been tion of styrene [112]. Analysis of MALDI TOF spectra of
studied [107,108]. polymers revealed, however, that some macromolecules
contain head-groups resulting from initiation by proton
4. Ionic polymerization (most probably formed by transfer). Observed molecular
weights differed from calculated values and dispersity was
4.1. Cationic polymerization rather broad. These observations indicated that although
ionization of C Cl bond indeed proceeded in IL solution
Ability of ionic liquids to dissolve wide range of even in the absence of coinitiators, full control of polymer-
inorganic compounds was exploited in the study in ization could not be achieved. Ionization of C Cl bond in
which organoborate acids (HBOB) (bisoxalatoboric acid, IL solution was confirmed by measurements of the rate
bissuccinatoboric acid and bisglutaratoboric acid) were of racemization of optically active 1-phenylethyl chloride
used as initiators of the cationic polymerization of (model of dormant species) [113]. Although racemization
styrene in pyrollidonium, imidazolium and phospho- (proceeding by reversible ionization of C Cl bond) was
nium bis(trifluromethanesulfonyl)amide ILs as shown in observed, its rate was relatively low as compared with the
Fig. 9 [109,110]. At relatively high initiator concentration rate of polymerization therefore the requirement of fast
ć%
([Styrene]0/[HBOB]0 in the range of 10 30) at 60 C poly- interconversion of active and dormant species was appar-
merization proceeded to practically complete conversion ently not fulfilled in the studied system. More recently it
giving polymers with dispersity <"1.3. Mn values were close has been shown that ionization of C Cl bond may proceed
to calculated up to DPn about 20. Above this value observed more efficiently in IL/SO2 mixtures (several ILs may dis-
Mns were considerably lower than calculated indicating solve up to 2 moles of SO2 per 1 mole of IL [114]) although
chain transfer. Authors described studied system as con- the rate of ionization was still not sufficient to achieve con-
trolled polymerization. trolled cationic polymerization of styrene [115].
Earlier belief that ILs are highly polar solvents stim- In another study cationic polymerization of styrene ini-
ulated another study of polymerization of styrene in the tiated with AlCl3 in IL ([bmim][PF6]), supercritical CO2 and
system in which equilibrium between dormant and active organic solvent (CH2Cl2) was investigated. The only conclu-
species governs the concentration of growing ionic species sion was that in ILs rates and molecular weights are higher
[111]. It has been expected that polarity of ionic liquids than in organic solvent [116].
would favor ionization of C Cl bond (cf. Fig. 10). There were also some attempts to apply ILs for cationic-
Indeed it has been shown that 1-phenylethyl chlo- ring-opening polymerization (ROP). Imidazolium IL with
ride even in the absence of coinitiators (Lewis acids such PF6- anion was used as solvent for ROP of lactones with
Fig. 10. Schematic representation of activation deactivation processes in controlled cationic polymerization of styrene.
1340 P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347
rare-earth metal triflates as catalysts and it was shown Anionic polymerization of styrene initiated by butyl
that catalyst may be recycled (at least three times) with- lithium (BuLi) or sodium acetate (NaAc) in phosphonium
out losing activity [117]. Considering cost of catalyst this type IL was also reported [124]. At relatively high initia-
approach may offer some advantage. It was also observed tor concentration (<"2 mol% with respect to styrene) BuLi
ć%
that ILs have accelerating effect on polymerization of initiated polymerization gave 20% yield after 70 h at 60 C,
caprolactone initiated with polymer supported scan- with NaAc yield was still lower (10%). Yields were improved
dium triflate [118]. In other attempt imidazolium IL was upon addition of butyl imidazolium butane sulfonate zwit-
used as solvent for cationic polymerization of 3-ethyl-3- terion (cf. the structure shown in Fig. 2) up to 75% in
hydroxymethyloxetane (EOX). In cationic polymerization 70 h and 94% in 140 h. Molecular weights were high (up
of EOX, leading to branched multihydroxyl polyethers in to 400,000) with dispersity in the range of 1.4 2.1. This
organic media intramolecular hydrogen bonding leads to indicates that transfer reactions are not important in this
intramolecular chain transfer to polymer. It was expected system thus perhaps phosphonium ionic liquids are more
that hydrogen bonding could be minimized in IL solution suitable as solvents for anionic polymerization than imi-
allowing preparation of higher molecular weight poly- dazolium ionic liquids (although imidazolium cation was
mers. Only a limited effect was, however, observed [119]. present in zwitterionic additive).
