Functional Poly(Ionic Liquid)s: Catalytic Conversion of CO2

TL;DR

This paper presents new polymeric ionic liquids that efficiently convert CO2 into useful products, showing potential for sustainable carbon capture and utilization.

Contribution

The study introduces block copolymer PILs with enhanced catalytic performance and self-assembly properties for CO2 conversion.

Findings

Block copolymers showed higher catalytic activity than homopolymers, with pS-b-p[HVIm][Cl] achieving 82.69% CO2 conversion.

The block copolymers formed ordered nanostructures, enabling a micellar catalytic effect that enhances reagent concentration near active sites.

The results suggest that PILs can be used in membrane reactors for simultaneous CO2 capture and conversion.

Abstract

This study reports the synthesis and catalytic evaluation of a series of imidazolium-based polymeric ionic liquids (PILs) for the cycloaddition of CO2 to epichlorohydrin (ECH). The synthesized catalysts include homopolymers, poly(3-hydroxyethyl-1-vinylimidazole chloride) (p[HVIm][Cl]) and poly(3-carboxymethyl-1-vinylimidazole chloride) (p[CMVIm][Cl]), and their block copolymers with polystyrene, synthesized for the first time, pS-b-p[HVIm][Cl] and pS-b-p[CMVIm][Cl]. Structural characterization by NMR, IR spectroscopy, and gel permeation chromatography confirmed the successful synthesis. The block copolymers exhibited a low polydispersity index (PDI 1.1–1.2), which is indicative of homogeneous chain lengths and the propensity to form ordered nanostructures, whereas the homopolymers showed higher PDI (2.4–2.9). Catalytic testing at 90 °C and 1 MPa CO2 for 4 h revealed a clear activity trend: p[CMVIm][Cl] < p[HVIm][Cl] < pS-b-p[CMVIm][Cl] < pS-b-p[HVIm][Cl], with conversions exceeding 75% for all catalysts and a maximum of 82.69% for pS-b-p[HVIm][Cl]. These results demonstrate that the catalytic performance of PILs is governed by a synergistic interplay between the local chemical functionality of the ionic moiety and the overall polymer architecture. Based on these results, the synthesized polymeric ionic liquids, particularly pS-b-p[HVIm][Cl], demonstrate strong potential for creating multifunctional materials. Their ability to self-assemble into ordered nanostructures with distinct hydrophobic and hydrophilic domains provides a foundational architecture for combined gas separation and catalysis. The observed “micellar catalytic effect”, which enhances local reagent concentration near active sites, could be leveraged in a membrane reactor to simultaneously capture and convert CO2 directly within the membrane. This integrated “separation–reaction” approach represents a promising strategy for advancing circular carbon economy technologies.

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Full text

1. Introduction

The escalating concentration of atmospheric carbon dioxide (CO_2_) is the most pressing issue of our time, as it is the primary driver of climate change. Renewable energy sources, highly efficient thermal power plants converting coal into syngas, with residual heat used in the steam cycle, and biofuels are among the best examples of efficient energy systems with low greenhouse gas emissions [1]. However, developing CO_2_ capture and storage technologies that would immediately reduce CO_2_ emissions remains a pressing challenge. Common CO_2_ separation technologies include sorption, a membrane and combined methods. The chemical absorption method is based on the selective chemical reaction of CO_2_ with a liquid absorbent, typically an aqueous solution of alkanolamines, to form chemical compounds [2,3,4,5,6]. In recent years, combined solutions of alkanolamines, including highly effective additives such as piperazine or ionic components, have been used as absorbents. The process involves absorption at 40–60 °C, followed by thermal regeneration of the absorbent (desorption) at 100–120 °C, which is accompanied by the release of CO_2_. This technology is the most mature among existing technologies, achieving high capture rates (>90%) and selectivity even at low CO_2_ partial pressures (12–15 vol%) typical of thermal power plant flue gases. However, this process is characterized by the corrosiveness of the regenerated solutions, thermal and oxidative degradation of amines, and the significant size of the equipment [7,8].

