Alternating Degradable Copolymers of an Ionic Liquid-Type Vinyl

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Alternating Degradable Copolymers of an Ionic Liquid-Type Vinyl Ether and a Conjugated Aldehyde: Precise Synthesis by Living Cationic Copolymerization and Dual Rare Thermosensitive Behavior in Solution Daichi Yokota, Arihiro Kanazawa, and Sadahito Aoshima*

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Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *

ABSTRACT: Alternating degradable copolymers consisting of a vinyl ether (VE) unit with an ionic liquid-type moiety and a conjugated aldehyde were synthesized by living cationic copolymerization and subsequent modification. The alternating copolymers exhibited upper critical solution temperature (UCST)-type phase separation behavior in water like the corresponding ionic liquid-type VE homopolymers. In addition, the copolymers exhibited lower critical solution temperature (LCST)-type phase separation behavior in acetone. Both the UCST- and LCST-type thermosensitive behaviors were most likely induced by the interpolymer electrostatic interactions among the pendant ionic liquid-type groups. The alternating copolymers were completely degraded by acid hydrolysis because of the cleavage of the acetal linkages, generated through the crossover reaction from VE to aldehyde, in the main chain.

1. INTRODUCTION The properties of thermosensitive polymers are tunable by elaborately designing polymer structures based on the ratios and types of functional groups. Copolymerization is one of the promising strategies for yielding thermosensitive polymers and for adjusting their thermosensitive behavior.1−4 Polymers that exhibit upper critical solution temperature (UCST)-type thermosensitive behavior in water were also obtained via the copolymerization of hydrophobic and hydrophilic monomers. The UCST-type polymer is more difficult to achieve compared with lower critical solution temperature (LCST)-type thermosensitive polymers,5−8 because the intra- or interpolymer interaction needs to be preferentially exerted at a lower temperature compared to the interaction between a polymer chain and water. The requisite for the UCST-type polymers is in sharp contrast to LCST-type polymers, which are soluble in water by hydration at low temperature and become insoluble because of dehydration on heating (dehydration at higher temperature is a common phenomenon because the hydrogen bonding interaction is weaker at higher temperature). For example, copolymers of acrylamide (AA) and acrylonitrile (AN) exhibit UCST-type phase separation behavior in water, whereas AA and AN homopolymers are soluble and insoluble, respectively, in water at 0−100 °C.1 In addition, the phase separation temperature of UCST-type thermosensitive polymers are adjusted by changing the composition ratio of the monomers. The typical UCST-type polymer in water is a sulfobetaine-type polymer. The phase separation behavior of copolymers consisting of the sulfobetaine moieties and hydrophobic comonomers depends on the structures of the hydrophobic units.2 The phase separation temperature of © XXXX American Chemical Society

copolymers composed of benzyl AA, which has an aromatic ring in the side chain, as the hydrophobic comonomer increased as the composition ratio of the hydrophobic units was increased. However, a copolymer with certain amounts of hydrophobic units containing a pentyl group in the side chain did not undergo the UCST-type phase separation in water in the whole temperature range examined, indicating that the structures of the hydrophobic units influence the phase separation behavior. The design of the structures in ionic polymers is also important for exerting LCST-type phase separation behavior in organic solvents. LCST-type behavior in organic solvents is rare because the LCST-type phase separation temperature of polymers is higher than the boiling points of organic solvents in most cases.9,10 Indeed, some polymers, such as polystyrene, exhibit LCST-type phase separation behavior in organic solvents under harsh conditions, such as at high temperature and high pressure.11,12 In contrast, some low-molecular imidazolium salts have been reported to exhibit LCST-type phase separation in acetone.13 The mechanism of the phase separation has also been explained theoretically. In addition, poly(benzyl methacrylate) undergoes LCST-type phase separation in ionic liquids.14 These examples suggest that a polymer with ionic liquid pendants can exert LCST-type thermosensitivity in organic solvents. Indeed, our group15,16 reported that poly(vinyl ether)s [poly(VE)s] with pendant imidazolium moieties exhibit LCST-type phase separation in Received: March 28, 2019 Revised: July 19, 2019

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DOI: 10.1021/acs.macromol.9b00634 Macromolecules XXXX, XXX, XXX−XXX

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subsequent chemical modifications. The thermosensitive properties of the obtained copolymers in solution were investigated in detail by turbidity measurements and dynamic light scattering (DLS) measurements of the polymer solutions. Interestingly, copolymers with appropriate substituents and counteranions showed LCST-type phase separation behavior in acetone, unlike the corresponding VE homopolymer. In addition, thermosensitive alternating copolymers consisting of imidazolium-type poly(ionic liquid)s were completely hydrolyzed under acidic conditions in solution because of the degradation of the acetal moieties.

