Intramolecular Folding or Intermolecular Self-Assembly of Amphiphilic

Apr 26, 2018 - pendants in aqueous or organic media.13,17,24−31. The amphiphilic .... analysis. In all cases, those copolymers homogeneously dissolv...
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Intramolecular Folding or Intermolecular Self-Assembly of Amphiphilic Random Copolymers: On-Demand Control by Pendant Design Motoki Shibata,† Mayuko Matsumoto,† Yuji Hirai,† Mikihito Takenaka,‡,§ Mitsuo Sawamoto,†,∥ and Takaya Terashima*,† †

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan § RIKEN SPring-8 Center, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan ∥ Institute of Science and Technology Research, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan ‡

S Supporting Information *

ABSTRACT: Amphiphilic random copolymers comprising different hydrophilic poly(ethylene glycol) (PEG, average number of oxyethylene units = 4.5 or 8.5) and hydrophobic butyl or dodecyl pendants were designed to investigate self-folding and self-assembly behavior in water. The copolymers with controlled composition and chain length were synthesized by ruthenium-catalyzed living radical copolymerization. We revealed that the pendant design was one of the most critical factors to selectively induce intramolecular self-folding or intermolecular selfassembly. In the case of 30 mol % hydrophobic monomers, random copolymers bearing short PEG (on average 4.5 oxyethylene units) and butyl pendants intramolecularly self-folded into unimer micelles in water, independent of chain length. The size of unimer micelles thus increased with increasing chain length. In contrast, random copolymers bearing long dodecyl pendants intermolecularly selfassembled into uniform multichain micelles; the size depended on composition and PEG length. Additionally, the polymer micelles showed thermoresponsive solubility in water. The cloud point temperature was effectively controlled by the pendant structure, composition, and chain length.

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INTRODUCTION

monomers. Therefore, in water, these random copolymers undergo chain folding via the local association of the hydrophobic (functional) pendants to form quite small unimer and/or multichain micelles (∼10 nm),24 though block counterparts generally induce multichain association into relatively large aggregates. The design of pendant and primary structure of random copolymers enables on-demand selfassembly, that is, selective formation of unimer, multichain, necklace, and multicompartment micelles.24,25,30 The representative scaffolds are methacrylate-based amphiphilic random copolymers bearing PEG (average number of oxyethylene units = 8.5) and dodecyl (−C12H25) pendants (Scheme 1): PEG8.5MA/DMA random copolymers are efficiently synthesized by living radical copolymerization of PEG methyl ether methacrylate (PEG8.5MA: Mn = 475) and dodecyl methacrylate (DMA). Owing to controlled composition and chain length, these copolymers show unique selffolding/self-assembly behavior and innovative size controll-

Self-assembly of amphiphilic copolymers in water via hydrophobic effects is a cardinal strategy to construct compartmentalized polymer materials with well-defined 3D structures and unique functions.1−20 Amphiphilic copolymers comprising hydrophilic and hydrophobic units induce the association of the hydrophobic segments in water, forming nanoaggregates such as micelles and vesicles. This self-assembly behavior relies on the design and balance of hydrophobic/hydrophilic segments, in addition to primary structure (e.g., chain length, sequence, and composition). For desired self-assembly objects, amphiphilic copolymers with various architectures and monomer sequences have been designed and synthesized as precursors by precision polymerizations including living radical polymerization:21−23 e.g., amphiphilic block,1−4 random,5−13 alternating,14 gradient,15−17 and star18 copolymers. Recently, we have developed self-assembly systems of amphiphilic random copolymers carrying hydrophilic poly(ethylene glycol) (PEG) and hydrophobic alkyl or functional pendants in aqueous or organic media. 13,17,24−31 The amphiphilic copolymers comprise the statistical sequence distribution of hydrophilic and hydrophobic (functional) © XXXX American Chemical Society