Adsorption methods rely on the physical sorption of CO_2_ molecules on the surface of microporous solid materials (zeolites, activated carbons, and metal–organic frameworks) followed by desorption by reducing the overall pressure, increasing the temperature, or a combination of these factors [9]. The absence of liquid corrosive media and potentially lower specific energy consumption compared to absorption allow adsorption methods to be used for capturing CO_2_ from streams with medium and high concentrations (steel mill process gases, converter gases, biogas) [10]. The key technological constraints are a decrease in adsorption effectiveness at low CO_2_ concentration, a reduction in adsorption capacity in the presence of water vapor, which requires feed gas dehydration, and adsorbent mechanical degradation due to cyclic load, which results in selectivity loss.

The gas hydrate-based CO_2_ capture method relies on the ability of gas molecules to integrate into clathrates formed by water molecules under specific thermobaric conditions (high pressure, low temperature) [11,12]. Then the resulting solid hydrates are separated and subjected to controlled destabilization, releasing CO_2_. Promising applications for gas hydrate-based methods include long-term geological storage, particularly in offshore environments, and in methane substitution projects. Hydrate-based capture technologies remain an area of active laboratory research, with potential applications including CO_2_ capture from associated gases on offshore platforms. Despite the potential of this technology, its technology readiness level is currently low—4. Furthermore, this process is characterized by slow hydrate formation kinetics and high operating pressures, which require the use of chemical promoters and high-power compression.

Membrane gas separation represents another effective and non-reactive approach for CO_2_ removal from gas mixtures, operating on differences in the partial pressures of the mixture components [13,14,15]. Membrane processes represent an energy-efficient alternative technology, characterized by low energy consumption, a relatively small footprint, modular configuration, and reagent- and heat-free operation.

Furthermore, an excellent solution is to combine processes to overcome their individual limitations and minimize overall energy costs. Examples of such technologies include hybrid gas hydrate and membrane technologies, hybrid batch distillation/membrane processing, and membrane-assisted gas absorption [16,17,18,19,20,21,22].

Although most of these CO_2_ capture methods are actively used in real chemical facilities, the current emphasis is shifting to the development of technologies for their subsequent conversion into valuable products. Converting carbon dioxide into feedstock for the production of fuels, polymers, or building materials closes the carbon cycle, transforming an environmental problem into an economic opportunity [23,24,25,26]. This allows not only to sequester carbon but to recycle it, which is consistent with the principles of the circular carbon economy and sustainable chemistry. The atom-economical cycloaddition of CO_2_ to epoxides to form cyclic carbonates forms is a highly attractive and industrially relevant process [27,28]. These products find extensive applications as polar aprotic solvents, electrolytes in lithium-ion batteries, and precursors for polymeric materials [29,30,31,32,33].

This reaction typically requires an efficient catalyst to overcome the kinetic and thermodynamic stability of CO_2_. As catalysts, zeolite imidazolate frameworks (ZIF), aluminum complexes, metal–organic frameworks (MOFs), and organocatalysts are used. However, many of these frameworks face fundamental limitations. The primary challenge for porous coordination polymers such as ZIFs and MOFs is their limited stability under extreme conditions. Many of these materials exhibit structural degradation in the presence of water vapor or at elevated temperatures, critically reducing their catalytic activity and lifetime. Many of traditional ZIFs do not have sufficient Lewis acid sites to activate the epoxide, resulting in low conversions, and often require high temperatures, high pressures, or cocatalysts to improve process efficiency [34,35,36,37,38]. The synthesis of these frameworks often involves expensive ligands and multistep processes, complicating their scale-up for industrial applications. Organocatalysts, such as tertiary amines like triethanolamine, typically exhibit high activity, but their homogeneous nature poses challenges in separation and recycling from the reaction mixture, increasing process costs and waste generation [39,40].

In this context, ionic liquids (ILs) represent an alternative class of catalytic media, whose properties can be tailored to overcome these drawbacks. A key advantage of ILs is their exceptional thermal and chemical stability, including resistance to hydrolysis and high temperatures, which enables long-term operation under harsh conditions [41,42,43,44,45,46,47,48]. A crucial property is their high ionic conductivity, making them ideal electrolytes and catalysts for the electrochemical reduction of CO_2_, paving the way for the use of renewable energy for the synthesis of valuable products [49,50,51].