organic solvents, such as acetone and chloroform, under mild conditions. An appropriate combination of alkyl groups on the imidazolium ring and a counteranion is indispensable for exerting thermosensitivity. Other examples of polymers that exhibit LCST-type phase separation behavior in organic solvents include a thiazolium-type polymer,17 chiral-polymeric ionic liquids,18 and poly(arylene ether sulfone)s.19 These LCST-type polymers have a site that suitably interacts with organic solvents above a certain temperature. Functional copolymers with a degradable moiety, such as esters and acetals, in the main chain are hydrolyzed into low molecular compounds via treatment under appropriate conditions. In recent years, the synthesis of degradable polymers has attracted considerable attention because of environmental pollution problems associated with the vast amount of plastic wastes.20−22 For example, poly(ester)s, such as poly(lactic acid), have been widely studied for their characteristics, such as acid-, alkaline-, or biodegradability, crystalline properties, and biocompatibility.23−25 In particular, the precise synthesis of poly(lactic acid)s via living polymerization of lactide was achieved using various initiators and catalysts, resulting in the application of poly(lactic acid)s to various materials, such as in engineering,26 carbon neutraltype,27 and biomedical materials.28 Copolymerization of different types of monomers is also an effective strategy for generating degradable moieties in polymer chains.29−32 For example, degradable alternating copolymers were synthesized via anionic living copolymerization of ethylphenylketene with aldehyde derivatives.29 The copolymer has ester linkages in the main chain. Degradable alternating polymers were also synthesized via living cationic copolymerization of VEs and conjugated aldehydes.30,31 The alternating copolymer had aciddegradable acetal linkages in the main chain. In addition, the incorporation of functional groups, such as oxyethylenic, carboxy, and amino groups, into the VE moieties allowed for the exertion of thermo- or pH-responsivity of the degradable alternating copolymers.32 In this study, degradable alternating copolymers composed of pendant imidazolium ionic liquid-type VE units and conjugated aldehydes were synthesized to examine the effects of alternatingly arranged aldehyde moieties on the solubility of copolymers in water and in organic solvents (Scheme 1). The alternating copolymers were synthesized by the living cationic copolymerization of 2-chloroethyl vinyl ether (CEVE) and aldehydes with different substituents at the p-position and

2. EXPERIMENTAL SECTION 2.1. Materials. CEVE (TCI; >97%) was washed with 10% aqueous sodium hydroxide solution and then with water, dried overnight over sodium sulfate, and distilled twice under reduced pressure over calcium hydride. Benzaldehyde (BzA, Wako; >98%), pmethoxybenzaldehyde (pMeOBzA, Nacalai Tesque; >99%), pmethylbenzaldehyde (pMeBzA, TCI; >98%), and p-chlorobenzaldehyde (pClBzA, TCI; 98%) were distilled over calcium hydride twice under reduced pressure prior to use. Toluene (Wako; >99.5%) was dried using solvent purification columns (Glass Contour; Solvent Dispensing System). 1,4-Dioxane (Wako; 99.5%) was distilled over calcium hydride and then lithium aluminum hydride. Ethanesulfonic acid (EtSO3H, Aldrich; 95%), hydrochloric acid (HCl, Nacalai Tesque), and 1,2-dimethoxyethane (DME, Nacalai Tesque; >99%) were used as received. A stock solution of GaCl3 in hexane was prepared from commercial anhydrous GaCl3 (Aldrich; 99.999%). All solvents except for the polymerization solvent were used without further purification. 2.2. Polymerization of the Precursor Alternating Copolymer. Polymerization was conducted in a dry nitrogen atmosphere in a glass tube equipped with a three-way stopcock into which toluene, 1,4-dioxane, and monomers were first added using different dry syringes. To the prechilled solution [3.20 mL; 1,4-dioxane (5.0 mmol), VE (3.0 mmol), and aldehyde (3.0 mmol)], which was cooled to 0 °C in advance, a prechilled 40 mM EtSO3H solution (0.4 mL; 0.80 mmol) in toluene was added using a dry syringe. The tube was placed into a cooling bath set at −78 °C for 5 min after EtSO3H addition. The polymerization reaction was initiated by the addition of a prechilled Lewis acid solution (0.40 mL; 0.80 mmol of GaCl3). The reaction mixture was stirred throughout the polymerization. The polymerization was quenched with prechilled methanol containing a small amount of aqueous ammonia solution (2.50 mL; 0.1%). The quenched reaction mixture was diluted with dichloromethane and then washed with water to remove the catalyst residues. The solvents were evaporated under reduced pressure, and the residue was vacuum-dried for at least 6 h at 60 °C. The monomer conversion was determined by a gravimetric method and 1H NMR analysis. 2.3. Substitution of Chlorine Atoms in the Copolymer with Imidazole Derivatives. A precursor copolymer (5 wt %) was dissolved in a dimethylformamide (DMF) solution containing 1,2dimethylimidazole and NaI (five times the molar amounts of CEVE units). The solution was stirred with a magnetic stir bar at 80 °C for 72 h. The reaction mixture was dialyzed against deionized water for at least 24 h to remove DMF, the unreacted imidazole, and residual salts. After purification, the polymer solution was dialyzed against deionized water in the presence of NaBF4 (five times the amounts of CEVE units) for at least 24 h to exchange the counteranions from I− to BF4−, followed by evaporation under reduced pressure. The obtained polymer was vacuum-dried at 60 °C for at least 6 h. The degree of substitution to the imidazolium moieties was determined by 1H NMR analysis. 2.4. Acid Hydrolysis of the Product Copolymers. The obtained copolymers were purified by reprecipitation in methanol to remove low-MW oligomers. The purified copolymer (30 mg) was dissolved in DME (3.4 mL), and the hydrolysis was performed by adding aqueous HCl-DME (1.0 M; 3.4 mL) solution at 30 °C. The

