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[[Chem.Commun.]From LCST to crystals Structural modulation of ionic liquids drives the phase transition of PiPrOx.pdf](https://mdr.nims.go.jp/filesets/e9ab073c-cb42-4714-b494-56adb3e185be/download)

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[Mai Kamiyama-Ueda](https://orcid.org/0009-0003-4029-893X), [Takeshi Ueki](https://orcid.org/0000-0001-9317-6280), [Yuji Kamiyama](https://orcid.org/0000-0001-9483-2112), [Ryota Tamate](https://orcid.org/0000-0002-1704-1058), [Keisuke Watanabe](https://orcid.org/0000-0001-6374-3918), [Yukiteru Katsumoto](https://orcid.org/0000-0001-7773-2558)

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[From LCST to crystals: structural modulation of ionic liquids drives the phase transition of poly(2-isopropyl-2-oxazoline)](https://mdr.nims.go.jp/datasets/92747813-8442-4981-ad1b-b363ab9e56aa)

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From LCST to crystals: structural modulation of ionic liquids drives the phase transition of poly(2-isopropyl-2-oxazoline) ChemCommChemical Communicationsrsc.li/chemcomm COMMUNICATION  Takeshi Ueki, Yukiteru Katsumoto  et al .  From LCST to crystals: structural modulation of ionic liquids drives the phase transition of poly(2-isopropyl-2-oxazoline) ISSN 1359-7345Volume 61Number 4921 June 2025Pages 8767–8948This journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 8843–8846 |  8843Cite this: Chem. Commun., 2025,61, 8843From LCST to crystals: structural modulationof ionic liquids drives the phase transitionof poly(2-isopropyl-2-oxazoline)†Mai Kamiyama-Ueda, ‡ab Takeshi Ueki, ‡*ab Yuji Kamiyama, aRyota Tamate, a Keisuke Watanabe c and Yukiteru Katsumoto *cPoly(2-isopropyl-2-oxazoline) (PiPrOx) exhibits UCST- or LCST-typephase transitions in ionic liquids (ILs) depending on the IL structure.In LCST-type ILs, where low solution entropy is favorable, PiPrOxcrystallizes immediately after phase separation, unlike in water. FT-IRreveals a trans-rich conformation with structural constraint, facilitatingboth LCST-type phase separation and rapid crystallization.Polymer crystallization from solution is a cornerstone processin materials science, underpinning diverse industrial applica-tions such as functional membrane productions and wet spin-ning for fibers. The choice of solvent critically impacts thecrystallization process, dictating crystal morphology, growthkinetics and the resulting material properties.1,2 Among varioussolvent systems, ionic liquids (ILs) have emerged as highlyversatile candidates for polymer processing due to their uniqueproperties,3 including negligible vapor pressure, high thermalstability, and the ability to dissolve a wide range of polymers.4–7Beyond these advantages, the structural tunability of ILs offersunparalleled flexibility to design solvent environments tailoredfor phase behavior of polymers.8–13 This structural tunabilityand environmental sustainability positions ILs as a promisingplatform for advancing polymer crystallization and applica-tions. Herein, we reveal the phase behaviors and crystalli-zation dynamics of poly(2-isopropyl-2-oxazoline) (PiPrOx) inimidazolium-based ILs. PiPrOx is a unique polymer thatexhibits lower critical solution temperature (LCST)-type phasebehavior in aqueous solutions, where it undergoes slow crystal-lization through prolonged annealing after phase separa-tion.14–17 Structurally, PiPrOx is an isomer of poly(N-isopro-pylacrylamide) (PNiPAm), the most widely studied LCST-typepolymer in water. PNiPAm is also known to exhibit oppositeupper critical solution temperature (UCST)-type phase behaviorin ILs18–21 (Fig. 1). PiPrOx demonstrates both UCST- andFig. 1 Chemical structure of poly(2-isopropyl-2-oxazoline) (PiPrOx) andpoly(N-isopropylacrylamide) (PNiPAm) and their phase behaviors in waterand ionic liquids (ILs). PiPrOx exhibits LCST-phase transitions in water,followed by slow crystallization upon prolonged heating. In ILs, PiPrOxdemonstrates UCST- and LCST-type phase transitions, depending on theIL structure, with rapid crystallization observed immediately afterLCST-type phase separation in BF4-based ILs. By contrast, PNiPAm under-goes LCST-type transitions in water and exclusively UCST-type transitionsin ILs, without exhibiting crystallization.a Research Centre for Macromolecules and Biomaterials,National Institute of Materials and Science, 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan. E-mail: UEKI.Takeshi@nims.go.jpb Graduate School of Life Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku,Sapporo, Hokkaido 060-0810, Japanc Department of Chemistry, Faculty of Science, Fukuoka University,8-19-1 Nanakuma, Johnan-ku, Fukuoka 814-0180, Japan.E-mail: katsumoto@fukuoka-u.ac.jp† Electronic supplementary information (ESI) available: Experimental details,GPC traces, and thermodynamic aspects of the phase transition of polymers inILs. See DOI: https://doi.org/10.1039/d5cc01017a‡ M. K.