Polymeric ionic liquids, while retaining the key advantages of traditional ionic liquids, additionally address the problem of high viscosity by improving the mass transfer of reagents and can be used both as individual catalysts for CO_2_ conversion reactions and as components of catalytic systems [52,53,54,55,56,57,58]. They can also be formed into rigid yet flexible membranes or porous structures, simplifying their integration into reactors and the regeneration process [59,60,61]. Moreover, the high density of Lewis acidic and basic sites that makes PILs excellent catalysts also makes them promising materials for gas separation membranes. This is particularly true for CO_2_ capture, as these sites exhibit a strong affinity for molecules with a significant quadrupole moment. While the catalytic performance of PILs in cyclic carbonate synthesis and their gas transport properties have been investigated independently, these two research streams remain largely disconnected. The design of a single bifunctional PIL material capable of simultaneously facilitating CO_2_ transport (separation function) and catalyzing its subsequent conversion (catalytic function) represents a groundbreaking integrative approach. This synergy could lead to the development of efficient catalytic membrane reactors, intensifying the process of CO_2_ utilization.

Ionic liquids based on imidazole cations are highly effective catalysts for the conversion of CO_2_, for example, into cyclic carbonates, due to the combination of high catalytic activity and low volatility. The key role is played by the cation, which, due to the acidic proton of the imidazole ring, carries out electrophilic activation of the substrate (epoxide), polarizing it and facilitating ring opening [62,63]. The catalytic properties can be significantly enhanced by introducing functional groups into the cation structure, such as OH or COOH, which act as internal co-catalysts, forming hydrogen bonds with the substrate and creating a bifunctional catalytic center [64,65]. The choice of anion is no less important: nucleophilic halide ions (Cl^−^, Br^−^, I^−^), which are directly involved in the reaction mechanism by attacking the activated epoxide, are effective for catalysis [66]. Thus, the high activity of catalytic systems will be due to the synergistic action of the imidazolium cation (often modified with functional groups) and the nucleophilic halide anion. Therefore, in this study, functionalized polymeric ionic liquids based on the N-vinylimidazolium cation with halide anions and various substituents were synthesized and characterized. Additionally, new copolymers based on these PILs and styrene were synthesized and characterized for the first time. Their ability to catalyze the conversion of CO_2_ to cyclic carbonates using epichlorohydrin as a substrate was assessed.

2. Materials and Methods

2.1. Materials

Table 1 presents the chemicals used in this study. The pre-treatment procedure for removing the stabilizer from the styrene included washing it five times with a 20% NaOH solution. Azobisisobutyronitrile was recrystallized in ethanol. All other chemicals were used without extra purification.

2.2. Synthesis and Characterization

The chemical structures of the synthesized polymeric ionic liquids were analyzed by ^1^H nuclear magnetic resonance (^1^H NMR) using an Ascend400 NMR spectrometer (Bruker, Switzerland, Zurich) with deuterated solvents including deuterium oxide, chloroform-d, and DMSO-d6.

IR spectra were recorded on a Bruker Equinox 55/s FTIR spectrometer (Ettlingen, Germany) in the range of 4000–550 cm^−1^ to characterize the synthesized polymeric ionic liquids (reveal their structures and functional groups).

Gel permeation chromatography (GPC) measurements were performed on a Stayer setup with Phenogel columns (1–500) × 10^3^ using DMF as eluent at 60 °C and a flow rate of 1 mL/min. Polystyrene standards with M = (30, 70, 150, 300) × 10^3^ were used as a standard. Data processing was performed using the MultiChrom Version 1.5X software.

Analysis of the morphology of homo- and block copolymer samples was performed using a JSM-IT500 scanning electron microscope (SEM) from JEOL (Japan, Akishima, Tokyo).