Scheme 1. Synthesis of Amphiphilic Alternating Copolymers Consisting of Conjugated Aldehydes and Ionic Liquid Structures with Various Substituents

B

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Macromolecules Table 1. Cationic Copolymerization of CEVE and Conjugated Aldehydesa Conv. entry

aldehyde

time (h)

VE (%)

Ald (%)

Mn × 10−3b

Mw/Mnb

aldehyde content (%)c

polymer/cyclic trimerc

1 2 3 4 5 6 7d 8d 9e 10f

BzA

1 25 2 48 2 48 24 72 72 48

18 73 34 84 37 77 49 84 70 52

15 62 35 81 36 75 42 70 98 ∼99

4.8 16.7 15.1 27.3 4.3 5.5 8.9 11.0 12.0 11.6

1.27 1.19 1.19 1.28 1.68 1.75 1.27 1.31 1.28 1.21

44 45 49 49 50 50 43 43 37 21

95/5 95/5 98/2 98/2 100/0 100/0 88/12 89/11 84/16 86/14

pMeBzA pMeOBzA pClBzA

a Polymerization conditions: [CEVE]0 = 0.60 M, [aldehyde]0 = 0.60 M for entries 1−8; [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M in toluene at −78 °C. bBy GPC (polystyrene calibration). cDetermined by 1H NMR. dSee Figures S1 and S2 for details. e[CEVE]0 = 0.80 M, [pClBzA]0 = 0.40 M; see Figure S3 for details. f[CEVE]0 = 1.0 M, [pClBzA]0 = 0.20 M; see Figure S3 for details.

Figure 1. (A) Time-conversion curves, (B) Mn and Mw/Mn for polymer peaks, and (C) MWD curves of products obtained by copolymerization of CEVE with pMeBzA (polymerization conditions: [CEVE]0 = 0.60 M, [pMeBzA]0 = 0.60 M [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4dioxane] = 1.0 M in toluene at −78 °C). aCyclic trimer ratio in products.

Conjugated Aldehydes. Controlled cationic copolymerizations of CEVE with conjugated aldehydes were conducted using an EtSO3H/GaCl3 initiating system in the presence of 1,4-dioxane as an added base in toluene at −78 °C. This initiating system is effective for the precise synthesis of alternating copolymers of VEs and aldehydes as reported in our previous work.33 In this study, BzA, pMeBzA, pMeOBzA, and pClBzA were used as conjugated aldehyde monomers. These aldehydes have different reactivities for cationic polymerization. The copolymerization of CEVE and pMeOBzA, which has the highest reactivity among the examined aldehydes, reached approximately 80% conversion within 48 h, yielding copolymers with broad MWDs (entries 5 and 6 in Table 1). The MWs of the copolymers did not increase with VE conversion, indicating that the copolymerization proceeded in an “uncontrolled”34 manner.35 The inappropriate balance of the reactivities of CEVE, which is a relatively low-reactive VE, and pMeOBzA is likely responsible for the uncontrolled copolymerization.30 Indeed, the copolymerization of CEVE and pMeBzA, which has a lower reactivity than pMeOBzA, proceeded in a controlled manner, yielding copolymers with narrow MWDs and MW values that increased with an increase in VE conversion (entries 3 and 4 in Table 1, Figure 1). 1H NMR analysis of the copolymers indicated that acetal moieties derived from a crossover reaction from CEVE to pMeBzA were observed at 3.8−4.7 ppm and that peaks for the pMeBzA− pMeBzA sequence were not observed at 5.0−6.0 ppm (Figure 2). In addition, the aldehyde content of the copolymer was 49%. Moreover, the copolymer was completely degraded into a low-MW compound via the acid hydrolysis reaction (discussed

