CHEMSUSCHEM REVIEWS DOI: 10.1002/cssc.201300864

Deep Eutectic Solvents in Polymerizations: A Greener Alternative to Conventional Syntheses Francisco del Monte,* Daniel Carriazo, Mara C. Serrano, Mara C. Gutirrez, and M. Luisa Ferrer[a]

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CHEMSUSCHEM REVIEWS The use of deep eutectic solvents (DESs) that act as all-in-one solvent–template–reactant systems offers an interesting green alternative to conventional syntheses in materials science. This Review aims to provide a comprehensive overview to emphasize the similarities and discrepancies between DES-assisted and conventional syntheses and rationalize certain green features that are common for the three DES-assisted syntheses described herein: one case of radical polymerization and two cases of polycondensations. For instance, DESs contain the precursor itself and some additional components that either provide certain functionality (e.g., drug delivery and controlled release, or electrical conductivity) to the resulting materials or direct their formation with a particular structure (e.g., hierarchical-type). Moreover, DESs provide a reaction medium, so poly-

1. Introduction The use of solvents in materials science is ubiquitous. Most synthetic processes for common organic, inorganic, and hybrid materials (e.g., polymers, nanoparticles, or porous materials) are performed in solution.[1–5] The homogenization of reagents and products within the reaction media is clearly one of the main roles played by solvents, but they can also influence the activation energetics and reaction thermodynamics.[6, 7] Aqueous, nonaqueous, or mixed solvents have been chosen, depending on the peculiarities of the synthesis. For scaling up and with increasing environmental status and regulatory pressure focusing on solvents, significant attention is paid to the use of green chemistry alternatives to traditional solvents in not only fundamental research but also in the chemical industry.[8] In 1998, Anastas and Warner defined the term green chemistry as “The invention, design, and application of chemical products and processes to reduce or to eliminate the use and generation of hazardous substances.”[9] However, controversy may arise when determining whether or not a particular synthetic process is green. Thus, the 12 Principles of Green Chemistry have become a widely accepted set of criteria for the rapid assessment of the “greenness” of a given chemical route or for comparison of the environmental acceptability of two rival processes (Figure 1). In materials science and from a green chemistry point of view, progress in the preparation of advanced materials with controlled structure and/or unprecedented functionality depends largely on the core competence of materials chemists to design and develop new synthetic strategies that, at any stage of the synthesis, limit the use of reducing and/or cross-linking agents, solvents, and surfactants. As mentioned above, solvent reduction is of paramount importance. It is worth noting that [a] Dr. F. del Monte, Dr. D. Carriazo, Dr. M. C. Serrano, Dr. M. C. Gutirrez, Dr. M. L. Ferrer Instituto de Ciencia de Materiales de Madrid (ICMM) Consejo Superior de Investigaciones Cientficas (CSIC) Campus de Cantoblanco, 28049 Madrid (Spain) E-mail: [email protected]

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www.chemsuschem.org merizations are ultimately carried out in a solventless fashion. This means that DES-assisted syntheses match green chemistry principles 2 and 5 because of the economy of reagents and solvents, whereas the functionality incorporated by the second component allows the need for any post-synthesis derivatization to be minimized or even fully avoided (principle 8). DESs also provide new precursors that favor more efficient polymerization (principle 6) by decreasing the energy input required for reaction progress. Finally, the use of mild reaction conditions in combination with the compositional versatility of DESs, which allows low-toxic components to be selected, is also of interest from the viewpoint of green chemistry because it opens up the way to design biocompatible and/or eco-friendly synthetic methods (principle 3).