2.2.1. Synthesis of Imidazolium-Based PILs

Reaction of 1-vinylimidazole polymerization was carried out at 70 °C under N_2_ atmosphere using toluene as the solvent and 2 mas. % AIBN as the initiator. After 12 h of stirring on a magnetic stirrer, the obtained light-yellow solid product was filtered and vacuum-dried for 12 h at 70 °C (the product was a white powder; the reaction yield was 71%). Then, 2-Chloroethan-1-ol or chloroacetic acid was dissolved with p[VIm] in acetonitrile (2:1 mol) for obtaining Poly (3-hydroxyethyl-1-vinylimidazole chloride) (p[HVIm][Cl]) or Poly(3-carboxymethyl-1-vinylimidazole chloride) (p[CMVIm][Cl]). Reactions were carried out at 70 °C under N_2_ atmosphere for 12 h. Obtained PILs were filtered and vacuum-dried for 24 h at 60 °C (light-yellow powders; the reaction yields were 76% for p[HVIm][Cl] and 74% for p[CMVIm][Cl]). The synthesis scheme is shown in Figure 1 (left). The photos of the synthesized p [HVIm][Cl] and p[CMVIm][Cl] are shown at the top of Figure 2.

[HVIm][Cl]: ^1^H NMR (300 MHz, deuterium oxide) δ 7.30–7.24 (m, 2H), 6.82–6.49 (m, 1H), 4.20 (s, 2H), 3.99–3.54 (m, 2H), 2.13 (d, 6H). [CMVIm][Cl]: ^1^H NMR (300 MHz, deuterium oxide) δ 2.02 (d, 6H), 7.36 (d, 1H), 7.44 (d, 1H), 8.70 (s, 1H), 4.74 (s, 2H), 4.28–4.06 (m, 1H).

2.2.2. Synthesis of Imidazolium Copolymer-Based PILs

Styrene and 1-vinylimidazole (1:1 mol) were combined in a flask with the addition of 2 mas. % (Sty+VIm) AIBN. Reaction was carried out at 60 °C for 12 h under N_2,_ using a magnetic stirrer. The reaction solution was then added dropwise into a cold mixture of diethyl ether/hexane (20/80 vol.%). Precipitated pS-b-p[VIm] was filtered, washed several times with cold hexane, and vacuum-dried for 24 h at 80 °C (the product was a white powder; the reaction yield was 53%). For obtaining imidazolium copolymer-based PILs 2-Chloroethan-1-ol or chloroacetic acid was dissolved with pS-b-p[VIm] in DMF (2:1 mol) for obtaining pS-b-p[HVIm][Cl] or pS-b-p[CMVIm][Cl]. Reactions were carried out at 70 °C under N_2_ atmosphere for 12 h. Obtained PILs were filtered and vacuum-dried for 24 h at 60 °C (light-yellow powders; the reaction yields were 71% for pS-b-p[HVIm][Cl], 70% for pS-b-p[CMVIm][Cl]). The synthesis scheme is shown in Figure 1 (right). Photos of the synthesized pS-b-p[HVIm][Cl] and pS-b-p[CMVIm][Cl] are shown at the bottom of Figure 2.

pS-b-p[HVIm][Cl]: ^1^H NMR (300 MHz, chloroform-d) δ 7.29 ppm (s, 1H), 7.27–6.86 ppm (br.s, 2H), 6.80–6.51 ppm (br.s, 4H), 6.49 ppm (s, 1H), 3.88 ppm (t, 2H), 3.79 ppm (m, 1H), 3.68 ppm (t, 2H), 3.08 ppm (m, 1H), 2.10–1.16 ppm (m, 7H), 0.91 ppm (m, 3H). pS-b-p[CMVIm][Cl]: ^1^H NMR (300 MHz, DMSO-d6) δ 6.80 (dd, J = 155.0 Hz, 2H), 4.52–4.48 (m, 2H), 4.46–4.37 (m, 3H), 4.27 (s, 1H), 4.07–3.80 (m, 1H), 3.63–3.54 (m, 1H), 2.54–2.48 (m, 4H), 2.00–1.07 (m, 6H).