reaction mixture was stirred with a magnetic stir bar and stirrer throughout the hydrolysis. After a predetermined reaction time, the reaction was quenched with aqueous sodium hydroxide (1.0 M, 3.4 mL). The quenched reaction mixture was diluted with dichloromethane and washed with distilled water at least five times to remove the resulting salt and evaporated under reduced pressure. The residue was vacuum-dried at room temperature. 2.5. Characterization. The molecular weight distribution (MWD) of polymers in chloroform was measured at 40 °C using gel permeation chromatography (GPC) with polystyrene gel columns [Tosoh TSKgel GMHHR-M × 2 (exclusion limit molecular weight = 4 × 106; bead size = 5 μm; column size = 7.8 mm i.d. × 300 mm; flow rate = 1.0 mL min−1)] connected to a Tosoh DP-8020 pump, a CO8020 column oven, a UV-8020 ultraviolet detector, and an RI-8020 refractive index detector. The number-average molecular weight (Mn) and polydispersity ratio [weight-average molecular weight/numberaverage molecular weight (Mw/Mn)] were calculated from the chromatograph with respect to 16 polystyrene standards (Tosoh; 577 to 1.09 × 106, Mw/Mn ≤ 1.1). 1H NMR spectra were recorded using a JEOL ECA500 (500 MHz) spectrometer at 30 °C. The particle size was measured by DLS (Otsuka Electronics FPAR1000HG, λ = 632.8 nm, scattering angle = 90°). The transmittance of the polymer solutions was measured at 500 nm at various temperatures using a JASCO V-500 UV−vis spectrometer equipped with a Peltier-type thermostatic cell holder (ETC-505) with heating or cooling at 1 °C/min.

3. RESULTS AND DISCUSSION 3.1. Alternating Copolymerization of CEVE with Conjugated Aldehydes. 3.1.1. Precise Synthesis of Precursor Degradable Copolymers via Controlled Alternating Cationic Copolymerization of CEVE with Various C

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oligomers because of the degradation of the acetal linkages in the main chain under mild acidic conditions. Acid hydrolysis of poly([Me2Im][BF4]-alt-pMeBzA) was conducted in DME with HCl aq for 2 h. Upon GPC analysis, a sharp peak appeared in the low-MW region (the lower curve, Figure 3A). 1 H NMR analysis of the hydrolysis products showed the generation of p-methylcinnamaldehyde derived from the cleavage of the [Me2Im][BF4]-pMeBzA-[Me2Im][BF4] sequence in the main chain of the alternating copolymer (Figure 3C). The imidazolium ionic liquid-type alcohol, which was obtained via the hydrolysis of the alternating copolymer, was removed during the purification process because of the transfer from the organic to the aqueous layer. All the obtained alternating copolymers (Table 2) were hydrolyzed into small molecules in a manner similar to that for poly([Me2Im][BF4]alt-pMeBzA). 3.2. Thermosensitive Behavior of Alternating Copolymers with Pendant Ionic Liquid-Type Moieties and Conjugated Aldehydes. 3.2.1. UCST-Type Phase Separation Behavior of Alternating Copolymers in Water. The copolymers consisting of pendant imidazolium ionic liquidtype VE units and conjugated aldehydes exhibited UCST-type phase separation behavior in water. The solubilities of the alternating copolymers with the dimethylimidazolium tetrafluoroborate ([Me2Im][BF4]) moieties and various conjugated aldehydes were investigated to elucidate the effects of the alternatingly arranged aldehyde moieties (Table 2). All the alternating copolymers showed UCST-type phase separation behavior in water in a manner similar to the [Me2Im][BF4] homopolymer (Figure 4A). The result indicates that the alternately arranged aldehyde moieties do not interfere with the UCST-type thermosensitivity. However, the phase separation temperature of the alternating copolymers was higher than that of the homopolymer (28 °C, black line). The copolymers with BzA, pMeBzA, pMeOBzA, or pClBzA as a hydrophobic comonomer exhibited phase separation at approximately 30 (blue line), 42 (red line), 45 (green line), and 59 °C (purple line), respectively. Woodfield et al.2 reported that the UCST-type phase separation temperature of a copolymer that consisted of sulfobetaine moieties and benzyl AA in water increases with an increase in compositional ratio of the hydrophobic units. The researchers explained that an entropic loss of water molecules or potential attractive interactions of the aromatic benzyl groups, such as π−π, cation−π, and anion−π interactions, were likely responsible for the change in the phase separation temperature. In this study,

Figure 2. 1H NMR spectrum of poly(CEVE-alt-pMeBzA) [Mn(GPC) = 23.2 × 103, Mw/Mn(GPC) = 1.31, aldehyde content = 49%; in CDCl3 at 30 °C] *solvent, methanol, acetone, and water.

