Ionic liquid – containing ionogels by thiol–ene photopolymerization. Kinetics and solvent effect

Abstract

Photopolymerization of monomers in ionic liquids (ILs) leads to the formation of conducting ionogels. In the frame of our continuing investigations in this field we report the first example of thiol–ene polymerization in ILs. The work focuses on the polymerization kinetics and reaction mechanism. The model system is based on a difunctional thiol and a divinyl ether (to exclude ene homopolymerization) with spacers built from oxyethylene units. The photo-initiated polymerization was carried out in the presence of various amounts of imidazolium-based ILs and was compared to the polymerization in non-ionic solvents. It was found that the addition of the solvents to the investigated system accelerates the reaction; however, the reaction occurs faster in non-ionic solvents than in ILs. The accelerating effect is associated with the additional stabilization of a partial charge separation in the transition state and its magnitude is closely related to the Kamlet–Taft β parameter which describes the hydrogen bond accepting ability of the solvent. In the case of ILs, the stabilizing effect is exerted by the IL's anion. However, the imidazolium cation interacts with the ether oxygen of the monomer decreasing its stabilizing influence on the transition state (which is related to the Kamlet–Taft α parameter representing the hydrogen bond accepting ability). The knowledge of the presented relationships provides a tool for designing thiol–ene systems containing ILs.

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1. Introduction

The thiol–ene reaction is the hydrothiolation of a C=C bond (reaction (1)) which can proceed under a variety of conditions via a radical pathway (including photochemical initiation), via catalytic processes mediated by nucleophiles, acids, bases or in the absence of an added catalyst, and also in highly polar solvents such as water or DMF.¹

The thiol–ene chemistry found wide applications in photo-curable polymers and resins for use in protective coatings and films. Photoinduced thiol–ene polymerizations are known to occur rapidly (can be completed in a matter of seconds, even at ambient temperature) with formation of low stress homogeneous polymer networks with narrow glass transitions and are able to achieve wide range of material properties. An important advantage of thiol–ene photopolymerization is that it can proceed in the presence of oxygen. The thiol–ene chemistry has been well established and described in many papers; it is worth to mention works of Hoyle and Bowman's group, devoted to thiol–ene polymerization (list of papers can be found in ref. 2) and some recent reviews^1,3–9 presenting also a wide range of applicability of thiol–ene reaction including “click” possibility.

Ionic liquids (ILs) are new solvents for polymerization reactions. Very important characteristics of ionic liquids are a high thermal stability, a wide liquid range in comparison to water and conventional organic solvents and the very low vapor pressure. ILs are known to accelerate the polymerization initiated both radically and ionically, including free-radical polymerization, living/controlled radical polymerization (e.g. ATRP), reversible addition-fragmentation transfer (RAFT), or ionic and coordination polymerizations.¹⁰ The radical polymerizations in ILs were initiated also photochemically.^11–15 It was proposed that the acceleration of the polymerization of acrylate monomers in ILs is associated both with the decrease of the bimolecular termination rate coefficient k^b_t (as a consequence of high viscosity of ILs)^11,16 as well as with a significant increase in the propagation rate coefficient k_p. IL polarity was suggested to be the most probable origin of the k_p enhancement (due to formation of charge separated monomer resonance structures which are favored by ionic liquids).^15,17,18

Our investigations in the field of photopolymerization in ILs concerned both the polymerization kinetics^11,13–15 as well as application of in situ formed polymer/IL ionogels as solid polymer electrolytes (SPE).^19–23 Very good results (high ionic conductivities) obtained for SPE based on polyacrylate matrices prompted us to test also the elastomers from thiol–ene family. Thiol–ene polymerization in ionic liquids can give a new generation of ionogels, which can find application in production of electrochemical devices, catalytic membranes or drug delivery systems. However, selection of starting materials and design of curing conditions necessitate the knowledge of the influence of solvents on the reaction mechanism. There are no reports about the thiol–ene polymerization in ILs (except of our first preliminary communication)²⁴ and only a few works appeared which describe the thiol–ene reaction of monofunctional compounds catalyzed by ILs^25–27 or in ILs.²⁸