2.3. Study of Catalytic Activity

The catalytic activity of the prepared polymeric ionic liquids was studied using the experimental setup shown in Figure 3. A stainless steel reactor was loaded with a fixed amount of substrate (epichlorohydrin) and 2 mol% catalyst (polymeric ionic liquid). At the beginning of the experiment, the reactor was evacuated using a Vacuubrand MZ 2C NT vacuum pump (Vacuubrand GmbH & Co. KG, Wertheim am Main, Germany), and then a fixed amount of CO_2_ was added (the set pressure was maintained constant using a back pressure regulator Bronkhorst P702CM (Ruurlo, The Netherlands)). The constant temperature (90 °C) of the gas lines and reactor was maintained using a thermostat. After reaction time (4 h) the pressure in the reactor was slowly released by back pressure regulator and then it was cooled. The liquid phase was sampled for GC analysis using a sampler through a special septum, preventing air from entering the reactor from the atmosphere. The reaction scheme of cycloaddition of CO_2_ to epichlorohydrin (Oxirane, (chloromethyl)-) with the formation of chloropropylene carbonate (4-Chloromethyl-1,3-dioxolan-2-one) is shown in Figure 4.

Qualitative analysis of the reaction mixture was performed using a Thermo Scientific Chromatec-Crystal 5000 chromatograph (Russia, Yoshkar-Ola) with an ISQ mass spectrometric detector to identify the resulting product. Quantitative analysis of the product was performed using a Chromatec Crystal 5000 chromatograph with a flame ionization detector (FID). Chromatec Analytic software (version 4.0.2601.30) was used to set the chromatography parameters. For quantitative analysis of samples, absolute calibration was performed using pure reagents. The GC system included an automatic liquid sample injector, a hydrogen generator, a compressed air compressor, and a catalytic gas filter. High-purity helium was used as the carrier and makeup gas, while hydrogen and air were used for flame combustion in the FID. The operating conditions and components characteristics of the GC system are given in Table 2.

3. Results and Discussion

The IR spectra of the synthesized polymeric ionic liquids are shown in Figure 5. All catalysts exhibit characteristic vibrational peaks at 1163 cm^−1^, which correspond to the in-plane C-H bond deformation vibration of the imidazole ring [67]. PILs with chloroacetic acid, attached to the imidazole ring, exhibit a stretching vibration peak of the C–O bond at 1741 cm^−1^, which indicates the successful attachment of halogen acetic acids to the imidazole ring [68]. PILs p[HVIm][Cl] and pS-b-p[HVIm][Cl] demonstrate an in-plane bending vibration peak of C-OH at 1352 cm^−1^, which indicates the successful attachment of 2-chloroethanol to the imidazole ring [68]. In addition, the C–H out-of-plane bending vibrations of ring of styrene fragment were found at 754 and 698 cm^−1^ [69]. The bands at 2925 and 2850 cm^−1^ were the C–H stretching vibrations of CH_2_ and CH groups on the main polystyrene chain [69]. In the range from 3005 to 3104 cm^−1^, bands corresponding to the C–H stretching vibrations of aromatic bonds in polystyrene were observed [69,70]. The absorption band at 3082 cm^−1^ corresponds to the C–H stretching vibrations of bonds in the imidazole ring [71]. The absorption bands in the region of 2946 cm^−1^ for p[HVIm][Cl] and 2916 cm^−1^ for p[CMVIm][Cl] correspond to asymmetric stretching vibrations of C–H bonds in methylene fragments (-CH_2_-) both in the polymer chain and in the N-CH_2_-CH_2_-OH and N-CH_2_-COOH bridges, respectively [72,73]. The absorption bands in the region of 758 cm^−1^ correspond to out-of-plane deformation vibrations of C–H bonds in the imidazole ring [74].

