Fluorous Gradient Copolymers via in-Situ Transesterification of a

Jan 12, 2018 - To create such well-defined nanostructures, unique proper- ties, and desired functions, controlling the sequence distribu- tion of segr...
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Fluorous Gradient Copolymers via in-Situ Transesterification of a Perfluoromethacrylate in Tandem Living Radical Polymerization: Precision Synthesis and Physical Properties Yusuke Ogura,† Mikihito Takenaka,†,‡ Mitsuo Sawamoto,†,§ and Takaya Terashima*,† †

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, 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: Perfluorinated gradient copolymers comprising fluorous units and hydrophobic or hydrophilic units were created as fluorous polymer materials with unique physical properties. The gradient copolymers were efficiently synthesized via the tandem catalysis of living radical polymerization and titanium alkoxide-mediated transesterification of 1H,1H,2H,2Hperfluorooctyl methacrylate (13FOMA) with alcohols. Owing to the electron-withdrawing perfluorooctyl unit, 13FOMA was efficiently transesterified into another methacrylate (RMA) with alcohols during the copolymerization of 13FOMA and RMA to afford well-controlled 13FOMA/RMA gradient copolymers. The gradient sequence gradually changed from 13FOMA to RMA-rich composition according to monomer composition varying in the polymerization solutions. The tandem catalysis is allowed to use various alcohols including 1-dodecanol, 1octadecanol, ethanol, and poly(ethylene glycol) methyl ether; the catalysis developed herein is thus one of the most versatile systems to produce fluorous/hydrophobic or fluorous/hydrophilic gradient copolymers. Additionally, physical properties of a fluorous/hydrophobic 13FOMA/dodecyl methacrylate gradient copolymer were investigated. The gradient copolymer exhibited broad glass transition temperature, microphase separation, self-assembly, surface tension, and water/oil repellency distinct from corresponding random or block counterparts.

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INTRODUCTION Perfluorinated compounds and polymers1−24 exhibit inherent physical properties such as “fluorous” nature, i.e., immiscibility with hydrophilic or hydrophobic molecules and common organic solvents,1−8 high thermal and chemical stability,6 and high oxygen affinity.2,3 Perfluorinated polymers therefore attract attention as functional materials for durable surface coating and water/oil repellency,9−12 fuel cell membranes,13 and bioapplications.3,14 In designing fluorinated polymer materials, partial or site-selective functionalization of polymers with perfluorinated units is rather intriguing because fluorinedriven physical properties can be introduced into mother polymers with distinct properties. In fact, various fluorinated copolymers comprising fluorous units and hydrophobic or hydrophilic units have been produced to induce unique micellization/self-assembly into nanostructured materials,14−18 efficient microphase separation,5,19 and selective molecular encapsulation.20,21 To create such well-defined nanostructures, unique properties, and desired functions, controlling the sequence distribution of segregating monomer pairs, i.e., fluorinated and nonfluorinated monomers, is essential. Gradient copolymers23−34 © XXXX American Chemical Society

are one class of sequence-controlled copolymers with the biased sequence distribution of two monomers; the composition of one monomer gradually and seamlessly increases from the initiating terminal (α-end) to the growing terminal (ω-end) along a chain. Owing to the unique sequence distribution, gradient copolymers are known to show physical properties different from corresponding random or block copolymers (e.g., broad glass transition temperature).25−34 Thus, perfluorinated gradient copolymers are expected to serve as fluorous materials with unique properties23,24 because segregating monomer pairs are forced to be sequenced in gradient composition along a chain. As a versatile strategy to synthesize functional gradient copolymers, we have developed tandem catalysis24,30−34 of living radical polymerization (LRP)35,36 and in-situ transesterification37 of methacrylates (R1MA, a starting monomer) with alcohols (R2OH). Here, R1MA is gradually transesterified with a metal alkoxide catalyst [e.g., Ti(Oi-Pr)4] into another Received: November 27, 2017 Revised: January 12, 2018

A

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Macromolecules Scheme 1. Synthesis of Fluorinated Gradient Copolymers via Tandem Living Radical Polymerization with in-Situ Transesterification with (a) a Perfluoromethacrylate (13FOMA) or (b) Fluoroalcohols