Cationic polymerization of 3,3-bis(chloromethyl)oxetane Complications arising from the presence of acidic pro-
in ILs has been also reported but molecular weights were ton at 2-position of imidazolium ring may be eliminated
limited [120]. if proton is replaced by other group. We have found
that when paraformaldehyde is dissolved in 1-alkyl-3-
methylimidazolium chloride slow reaction proceeds by
4.2. Anionic polymerization
which C(2) H group is quantitatively converted into
C(2) CH2 OH group [23]. By this reaction not only acidic
As mentioned in Section 1, under basic conditions ILs
proton is removed but additionally functional group is
are not entirely stable therefore they do not seem to
introduced into imidazolium ring of IL. By reaction with
be suitable solvents for anionic polymerization. In spite
NaH alkoxide ions were formed which were used to ini-
of that there are a few reports on anionic polymeriza-
tiate anionic polymerization of ethylene oxide (EO) [125].
tion in IL. Group transfer polymerization (GTP) of methyl
Up to DPn <"30 polymerization proceeded without any side-
methacrylate (MMA) in IL was studied [121]. GTP is not a
reactions giving with quantitative yields imidazolium ionic
typical anionic polymerization but at present it is believed
liquids containing short polyoxyethylene chains attached
that propagation proceeds on anionic species that are
at 2-position, as shown in Fig. 12.
reversibly deactivated [122]. Although reported yields
Blending of those materials with high molecular weight
were not always high (between 20 and 99%) and dis-
polyoxyethylene (POE) resulted in the reduction of crys-
persities were rather high (1.7 2.4) authors claimed that
tallinity of POE. Because addition of ILs to polymer
polymerization proceeded as living process [121]. More
electrolytes leads to enhanced ionic conductivity, ILs with
recently anionic polymerization of MMA initiated with
chemically bound POE chains may find application in the
alkyl lithium initiators was reported [123]. Observed lim-
field of solid polymer electrolytes [30].
ited yields (<"10%) and high dispersities (PDI <"2) were
An accelerating effect of ILs on polymerization of
attributed to deactivation of initiator by acidic proton in
propylene oxide in the presence of double metal cyanide
2-position of imidazolium ring. Another side-reaction in
catalysts has been observed. Addition of IL led not
anionic MMA polymerization initiated by alkyl lithium
only to about 10-fold increase of reaction rate but
initiators was observed in our laboratory [24]. Analyzing
also to significant reduction of the content of terminal
the end-group structure by MALDI TOF we have found
unsaturation [126]. ILs were used also as solvents for poly-
that chain transfer to ionic liquid occurs at the early
merization of N-carboxyanhydrides (NCA) initiated with
stages of polymerization according to a scheme shown in
amines. Poly(amino acid)s having low dispersity, molec-
Fig. 11.
ular weights close to the theoretical values, and helical
Although this reaction puts a limit on molecular weights
secondary structures were obtained [127].
(Mn < 2000) yields are high (up to 98%) and practically all
the macromolecules contain ionic end-groups derived from
5. From ionic liquids to supramolecular polymers
IL, which may be of synthetic interest.
In very recently published Highlight article an inter-
esting possibility of formation of supramolecular ionic
networks in ILs composed of multivalent cations and anions
is discussed [128]. By combining dication (two covalently
linked tetraalkylphosphonium cations) and tetraanion
(ethylenediaminetetraacetate anion) ionic liquids show-
ing high dynamic viscosity more than order of magnitude
higher than simple phosphonium type ILs were formed.