-U. and T. U. contributed equally.Received 25th February 2025,Accepted 9th April 2025DOI: 10.1039/d5cc01017arsc.li/chemcommChemCommCOMMUNICATIONhttps://orcid.org/0009-0003-4029-893Xhttps://orcid.org/0000-0001-9317-6280https://orcid.org/0000-0001-9483-2112https://orcid.org/0000-0002-1704-1058https://orcid.org/0000-0001-6374-3918https://orcid.org/0000-0001-7773-2558http://crossmark.crossref.org/dialog/?doi=10.1039/d5cc01017a&domain=pdf&date_stamp=2025-04-17https://doi.org/10.1039/d5cc01017ahttps://rsc.li/chemcomm8844 |  Chem. Commun., 2025, 61, 8843–8846 This journal is © The Royal Society of Chemistry 2025LCST-type phase behaviors, depending on the anion structureof the IL. Specifically, PiPrOx transitions via a UCST-type phaseseparation in [Cnmim][TFSI] systems, while LCST-type phaseseparation are observed in 1-hexyl-3-methylimidazolium tetra-fluoroborate ([C6mim][BF4]) and in 1-butyl-3-ethylimidazoliumtetrafluoroborate ([C2C4im][BF4]) (Fig. 2). In only the latter case,PiPrOx undergoes rapid crystallization immediately followingLCST-type phase separation—a phenomenon sharply contrast-ing the slower crystallization observed in aqueous solutions.FT-IR spectroscopy revealed that PiPrOx adopts a trans-richconformation when dissolved in LCST-type ILs, effectivelyrendering the polymer ‘ready-to-crystallize’.We initially screened the solubility of PiPrOx in 43 kinds ofILs, covering a temperature range from 4 1C to 120 1C (Table S1in the ESI†). Among these, ILs containing [BF4] and trifluor-omethylsulfonylimide ([TFSI]) anions were selected. A detailedrationale for this selection is available in the ESI.†Fig. 3 showsthe transmittance curve for the PiPrOx (Mn = 7700 g mol�1,Mw/Mn = 1.30) in [C6mim][BF4] and [C2C4im][BF4], respectively.We found that the PiPrOx exhibited LCST-type phase separationboth in [C6mim][BF4] and [C2C4im][BF4]. Table 1 compares thesolubility of PiPrOx and PNiPAm in ILs. PiPrOx exhibitedLCST-type phase separation in [C6mim][BF4] and [C2C4im][BF4],while PiPrOx showed UCST-type phase separation in [C2mim][TFSI] and [C4mim][TFSI]. In contrast, PNiPAm showed onlyUCST-type phase separation in both [Cnmim][BF4] with an alkylcarbon number in the imidazolium cation of n = 2, 3 or 4 and[Cnmim][TFSI] with n = 2, 4, 6, 8, 10 and 12. Fig. 4(a) shows therelationship between the Tc, defined as the temperature corres-ponding to 50% transmittance, of the polymers (2 wt% in ILs)and n of the imidazolium cation. It is revealed that theUCST-type phase separation temperature (Tc,U) monotonicallydecreases with an increase in alkyl chain length. It is widelyreported that the mutual solubility in ILs improves as thelength of the alkyl chain attached to the cation increases bothin UCST-22,23 and LCST-systems.24,25 A solubility improvementof the polymer following an increase in the alkyl chain lengthhas also been reported in certain LCST-type phase separationsof polymethacrylates11,13,26 and polyethers.12,20,27 A similartendency was observed to increase the Tc,U of the presentsystem. As can be seen in Table 1 and Fig. 4(a), LCST-typephase separation of PiPrOx occurred only when [C6mim][BF4]was used as the solvent. PiPrOx was incompatible with[Cnmim][BF4] at n = 2, 3, and 4, and gave a completelycompatible system at n = 8. This also indicates that extendingthe alkyl chain length works to increase solubility.Fig. 4(b) highlights the phase separation temperature ofPiPrOx and PNiPAm in [C4mim][BF4], [C6mim][BF4], and[C2C4im][BF4] as solvents. For [C2C4im][BF4], where the cationcarries four carbons (butyl group) at position 1 and two carbons(ethyl group) at position 3, the solubility of PiPrOx is lowercompared to [C6mim][BF4], which carries six carbons (hexylgroup) only at position 3. Consequently, the LCST-type phaseseparation temperature of PiPrOx in [C2C4im] is nearly 30 1Clower than that in [C6mim][BF4]. In contrast, the UCST-typephase separation temperature of PNiPAm decreases by only2 1C when the cation changes from [C2C4im] to [C6mim]. Thegreater shift observed in the LCST-type phase separation tem-perature compared to the UCST-type phase separation tempera-ture suggests that the introduction of alkyl chains at bothpositions 1 and 3 of the imidazolium ring significantly impactsthe thermodynamics of the solution. Detailed discussions onthe phase behavior of PiPrOx and PNiPAm in ILs from thermo-dynamic aspects are available in the ESI.