Generally the addition of the thiol group to an ene compound can occur as a catalyzed thiol Michael addition to electron deficient carbon–carbon double bond and free-radical thiol addition to electron-rich/electron-poor carbon–carbon double bonds.⁵ In the work²⁵ 1-pentyl-3-methylimidazolium bromide was used as the catalyst for Michael addition of thiols to a variety of conjugated alkenes. The conclusion was that this IL catalyzes the reaction because no products were observed in its absence; it was speculated that the bromide ion is hydrogen bonded to the thiol increasing the nucleophilicity of sulfur atom. The Michael-type addition was also the subject of another work devoted to reaction of thiophenols with chalcones in ILs with and without organocatalyst.²⁶ The rates of the reactions were similar indicating that nucleophilicity and possibly also the dissociation constant of thiols is higher in ILs than in organic solvents. The next work concerning thiol-Michael addition catalyzed by ILs emphasized the role of hydrogen bonding (H-B) in induced reactivity and selectivity control.²⁷ The catalytic efficiency of ILs was found to be strongly influenced by the counter anion; the role of C-2 hydrogen in 1-butyl-3-methyl imidazole moiety in imparting catalytic power also was proved. The overall role of ILs was indicated as H-B induced “electrophile–nucleophile dual activation”.

Only one paper discussed radically-initiated thiol–ene reaction; the system consisted of monofunctional compounds.²⁸ Both thermal and photochemical initiators were used. The results showed that the radical hydrothiolation of alkenes can be favored by ILs and can occur with higher yield than in non-ionic solvent. The proposed explanation was that hydrogen transfer reactions are accelerated by polar factors (and substituents) due to stabilization of a partial charge separation in the transition state. The same may concern the reaction in IL, which could stabilize charge separation and lower in this way the activation energy of H-atom donation from the electrophilic thiol to the intermediate thioalkyl radical.

In this work we present preliminary results of investigation of the thiol–ene photopolymerization in ILs. The polymerization in two ILs was compared to the polymerization in several non-ionic solvents. Both the polymerization kinetics and intermolecular interactions were studied. As a model system we selected a difunctional thiol and a divinyl ether (to exclude ene homopolymerization), structurally similar, with spacer groups built from oxyethylene units. To the best of our knowledge this is the first report on the thiol–ene photopolymerization in IL.

2. Experimental

2.1. Materials

The monomers: tri(ethyleneglycol)divinylether (DVE) and 2,2′-(ethylenedioxy)diethanethiol (T2), as well as non-ionic solvents: diethylene glycol dimethyl ether (DG, 99.5%), tetramethylene sulfone (TMS, 99%), propylene carbonate (PC, 99.7%) and tricresyl phosphate (TCP, 90%) were purchased from Sigma-Aldrich and dried before use. The ionic liquids: 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMImNTf₂) and 1-butyl-3-methylimidazolium tris(penta-fluoroethyl)trifluorophosphate (BMImFAP), delivered by Merck, were of high purity (≥99.0%; water content ≤100 ppm; halide content ≤100 ppm). All the solutions in ILs were prepared in an Ar-filled glovebox. Chemical structures of the reagents are shown in Fig. 1. As a photoinitiator 2,2-dimethoxy-2-phenylacetophenone (Sigma-Aldrich) in amount 0.2 wt% on the whole composition was used.

Fig. 1 Chemical structures of monomers.

2.2. Methods

FTIR studies (kinetics and interactions) were conducted with Nexus Nicolet model 5700. Investigated compositions (with stoichiometric thiol to ene ratio) were laminated between KBr salt plates and placed in a horizontal sample accessory. Series of scans were recorded at a resolution of 2 cm⁻¹ with spectra taken at the rate of approximately 5.5 scans per second. Polymerization was initiated via Hamamatsu Lightningcure LC8 UV spot light source (equipped with 365 nm narrow bandpass filter) at a light intensity of 1 mW cm⁻². Irradiation intensity was measured with Dymax ACCU-CAL-50 radiometer. Functional group conversion was monitored using S–H absorption peak at 2556 cm⁻¹ and carbon–carbon double bond absorption peak at 1620 cm⁻¹ and was calculated from the change of the corresponding peak height (practically the same results were obtained both from –SH and C=C peak monitoring). All reactions were performed at 25 °C.