Received: March 16, 2018 Revised: April 26, 2018

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Given these backgrounds, we herein designed amphiphilic random copolymers bearing various hydrophilic PEGs and hydrophobic alkyl pendants to investigate effects of the pendant structures on self-folding and self-assembly behavior in water (Scheme 1). The following four types of PEGMA/RMA random copolymers were synthesized by ruthenium-catalyzed living radical copolymerization of PEGMA (PEG4.5MA: average number of oxyethylene units = 4.5; Mn = 300, PEG8.5MA) and hydrophobic butyl or dodecyl methacrylate (BMA or DMA): (1) PEG4.5MA/BMA, (2) PEG4.5MA/ DMA, (3) PEG8.5MA/BMA, and (4) PEG8.5MA/DMA. (1) PEG4.5MA/BMA random copolymers have PEG and butyl pendants, both of which are shorter than (4) conventional PEG8.5MA/DMA counterparts.13,24 (2) PEG4.5MA/DMA or (3) PEG8.5MA/BMA copolymers are more hydrophobic or hydrophilic than (1) PEG4.5MA/BMA copolymers, respectively. Self-assembly of those PEGMA/RMA copolymers in water was evaluated by size-exclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) and dynamic light scattering (DLS), while the structure of resulting micelles was analyzed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Interestingly, PEG4.5MA/ BMA copolymers bearing short hydrophilic and hydrophobic pendants (BMA: 30 mol %) “intramolecularly” self-folded in water, independent of their chain length, to form unimer micelles; the micelle size thus increased with increasing the chain length. In contrast, the copolymers bearing long dodecyl pendants intermolecularly self-assembled in water to produce multichain micelles; their size was determined by composition and PEG length. Additionally, all the copolymers provided thermoresponsive micelles in water.

Scheme 1. Intramolecular Self-Folding and Intermolecular Self-Assembly of Amphiphilic Random Copolymers with Hydrophilic and Hydrophobic Pendants in Water

ability of micelles in water. The copolymers have the threshold degree of polymerization (DPth) that is critically suitable for self-folding into unimer (unicore) micelles.24 The copolymers with DP less than DPth induce “intermolecular” self-assembly into multichain micelles with constant size. This size (molecular weight, Mw,H2O) is identical to that of unimer micelles of copolymers with DPth; the size increases with increasing hydrophobic DMA content (only dependent on copolymer composition), while it is independent of DP (C4H9) and/or (2) the weight fraction of the hydrophobic part (HWF) is kept relatively high (HWF: >30 wt %). Importantly, the size of the multichain micelles is controlled by HWF: e.g., PEG8.5MA/ BMA (butyl methacrylate, 60 mol %) and PEG8.5MA/DMA (40 mol %) random copolymers with almost the same HWF (45 and 42 wt %, respectively) form multichain micelles with almost identical size (Mw,H2O = ∼100 000). However, the effects of hydrophilic PEG length on self-folding/self-assembly have not been elucidated yet. Since hydrophilic PEG units serve as shell layers to stabilize and cover hydrophobic cores in water, random copolymers bearing shorter hydrophilic PEGs would show different self-assembly behavior.

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RESULTS AND DISCUSSION Polymer Design: Different Pendant Length. Amphiphilic PEGMA/RMA random copolymers with different hydrophilic/hydrophobic pendants, composition, and chain length (degree of polymerization: DP) (Chart 1) were designed to investigate the effects of their pendant structures, RMA contents, and DP on self-folding and self-assembly behavior in

Chart 1. Design of PEGMA/RMA Random Copolymers: Hydrophobic Monomer Content and Weight Fraction and Chain Length