†Previous work demonstrated that PiPrOx exhibited LCST-type phase separation in aqueous solution, similar to PNiPAm.However, unlike PNiPAm, PiPrOx undergoes crystallization inthe polymer-rich phase after prolonged heating. In water, thecrystallization of PiPrOx follows LCST-type phase separation. Attemperatures above the phase transition, polymer-rich dropletsform, initially smooth but slowly transforming into ruggedcrystalline structures upon prolonged heating.28 The processrequires B20 hours at Tc + 2 1C, indicating slow kinetics. PiPrOxchains must overcome rotational energy barriers in the backboneC–C and C–N bonds to adopt the trans-conformation needed forcrystallization. Prior studies revealed that PiPrOx can be kineticallytrapped in metastable gauche-conformers before reaching the mostenergetically favorable trans-conformer, making conformationalalignment a rate-determining step for nucleation.28 We discoveredthat PiPrOx crystallizes rapidly in IL systems, immediately afterundergoing LCST-type phase separation. Notably, after cooling theFig. 2 Chemical structures and names of the cations and anions of the ILsused in this study. The cations include 1-alkyl-3-methylimidazolium([Cnmim]) and 1-butyl-3-ethylimidazolium ([C2C4im]), while the anionsconsist of trifluoromethylsulfonylimide ([TFSI]) and tetrafluoroborate([BF4]).Fig. 3 Transmittance curves of PiPrOx in (a) [C6mim][BF4] and (b)[C2C4im][BF4].Communication ChemCommThis journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 8843–8846 |  8845solution following LCST-type phase separation, the solution didnot return to its original transparent state. Further investiga-tion revealed that the polymer-rich phase, once washed withmethanol, was insoluble, and microscopic analysis of theprecipitate confirmed the formation of crystals (Fig. 5(A)). Inother PiPrOx/IL combinations exhibiting UCST-type phaseseparations, the polymer-rich phase precipitated at low tem-peratures did not show any detectable crystallization under thesame experimental conditions. FT-IR measurements revealedthat the rapid crystallization of PiPrOx is attributed to itsmolecular conformation in [C6mim][BF4] (Fig. 5(B)). The C–Nstretching band between 1400 and 1450 cm�1 indicates thetrans-rich conformation of PiPrOx, confirmed by DFT calcula-tions with model compounds. In the FT-IR spectrum of PiPrOxdissolved in [C6mim][BF4] (Fig. 5(B)(d)), the C–N stretchingband shows high intensity, suggesting a trans-rich conforma-tion even in the dissolved state. In contrast, the FT-IR spectrumof PiPrOx aqueous solution exhibits weaker C–N stretchingband intensity (Fig. 5(B)(a)). This indicates that the transforma-tion toward a trans-rich conformation occurs gradually(Fig. 5(B)(b)), contributing to slower crystallization. The inher-ent trans-rich conformation in the ILs closely resembles itscrystalline state (Fig. 5(B)(c)), lowering the energetic barrier forcrystallization and facilitating rapid crystal formation afterLCST-type phase separation. By contrast, in aqueous systems,PiPrOx requires significant conformational adjustments post-phase separation, delaying crystallization onset. In UCSTsystems such as PiPrOx in [C2mim][TFSI], the FT-IR spectraconfirm no trans-rich conformation in the dissolved state(Fig. S6 and S7, ESI†). This lack of pre-alignment likely explainsthe absence of crystallization in UCST-type phase separationsand underscores the critical role of the trans-rich conformationin facilitating rapid crystallization in LCST-type phase separa-tion systems. A noteworthy aspect of the trans-rich PiPrOxconformer is its potential influence on solution entropy. Thisconformational restriction may reduce mixing entropy, servingas a thermodynamic requirement