3. Results and discussion

Selection of the model systems was dictated by following factors: (i) simplicity of the monomers and similarity of their structure, (ii) solubility of the monomers in the solvents used and (iii) macroscopic compatibility of the polymer formed with these solvents (full transparency). The molecular structure of DG ensured both good solubility of the monomers and the arising polymer in the solvent as well as no significant change in the polarity of the medium after the solvent addition. Other non-ionic solvents (tetramethylene sulfone, propylene carbonate, tricresyl phosphate) were also compatible with the monomers and with the polymers formed. The problem was rather poor compatibility of the polymer with the majority of popular ionic liquids. From the ILs compatible with the polymer we selected BMImNTf₂ and BMImFAP. These ILs were shown to exert an accelerating effect on the radical polymerization of methacrylates.¹⁵

Taking into account the literature reports we could expect that the thiol–ene polymerization in ILs should be faster than in non-ionic solvents. However, the results were somewhat surprising. First, the both types of solvents (non-ionic and ILs) accelerated the polymerization; secondly, much better results were obtained in non-ionic solvents, and thirdly, final conversion was reduced, very slightly in non-ionic solvents and visibly in ILs. The results for the polymerization of DVE/T2 system are shown in Fig. 2–4: exemplary polymerization rate (R_p) – irradiation time (t) and R_p – conversion (p) curves in BMImNTf₂, DG and TCP (Fig. 2), comparison of the kinetic curves obtained in the presence of 50 wt% of the solvents used (Fig. 3), the dependence of the maximum polymerization rate (R^max_p) and the rate at 20% conversion (R_p^0.2) on the solvent concentration (Fig. 4a) and final conversion (p^f) as a function of solvent concentration (Fig. 4b).

Fig. 2 Polymerization rate R_p as a function of irradiation time t and conversion p for the DVE/T2 system polymerized in BMImNTf₂, DG and TCP at different solvent concentrations.

Fig. 3 Polymerization rate vs. conversion for the DVE/T2 system polymerized in the presence of 50 wt% of various solvents.

Fig. 4 Solvent influence on thiol–ene polymerization: (a) maximum polymerization rate R^max_p and the rate at 20% conversion R_p^0.2, and (b) final conversion p^f.

It should be noted that all the non-ionic solvents used, except of TCP, give very similar results, whereas the influence of the ionic liquids strongly depends on their structure. The accelerating effect exerted by ILs is comparable to that of non-ionic solvents at low conversions (Fig. 3) but as the reaction proceeds it becomes less marked leading to lower maximum polymerization rates (R^max_p) compared to those in molecular solvents; in the case of BMImFAP R^max_p becomes even lower than in the solvent-free polymerization (Fig. 3 and 4a). Generally, the accelerating effect of the solvents used decreases in the following order: TCP > TMS ∼ PC ∼ DG > BMImNTf₂ > BMImFAP.

Conversion of functional groups decreases with the increasing amount of the solvent added as shown in Fig. 4b (these values are most uncertain and should be treated as approximations). Interestingly, the conversions are very similar in all the non-ionic solvents; in the case of ILs the conversions become visibly lower compared to molecular solvents above 50 wt% content.

Typically, thiol–ene polymerizations are carried out in bulk; works which mention radically mediated thiol–ene polymerization in solvents are rather scarce^29,30 and do not discuss the effect of solvent on the polymerization mechanism. Thus, in our work we need to discuss also the accelerating effect of the non-ionic solvents used.

The mechanism of radical thiol–ene click chemistry involves the addition of a thiol across carbon–carbon double bond to give a thioether as an anti-Markownikov addition product. The propagation mechanism of the thiol–ene photopolymerization is an alternation of addition and chain transfer reactions: a thiyl radical adds to an alkene, generating a carbon-centered radical intermediate and new-created carbon radical undergoes a chain-transfer step by abstracting a hydrogen atom from another thiol (step-growth) forming addition product and thiyl radical (Fig. 5). For systems where ene monomers do not homopolymerize, this addition/chain transfer process continues cyclically forming the basis of the thiol–ene step growth polymerization mechanism.⁶

Fig. 5 Thiol–ene reaction mechanism with no ene homopolymerization.

The most important factor governing the overall kinetics of thiol–ene polymerization is the ratio of the propagation rate constant (k_p) to the chain-transfer rate constant (k_CT)^5,29,31–36 which depends on structure of the reactants. When k_p/k_CT is much greater than 1.0 the chain-transfer step is rate limiting; when k_p/k_CT is much less than 1.0 the propagation step is rate limiting. Another possibility is when k_p/k_CT is equal or close to 1.0, then the both propagation steps control the polymerization rate. The latter case describes our system, since for vinyl ethers, k_p/k_CT is about 1.2.³⁷ Vinyl ethers belong to electron-rich alkenes, which are the most reactive in thiol–ene reaction.^5,37 It has been proposed that the propagation rate constant (k_p) is controlled by the electron density of the reacting ene while the chain-transfer rate constant (k_CT) is controlled by the stability of the carbon-centered radical intermediate.^3,32 Hydrothiolation occurs by a transition state in which the nucleophilic alkyl radical (in our case from vinyl ether) rapidly abstracts the thiol hydrogen through a transition state with partial charge separation in which the carbon radical donates an electron to sulfur (Fig. 6). Because hydrogen transfer reactions are known to be highly affected by polar solvents, the substituents capable of stabilizing a partial charge separation in the transition state typically increase the reaction rate. It was proposed that polar media, including possibly ILs, can stabilize charge separation and lower the activation energy of H-atom donation from the electrophilic thiol to the nucleophilic β-sulfanyl alkyl radical.²⁸ Such stabilization could explain the acceleration of the thiol–ene polymerization by polar solvents and ILs; however, the question remains why in the system studied the non-ionic solvents accelerate the reaction stronger than the ILs used.