R2MA during LRR to change monomer composition (the ratio of R1MA and R2MA) in the polymerization solutions. Resulting copolymers are not transesterified because the quaternary carbon substituents (polymethacrylate backbones with αmethyl groups) sterically hinder the transesterification of the pendant esters.31,38 As a result, R1MA/R2MA gradient copolymers are obtained in one-pot, where the gradient sequence varies from R1MA- to R2MA-rich composition according to the instantaneous monomer composition in the solutions. Importantly, desired properties (e.g., soft/hard,32 amphiphilic,33 and hydrogen bonding34) can be programmed into gradient copolymers by selecting methacrylates and diverse alcohols (e.g., ethanol,30,31 dodecanol,32 poly(ethylene glycol) methyl ether (PEG-OH),33 fluoroalcohols,24 an amidefunctionalized alcohol34). To produce fluorinated gradient copolymers by tandem LRP/transesterification, we have two potential pathways using different fluorine sources: one is tandem LRP of MMA with fluoroalcohols (RFOH),24 while another is tandem LRP of perfluoromethacrylates (RFMA) with hydrophobic or hydrophilic alcohols (Scheme 1). Recently, we found that MMA was transesterified into fluorinated methacrylates (RFMA) with fluoroalcohols (CF 3 CF 2 CH 2 CH 2 CH 2 OH, CF 3 CF 2 CF 2 CF2CH2CH2OH, etc.) in the presence of Ti(Oi-Pr)4 and molecular sieves to successfully produce fluorinated MMA/ RFMA gradient copolymers (Scheme 1b).24 However, due to less nucleophilicity and high acidity, fluoroalcohols available in this system are limited to alcohols of relatively small number of fluorine (−CnF2n+1: n ≤ 4).

Given these backgrounds, we herein report the precision synthesis of fluorous, perfluorinated gradient copolymers via the in-situ transesterification of a perfluoromethacrylate (1H,1H,2H,2H-perfluorooctyl methacrylate: 13FOMA)18 with hydrophobic or hydrophilic alcohols [ROH: 1-dodecanol, 1octadecanol, ethanol, and poly(ethylene glycol) methyl ether (Mn = 350)] in tandem LRP (Scheme 1a). In contrast to tandem LRP of MMA with fluoroalcohols,24 tandem LRP of 13FOMA with alcohols developed herein effectively gave fluorous/hydrophobic or fluorous/hydrophilic 13FOMA/RMA gradient copolymers bearing perfluorooctyl pendants [−CH2CH2(CF2)5CF3]. This is because the transesterification of 13FOMA smoothly proceeds owing to the electronwithdrawing perfluorooctyl group and the less nucleophilicity of a perfluorooctanol [HOCH2CH2(CF2)5CF3] generating therefrom.39,40 Typically, 13FOMA was transesterified with dodecanol and Ti(Oi-Pr)4 into dodecyl methacrylate (DMA) during tandem LRP to give well-controlled 13FOMA/DMA gradient copolymers. The gradient sequence was catalytically controlled by tuning the concentration of Ti(Oi-Pr)4 and 13FOMA. Owing to the fine controllability of gradient sequence and the wide applicability of alcohols, tandem LRP of 13FOMA is regarded as one of the most versatile systems to produce fluorous gradient copolymers. Additionally, physical properties of a fluorous/hydrophobic 13FOMA/DMA gradient copolymer were investigated to clarify effects of the sequence distribution of perfluorinated segments (random, gradient, and block) on thermal properties, phase separation, surface tension, micellization/aggregation, and water/oil repellency. B

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Macromolecules Table 1. Fluorous Gradient Copolymers via Concurrent Tandem Living Radical Polymerizationa entry

alcohol (ROH)

13FOMA (M)

Ti (mM)

ROH (M)

time (h)

convb (%)

Mnc (GPC)

Mw/Mnc

Fcum.DMAd (%)