of LCST-type phase separa-tion. This interpretation agrees with the lack of crystallizationin UCST-type PiPrOx/IL systems, where phase transitions do notrely on entropy-lowering solvation. To further explore thestructural origin of the trans-rich state, we examined the con-formation of PiPrOx in [C6mim][TFSI] and [C8mim][BF4](Fig. S8, ESI†). The latter, despite being fully miscible, showsstrong C–N stretching bands similar to [C6mim][BF4], whereas[C6mim][TFSI] exhibits much weaker intensity. This suggeststhat the [BF4] anion plays a key role in stabilizing the trans-conformation. The absence of LCST behavior in [C8mim][BF4],despite the trans-rich state, is likely due to enhanced polymer–solvent interactions caused by longer alkyl chains, elevating theLCST above the experimental range. Furthermore, to assessdifferences among [BF4]-based ILs, we compared IR spectra ofPiPrOx in [C6mim][BF4] and [C2C4im][BF4] (Fig. S9, ESI†). Bothdisplay comparable C–N band intensities, indicating that thetrans-rich conformation exists in both. Therefore, the B30 1Cdifference in LCST-type transition temperature between themlikely arises not from differences in the conformation, but froma dissimilarity in solvation-related factors, potentially linkedto the degree of nanostructuring in longer-chain ILs like[C6mim][BF4]. Finally, we note that the trans-rich conformationobserved in ILs may reduce not only the conformationalTable 1 Summary of the solubility test for the PiPrOx and PNiPAm in [Cnmim][BF4], [Cnmim][TFSI], and [C2C4im][BF4]. Polymer concentration is 2 wt%.‘‘Yes’’ and ‘‘No’’ mean the combination gives compatible and incompatible systems, respectively. The temperature conditions are from 4 1C to 120 1CAnion: [TFSI] C2mim C4mim C6mim C8mim C10mim C12mimPiPrOx UCST UCST Yes Yes Yes YesPNiPAm UCST UCST UCST UCST UCST UCSTAnion: [BF4] C2mim C3mim C4mim C2C4im C6mim C8mim C10mimPiPrOx No No No LCST LCST Yes YesPNiPAm UCST UCST UCST UCST Yes Yes YesFig. 4 (a) Relationship between phase transition temperature and alkylchain length of the imidazolium cation. (b) Phase transition temperature ofPiPrOx and PNiPAm in [C4mim][BF4], [C2C4im][BF4], and [C6mim][BF4] assolvents.Fig. 5 (A) A photograph of the PiPrOx crystal deposited from[C6mim][BF4]. (B) FT-IR spectra of PiPrOx (a) in D2O homogeneoussolution before phase transition, (b) soon after phase separation fromD2O, (c) after complete phase separation from D2O, (d) in [C6mim][BF4]homogeneous solution before phase transition, and (e) soon after phaseseparation from [C6mim][BF4].ChemComm Communication8846 |  Chem. Commun., 2025, 61, 8843–8846 This journal is © The Royal Society of Chemistry 2025entropy but also the configurational entropy, as the spatialfreedom in the configuration of the extended chains should behighly limited compared to that of flexible random coils. Thisdual entropy reduction could serve as an additional drivingforce for the LCST-type phase separation.In this study, we described the unique phase transitionbehaviors of PiPrOx in ILs, revealing its ability to exhibit eitherUCST- or LCST-type phase separation depending on the ILstructure. The rapid crystallization of PiPrOx in ILs, observedimmediately after LCST-type phase separation, marked a sig-nificant departure from the slow crystallization kinetics inaqueous systems. Our findings unveil novel insights intopolymer-IL interactions, emphasizing the role of IL structurein modulating polymer phase behavior and offering a founda-tion for designing advanced functional materials.This study was financially supported by JSPS KAKENHIgrants (20H02804, 20K21229, and 23H02030 to T. U.). T. U.thanks Sadaki Samitsu for fruitful discussion on the crystal-lization of the polymer.Data availabilityThe data supporting this article have been included as part ofthe ESI.†Conflicts of interestThere are no conflicts to declare.Notes and references1 B. Wunderlich, Macromolecular Physics, Academic Press, New York,1973.2 L. Mandelkern, Crystallization of Polymers, Cambridge UniversityPress, Cambridge, 2004.3 N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37,123–150.4 S. Singh, B. A. Simmons and K. P. Vogel, Biotechnol. Bioeng., 2009,104, 68–75.5 S. Zhu, Y. Wu, Q. Chen, Z. Yu, C. Wang, S. Jin, Y. Ding and G. Wu,Green Chem., 2006, 8, 325–327.6 H. Xie, S. Li and S. 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