Fig. 6 Transition state in the thiol–ene reaction (according to ref. 38).

Taking into account polar effects we brought the observed polymerization rates into relation to the Kamlet–Taft solvent parameters, which allow a differentiated polarity assignment to be made.^39,40 The Kamlet–Taft parameter α describes the hydrogen bond donating ability (acidity), β represents the hydrogen bond accepting ability (basicity) and π* is the dipolarity/polarizability of the solvent. The Kamlet–Taft parameters for the solvents used in this work and for tributyl phosphate (instead of unavailable for TCP) are listed in Table 1. The β values of these solvents allow classifying them to moderately strong hydrogen bond accepting solvents.

Table 1 Kamlet–Taft parameters and DN values for the solvents used

Solvent	α	β	π*	DN [kcal mol^−1]
TMS	0 (ref. 42)	0.39 (ref. 42)	0.93 (ref. 42)	14.8 (ref. 42, 45 and 47)
PC	0 (ref. 39 and 41)	0.40 (ref. 39 and 41)	0.83 (ref. 39 and 41)	15.1 (ref. 46 and 48)
DG	0 (ref. 39 and 41)	0.40 (ref. 41)	0.64 (ref. 39 and 41)	24 (ref. 47)
Tributyl phosphate	0 (ref. 39 and 41)	0.77 (ref. 43)	0.65 (ref. 43)	23.7 (ref. 42 and 45)
BMImNTf2	0.55 (ref. 40)	0.42 (ref. 40)	0.83 (ref. 40)	10.2 (ref. 44)
BMImFAP	0.59 (ref. 40)	0.25 (ref. 40)	0.78 (ref. 40)

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Considering the accelerating effect of the solvents on the overall reaction rate: TCP > TMS ∼ PC ∼ DG > BMImNTf₂ > BMImFAP we can see that (i) the reaction is faster in solvents with higher β and (ii) the ability of the solvent to donate hydrogen bond seems not to be advantageous for the reaction. The latter statement results from the observation that (i) although the β parameter of BMImNTf₂ is higher than that of the majority of the non-ionic solvents, the polymerization rate is lower, and (ii) the α parameter is higher for BMImFAP than for BMImNTf₂, but the polymerization rate in the former IL is lower. This is in contrast with previous reports about the accelerating influence of ILs which was attributed mainly to the H-bond interactions of the imidazolium cation. Thus, our results indicate rather that in thiol–ene reaction the transition state is promoted by solvents able to donate electrons which stabilize the partial positive charge on the carbon atom (Fig. 6).

Noteworthy is the highest polymerization rate in TCP; this result seems to confirm our supposition. Although β parameter for this solvent was unavailable, we found that the β value for an analogous solvent, tributyl phosphate, is twice higher than for all the solvents used in this work (Table 1). Therefore, we may expect a similar high β value for TCP, which could explain the observed highest accelerating effect of this solvent.

However, the role of the imidazolium cation should be explained. To elucidate the influence of solvents on reactions, interactions between the substrates and the solvents are often followed. This was made e.g. in the case of the radically^11,18,49,50 or imine base initiated⁵¹ methacrylate polymerization in ILs and a good correlation of the reaction rate or rate coefficients with changes in IR vibration frequencies of C=O or C–O groups was found. In our case the interactions can be observed mainly between the thiol and the solvent (the effect of solvents on the position of the C=C absorption band was found to be very small). Thus, the IR shifts (Δν) of the absorption bands of the SH group and the imidazolium cation were followed as a function of the solvent fraction. The results are shown in Fig. 7a and b.

Fig. 7 Shifts of the IR absorption bands of: (a) SH group and (b) C(4,5)H groups as functions of solvent concentration.