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Macromolecules water. Various hydrophilic PEGMAs (PEG4.5MA: Mn = 300, PEG8.5MA: Mn = 475) and hydrophobic alkyl methacrylates (RMA, butyl methacrylate: BMA; C4, dodecyl methacrylate: DMA; C12) were employed to prepare four kinds of random copolymers: (1) PEG4.5MA/BMA, (2) PEG4.5MA/DMA, (3) PEG8.5MA/BMA, and (4) PEG8.5MA/DMA. For PEG4.5MA/BMA random copolymers, their BMA contents and DP were systematically varied: BMA = 22, 30, and 40 mol %; DP = 24−468. For the other three types of copolymers, the content of BMA or DMA was set as 30 mol %, while DP was changed between 23 and 221. Self-assembly of amphiphilic copolymers in water via hydrophobic effects depends on not only the molar ratio of hydrophilic and hydrophobic monomers (i.e., PEGMA/BMA or DMA) but also the weight fraction of hydrophobic segments. Assuming that hydrophobic segments consist of alkyl pendants plus polymethacrylate backbones, the weight fraction of the hydrophobic segments of the copolymers (HWF, wt %) was calculated (see Supporting Information, Chart S1): (1) PEG4.5MA/BMA (22 mol %: 41 wt %, 30 mol %: 44 wt %, 40 mol %: 49 wt %); (2) PEG4.5MA/DMA (30 mol %: 51 wt %); (3) PEG8.5MA/BMA (30 mol %: 30 wt %); (4) PEG8.5MA/DMA (30 mol %: 36 wt %). All the copolymers were synthesized by ruthenium-catalyzed living radical copolymerization of PEGMA and BMA or DMA with a chloride initiator (Figure 1 and Figure S1, Tables S1− S4). Typically, PEG4.5MA and BMA were efficiently copolymerized with Ru(Ind)Cl(PPh3)2/n-Bu3N and ethyl 2chloro-2-phenylacetate (ECPA) in toluene at 80 °C to produce well-controlled PEG4.5MA/BMA random copolymers [Mw/Mn = 1.1−1.5, determined by SEC in N,N-dimethylformamide (DMF) with PMMA standard calibration]. Both of the monomers were simultaneously consumed at the same speed (Figure 1a and Figure S1), independent of the feed ratio of the monomers ([PEG4.5MA]0/[BMA]0) and the target DP (([PEG4.5MA]0 + [BMA]0)/[ECPA]0). Such simultaneous, synchronized consumption was also observed for the other combination of monomers (PEG4.5MA/DMA, PEG8.5MA/ BMA, and PEG8.5MA/DMA) (Figure 1c and Figure S1).24 These results importantly demonstrate that all the copolymers have statistical sequence distribution of PEGMA and BMA or DMA. The composition and number-average molecular weight of the copolymers determined by 1H NMR were in good agreement with those calculated from the feed ratio of monomers and the conversion. Self-Folding and Self-Assembly in Water. Self-folding and self-assembly behavior of PEGMA/RMA random copolymers in water and the size of micelles were evaluated with SEC coupled with multiangle laser light scattering detector (SECMALLS) and dynamic light scattering (DLS). Discussion is especially focused on the effects of (1) pendant length (PEG or alkyl groups), (2) main chain length (DP), (3) composition, and (4) total polymer concentration. For these experiments, the aqueous solutions of the random copolymers were prepared according to the following procedures: In vials, the copolymers were vigorously mixed with water and instantaneously dissolved in water. The obtained solutions were sonicated at 25 °C for 5 min (Sonicator: Branson, Bransonic 1510) and filtrated with PTFE membrane filter (0.45 μm, Merck Millipore) before analysis. In all cases, those copolymers homogeneously dissolved in water without any specific process. Apparent Size in Water. The copolymers with 30 mol % hydrophobic BMA or DMA were analyzed by SEC in water and

Figure 1. Synthesis of (a, b) PEG4.5MA/BMA or (c, d) PEG4.5MA/ DMA random copolymers via Ru-catalyzed living radical copolymerization: [PEGMA]0/[RMA]0/[ECPA]0/[Ru(Ind)Cl(PPh3)2]0/[nBu3N]0 = (a, b) 490/210/5.6/2.0/20, (c, d) 350/150/4.0/2.0/20 mM in toluene at 80 °C. (a, c) Time−conversion curves and (b, d) SEC curves of the crude products. The final products in (b) and (d) were purified to give polymer samples of DP = 116 (BMA: 30 mol %) and DP = 109 (DMA: 30 mol %), respectively.