below; Figure 3). These results indicate that the copolymers have alternating sequences of CEVE and pMeBzA. A cyclic trimer was also formed from two pMeBzA units and one CEVE unit (Scheme S1).33 The amount of this byproduct was approximately 2% of the obtained copolymers as calculated by 1 H NMR (Figure S2). 3.1.2. Introduction of Imidazolium Moieties into the Side Chain via Chemical Modifications of Various Alternating Precursor Copolymers. The chlorine atoms in the CEVE units of the alternating precursor copolymers were substituted with 1,2-dimethylimidazole in the presence of NaI, which accelerates the chemical modification of the side chain,36 in DMF at 80 °C for 72 h. The reaction proceeded to yield alternating copolymers with dimethylimidazolium cations and iodide as the counteranion. Subsequently, the copolymers were treated with five equivalents of NaBF4 in water for 24 h, resulting in exchange of the counteranions from I− to BF4−. The 1H NMR analysis of the obtained copolymers indicated that the introduction of 1,2-dimethylimidazole (96−100%; Table S1; in Figure 3B, the value was calculated from the integral ratios of peaks g, h, j, and a) and the subsequent counteranion exchange quantitatively proceeded. In addition, the peaks derived from the acetal linkage and conjugated aldehydes were observed in the 1H NMR spectra of any of the copolymers with different conjugated aldehydes. Integral ratios of these peaks were unchanged compared to the original polymer. Moreover, all peaks were correctly assigned to the expected structure.37 The results indicated that the alternatingly arranged acetal moieties in the copolymers remained intact after the introduction of ionic liquid-type moieties into the side chain and that undesired reactions did not occur during the polymer reaction (Figure 3B). 3.1.3. Acid Hydrolysis of the Alternating Copolymers that Have Ionic Liquid-Type Moieties. The alternating copolymers that have imidazolium ionic liquid-type moieties were completely decomposed into small molecules without residual

Figure 3. (A) MWD curves of poly(CEVE-alt-pMeBzA) (upper) and hydrolysis product of poly([Me2Im][BF4]-alt-pMeBzA) (lower), 1H NMR spectra of (B) poly([Me2Im][BF4]-alt-pMeBzA) in DMSO-d6 at 100 °C and the (C) hydrolysis product in CDCl3 at 30 °C [hydrolysis conditions: 0.50 M aqueous HCl-DME at 30 °C for 2 h; 0.50 wt % polymer solution]. Number written in green: integral ratio. *Water, DMSO, DME, CH2Cl2, and CHCl3. D

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Table 2. Solubility of Alternating Copolymers Consisting of Ionic Liquid-Type Vinyl Ether Units and Conjugated Aldehydesa P([Me2Im][BF4]-alt-X) solvent

P([Me2Im][BF4])

X = BzA

pMeBzA

pMeOBzA

pClBzA

DMSO water methanol acetone chloroform DCM

S UCST (28 °C) I I I I

S UCST (30 °C) I I I I

S UCST (42 °C) I LCST (21 °C) I I

S UCST (45 °C) I LCST (21 °C) I I

S UCST (59 °C) I LCST (20 °C) I UCSTb

S: Soluble, I: Insoluble, UCST: upper critical solution temperature-type phase separation behavior (4 wt % solutions, from 10 to 70 °C in water), LCST: lower critical solution temperature-type phase separation behavior (0.4 wt % solution, from −5 °C to solvent boiling point). bThe polymer is partially insoluble in dichloromethane. In addition, the phase separation occurred in a broad temperature range.

a

Figure 4. Thermosensitive behavior of 4 wt % aqueous solutions of (A) poly([Me2Im][BF4]) (black line), poly([Me2Im][BF4]-alt-BzA) (aldehyde content = 41%, blue line), poly([Me2Im][BF4]-alt-pMeBzA) (aldehyde content = 49%, red line), poly([Me2Im][BF4]-alt-pMeOBzA) (aldehyde content = 50%, green line), and poly([Me2Im][BF4]-alt-pClBzA) (aldehyde content = 43%, purple line), (B) poly([Me2Im][BF4]-alt-pClBzA) (aldehyde content = 43%, red line), poly([Me2Im][BF4]-co-pClBzA) (aldehyde content = 37%, green line; see Figure S3), and poly([MeIm][BF4]co-pClBzA) (aldehyde content = 21%, blue line; see Figure S3) (Mn = 5.5−23.2 × 103 for precursor copolymers; see Tables S2 and S3 for the values), and (C) Mn dependence on cloud points of 4 wt % aqueous solutions of poly([Me2Im][BF4]-alt-pMeBzA)s [Mn(GPC) = 13 to 27 × 103 for precursor copolymers].

