Elsevier

Polymer

Volume 134, 3 January 2018, Pages 20-23
Polymer

Design of thermoresponsive poly(ionic liquid) gels containing proline units to catalyse aldol reaction in water

https://doi.org/10.1016/j.polymer.2017.11.047Get rights and content

Highlights

  • Thermo-responsive ionic liquids were copolymerised with proline monomer.

  • The hydration degree of the gels depended on temperature.

  • Proline catalysed aldol reaction was affected by water content.

  • The catalytic reaction in this gel can be controlled by temperature.

Abstract

Ionic liquid monomers and a proline-containing monomer were copolymerised in the presence of cross-linker to form ionic liquid polymer gels that show lower critical solution temperature-type phase change in water. The obtained gels were utilised as catalysts for aldol reaction in water. Since proline-catalysed aldol reaction is composed of three equilibrium steps including dehydration condensation reaction and hydrolysis reaction, yield of product was affected by the water content around the proline unit in the gel. Since thus prepared ionic liquid polymer gels showed the thermo-responsive change in water content, the product yield was found to be affected by the temperature around the phase transition temperature.

Introduction

Some ionic liquids (ILs) were found to show a lower critical solution temperature (LCST)-type phase change after mixing them with water [1], [2], [3], [4], [5], [6], [7], [8], [9]. Systematic studies done by Kohno et al. revealed that LCST behavior of IL/water mixtures was seen in ILs having finely balanced hydrophobicity/hydrophilicity [3], [4], [5]. These ILs were prepared by the combination of cations and anions to show moderate hydrophobicity, as confirmed by hydrophilicity index value [5]. The transition temperature between miscible state and immiscible state of IL/water mixtures was also controlled by the hydrophobicity/hydrophilicity balance of the ion structures [1], [3], [5], [6], [7], [8], [9]. This phase transition is comprehended as change in hydration degree of the component ions. It is interesting that these ILs can be polymerised after introducing polymerisable groups on the charged sites, and thus obtained polymerised ILs also show the LCST-type phase change in water [10], [11], [12], [13], [14], [15], [16], [17]. Similar to non-polymerised ILs showing the LCST behavior, the total hydrophobicity/hydrophilicity balance of the IL monomers was the major factor to govern the transition temperature of the resulting polymerised ILs. Accordingly, the transition temperature was freely controlled by the copolymerisation of IL monomers with different hydrophobicity [10], [11]. Significant change in hydration degree was also detected as temperature-dependent and reversible water absorption/desorption behavior in chemically cross-linked IL polymer gels (IL gels) derived from the IL monomers [18], [19], [20], [21], [22], [23], [24], [25], [26]. This study enabled to provide IL gels having ability to pump water by small temperature change [19], [21]. We have also tried to pump pure water from sea water with the IL gels, but the salt concentration was found to considerably affect the thermal response [21]. In spite of the limited number of examples as mentioned above, there are a wide range of potential applications with these thermo-responsive IL gels. Little reports exist on the dynamic change in water content of the IL gels driven by a small temperature change.

In the present paper, we have focused on the proline-catalysed aldol reaction [27], [28], [29], [30]. Proline-catalysed aldol reaction is known to be composed of three steps. Among these, there are two steps that water plays a key role, i.e., initial dehydration condensation reaction and subsequent hydrolysis reaction [31], [32]. These steps are susceptible to water content in the reaction system. Accordingly, the yield of products depended on the hydrated state around the proline moiety. Addition of water into the system containing small amount of water should be good to obtain high product yield for one-pot reaction systems. However, strict and reversible control of water content in the systems should be required for entire catalytic cycling steps. Our challenge is to evaluate the effect of water content on the overall reaction yield of proline-catalysed aldol reaction with the aid of IL gels showing the LCST-type phase change.

Section snippets

Materials

Tributylphosphine (>95.0% purity), sodium 1-pentane sulfonate (>98.0% purity), 4-chloromethylstyrene (>95.0% purity), trans-4-hydroxy-l-proline (>99.0% purity), trifluoroacetic acid (>99.0% purity), acryloyl chloride (>98.0% purity), and 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (>98.0% purity) were purchased from Tokyo Chemical Industry Co. Polyethylene glycol (PEG) dimethacrylate (average Mn = 550), cyclohexanone (>99.8% purity), and p-nitrobenzaldehyde (>99% purity) were purchased

Thermal phase behavior of proline-functionalised IL gels

First, we have examined the effect of temperature on the swelling degree of PP4 gel. The swelling degree of gels was defined here to be the weight ratio of absorbed water and the gel. Large swelling degree means well-hydrated condition of the gels. The swelling degree of PP4 gel was 12.8 at 10 °C, and it gradually decreased with heating. It dropped at around 50 °C to reach 2.9 at 60 °C as shown in Fig. 2. As shown in this figure, considerable change of the swelling in the gel by a small

Conclusion

Proline-units were fixed in the ionic liquid polymer gels that showed the LCST-type phase change to be used as proline-catalysed aldol reaction in an aqueous medium. The major advancement of the gel is the controllability of water content by temperature. This property was applied to the proline-catalysed aldol reaction, because the reaction is composed of multi-step reactions including dehydration condensation reaction and hydrolysis reaction. These are proceeded by the different water content.

Acknowledgment

Authors acknowledged Hokko Chemical Industry Co. for kind donation of tripentylphosphine. Financial support for this work was provided by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (KAKENHI, No. 17H01225 and 16K17954).

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