1 2 3 4 5 6 7

1-dodecanol 1-dodecanol 1-dodecanol 1-dodecanol ethanol 1-octadecanol PEGOH

1.0 1.0 1.0 2.0 2.0 1.3 1.0

15 30 40 30 20 26 20

1.5 1.5 1.5 1.5 3.3 0.98 1.0

29 24 24 24 22 25 49

91 88 89 95 97 90 77

34800 43500 68400 36300 16700 27000 51800

1.33 1.24 1.17 1.24 1.23 1.56 1.19

57 61 68 41 64 53 65

a

Conditions: [13FOMA]0/[ECPA]0/[Ru(Ind)Cl(PPh3)2]0/[Ti(Oi-Pr)4]0 = 1000/5/1/15 (entry 1), 30 (entry 2), 40 (entry 3) mM in toluene/1dodecanol (1/1, v/v), (entry 4) 2000/10/1/30 mM in toluene/1-dodecanol (1/5, v/v), (entry 5) 2000/20/2/20 mM in toluene/ethanol (1/1, v/v), (entry 6) 1300/13/1.3/26 mM in toluene/1-octadecanol, and (entry 7) 1000/5/1/20 mM in toluene/PEG-OH (CH3(OCH2CH2)nOH; Mn = 350) (1/1, v/v) at 80 °C. bTotal monomer conversion determined by 1H NMR with an internal standard (tetralin). cDetermined by SEC in CHCl3 with a PMMA standard calibration. dCumulative RMA content of products determined by 1H NMR. RMA: dodecyl methacrylate (DMA, entries 1−4); ethyl methacrylate (EMA, entry 5), octadecyl methacrylate (ODMA, entry 6); poly(ethylene glycol) methyl ether methacrylate (PEGMA, entry 7).

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RESULTS AND DISCUSSION Synthesis of Fluorous Gradient Copolymers with 13FOMA. We employed a perfluoroalkyl methacrylate (13FOMA) as a fluorous monomer to produce fluorous/ hydrophobic or fluorous/hydrophilic gradient copolymers via concurrent tandem LRP with in-situ transesterification of 13FOMA in the presence of various alcohols (ROH) (Table 1). We first investigated tandem LRP of 13FOMA with 1dodecanol (C12H25OH) and Ti(Oi-Pr)4 (catalyst) to synthesize fluorous/hydrophobic gradient copolymers via the in-situ transesterification of 13FOMA into dodecyl methacrylate (DMA). For this, 13FOMA was polymerized with Ru(Ind)Cl(PPh3)2 (polymerization catalyst) and ethyl 2-chloro-2-phenylacetate (ECPA, a chloride initiator) in the presence of Ti(Oi-Pr)4 in a toluene/1-dodecanol (1/1, v/v) mixture at 80 °C (entry 1 in Table 1 and Figure 1). The concentration of 13FOMA, 1dodecanol, and Ti catalyst was set as 1000, 1500, and 15 mM, respectively. The extra amount of 1-dodecanol to 13FOMA was used to efficiently induce transesterification of 13FOMA into DMA. The target degree of polymerization (DP = [13FOMA]0/[ECPA]0) was 200. The content of generating DMA in monomer [DMA/(13FOMA + DMA)] and the total conversion (olefin consumption) of 13FOMA and DMA by copolymerization were estimated by 1H nuclear magnetic resonance (NMR) measurement of the polymerization solutions that are sampled at predetermined periods. The polymerization smoothly proceeded to reach high conversion (91%) in 29 h (black line in Figure 1a). 13FOMA (monomer) was simultaneously transesterified into DMA to gradually increase DMA content in monomer along with monomer consumption (blue line in Figure 1a); the transesterification of 13FOMA into DMA was well synchronized with the copolymerization of the two monomers. The reaction mixture was kept homogeneous throughout the tandem polymerization in spite of the in-situ generation of a fluoroalcohol (HO− CH2CH2(CF2)5CF3) and DMA via the transesterification. Confirmed by size exclusion chromatography (SEC), the molecular weight of products increased with increasing monomer conversion, resulting in the formation of a wellcontrolled copolymer with narrow molecular weight distribution (Mn = 34 800, Mw/Mn = 1.33, Figure 1b). Cumulative DMA content in polymer (Fcum.DMA) was further estimated from 1H NMR measurement of the polymerization solutions that were sampled at predetermined periods. Fcum.DMA against total monomer conversion was first determined with the

Figure 1. Synthesis of a fluorous 13FOMA/DMA gradient copolymer via concurrent tandem living radical copolymerization of 13FOMA with 1-dodecanol: [13FOMA]0/[ECPA]0/[Ru(Ind)Cl(PPh3)2]0/[Ti(Oi-Pr)4]0 = 1000/5/1/15 mM in toluene/1-dodecanol (1/1, v/v, [1dodecanol]0 = 1500 mM) at 80 °C (entry 1 in Table 1). (a) Total monomer conversion (black) and dodecyl methacrylate (DMA) content (blue) in monomer as a function of time. (b) SEC curves of products obtained at 2 h (gray) and 29 h (black). (c) DMA content in monomer as a function of total monomer conversion. (d) Cumulative and instantaneous DMA content in products (Fcum.DMA: blue; Finst.DMA: red) as a function of normalized chain length.