Replacement of ethylenediaminetetraacetate anion with
a porphyrin tetracarboxylate gave materials that could be
pulled into fibers or molded into shape-persistent objects
Fig. 11. Chain transfer to ionic liquid in anionic polymerization of methyl
methacrylate. in which porphyrine moiety retained its fluorescent prop-
P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347 1341
Fig. 12. Synthesis of imidazolium ionic liquid containing  CH2OH substituent and its application as initiator of anionic polymerization of ethylene oxide.
erties which is important for possible applications as e.g. Without using any catalyst, cellulose derivatives with high
sensors [129]. This observation may pave a way for manu- degree of substitution could thus be prepared in IL solutions
facturing of a new type of materials combining mechanical [138,139].Graft copolymers of cellulose were obtained by
properties of ionomers with the homogeneity and high ATRP of methacrylates or styrene after functionalization of
charge density typical for ionic liquids. HO- groups in cellulose with 2-bromopropionyl bromide
Recently dendritic ionic liquids that self-assemble into [140,141], as shown in Fig. 13.
supramolecular columns and spheres undergoing self- Graft copolymers were also prepared by ring-opening
organization into liquid crystalline and crystalline lattices polymerization of cyclic esters initiated by HO- groups of
has been reported. cellulose in IL solution [142 145].
These supramolecular structures contain the ionic liq- Taking advantage of cellulose solubility in IL cellu-
uid part segregated as a core forming thus nanoreactors lose functionalized with acrylate group was polymerized
that may be used to perform reactions in confined ionic in solution in IL. The isolated product was a composite
liquids geometries [130]. consisting of cellulose and the polymerized ionic liquid
[146]. Dissolution of cellulose in imidazolium IL containing
polymerizable group (<"10 wt% solution) and subsequent
6. Ionic liquids as solvents for cellulose
polymerization led to IL/cellulose composite in which both
The application of ILs as solvents in carbohydrate chem- components were efficiently compatibilized [146]. Solu-
tion of cellulose (microcrystalline cellulose or even wood)
istry has recently been reviewed [131 133]. Some ionic
liquids, especially those containing Cl- anion, are dissolv- in IL in the presence of solid acid catalysts could be selec-
tively depolymerized first to oligocellulose and than to
ing cellulose including lignocellulosic biomass [134 137].
simple sugars [147].
Cellulose with a degree of polymerization in the range
Ability of ILs to dissolve materials that are strongly
from 290 to 1200 could be dissolved in [bmim][Cl] to
relatively high concentration (up to <"20%) without degra- hydrogen bonded and thus hardly soluble has been
employed also in other systems. Nylon 6 was depolymer-
dation although solubility decreased with increasing DPn.
Fig. 13. Functionalization of cellulose in ionic liquid and synthesis of graft copolymer.
1342 P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347
chemical polymerization leading to specialty conducting
polymers [30,31,155].
Less obvious is an answer to the question whether it
is justified to study conventional polycondensation (syn-
Fig. 14. Formation of carbene from imidazolium ionic liquid.
thesis of polyesters or polyamides) or polymerization
processes (leading e.g. to polystyrene, polymethacrylates
ized in IL in high yield (>85%) to caprolactam and recycled
or polyacrylates). Certainly the mass production of e.g.