interpolymer electrostatic interactions among the pendant ionic groups.15 Regarding the cloud point, the homopolymer is independent of MW, but dependent on the polymer concentration. Then, we examined the polymer concentration and the MW dependency of the phase separation behavior on the alternating copolymers in water. First, the turbidity measurements of various poly([Me2Im][BF4]-alt-pMeBzA)s with Mn = 27.7, 23.2, 16.6, and 12.8 × 103 were conducted in water. The data in Figure 4C indicate that the cloud point increases only slightly with increasing MW from 1.28 × 104 to 2.77 × 104, which indicates that the dependence of the phase separation behavior on MWs is negligible. In contrast, a 2 wt % aqueous solution of the poly([Me2Im][BF4]-alt-pMeBzA)s underwent phase separation at a much lower temperature (25 °C) than that of a 4 wt % aqueous solution (42 °C), suggesting considerable dependence on the polymer concentration. Therefore, the alternating copolymers with conjugated aldehydes also exhibited UCST-type phase separation behavior in water via interpolymer electrostatic interactions like the homopolymers possessing pendant imidazolium ionic liquidtype moieties. The copolymer with ionic liquid-type moieties and conjugated aldehydes dissolve as a unimer at high temperature and aggregates at low temperature in water, which was indicated by DLS measurements using a 0.5 wt % aqueous solution at various temperatures (blue symbols in Figure 5). Poly([Me2Im][BF4]-alt-pClBzA) has a diameter of approximately 3 nm at 60 °C in water. Interestingly, the diameter considerably increased at a low temperature. A large diameter of approximately 90 nm was detected at 45 °C, and the size did not change at lower temperatures. The results indicated that

the phase separation temperature of the copolymers that had aromatic aldehydes appeared to change for a similar reason. In particular, the electron-donating and -withdrawing substituents on the aromatic ring potentially affect the degree of π−π, cation−π, and anion−π interactions, which may be responsible for the difference of the phase separation temperature. The aldehyde content of the copolymers influenced the UCST-type phase separation behavior in water. To examine the effect of aldehyde content on the copolymers, we conducted a turbidity measurement with 4 wt % aqueous solutions of various poly([Me2Im][BF]4-co-pClBzA)s with different aldehyde contents (Figure 4B). These copolymers with different aldehyde contents were synthesized at different feed ratios of CEVE and pClBzA (entries 7−10 in Table 1; Figures S1 and S3). Both copolymers of 43% (red line in Figure 4B) and 37% (green line) aldehyde contents exhibited sharp phase separation behavior in water. The phase separation temperature of the copolymer of 37% aldehyde content was lower than that of the copolymer with 43%. In addition, the copolymer with 21% (blue line) aldehyde content, which had a gradient-like sequence, showed phase separation behavior at lower temperatures in a broader temperature range than the copolymers of higher aldehyde contents. Variations in the interactions among polymer chains may occur because of the gradient-like sequence of the copolymer with 21% aldehyde content. Interpolymer electrostatic interactions were mainly responsible for the UCST-type phase separation behavior of the alternating copolymers in water. In a previous work, poly(VE)s with pendant imidazolium ionic liquid-type moieties exhibited UCST-type phase separation behavior in water because of E

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UCST-type and LCST-type phase separation in water and organic solvents, respectively. An interpolymer interaction is likely responsible for the LCST-type phase separation behavior of the alternating copolymer in acetone. We conducted a turbidity measurement of acetone solutions of the alternating copolymer to investigate the effects of the polymer concentration and the MWs. Indeed, the polymer concentration of the alternating copolymer in acetone had a considerable effect on the cloud point (Figure 6A,B). The cloud point increased from 7 °C to approximately 50 °C with a decrease of the polymer concentration from 1 to 0.2 wt %. The data suggest that the phase separation is not because of intrapolymer interactions but interpolymer interactions. The independence of MWs on the phase separation behavior also supported this mechanism (Figure 6C). The occurrence of phase separation by temperature change would be because of the change of the degree of interaction between acetone and the polymer. The carbonyl group of acetone and the imidazolium moieties potentially interacts through the dipole−dipole interaction.39−41 However, such interactions become weaker at a higher temperature because of larger thermal motion of acetone, which would lead to the preferential interpolymer interaction to result in phase separation. In addition, additional salts and ketones with different alkyl groups affected the phase separation behavior of the alternating copolymer (Figure S5). The solubilities of alternating copolymers in various solvents were also affected by the substituents and the counteranions of the ionic liquid-type moieties. The details are summarized in the Supporting Information (Table S4, Figure S6). The aldehyde contents of the copolymers influenced the solubility. Copolymers with 21 or 13% aldehyde contents were insoluble in the whole concentrations and temperature range examined in acetone unlike copolymers with higher aldehyde contents. Copolymers with 30% (blue line, Figure 7) and 37% (green line) aldehyde contents underwent LCST-type phase separation in a broader temperature range than that of the alternating copolymer of a 43% (red line) aldehyde content. These results indicate that the solubility of copolymers of low aldehyde contents in acetone decreases because of the large consecutive [MeIm][BF4]−[MeIm][BF4] sequence, which has a low affinity for acetone. 3.3. Block Copolymers: Synthesis and Thermosensitive Behavior. 3.3.1. Precise Synthesis of Block Copolymers with Alternating Sequences via Living Cationic Copolymerization. Precursor block copolymers composed of poly(IBVE) and degradable poly(CEVE-alt-pMeBzA) segments