In almost all solvents used the position of the SH groups (2556 cm⁻¹ for the neat compound) shifts to higher wavenumbers (positive Δν) with the increasing solvent concentration (Fig. 7a). The only exception is DG, in which the position of the SH band remains practically unchanged. This clearly suggests that there is no interaction between the thiol and this solvent, which is in agreement with the observation that the thiol group has little tendency to form S–H⋯O (ether) –type hydrogen bonds.⁵²

The influence of aprotic solvents on the stretching frequency ν(SH) was described in ref. 53 and a linear correlation between the donor number (DN) and the SH band position (decrease of ν(SH) with the increase in DN) was found (for polar solvents there was also correlation of IR data with NMR shifts). Interestingly, for weakly hydrogen-bonding and non-polar solvents ν(SH) was shifted to higher frequencies relative absorption of the neat thiol (compare Fig. 1 in ref. 53), still in agreement with the DN value. This was explained by very weak interactions with these solvents and an increase in the contribution of non-associated thiol molecules. Taking into account available DN values for our solvents (Table 1) and DN values of acetone (∼16) and acetonitrile (∼14) in which ν(SH) is blue shifted compared to the neat thiol by ca. 13 and 21 cm⁻¹, resp.,⁵³ we can expect also a blue shift of the SH band position in our solvents. This shift is somewhat higher in the case of the non-ionic solvents than in BMImNTf₂ (up to about 40% of the IL content), again in agreement with their DNs. On the other hand, ν(SH) seems to correlate with the Kamlet–Taft π* parameter. The highest and very similar shifts are observed in TMS, PC and BMImNTf₂ (the solvents with very close π* values, Table 1); the shift decreases with decreasing π* value, as for BMImFAP, DG and phosphates. Thus, whereas polymerization rates correlate with hydrogen bond accepting ability β, interaction of the thiol groups with the solvents seems to be associated rather with the dipolarity/polarizability parameter. The behavior of ν(SH) in 3-component systems (in the presence of both the solvents and the ene) is qualitatively analogous. After diene addition the position of the SH absorption band shifts to higher frequencies by 8 cm⁻¹ indicating on partial disruption of SH⋯SH associates. Dilution with the solvents causes a stepwise shift of the SH absorption band position tending to achieve the shift as for 2-component system. Therefore, we can conclude that in solvents with highest π* values disruption of thiol associates is strongest.

Changes in the absorption of the imidazolium cation were observed for the C(4,5)H (Fig. 8) stretching vibrations which for the neat ILs occur (main peaks) at 3159 cm⁻¹ (BMImNTf₂) or at 3177 cm⁻¹ (BMImFAP). These initial values indicate on stronger cation–anion interactions in the former IL. It was not possible to observe the shift of the C(2)H absorption band due to its overlap with the absorption band of =C–H stretching vibrations. However, usually the behavior of the C(4,5)H and C(2)H bands is qualitatively analogous, with larger shifts in the case of C(2)H band due to its higher acidity and resulting stronger interactions. The behavior of the C(4,5)H absorption band was observed both in 2- and 3-component systems, i.e. for IL + thiol, IL + ene and IL + thiol–ene mixtures as a function of the IL content (Fig. 7b). The shifts for a given IL are very similar in all the systems indicating that there is no significant interaction between the imidazolium cation and the SH (and C=C) group. The very weak interactions between the –SH groups and imidazolium-based ILs were explained by different types of molecular forces (dispersive and weak dipol–dipol in thiols vs. electrostatic and H-bonding in ILs) which allows only for van der Waals and dipol–dipol interaction between the two groups of compounds.⁵⁴ Therefore, the decrease of the ν(C(4,5)H) stretching frequency with decreasing IL concentration (increasing monomer concentration) in the mixture can result from two facts: (i) interaction of the Im⁺ cation with the ether groups in the reactants (such interaction has been described in details in ref. 55) (ii) dilution – promoted transition from larger H-bond networks to ion-pairs which leads to red-shifted CH vibrational modes.⁵⁶ This conclusion is supported by the fact that the same shifts of ν(C(4,5)H) are observed after dilution of the ILs with DG only (not shown). Larger shifts in the case of FAP-containing IL result from weaker cation–anion interactions in this IL and resulting greater susceptibility of the Im⁺ cation to interactions with other molecules.

Fig. 8 Imidazolium ring proton numbers.