they typically represent over 85 % of mass utilization in a typical chemical manufacturing process; considering that recovery efficiency is far from satisfactory they are major contributors to environmental pollution. The development of solvent-free alternative processes would be the ideal situation. Otherwise (i.e., if none of the reagents is a liquid that can be used as the reaction medium), replacement of volatile organic solvents by nonflammable, nonvolatile, nontoxic, and inexpensive “green solvents” would be desired. In addition to solvent reduction, the design of synthetic strategies in which the use of any of the above-mentioned chemical reagents is reduced or even fully eliminated is a challenge as well. Within this context, ionic liquids (ILs) have been intensively investigated as an alternative reaction medium. They are defined as salts with a melting point below the boiling temperature of water (100 8C). ILs exhibit exceptional properties, such as low flammability; stability against air and moisture; excellent solvation potential; low water content; chemical and thermal stability; and high heat capacity, density, and electrical conductivity. Nonetheless, their negligibly low vapor pressure is the most prominent feature and why they are considered to be green. However, the current consensus is that ILs cannot be generalized as either green or toxic, but that their environmental impact strongly depends on the type of cation and anion used to produce the IL.[10, 11] Recycling is therefore mandatory also when considering their economical cost. At this stage, it is worth discussing a related class of ILs named deep eutectic solvents (DESs). DESs are molecular complexes typically formed between quaternary ammonium salts and hydrogen-bond donors. The charge delocalization that occurs through hydrogen bonding between the halide anion and the hydrogen-donor moiety is responsible for the decrease in the melting point of the mixture relative to the melting points of the individual components. DESs share many characteristics of conventional ILs (e.g., nonreactive with water, nonvolatile, and biodegradable) and offer certain advantages. For instance, the preparation of eutectic mixtures in a pure state can be accomplished more easily than that of ILs with no need for postsynthetic purification because the purity of the resulting DES simply depends on the purity of the individual compoChemSusChem 2014, 7, 999 – 1009

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Figure 1. Green Chemistry is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances. Anastas and Warner developed a list of the 12 Principles of Green Chemistry, which help to explain what the definition means in practice. Adapted with permission from Ref. [9].

nents. Moreover, the low cost of eutectic mixtures based on readily available components makes them particularly desirable (more so than conventional ILs) for large-scale synthetic applications. Examples of low-cost eutectic mixtures include DESs based on choline chloride (ChCl) first described by Abbott and co-workers;[12–15] low-melting eutectic mixtures of sugar, urea, and salts first described by Kçnig and co-workers;[16–18] natural deep eutectic solvents (NADES) first described by Choi et al.;[19, 20] and low-transition–temperature mixtures (LTTMs) described by Kroon and co-workers.[21] Interestingly, the use of ChCl, natural carboxylic acids, amino acids, different sugars, and even water provide certain biodegradable and renewable features to the resulting eutectic mixtures.

Francisco del Monte is a Scientist at the Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC) in Spain. He received a BSc degree in organic chemistry in 1991, an MSc degree in polymer science in 1992, and a PhD in chemistry in 1996. He then spent two years as a post-doctoral fellow at the University of Los Angeles, California. Since 2004, he has led the Group of Bioinspired Materials at the ICMM-CSIC. His current scientific interest is in the use of biomimetic chemistry and deep eutectic solvents for the preparation of hierarchically organized materials.

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Organic syntheses,[22–28] separation processes,[29–32] biomass processing,[33–38] and carbon dioxide adsorption[39, 40] constitute the principal fields in which DESs have been applied. Moreover, the eco-friendly and low-toxic character of some DESs has allowed their use as solvents for both natural and synthetic membrane-like structures[41] and proteins,[42, 43] and even for microorganism[44] with preservation of their intrinsic activity. Thus, Gorke et al. were the first to use DESs as reaction media for enzyme-based biotransformations by assessing the activity of different hydrolases in ChCl/urea mixtures.[42] They found that the conversion of styrene oxide into the corresponding diol with epoxide hydrolase was 20-fold enhanced by using this DES as a co-solvent, despite the denaturing character that urea typically exhibits in aqueous dilutions. Whether or not urea maintains any denaturing activity in a DES was recently studied in DESs of ChCl/urea and ChCl/glycerol by comparison of the lysozyme thermal unfolding and refolding process,[45] the reversibility of which was more impeded in the former than in the latter DES. This feature was anticipated by Lindberg et al. who, using different DESs (e.g., mixtures of ChCl with urea, ethylene glycol, or glycerol) as co-solvents, studied the hydrolysis of chiral (1,2)-trans-2-methylstyrene oxide enantiomers by potato EH StEH1 hydrolase.[46] Thus, DESs of ChCl/glycerol were subsequently used by Zhao et al. in the protease-catalyzed transesterification of N-acetyl-1-phenylalanine propyl ester from the corresponding ethyl ester in 1-propanol.[47] Further interesting examples of the use of DESs in organic synthesis, separation processes, biomass processing, carbon dioxide adsorption, and biotransformations exist, but, while recognizing the tremendous relevance of this topic, we will not continue to describe ChemSusChem 2014, 7, 999 – 1009