DMF (Figure 2). The apparent sizes of those copolymers and/ or the micelles were evaluated by SEC in water or DMF with poly(ethylene oxide) (PEO) calibration. All the PEG4.5MA/ BMA random copolymers exhibited unimodal SEC curves (refractive index detector) in both water and DMF (Figure 2a). The peak-top molecular weight increased with increasing the DP, while the peak-top molecular weight for each polymer in water was smaller than that in DMF. This indicates that the copolymers bearing butyl (C4) groups “intramolecularly” selffolded in water via hydrophobic effects, independent of chain length (DP: 56−468) to result in more compact structures than those in DMF. The local self-assembly of hydrophobic butyl pendants in water was supported by 1H NMR: A PEG4.5MA/ BMA random copolymer (DP = 116) exhibited broad methylene and methyl proton signals of the butyl groups in D2O (Figure S2). Similarly, PEG8.5MA/BMA random copolymers bearing longer hydrophilic PEG pendants showed unimodal SEC curves in both water and DMF, whereas the peak-top molecular weight in water was almost identical to that in DMF (Figure 2c). This suggests that the copolymers fail to induce effective self-folding in water and thus have apparent sizes almost identical to those in DMF. C

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water, absolute weight-average molecular weight (Mw) of the copolymers was determined by SEC-MALLS in water or DMF (Figure 3). In random copolymers carrying 30 mol % hydrophobic butyl pendants (PEG4.5MA/BMA and PEG8.5MA/BMA), both Mw in water and that in DMF increased with increasing DP; Mw in water almost agreed with Mw in DMF (Figure 3a,c). Additionally, Mw in DMF was close to the weight-average molecular weight calculated with Mn by 1 H NMR and Mw/Mn by SEC in DMF [Mw(calcd) = Mn(NMR) × Mw/Mn] (Tables S1 and S3). The aggregation number in water [Nagg = Mw(H2O)/Mw(DMF)] was thus estimated to be almost 1. These results indicate that the random copolymers bearing butyl pendants exist as unimer in water under diluted and flow conditions of SEC measurement. PEG4.5MA/BMA random copolymers (BMA = 30 mol %, DP = 24−468) were further analyzed by DLS in water at 25 °C (Figure 4a and Table S1). The copolymers exhibited unimodal light intensity size distribution in water ([polymer] = 10 mg/ mL). Hydrodynamic radii (Rh) in water gradually increased from 3 to 7 nm with increasing DP. Importantly, for each polymer, Rh in water was smaller than that in DMF (Figure 5a), indicating that PEG4.5MA/BMA copolymers have more compact structure in water than those in DMF. This tendency was fully consistent with the fact that the copolymers showed the peak-top molecular weight by SEC in water smaller than that in DMF (Figure 2a). Thus, PEG4.5MA/BMA copolymers were found to intramolecularly self-fold into unimer micelles in water via self-assembly of the hydrophobic butyl pendants, independent of DP. As for PEG8.5MA/BMA random copolymers bearing longer hydrophilic PEG pendants, Rh increased with DP in water (Figure 4c), whereas each of the copolymers had almost the same size (Rh by DLS, peak-top molecular weight by SEC) in both water and DMF (Table S3 and Figure 2c). In contrast to those BMA-based copolymers, PEG4.5MA/ DMA (30 mol %) random copolymers with dodecyl pendants

Figure 2. SEC curves (by refractive index detector) of (a) PEG4.5MA/BMA (30 mol %), (b) PEG4.5MA/DMA (30 mol %), and (c) PEG8.5MA/BMA (30 mol %) random copolymers in DMF (10 mM LiBr, black lines) or H2O (blue lines). DP: degree of polymerization.