Figure 5. DLS measurement for 0.5 wt % aqueous solution (blue symbol) and 0.1 wt % acetone solution (red symbol) of poly([Me2Im][BF4]-alt-pClBzA) [Mn(GPC) = 9.0 × 103, Mw/Mn(GPC) = 1.31, aldehyde content = 43% for precursor copolymer] at different temperatures.

the copolymer aggregates at low temperatures in water. This thermosensitive behavior in a dilute aqueous solution corresponds with the UCST-type phase separation behavior that was confirmed by the turbidity measurement of the alternating copolymer (purple line, Figure 4A). 3.2.2. LCST-Type Phase Separation Behavior of Alternating Copolymers in Acetone. An alternating copolymer consisting of [Me2Im][BF4] units and conjugated aldehydes, which exhibits UCST-type phase separation behavior in water, also exhibited LCST-type phase separation behavior in acetone. We investigated the solubility of the alternating copolymers in various organic solvents (Table 2). The copolymers were insoluble in polar protic and nonpolar solvents, such as methanol, ethanol, and toluene. However, the copolymer exhibited LCST-type phase separation behavior in acetone (Figure 6A). The LCST-type thermosensitivity in acetone is very interesting because the [Me2Im][BF4] homopolymer is insoluble in acetone at all temperatures. To examine the thermosensitivity of the alternating copolymer in acetone, we conducted DLS measurements of a 0.1 wt % acetone solution at various temperatures.38 Poly([Me2Im][BF4]-alt-pClBzA) chains were molecularly dispersed, showing a diameter of ca. 5 nm at 10 °C, but formed aggregates of ca. 110 nm in diameter at 50 °C (red symbols in Figure 5). The thermosensitive behavior corresponds to the LCST-type phase separation behavior confirmed by the turbidity measurement of the copolymer in acetone (Figure S4). To the best of our knowledge, this is the first example of a thermosensitive polymer that exhibits both

Figure 6. (A) Turbidity measurement of poly([Me2Im][BF4]-alt-pMeBzA) [Mn(GPC) = 12.8 × 103, Mw/Mn(GPC) = 1.23, aldehyde content = 49% for precursor copolymer] (scan rate: 1.0 °C/min, solid line: heating, broken line: cooling), (B) dependence of the polymer concentration on cloud points, and (C) dependence of Mn values (0.4 wt %) on cloud points [Mn(GPC) = 13 to 27 × 103 for precursor copolymers] in acetone. F

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Macromolecules

curves in Figure 8C). For the block copolymer synthesis, the second monomers, CEVE and pMeBzA, were added sequentially during the late stage of the IBVE polymerization. The second-stage polymerization was completed in 96 h. As shown in Figure 8C, the peak shifted toward the higher molecular weight region after the addition of CEVE and pMeBzA.42 In addition, the peak detected via UV adsorption (dotted curves; 254 nm) was in accordance with the peak detected with the RI detector (solid curves), indicating the incorporation of pMeBzA units into the polymer chain. The MW of the copolymer obtained increased with the CEVE conversion. The acid hydrolysis product of the copolymer had a MW that agreed with the MW before the addition of CEVE and pMeBzA. 1H NMR analysis of the hydrolysis product showed the generation of a cinnamaldehyde derivative, which was derived from the degradation of the alternating segment, and poly(IBVE) (Figure S7). These results indicate that the block copolymer was successfully synthesized via the homopolymerization of IBVE, followed by the alternating copolymerization of CEVE and pMeBzA. One-shot copolymerization of CEVE with a small amount of pMeBzA was effective for synthesis of other block copolymers consisting of a poly(CEVE-alt-pMeBzA) segment and a poly(CEVE) segment (Scheme S2). The details of the synthesis and thermosensitive behavior of the copolymers are summarized in the Supporting Information (Figures S8−S10). 3.3.2. Thermosensitive Behavior of the Block Copolymers in Acetone. To examine the influence of the hydrophobic segment on the thermosensitivity in acetone, we conducted a turbidity measurement of a 0.4 wt % acetone solution of the block copolymer consisting of the poly([Me2Im][BF4]-altpMeBzA) segment, which exhibits LCST-type thermosensitive behavior in acetone, and a poly(IBVE) segment. The ratio of poly(IBVE) segment/poly([Me2Im][BF4]-alt-pMeBzA) segment was 1.0/4.6. The block copolymer underwent the LCST-type phase separation in acetone at a higher temperature than that for the alternating copolymer of [Me2Im][BF4] and pMeBzA (Figure 9). An increase in the hydrophobicity of the copolymer because of the IBVE block segment was likely responsible for the increase in the phase separation temperature. Notably, transmittance of the block copolymer solution became 0% at a temperature higher than the phase separation temperature of poly([Me2Im][BF4]-alt-pMeBzA). This phe-