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CHEMSUSCHEM REVIEWS them herein. We consider that there are already excellent reviews that cover most of the recent advances in this area, so readers are referred to them for detailed information.[48–53] We focus now on the use of DESs in the synthesis of materials. Within the context of the synthesis/preparation of materials, two of the most successful applications of DESs are as 1) electrolytes for the electrodeposition of metals and 2) solvents for electrochemistry reactions and electropolishing (metal dissolution).[54–56] The use of DESs as versatile tools capable of playing a structure-directing role in the formation of different framework materials is also interesting. In these cases, and in a similar manner to some ILs,[57] DESs act as all-in-one solvent–template–reactant systems, that is, at the same time DESs are the precursor, template, and reactant medium for the fabrication of a desired material with a defined morphology or chemical composition. This is an interesting situation from a green chemistry point of view. As mentioned above, if none of the reagents is a liquid that can be used as the reaction medium, the development of solvent-free alternative processes allows the replacement of volatile organic solvents by nonflammable, nonvolatile, nontoxic, and inexpensive green solvents. Examples of materials prepared from all-in-one DESs include metal phosphates,[58] oxalatophosphates,[59] aluminophosphates,[60, 61] carboxymethylphosphonates,[62] oxalato[63] phosphonates, polyoxometalate-based hybrids,[64] zeolites,[65] or metal–organic frameworks (MOFs).[66] Readers are referred to a review published by Morris and Parnham for further information on ionothermal syntheses of zeolites, MOFs, and inorganic–organic hybrids.[67] Moreover, another excellent, recent review published by Jrme and co-workers highlighted the most interesting work with DESs not only in the these fields but also in electrochemistry.[53] Recently, we also reviewed the use of DESs in the synthesis of polymers and related materials.[68] Unfortunately, at that stage, we were not able to provide a comprehensive view of the green aspects of these synthetic processes. Now, with some further experience in the field gained from our most recent works, we aim to demonstrate the universality of DES-assisted syntheses, to emphasize their similarities and discrepancies with conventional syntheses, and to rationalize their most characteristic green features. We focused on three polymerization processes: one case of radical polymerization and two cases of polycondensations. These polymerizations were selected as model systems because they already meet some of the above-mentioned green principles. Thus, our main objective was to further enhance the greenness features of these model systems upon the use of DESs. We consider that the following paragraphs provide an unprecedented rationalization of the green aspects of DES-assisted syntheses that may help to develop further interesting research in this field.

2. Green Acrylic Polymerizations Among different radical polymerizations, we selected frontal polymerization (FP) as the model reaction. FPs were first described by Chechilo and co-workers.[69] in the 1970s and later, in the 1990s, extended by Pojman and co-workers to different  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 2. a) Pictures (and scheme) show the experimental setup used for FP. The bottom of the sample was immersed into an oil bath at 130 8C. Thermocouples were placed at different height positions of the sample (T1–T4 were located on the outer part of the glass that contained the monomeric solution, so as not to disturb propagation, whereas T4’ was located on the inner one because FP ended at that position) to monitor propagation of the heat front. Plot of temperature versus time for the temperatures experimentally recorded by the different thermocouples for FPs performed in b) DES composed of acrylic acid (AA) and ChCl, and c) regular solvents (e.g., DMSO) are also given, along with pictures of the monolithic polyacrylates that result from FPs performed in d) DES and e) DMSO. Adapted with permission from Ref. [73].

monomers and systems.[70–72] FPs are performed in long reactors in which initiation (either thermal or light induced) is triggered at one end of the reactor (Figure 2 a). Thus, a well-defined polymerization front propagates axially along the reaction vessel (from one end of the reactor to the other) taking advantage of the exothermic nature of acrylic polymerizations. Consequently, FPs exhibit a particularly interesting energy efficiency (green chemistry principle 6) and may be considered as a greener alternative to conventional radical polymerizations. Unfortunately, its practical application is by no means trivial. The solvent of choice has to control the progressive propagation of the heat front along the reactor. Otherwise, the temperature tends to experience a sharp and simultaneous increase in the whole reactor (Figure 2 c) that promotes the appearance of bubbles, owing to boiling of solvent and/or monomer, and even the “burnout” of the initiator, the occurrence of which (either separately or in a combined fashion) disrupts the polyChemSusChem 2014, 7, 999 – 1009