In contrast, PEG4.5MA/DMA random copolymers bearing more hydrophobic dodecyl (C12) pendants uniquely showed the constant peak-top molecular weight in water, regardless of DP (36−214) (Figure 2b). This indicates that the copolymers “intermolecularly” self-assembled in water to form uniform size micelles. A similar tendency was also observed for PEG8.5MA/ DMA random copolymers bearing longer hydrophilic PEG pendants.24 These results importantly demonstrate that the length of both hydrophobic alkyl pendants and hydrophilic PEG units critically affect self-folding/self-assembly behavior of the copolymers in water. Molecular Weight and Hydrodynamic Radius in Water. To quantitatively evaluate self-folding/self-assembly behavior in

Figure 3. Absolute weight-average molecular weight of (a) PEG4.5MA/BMA (30 mol %), (b) PEG4.5MA/DMA (30 mol %), (c) PEG8.5MA/BMA (30 mol %), and (d) PEG8.5MA/DMA (30 mol %) random copolymers determined by SEC-MALLS in DMF (10 mM LiBr, black) or H2O (blue). D

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copolymers bearing longer hydrophilic PEG units (Mw = ∼53 000; DP < 100, Figure 3d).24 This importantly means that the balance of hydrophilic and hydrophobic segments determined the size of micelles formed via intermolecular self-assembly in water. The micelle size apparently increased with increasing the weight fraction of hydrophobic segments [HWF = 51 wt % (PEG4.5MA/DMA), 36 wt % (PEG8.5MA/ DMA)]. The molar ratio of hydrophilic PEGMA and hydrophobic BMA also affected the self-assembly behavior in water. PEG4.5MA/BMA random copolymers with 22 and 30 mol % BMA (HWF = 41 and 44 wt %) existed as unimers in water (Table S1 and Figure S3). However, the copolymers with 40 mol % BMA (HWF = 49 wt %) induced intermolecular selfassembly into multichain micelles in water (e.g., DP = 151, Mw = ∼61 000 in water, Table S1). Interestingly, PEG8.5MA/DMA (30 mol %) random copolymer micelles (DP < 100) with longer PEG and alkyl pendants had size close to PEG4.5MA/ BMA (40 mol %) counterparts with shorter pendants, although the HWF of the former polymer (36 wt %) was smaller than that of the latter one (49 wt %). This suggests that alkyl pendants more critically serve as hydrophobic units to induce intermolecular self-assembly in water than polymethacrylate backbones. The Rh values of PEG4.5MA/BMA, PEG4.5MA/DMA, and PEG8.5MA/BMA random copolymers with 30 mol % hydrophobic monomers (DP = ∼100) were measured by DLS in water at various total polymer concentrations (Figure 5b). Those copolymers effectively maintained small size (Rh < 5 nm) in wide range of polymer concentration (1−70 mg/mL), independent of PEG and alkyl pendant length. Typically, a PEG4.5MA/BMA copolymer (DP = 116) and a PEG4.5MA/ DMA copolymer (DP = 143) induced intramolecular selffolding into unimer micelles and intermolecular self-assembly into multichain micelles (Nagg = 2.4) in water, respectively, each of which had the almost constant Rh (about 4 and 5 nm, respectively) without producing larger aggregates (Figure 5b). The structure of PEGMA/RMA (30 mol %) random copolymer micelles was evaluated with transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). For TEM measurement, a PEG4.5MA/BMA random copolymer (DP = 468, Mw = 147 000 in water, Nagg = 1.1) was cast on carbon coat grids from the aqueous solution, followed by staining with OsO4. The sample clearly exhibited black dots of globular nanoparticles in the TEM image (Figure 6). To clarify the structure of copolymers in water, SAXS measurement was carried out on the aqueous solutions of PEGMA/RMA random copolymers (PEG4.5MA/BMA: DP = 230, 468, PEG4.5MA/DMA: DP = 214, PEG8.5MA/BMA: DP = 221) ([polymer] = 1 mg/mL, Figure 7 and Figure S4). Confirmed by SAXS, all the copolymers formed globular structures in water, while their detailed structure and size depended on the pendant structures. The SAXS profile of the PEG4.5MA/DMA copolymer had a minimum and a maximum at ca. 1 nm−1 and 1.5 nm−1, respectively (Figure 7b). The SAXS profile was close to that of spherical micelles obtained from sodium dodecyl sulfate (surfactants)32 or other dodecyl-bearing amphiphilic random or alternating copolymers.24,33 These results demonstrate that the DMA-based copolymers formed micelles with a hydrophobic core comprising the dodecyl pendants with low electron density. In contrast, BMA-based copolymers did not so clearly show such minimum and maximum in their SAXS profiles (Figure 7a and Figure S4).