Summing up, at the first glance the FTIR results seem to suggest that there is no correlation between the polymerization rate and interactions of the monomer functional groups with the solvents. However, they clearly show that interactions of the IL cation with the ether groups of the reactants must be important; this will be discussed below.

Considering the effect of solvents on the thiol–ene polymerization of vinyl ethers one should take into account that the reversibility of the propagation step for vinyl ethers is high. It is due to the relatively low stabilization of the carbon-centered radical by the ether substituent and low propagation barrier and is reflected in the rate constants values: whereas both propagation and chain-transfer rate constant are of the same order of magnitude (10⁶ M⁻¹ s⁻¹), depropagation rate constant is 2 order of magnitude higher (10⁸ M⁻¹ s⁻¹).³⁷ However, the ether oxygen stabilizes the partial charge separation in the transition state increasing k_CT (compare Table 4 in ref. 37). Further facilitation of the chain-transfer process by additional stabilization of the charge separation by the solvent would change the ratio of the depropagation to chain-transfer reaction rates leading to the acceleration of the overall polymerization process. This is schematically shown in Fig. 9a. The stabilization effect increases with the nucleophilicity of the solvent, described by the β parameter.

Fig. 9 Interaction of solvents with the transition state of thiol–ene reaction: (a) non-ionic solvent, (b) ionic liquid.

When the solvent is the IL, the stabilizing effect is exerted by the IL's anion (the stronger, the higher the β values is). However, the imidazolium cation interacts with the monomer's oxygen atom decreasing its stabilizing effect on the transition state (Fig. 9b). When the basicity of the IL's anion is lower (lower β), its interaction with the Im⁺ cation is also lower; the cation has more acidic character (higher α) and can stronger interact with the monomer in the transition state (BMImNTf₂ vs. BMImFAP). This will lead to a reduction of the IL's accelerating effect. Such an interaction mechanism can explain the observed influence of the ionic and non-ionic solvents in the thiol–ene polymerization with the participation of vinyl ethers. However, a different situation might be observed when other enes (and thiols) would be applied; further work is in progress.

When using polar solvents we should take also into account a solvent promoted process consisting in the formation of thiolate anion which would initiate the anionic chain reaction (such a possibility exists in solvents of high dielectric constant, such as DMSO and DMF that some degree of spontaneous dissociation of thiol into thiolate occurs¹). If so, the highest rates should be observed in PC (ε = 65.5)⁵⁷ and TMS (ε = 43),⁵⁸ much lower in BMImNTf₂ (ε = about 10)⁵⁹ and the lowest in TCP (ε = about 7).⁶⁰ However, as results from Fig. 4a, the polymerization in TCP is the fastest. Thus, at this stage of investigations, the most probable explanation of the presented results seems to be the mechanism shown in Fig. 9.

4. Conclusions

The work presents the first example of the thiol–ene photopolymerization in ILs. Preliminary investigations described in the work focused on the polymerization kinetics and the mechanism of solvent influence. The model system was based on a difunctional thiol and a divinyl ether (to exclude ene homopolymerization) which are structurally similar, with spacers built from oxyethylene units. The polymerization in ILs was compared to the polymerization in non-ionic solvents. In the considered case, when the monomer is the vinyl ether, the addition of solvents to the thiol–ene system accelerates the reaction; however, the reaction occurs faster in non-ionic solvents. The magnitude of the accelerating effect has been found to be closely related to Kamlet–Taft parameters α and β with the decisive role of the β parameter (describing the hydrogen bond accepting ability). Our results suggest that in thiol–ene reaction the transition state is promoted by solvents able to donate electrons which stabilize the partial positive charge on the carbon atom, thus, the higher β value is, the faster is the polymerization. This rule is true both for ionic, as well as for non-ionic solvents. When the solvent is an IL, the stabilizing effect is exerted by the IL's anion, but the imidazolium cation interacts with the ether oxygen of the monomer decreasing its stabilizing influence on the transition state. Such a destabilizing effect is associated with the hydrogen bond donating ability (described by the α parameter) of the imidazolium cation. This is the reason of lower reaction rates in ILs than in non-ionic solvents, even if the both solvents have comparable values of the β parameter. Solvent influence can be somewhat different for systems containing monomers other than vinyl ether; therefore, further work is in progress. The present work is part of our continuing investigations in the field of IL-containing ionogels; such materials with the thiol–ene matrix can give a new generation of ionogels, which can be used in many modern applications.

Acknowledgements

This work was supported by Polish Ministery of Science and Higher Education (grant 03/32/DSPB/0504).

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