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www.chemsuschem.org mer, forms the original DES will be the only byproduct after FP. If this second component lacks functionality, it can be fully recovered after FP and reutilized in subsequent syntheses.[73] If the second component is an active molecule (e.g., lidocaine), it will be capable of providing certain functionality (e.g., drug delivery) to the resulting material (Figure 3).[74] Moreover, one can take advantage of the excellent solvent properties of DESs to prepare added-value materials. For instance, DESs are capable of homogeneously dispersing carbon nanotubes (CNTs), the incorporation of which into the resulting polymer provided composites with some particular mechanical properties when compared with those of regular polyacrylates.[75] In summary, DESs not only further optimized the intrinsic energy efficiency of FP (green chemistry principle 6), but also allowed FPs to proceed in a solventless manner (green chemistry principle 5). Moreover, DESs offered the possibility of increasing FP conversions (green chemistry principle 2), allowed full recovery and reutilization of the resulting byproducts in a subsequent FP (green chemistry principle 1), or introduced certain functionalities to the resulting material (green chemistry principle 8). Based on this, DES-assisted FPs provided a remarkable enhancement of the green features of conventional FPs.

3. Green Polycondensations Figure 3. Top: Scheme representing the use of a DES composed of lidocaine and acrylic acid in FP. Full acrylic acid conversion into polyacrylic acid allows the achievement of drug-eluting materials. Bottom: Cumulative lidocaine release (measured by absorbance at l = 263 nm) in phosphate buffer at pH 7 and 22 8C under Sink conditions from the polyacrylic acid monolith obtained after FP (see inset). Adapted with permission from Ref. [74].

merization front and the ultimate formation of a useful al (Figure 2 e). The use of eutectic mixtures of acrylic or methacrylic acid with ChCl opened up interesting perspectives in FPs. For instance, the resulting polymer is always free of bubbles because DESs are nonboiling solvents. DESs also offer great versatility in compositional terms, so that the reaction medium can exhibit an excellent compromise between viscosity and density of double bonds. It is worth noting that the use of eutectic mixtures with viscosities of around 100– 150 cP allows the heat front to propagate progressively. This helps to control the temperature at the front (Figure 2 b), which avoids problems associated with initiator “burnout” (Figure 2 d). Moreover, stabilization of the temperature at the polymerization front minimized convection instabilities that could eventually result in collapse of the front and enhanced polymer conversions up to, in the first approach, about 80 %.[73] Interestingly, this conversion could be improved up to 100 %.[74] This is a remarkable result from a green chemistry point of view. Thus, the second component that, besides the acrylic mono 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

materi-

3.1. Polyesters obtained from citric acid and octanediol In this second case, the reaction model of choice was the polycondensation of citric acid with octanediol for the formation of poly(diol-co-citrates) (PDCs). PDCs were first described by Yang et al.[76, 77] in 2004 and, similar to other biodegradable and biocompatible polyesters, have demonstrated tremendous versatility as polymeric networks for regenerative medicine. The conventional synthetic process first consists of citric acid melt-

Figure 4. Scheme representing the synthesis of poly(octanediol citrate) from a) 1,8-octanediol dissolved in molten citric acid (at 160 8C), and b) a eutectic mixture composed of 1,8-octanediol and lidocaine, the low melting point of which allows the dissolution of citric acid and its subsequent polycondensation at temperatures as low as 90 8C. Adapted with permission from Ref. [78].