Figure 4. DLS intensity size distribution of (a) PEG4.5MA/BMA (30 mol %), (b) PEG4.5MA/DMA (30 mol %), and (c) PEG8.5MA/BMA (30 mol %) random copolymers in water at 25 °C: [polymer] = 10 mg/mL.

Figure 5. (a) Rh of PEG4.5MA/BMA (30 mol %) random copolymers in DMF (black) or water (blue) as a function of DP. (b) Effects of the concentration of random copolymers on Rh in water (PEG4.5MA/ BMA: DP = 116; black, PEG4.5MA/DMA: DP = 143; red, PEG8.5MA/BMA: DP = 107; blue).

formed uniform size multichain micelles, independent of DP. The molecular weight (Mw) by MALLS and Rh by DLS in water were almost constant at least in the range of DP from 36 to 214: Mw = ∼130 000; Rh = ∼5 nm (Figures 3b and 4b). As a result, aggregation number was also effectively controlled by changing DP as follows: Nagg(DP) = 13 (36), 6.4 (59), 5.1 (78), 3.3 (109), 2.4 (143), and 1.8 (214). It should be noted that the constant size of the micelles was larger than that of micelles obtained with PEG8.5MA/DMA (30 mol %) E

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Figure 6. A TEM image of a PEG4.5MA/BMA random copolymer (DP = 468) cast on a carbon coat grid from the aqueous solution (10 mg/mL). The sample was stained with vapor on the aqueous solution of OsO4.

Figure 7. SAXS profiles of (a) PEG4.5MA/BMA (30 mol %, DP = 230) and (b) PEG4.5MA/DMA (30 mol %, DP = 214) random copolymers in water (red) or DMF (blue) at 25 °C: [polymer] = 1 mg/mL in water or 10 mg/mL in DMF (blue).

Figure 8. (a) Transmittance of the aqueous solutions of PEGMA/ RMA (30 mol %) random copolymers (PEG4.5MA/BMA: DP = 116; black, PEG4.5MA/DMA: DP = 109; red, PEG8.5MA/BMA: DP = 107; blue, PEG8.5MA/DMA: DP = 103; green) monitored at 670 nm by changing temperature between 30 and 90 °C: heating (solid lines)/ cooling (dashed lines) rate: 1 °C/min; [polymer] = 4 mg/mL. (b) Effects of DP on cloud point (Cp) temperature of the aqueous solutions of PEGMA/RMA (30 mol %) random copolymers (PEG4.5MA/BMA: black; PEG4.5MA/DMA: red; PEG8.5MA/ BMA: blue; PEG8.5MA/DMA: green).

This indicates that those copolymers formed micelles with relatively loose hydrophobic cores; the butyl pendants would more dynamically self-assemble within polymer chains than the dodecyl pendants. The influence of hydrophobic or hydrophilic pendant structure and DP on the micelle size was further evaluated by radius of gyration (Rg) estimated with Guinier plot (Figure S5). In unimer micelles of PEG4.5MA/BMA copolymers, Rg increased with increasing DP: Rg(DP) = 5.2 nm (230), 7.0 nm (468). A PEG4.5MA/DMA copolymer with dodecyl pendants formed multichain micelles in water (DP = 214, Nagg = 1.6) to have Rg (7.7 nm) larger than a PEG4.5MA/ BMA copolymer (DP = 230, unimer micelle). A PEG8.5MA/ BMA copolymer bearing long hydrophilic PEG chains (DP = 221) also had Rg (7.7 nm) larger than a PEG4.5MA/BMA counterpart. Thermoresponsive Properties in Water. Thermoresponsive properties of PEG4.5MA/BMA, PEG4.5MA/DMA, PEG8.5MA/BMA, and PEG8.5MA/DMA random copolymers in water were evaluated by the cloud point (Cp) measurement of the aqueous solutions monitored at λ = 670 nm. Cp was defined as a temperature at which the transmittance of the aqueous solutions turned 90% upon heating from 30 to 90 °C. Upon heating, all the copolymers sharply and reversibly showed LCST-type phase separation in water (Figure 8a). Cp