IL could be used in subsequent reaction cycles [148].
polystyrene or poly(meth)acrylates involving application
of ionic liquids as solvents (even considering environmen-
7. Miscellaneous application of ionic liquids in tal issues) is difficult to imagine.
polymer chemistry Ionic liquids, however, may offer significant advantage
as model solvents for elucidating the details of polymer-
Owing to their specific physical and chemical properties ization mechanism in general and this aspect may be
ILs may find specific applications in polymer chemistry. ILs important for the further progress of polymer chemistry.
have been found to be effective in reducing the exothermic Radical polymerization (conventional as well as controlled)
self-heating in thermal polymerization of styrene and acry- is a good example of possibilities offered by application of
lonitrile and reducing the thermal product decomposition ionic liquids as solvents. Rate constants of elementary reac-
[149]. tions in radical polymerization depend on viscosity. This is
Chemical instability of ILs under specific conditions was especially pronounced for the rate constant of termination
employed in the system in which imidazolium IL was used [156,157] which requires the encounter of two growing
as a precatalyst reservoir in a phase-transfer polymeriza- radicals but there is also evidence that rate constants of
tion with an immiscible THF solution of monomer (lactide) other elementary reactions may depend on viscosity [158].
and initiator (BuOK). In situ activation of the ionic liquid ILs have a very broad range of viscosities. Depending
produced carbene (cf. Fig. 14) that migrated to the organic on the structure of cation and anion viscosity may vary
phase initiating polymerization of lactide [22]. between 20 and 40,000 cP as compared with viscosities of
ILs are suitable solvents for preparing polymer stere- typical organic solvents which are in the range between
ocomplexes and studying their properties. Isotactic and 0.2 and 100 cP [153]. Viscosity of particular ionic liquid
syndiotactic poly(methyl methacrylate) (PMMA) formed a may be tuned additionally by even minute amounts of
stereocomplex in ionic liquids ([bmim][PF6]). The stereo- water or organic solvents [159]. In the course of polymer-
complex formation brought about the gelation of IL and was ization viscosity of reaction medium (especially for bulk
fully thermoreversible. Due to non-volatility and thermal polymerization) may increase significantly and the depen-
stability of IL rapid stereocomplex formation and its disso- dence of rate constants of elementary reactions on viscosity
ciation could be achieved. The possibility of preparing two- is studied by analyzing the kinetics at different stages of
and three-dimensional arrangements of C60 molecules by polymerization. Viscosity, however, is not the only param-
stereocomplexation of fullerene end-capped poly-MMA eter that changes with conversion thus other factors may
has been indicated [150]. possibly contribute to observed effects. Ionic liquids, in
Aqueous solutions of ionic liquids have been used as which only minor change of structure may result in sig-
novel and environmentally friendly reaction media to syn- nificant change of viscosity, offer thus a unique possibility
thesize and  control the size of different cross-linked of studying the effect of viscosity alone, under otherwise
polymer beads by suspension polymerization reactions. identical conditions. As described earlier in this review,
The average size of polymer beads can be varied from the Haddleton et al. studying the polymerization of methyl
macro- to the nanoscale [151]. methacrylate in imidazolium ionic liquid at different pro-
portion of monomer to IL found that termination rate
8. Outlook constant decrease significantly with increasing fraction of
IL in the mixture and related this effect to increasing viscos-
Ionic liquids are no longer laboratory curiosity, finding ity [66,67]. Studies of systems at constant ratio of monomer
already industrial applications [152,153]. The first indus- to structurally similar ionic liquids of different viscosity
trial process using ionic liquid is so-called BASILTM process could provide more straightforward information of the
(Biphasic Acid Scavenging using Ionic Liquids) developed dependence of termination rate constant on viscosity.
by BASF, other processes are also in operation or in the The enhancement of rate constant of propagation in ILs
development stage [154]. has also been observed. The origin of this phenomenon is
Still it is rather difficult today to imagine wider appli- still a matter of dispute. In the first paper in which this
cation of ionic liquids as solvents for mass production of observation was reported [67], possible causes have been
commodity polymers. In polymer science ionic liquids may discussed and it was concluded that the observed accel-
rather find application as components of polymeric system eration is due to the increased polarity of the medium,
(plasticizers, ion-conducting components). which allows a greater contribution from charge-transfer
Is it therefore purposeful to study and develop typical structures, lowering the energy of the transition state.