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CHEMSUSCHEM REVIEWS ing by thermal treatment at 160–165 8C for a few minutes (Figure 4 a). Octanediol is then dissolved in this molten phase and polycondensation starts. The temperature is maintained at 140 8C over 60 min, and then decreased to 80 8C to allow the reaction to proceed over several days until completion. The synthesis fits well with green chemistry principles 1 and 2, which refer to the high conversion and biodegradable character of any eventual byproduct; principle 5, which refers to the absence of solvents in which the reaction is performed; and principle 7, which refers to the use of feedstock as precursors (e.g., citric acid). However, considering the biodegradable character of these polymers and their common use in biomedicine, their processing in the form of drug-eluting materials would indeed be of interest. In these cases, the entrapment of the active ingredient into the polyester becomes an issue because the exposition of these substances to the relatively high temperature used for synthesis could result in partial decomposition and subsequent loss of activity. The alternative would be postsynthetic functionalization, which is not an ideal situation in terms of green chemistry. Within this context, DES-assisted syntheses offer an interesting opportunity to circumvent this problem (Figure 4 b). For instance, the mixture of 1,8-octanediol and lidocaine, which is a local anesthetic, in stoichiometric molar ratios forms a eutectic with a melting point of about 40 8C. Citric acid can easily be dissolved in this eutectic mixture and polycondense with 1,8octanediol at temperatures far below those described above for regular polycondensations (e.g., 90 8C and even lower, if one assumes that the long reaction time is the penalty for the low input of energy). Lidocaine was finally entrapped—with a high loading because of the stoichiometric in which it formed part of the eutectic mixture—into the resulting polyesters. Thus, we were able to obtain drug-eluting materials, the controlled release of which into an aqueous medium was simply based on the biodegradable character of the polyester network. It is worth noting that lidocaine would decompose (at least, partially) following regular polycondensation (the T5%onset is ca. 155 8C).[78] This synthetic approach can be extended upon the use of different compounds capable of forming eutectic mixtures, depending on the functionality that one desires to introduce into the resulting polyester. For instance, quaternary ammonium and phosphonium salts have demonstrated certain antibacterial activity, so that a mixture of 1,8-octanediol with either of them can provide, after condensation with citric acid, polyesters with antibacterial properties (Figure 5).[79] Thus, the results described in this section demonstrate that the use of DESs makes the polycondensation of citric acid with 1,8-octanediol greener. DES-assisted polycondensations allowed not only matching of the principles that conventional ones originally matched (e.g., principles 1, 2, 5, and 7) but also some additional ones, such as principles 6 (more energy efficient processes upon lowering of the reaction temperature) and 8 (functionalization without postsynthetic derivatization).

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Figure 5. Top: Scheme representing combinations of 1,8-octanediol with different ammonium and phosphonium salts (C: choline chloride, T: tetraethyl ammonium bromide, H: hexadecyltrimethylammonium bromide, and M: methyltriphenylphosphonium bromide) that result in the formation of eutectic mixtures. Bottom: Plot representing the microbial activity of the poly(octanediol citrates) (POCs) that result from the different eutectic mixtures (POC-C, POC-T, POC-H, POC-M). The antibacterial properties of a poly(octanediol citrate) containing neither ammonium nor phosphonium salts were also used as a negative control. The antibacterial properties of POC-M were analyzed at two different concentrations: the concentration used in every case and one lower (0.75-fold) than that. Adapted with permission from Ref. [79].

3.2. Carbons obtained from resorcinol and formaldehyde and subsequent carbonization One of the most common synthetic processes for carbon preparation is the carbonization of polymers obtained by polycondensation between resorcinol and formaldehyde (Figure 6).[80–82] This was the reaction model of choice in this latest case. In aqueous media and in the presence of either acid or basic catalysts, this well-known polycondensation reaction is typically conducted with high conversion yields. Thus, it matches principles 7, because of the use of phenol-based feedstocks as precursors, and 2, because of the high conversion obChemSusChem 2014, 7, 999 – 1009