depended on the hydrophilic PEG length (Figure 8b). PEG8.5MA-based copolymers with long hydrophilic PEG units had Cp much higher than PEG4.5MA-based counterparts. In detail, PEG8.5MA/DMA (30 mol %) random copolymers (Cp = ∼80 °C) showed Cp higher than PEG8.5MA/BMA (30 mol %) counterparts (Cp = ∼75 °C); the Cp temperatures were independent of DP.24,26 PEG4.5MA-based copolymers had low Cp between 30 and 42 °C. The Cp depended on hydrophobic monomers (BMA vs DMA) and composition. In PEG4.5MA/BMA copolymers with about 170 DP, Cp increased with increasing BMA content: Cp(BMA) = 44 °C (22 mol %), 39 °C (30 mol %), and 30 °C (40 mol %) (Table S1). Uniquely, Cp of PEG4.5MA/BMA (30 mol %) copolymers also depended on DP: Cp gradually increased from 31 to 39 °C with increasing DP from 24 to 131 and kept constant at 39 °C up to DP of 468. This suggests that upon heating the random copolymers with short DP tend to form large aggregates in water via dehydration of the PEG units. It should be noted that by tuning composition and chain length, PEG4.5MA/BMA or DMA copolymers afford fine control of Cp in every 1 °C level around human body temperature. Thus, PEG4.5MA/BMA or DMA copolymer micelles would be useful as thermoresponsive nanocarriers and materials for bioapplications. F

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CONCLUSION In summary, self-folding and self-assembly of PEGMA/RMA random copolymers in water were found to critically depend on pendant structure, in addition to composition and chain length. In the case of 30 mol % hydrophobic monomers, PEG4.5MA/ BMA copolymers bearing short hydrophilic and hydrophobic pendants selectively induced intramolecular self-folding in wide range of chain length to form unimer micelles in water (Chart 2). Thus, the micelle size can be controlled by tuning the chain

by The Mazda Foundation, by The Sumitomo Electric Group Social Contribution Foundation, by The Ogasawara Foundation for the Promotion of Science & Engineering, and by The Noguchi Institute. We also thank Prof. Kazunari Akiyoshi, Dr. Yuta Koda and Mr. Goki Hattori (Kyoto University) for TEM and DLS measurement. The SAXS measurement was performed at BL45XU in SPring-8 with the approval of RIKEN (Proposal Nos. 20160005, and 20170020).

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Chart 2. Intramolecular Self-Folding or Intermolecular SelfAssembly of PEG4.5MA/RMA Amphiphilic Random Copolymers into Size-Controlled Micelles

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length. In contrast, PEG4.5MA or PEG8.5MA/DMA copolymers bearing long dodecyl pendants intermolecularly selfassembled in water to form multichain micelles; the micelle size is determined by composition and PEG length if the chain length of the copolymers is below a threshold value critically suitable for unimer micelles. The three-dimensional structure of resulting micelles was also dependent on the alkyl pendant structure. The copolymers showed LCST-type phase separation in water to serve as thermoresponsive micelles. The cloud point temperature was controlled by the pendant structure, composition, and chain length. Thus, this work provides novel strategies and important knowledges to design selfassembly polymer materials with desired sizes, morphologies, and thermoresponsive properties that could be widely applied to various research fields.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00570. Experimental details, SEC curves, characterization by MALLS, DLS, SAXS, and 1H NMR (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.T.). ORCID

Mitsuo Sawamoto: 0000-0003-0352-9666 Takaya Terashima: 0000-0002-9917-8049 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS through Grants-in-Aid for Scientific Research KAKENHI Grants JP26410134, JP17H03066, and JP17K19159, by Sekisui Chemical through “Innovations Inspired by Nature” Research Support Program, G

DOI: 10.1021/acs.macromol.8b00570 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.8b00570 Macromolecules XXXX, XXX, XXX−XXX