polymerization processes in ionic liquids? The alternative hypothesis was recently formulated [74]
There are certain applications in which ionic liquids may assuming the presence of protected radical (through for-
offer significant advantage. Wide electrochemical window mation of radical IL adduct) and the presence of monomer
(5 6 V) may justify their application as solvents for electro- domains within IL. In the report on RAFT polymerization in
P. Kubisa / Progress in Polymer Science 34 (2009) 1333 1347 1343
Fig. 15. Schematic representation of counterion exchange in ionic polymerization in ionic liquids.
pyridinium ILs, however, it was concluded that either pro-
tected radicals are not present or they have very little or no
influence on the kinetics [75]. It has been pointed out that
interaction between cations and anions in ILs may signifi-
cantly affect the course of reaction involving radicals with
dipole moment. Radical reactivity may depend on minor
changes in the media and until now these phenomena for
radicals in ILs have not been described and compared with
organic solvent [160]. Thus it seems that more kinetic stud-
ies of radical polymerization in ILs are needed to explain the
effect of IL on reactivity of radicals in propagation, which
may shed some light on the problem of radical reactivity in
Fig. 16. Examples of chiral ionic liquids.
general.
There is much less interest in ionic polymerization in
ILs. ILs cannot be treated as neutral solvents for ionic poly- [171]. Thus only very preliminary information concerning
merization because they are composed of ions. Cations or the effect of chiral ILs on the stereochemistry of polymer-
anions are counterions for growing species in anionic or ization is available until now.
cationic polymerisation, respectively. If ionic polymeriza- There is much more activity in this area among organic
tion is conducted in IL solution, there is a large excess of ions chemists and it is believed that although the use of chiral
that constitute ionic liquid. It is therefore not obvious which ILs is still in its infancy, it is an area with great potential that
ion is really the counterion for growing species. This may will expand in coming years [172]. It remains to bee seen
be especially important if ion-pairs are involved in propa- whether this optimistic view can be shared by polymer
gation. Polarity of ILs is not as high as previously suggested chemists.
therefore propagating species may exist predominantly in ILs may be useful solvents for the synthesis of inorganic
form of ion-pairs. To what extent an exchange of coun- coordination polymers and stabilization of their nano-sized
terions between ion-pairs (according to scheme shown in objects such as cyano-bridged metallic nanoparticles of dif-
Fig. 15) could occur cannot be easily predicted. Such possi- ferent size. ILs act in such systems both as the stabilizing
bility should certainly be considered. agent and solvent, so that no additional ligand is required
Exchange of anions was observed recently in systems to obtain stable colloidal solutions [173,174].
in which two different ILs (each liquid at room tempera-
ture) were mixed together. After mixing, gelation occurred
9. Conclusion
which was attributed to exchange of ions and formation of
ion-pairs of IL that was solid at room temperature [161].
Ionic liquids are intriguing solvents and their unique
It has to be remembered also that interactions between
properties, although still not fully understood, will
cationic and anionic components in ILs are quite complex
undoubtedly stimulate curiosity driven research in the area
and the nature of those interactions may depend on the
of polymer chemistry. We have already reached a stage
fraction of IL in monomer/IL solution, i.e. on concentration
when some basic features of polymerization processes in
of monomer in polymerizing mixture [162 165].
ILs have been established. It seems that further progress
An interesting albeit still unexplored area in which
in this field can be achieved by careful selection of studied
application of ILs may be fruitful is stereospecific poly-
systems in which application of ILs may either offer real
merization in chiral ILs. Chiral solvents have been used
synthetic advantage or provide new insight into polymer-
by organic chemists as inducers of chirality in asymmetric
ization mechanisms rather than by showing just another
synthesis but limited enantioselectivity coupled with high
examples of processes that may be conducted in ILs.
cost of chiral solvents hampered the progress in this area.
With developing of relatively simple methods of synthesis
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