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nonhierarchical ones in most applications in which carbons have been used (e.g., as adsorbents, filters, or electrodes).[85–87] It is widely accepted that the use of additional additives is not recommended in green terms (principle 5). Additionally, the recovery (e.g., by washing out) of these block-copolymers after polycondensation is not easy because of the impeded diffusion of pseudo-high-molecularweight substances through such small pores. Clearly, this lack of recovery has detrimental implications in terms of both atom economy (principle 2) and generation of waste after carbonization (principle 1). The preparation of resorcinolbased DESs opened up interesting perspectives in this field. For instance, Carriazo et al. reported the preparation of DESs based on mixtures of resorcinol and ChCl or urea, resorcinol, and ChCl (Figure 7), which, upon polycondensation with formaldehyde, resulted in the formation of monolithic carbons with a bimodal porosity that comprised both micropores and large mesopores of about 10 and 23 nm, respectively.[88] The morphology of the resulting carbons consisted of a bicontinuous porous network composed of highly crosslinked clusters that aggregated and assembled into a stiff, interconnected structure. This type of morphology is typical of carbons obtained by spinodal decompoFigure 6. Top: Scheme representing the polycondensation between resorcinol and formaldehyde. Bottom: Schesition processes, in which the matic representation of block-copolymer-assisted colloid growth by hydrolysis and condensation of resorcinol and formaldehyde. a) SEM micrograph of a monolith that exhibits a three-dimensional network of interconnected formation of a polymer-rich carbon colloids (bar is 1 mm). The inset shows a picture of two cylindrical carbon monoliths. b) TEM micrograph of phase by polycondensation is sintered carbon colloids of about 200 nm (bar is 100 nm). c) TEM micrograph of a single carbon colloid that exhibaccompanied by the segregation its microporosity. d) Plot of cumulative (open circles) and incremental volume (filled circles) versus pore mean diof the noncondensed matter ameter obtained from mercury porosimetry. PPO15–PEO22–PPO15 is poly(propylene glycol)15-block-poly(ethylene glycol)22-block-poly(propylene glycol)15. Adapted with permission from Ref. [84]. (creating first a polymer-poor phase that, ultimately, becomes a polymer-depleted phase), the elimination of which (either before carbonization by washing tained. Unfortunately, this process requires the presence of ceror during carbonization by thermal decomposition) results in tain additives (e.g., block copolymers) if one desires to control the formation of the above-mentioned bicontinuous porous the porous structure of the resulting carbon (Figure 6).[83, 84] structure. In our case, Carriazo et al. hypothesized that one of This is by no means a trivial issue given that hierarchical structhe components that formed the DES (e.g., resorcinol) acted as tures, which comprise of both small (e.g., micro- and meso-) precursor of the polymer phase, whereas the second one (e.g., and large (e.g., macro-) pores, prove to be more effective than  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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densation reaction (Figure 8).[89] Moreover, the preparation of carbon–CNT composites with intriguing structures can also be achieved upon the use of lowviscosity DESs that allow the homogeneous dispersion of high loadings of CNTs before polycondensation begins (Figure 9).[90] Finally, either nitrogen- or phosphorus-doped hierarchical carbons can also be obtained upon the use of DESs containing nitrogen-rich precursors (e.g., hydroxypyridine) or phosphorous-based catalysts (e.g., phosphoric acid), respectively (Figure 8).[91, 92] Thus, compared with conventional polycondensation processes used for the preparation of hierarchical carbons, the use of DESs offers a greener alternative with regard to green chemistry principles 1, 2, and 5 by reducing or even eliminating the residues and/or byproducts eventually released after the synthetic process, that is, one of the components forming the DES (e.g., resorcinol or mixtures of resorcinol with other precurFigure 7. Top: Picture shows the liquid aspect of the eutectic mixtures based on urea, resorcinol, and ChCl (left) and resorcinol and ChCl (right). Bottom: Scheme representing the formation of hierarchical carbon monoliths sors) becomes the material itself upon the addition of formaldehyde to the eutectic mixture of resorcinol and ChCl, followed by eutectic rupture, with high yields of conversion, phase segregation in a spinodal-decomposition-like process, and carbonization. Adapted with permission from whereas the second one (e.g., Ref. [88]. ChCl) is fully recovered and can be reused in subsequent reactions. Interestingly, the green principles observed in the origiChCl) was segregated into the polymer-depleted phase nal polycondensation reaction (e.g., principles 2 and 7) were (Figure 7). This mechanism resembles that mentioned above fully preserved in the DES-assisted ones, following the trend for the synthesis of zeolites,[65] except that DES rupture results described in Sections 1 and 2.1 for other polymerizations. from resorcinol–formaldehyde polycondensation rather than from thermal decomposition (i.e., via ionothermal synthesis). Interestingly, simple washing with water of the polymer result4. Summary and Outlook ing from polycondensation allowed full recovery of ChCl—as demonstrated by