Table 3 shows a diagram of the distribution of the weight-average molecular weight (Mw) and number-average molecular weight (Mn) of PILs. Block copolymers pS-b-p[HVIm][Cl] and pS-b-p[CMVIm][Cl] are characterized by low polydispersity index (PDI) (1.2 and 1.1, respectively), indicating chain homogeneity and likely forming ordered nanostructures (micelles, spheres, cylinders) with a distinct boundary between the hydrophobic and hydrophilic blocks. This may lead to a “micellar catalytic effect”, which is likely to provide increased catalytic activity. The block copolymer pS-b-p[HVIm][Cl] has a Mw approximately 2-fold higher (15.3 kDa) than pS-b-p[CMVIm][Cl] (7.4 kDa). This may indicate different reactivity of the monomers. The carboxymethyl substituent in p[CMVIm][Cl] may exert a steric or electronic influence, slightly slowing polymerization or resulting in shorter blocks under the same synthesis conditions. Homopolymers (p[HVIm][Cl] and p[CMVIm][Cl]) are characterized by higher PDI (2.4 and 2.9, respectively), suggesting that they form coils in solution.

As seen from SEM microphotographs given in Figure 6a, the p[HVIm][Cl] polymer is characterized by irregular particles with a broad size distribution. The particles form loose agglomerates, with a rough, folded surface but no signs of ordered nanostructures (pores, channels, or regular domains). The morphology of the p[CMVIm][Cl] (Figure 6b) is similar: irregularly shaped particles with a pronounced tendency to agglomerate, a broad size distribution, and a developed rough surface. The formation of large conglomerates from adhered small particles is observed. Ordered structures are also absent.

SEM of the p-S-b-p[HVIm][Cl] (Figure 6c) block copolymer at ×50 magnification reveals large agglomerates with a distinct surface microrelief, including folds and layers. At ×350 magnification (in the green square at the top of Figure 6c), a complex hierarchical structure is clearly visible: the surface is covered with folds, grooves, and “flaky” formations reminiscent of lamellar morphology. The observed relief indicates microphase separation of the blocks, characteristic of block copolymers, and differs fundamentally from the smooth or irregular surface of homopolymers.

The block copolymer p-S-b-p[CMVIm][Cl] (Figure 6d) exhibits a complex hierarchical organization: large agglomerates (50–500 µm) with a developed surface. Signs of internal porosity (indentations, cavities) are observed, which may be a consequence of microphase separation. The morphology is fundamentally different from the dense, unstructured agglomerates of the p[CMVIm][Cl] homopolymer.

The results obtained by GC-MS are shown in Figure 7. The main reaction product is chloropropylene carbonate (4). Possible byproducts of the reaction are 2,3-dichloropropan-1-ol (2) and vinyl chloroacetate (3), the content of which in the reaction mixture was insignificant.

Table 4 presents the average results of determining the catalytic activity of the synthesized polymer catalysts (three experiments for p[HVIm][Cl] and p[CMVIm][Cl] and four experiments for pS-b-p[HVIm][Cl] and pS-b-p[CMVIm][Cl]). As a result of carrying out the reaction at 90 °C and 1 MPa of CO_2_ for 4 h, a conversion of epichlorohydrin above 75% was achieved for all studied samples. The conversion ECH values increased in the series p[CMVIm][Cl] < p[HVIm][Cl] < pS-b-p[CMVIm][Cl] < pS-b-p[HVIm][Cl]. This trend indicates that the catalytic activity is governed not only by the presence of the imidazolium–chloride ion pair, but also by secondary functional groups and polymer morphology effects. Selectivity was determined as the ratio of the amount of the main product formed to the total amount of all products, including byproducts. Therefore, the slight decrease in selectivity with increasing ECH conversion is caused by the appearance of small amounts of byproducts in the case of block copolymers.