Figure 7. Thermosensitive behavior of 0.4 wt % acetone solutions of poly([MeIm][BF4]-alt-pClBzA) [Mn(GPC) = 10.8 × 103, Mw/ Mn(GPC) = 1.28, aldehyde content = 43% for precursor copolymer] (red line), poly([MeIm][BF4]-co-pClBzA) [Mn(GPC) = 12.4 × 103, Mw/Mn(GPC) = 1.26, aldehyde content = 37% for precursor copolymer] (green line), and poly([MeIm][BF4]-co-pClBzA) [Mn(GPC) = 7.4 × 103, Mw/Mn(GPC) = 1.23, aldehyde content = 30% for precursor copolymer] (blue line).

were synthesized via living cationic polymerization of IBVE, followed by the addition of CEVE and pMeBzA (Scheme 2). Scheme 2. Synthesis of a Block Copolymer Consisting of Poly(IBVE) and Poly([Me2Im][BF4]-alt-pMeBzA) Segments

First, homopolymerization of IBVE proceeded using the EtSO3H/GaCl3 initiating system in the presence of 1,4dioxane as an added base in toluene at −78 °C. The polymerization was completed in approximately 1 h, yielding poly(IBVE) with a narrow MWD (Figure 8A,B; the upper

Figure 8. (A) Time-conversion curves, (B) Mn for polymer peaks, and (C) MWD curves of products obtained by homopolymerization of IBVE and copolymerization of IBVE and CEVE with pMeBzA (top and middle curves) and the hydrolysis product of the copolymer (bottom curve) (polymerization conditions: [IBVE]0 = 0.20 M, [CEVE]add = [pMeBzA]add = 1.2 M, [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M in toluene at −78 °C; hydrolysis conditions: 0.50 M aqueous HCl-DME at 30 °C for 2 h; 0.50 wt % polymer solution). G

DOI: 10.1021/acs.macromol.9b00634 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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Polymerization data, NMR spectra, turbidity measurements, and DLS measurements (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Daichi Yokota: 0000-0002-8177-3026 Arihiro Kanazawa: 0000-0002-8245-6014 Sadahito Aoshima: 0000-0002-7353-9272 Notes

Figure 9. Thermosensitive behavior for 0.4 wt % acetone solutions of poly([Me2Im][BF4]-alt-pMeBzA) [Mn(GPC) = 12.8 × 103, Mw/ Mn(GPC) = 1.23, aldehyde content = 49% for precursor copolymer] and poly(IBVE)-b-poly([Me2Im][BF4]-alt-pMeBzA) [Mn(GPC) = 17.5 × 103, Mw/Mn(GPC) = 1.63, aldehyde content = 47% for precursor copolymer] (scan rate: 1.0 °C/min).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI grant 17H03068.

nomenon possibly indicates that the block copolymer could form micelles above the thermosensitive temperature of the poly([Me2Im][BF4]-alt-pMeBzA) segment. The result of DLS measurements of the block copolymer was consistent with the turbidity measurement (Figure S11). The copolymer has a diameter of approximately 2 nm at 30 and 40 °C, which indicates that the polymer is soluble in acetone as a unimer. However, a large diameter of 160 nm was detected at 50 °C. The result indicates that the block copolymer aggregated at high temperature in acetone because of thermosensitivity of the poly([Me2Im][BF4]-alt-pMeBzA) segment. The aggregation may consist of several micelles, which have the IBVE segment as a corona and the poly([Me2Im][BF4]-alt-pMeBzA) segment as a core.

4. CONCLUSIONS Alternating copolymers with imidazolium ionic liquid-type moieties and conjugated aldehydes were synthesized via living cationic copolymerization and subsequent chemical modification. The alternating copolymers exhibited UCST-type phase separation behavior in water. Interestingly, the copolymers exhibited LCST-type phase separation behavior in acetone unlike the corresponding ionic liquid-type homopolymers that are insoluble in acetone. Both the UCST- and LCST-type thermosensitive behaviors were most likely induced by the interpolymer electrostatic interactions among the pendant ionic liquid-type groups. In addition, the aldehyde contents of the alternating copolymers considerably influenced the thermosensitive behavior. The obtained copolymers that had pendant ionic liquid-type moieties were decomposed into a single small molecule under acidic conditions in solution. Block copolymers composed of UCST-type alternating and hydrophobic segments were also synthesized and exhibited different phase separation behaviors than the corresponding alternating copolymers. The results obtained in this study can contribute to the development of degradable thermosensitive polymers as smart functional materials for versatile applications.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00634. H

DOI: 10.1021/acs.macromol.9b00634 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00634 Macromolecules XXXX, XXX, XXX−XXX