gravimetric analysis and NMR spectroscoThe DESs described herein were composed of the precursor py—as the only byproduct of the reaction, which opens up itself and some additional components that either provided the path to ChCl reutilization in subsequent reaction certain functionality (e.g., drug delivery and controlled release, processes. or electrical conductivity) to the resulting materials or directed The wide range of DESs that can be prepared provides a retheir formation with a particular structure (e.g., hierarchicalmarkable versatility, in both structural and compositional type). Moreover, DESs provided a reaction medium, so that terms, to carbons that can be obtained through this synthetic polymerizations were ultimately performed in a solventless approach. For instance, one can control the dimensions of manner. The use of precursors in a eutectic form also allowed both the macro- and micropores of the resulting carbon. The principles 2 and 5 to be fulfilled because of the economy of reformer was achieved by using a ternary DES composed of reagents and solvents, whereas the functionality incorporated by sorcinol, urea, and ChCl,[88] whereas the latter involved DESs the second component allowed the minimization or even complete avoidance of any postsynthetic derivatization (princicomposed of not only resorcinol but also an alkyl-modified reple 8). DESs also provided new precursors that favored more sorcinol (e.g., hexylresorcinol), as precursors for the polycon 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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materials obtained from less green ones. We did not go into a lot of detail about the applications of the polymers described herein.[68] Nonetheless, it is worth mentioning that many applications fall in the area of energy and environmental science, the green character of which is indeed not in question. For instance, hierarchical porous carbons are suitable not only as electrodes in supercapacitor cells[90, 92] but also as adsorbents for CO2 capture.[89, 91] Moreover, the use of nontoxic reagents and the absence of undesired byproducts is of great interest for the preparation of materials suitable for catalytic/biocatalytic applications for which limiting or fully avoiding catalyst poisoning because of the presence of impurities is clearly a challenge. Finally, the combination of the features mentioned above— those are low toxicity, lack of waste, and the possibility of designing more biocompatible processes—opens up interesting perspectives for the application of these materials in biomedical devices (e.g., for drug delivery/ controlled release and tissue engineering). These encouraging results Figure 8. Top: Eutectic mixtures of urea, resorcinol, and ChCl (left), and resorcinol, hexylresorcinol, and tetraethysuggest that DES-assisted polylammonium bromide (right), as well as SEM micrographs of the respective carbons. Bottom: Eutectic mixtures of merizations can help to provide resorcinol, 3-hydroxypiridine, and ChCl (left), and resorcinol, glycerol, and ChCl (right), as well as SEM micrographs synthetic designs and products of the respective carbons. Adapted with permission from Refs. [88, 89, 91, 92]. with a greener character. Undeniably, the challenge remains in the development of processes to incorporate all twelve princiefficient polymerizations (principle 6) by decreasing the energy ples of green chemistry described by Anastas and Warner.[9] input required for the reaction to proceed. Representative examples included 1) DES-assisted FPs that self-propagated after Nonetheless, it is worth noting that the use of DESs in polymer application of energy at the initial stages of the reaction only; syntheses is still in its infancy and exciting developments can and 2) DES-assisted formation of polyesters, for which there be easily envisaged in forthcoming years. was a significant decrease in the reaction temperature. The use of mild reaction conditions in combination with the composiAcknowledgements tional versatility of DESs, which allows the selection of lowtoxic components, is also of interest from the viewpoint of This work was supported by MINECO (grant reference numbers green chemistry because it opens up the way to the design of MAT2009-10214, MAT2011-25329, and MAT2012-34811). M.C.S. biocompatible and/or eco-friendly synthetic methods (princiacknowledges MINECO for a Juan de la Cierva fellowship and ple 3). In every case, we observed that DES-assisted syntheses D.C. acknowledges CSIC for a JAE-doc fellowship. met more green principles than those of conventional ones (Figure 10). Keywords: eutectic solvents · green chemistry · ionic liquids · Clearly, materials obtained from greener processes have materials science · polymerization more opportunities to be feasible for green applications than  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 9. Scheme representing the catalytic condensation of furfuryl alcohol by the eutectic mixture of p-toluenesulfonic acid and ChCl for the formation, after carbonization, of carbon–CNT composites. SEM (a, c, and d) and TEM (b) micrographs show the hierarchical structure of the resulting monoliths. Adapted with permission from Ref. [90].

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Figure 10. Scheme representing the green principles fulfilled by the original syntheses and those achieved upon the use of DES-assisted ones.

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Received: August 14, 2013 Published online on December 27, 2013

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