A comparison between the homopolymeric catalysts shows a clear enhancement in epichlorohydrin conversion when the carboxymethyl-functionalized polymer (p[CMVIm][Cl]) is replaced by the hydroxyethyl analog (p[HVIm][Cl]). The lower activity of p[CMVIm][Cl] can be attributed to the presence of carboxylic acid groups, which are capable of forming strong hydrogen-bonding and ionic interactions with chloride anions. Such interactions reduce the effective nucleophilicity and mobility of Cl^−^, which is the key species responsible for the nucleophilic ring opening of the epoxide. In contrast, hydroxyl groups in p[HVIm][Cl] provide moderate hydrogen bonding interactions that preferentially stabilize and activate the epoxide ring without significantly suppressing the nucleophilicity of the chloride anion. As a result, the cooperative activation of epichlorohydrin via hydrogen bonding and nucleophilic attack by Cl^−^ is more efficient, leading to higher substrate conversion. A substantial increase in catalytic performance is observed upon introducing a polystyrene block into the polymeric ionic liquid structure. Both block copolymers, pS-b-p[CMVIm][Cl] and pS-b-p[HVIm][Cl], exhibit higher epichlorohydrin conversions compared to their corresponding homopolymers. Apparently, the hydrophobic polystyrene segments enhance the local concentration of epichlorohydrin near the catalytic sites. Among all catalysts studied, pS-b-p[HVIm][Cl] demonstrates the highest epichlorohydrin conversion, highlighting a pronounced synergistic effect between the hydroxy-functionalized imidazolium units and the block copolymer architecture. In this system, hydroxyl groups effectively activate the epoxide ring through hydrogen bonding, and the amphiphilic nature of the block copolymer may facilitate local CO_2_ accumulation in the vicinity of the imidazolium domains, thereby accelerating the cyclization step leading to cyclic carbonate formation. The simultaneous optimization of anion nucleophilicity, epoxide activation, and polymer morphology results in the most favorable catalytic environment, explaining the superior performance of pS-b-p[HVIm][Cl].

Overall, the observed activity trend underscores the importance of balancing chemical functionality and macromolecular design in polymeric ionic liquid catalysts. While strongly acidic groups can hinder catalytic activity by immobilizing nucleophilic anions, moderately hydrogen-bonding functionalities, combined with block copolymer-induced microphase separation, provide an optimal platform for efficient CO_2_–epoxide cycloaddition. These results demonstrate that rational tuning of both the local chemical environment and polymer architecture is a powerful strategy for enhancing the catalytic conversion of epoxides into cyclic carbonates.

The efficiency of catalyst recycling was assessed over three cycles (Figure 8). Reusing the catalyst under the same conditions (90 °C; 1 MPa; 4 h) resulted in only a slight efficiency decrease. Thus, over three extra cycles, epichlorohydrin conversion decreased by 9.2% when using p[HVIm][Cl], by 8% for p[CMVIm][Cl], by 8.5% for pS-b-p[HVIm][Cl], and by less than 9% for pS-b-p[CMVIm][Cl].

4. Conclusions

In this study, a series of imidazolium-based polymeric ionic liquids (PILs) was designed and synthesized, in which variation of the functional group (hydroxyethyl vs. carboxymethyl) and polymer architecture (homopolymer vs. polystyrene block copolymer) directly controlled catalytic efficiency for the model reaction of CO_2_ with epichlorohydrin. The excellent performance of the block copolymer pS-b-p[HVIm][Cl], which provides 82.69% epichlorohydrin conversion at 90 °C and 1 MPa CO_2_ in 4 h, can probably be attributed to the synergistic triple effect:

• (1)The moderate hydrogen-bonding ability of the hydroxyl group, which activates the epoxide ring without deactivating the nucleophilic chloride anion;
• (2)The amphiphilic block structure (PS and PIL blocks), which drives microphase separation into ordered nanostructures with low polydispersity (PDI 1.2);
• (3)The resulting “micellar catalytic effect,” which enhances the local concentration of both the hydrophobic substrate and CO_2_ at the hydrophilic imidazolium-chloride catalytic sites.

In such a membrane, continuous hydrophobic domains of polystyrene will likely act as a selective matrix for the permeation and concentration of CO_2_ or light hydrocarbons, while dispersed, nanoscale hydrophilic domains of PIL will function as immobilized catalytic reactors for the continuous conversion of the permeant gas—for example, into cyclic carbonates.

The results of molecular weight determination, catalytic activity, and SEM micrographs are consistent with the hypothesis that the high catalytic activity of block copolymers in the CO_2_ conversion reaction may be due to the formation of a nanostructured environment leading to a synergistic triple effect. However, more complex studies are needed to more accurately determine the type of ordering or the characteristic domain sizes, which are beyond the framework of the present study.

Thus, this work demonstrates the potential for further comprehensive study of synthesized polymeric ionic liquids and creating membranes based on them for the production of bifunctional materials.