area ratio of the methylene protons of polymerized DMA adjacent to the ester units (4.0−3.8 ppm) and those of polymerized 13FOMA (4.4−4.1 ppm). To express variation of Fcum.DMA along polymer chain, Fcum.DMA values were plotted as a function of normalized chain length (Figure 1d). The normalized chain length is defined as DPt/DPfinal for living copolymers; DPt = [RMA]0 × (total conversion/100)/ [ECPA]0; DPfinal = [RMA]0 × (final total conversion/100)/ [ECPA]0. Fcum.DMA and instantaneous DMA content (Finst.DMA) calculated therefrom (see Supporting Information) increased with normalized chain length (Figure 1d). Additionally, no transesterification of the pendant esters of 13FOMA-based copolymers was also confirmed (Figure S1). These results support the formation of a 13FOMA/DMA gradient copolymer, where the fluorous 13FOMA units decrease, and C

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content in monomer because of the extra amount of 13FOMA against 1-dodecanol (1500 mM). Thus, the resulting gradient copolymer has smaller DMA content (Fcum.DMA = 41%) than the other gradient copolymers obtained with 1000 mM 13FOMA. We further applied various alcohols [ethanol, 1-octadecanol, and poly(ethylene glycol) methyl ether (PEG-OH, CH3(OCH2CH2)nOH, n = 7.2 (average), Mn = 350)] to tandem LRP of 13FOMA to prepare fluorous/hydrophobic or hydrophilic gradient copolymers (Figure 3). In all the cases, the transesterification of 13FOMA into corresponding monomers (RMAs) was synchronized with the polymerization of the two monomers to provide 13FOMA/RMA gradient copolymers with narrow molecular weight distribution (entries 5−7 in Table 1). Interestingly, 13FOMA (1000 mM) was efficiently transesterified even with the equal amount of PEG-OH (1000 mM) to produce poly(ethylene glycol) methyl ether methacrylate (PEGMA) in high yield (∼75%) in tandem LRP (Figure 3c). The resulting fluorous/hydrophilic 13FOMA/ PEGMA gradient copolymer contained a large amount of PEGMA (Fcum.PEGMA = 65%). This is owing to the less nucleophilic character of a 1H,1H,2H,2H-perfluorooctanol that generates from the transesterification of 13FOMA with PEGOH. Thus, tandem LRP/transesterification of 13FOMA coupled with alcohols is one of the most efficient and versatile systems to design fluorous and perfluorinated gradient copolymers with various functional pendants. Physical Properties of 13FOMA/DMA Copolymers with Different Sequence Distributions. Physical properties of fluorous/hydrophobic 13FOMA/DMA random, gradient, and block copolymers were investigated to reveal the effects of the monomer sequence distribution on thermal properties, phase separation, micellization/aggregation properties, surface tension of polymer solutions, and water/oil repellency on polymer films. A 13FOMA/DMA gradient copolymer (Fcum.DMA = 57, DP = 177, Mn = 34 800, Mw/Mn = 1.33, entry 1 in Table 1 and Figure 4b) was employed as a gradient sample. Corresponding random or block copolymers were prepared according to Scheme 2. A 13FOMA/DMA random copolymer was synthesized by Ru(Ind)Cl(PPh3)2/n-Bu3N-catalyzed living radical copolymerization of 13FOMA and DMA with ECPA (Figures S3 and S4). Both 13FOMA and DMA were simultaneously consumed at almost the same speed to give a well-controlled random copolymer (Fcum.DMA = 53, DP = 170, Mn = 29 700, Mw/Mn = 1.25, Figure 4b). A well-controlled 13FOMA/DMA block copolymer (Fcum.DMA = 57, DP = 170, Mn = 38 600, Mw/Mn = 1.15, Figure 4b) was efficiently obtained via Ru(Ind)Cl(PPh3)2/n-Bu3N-catalyzed living radical polymerization of DMA with ECPA, followed by the direct addition of 13FOMA at the later stage of the DMA polymerization (at 84% conversion) (Scheme 2, Figures S3 and S4). The sequence distribution of hydrophobic DMA in the random, gradient, and block copolymers is shown in Figure 4a. Thermal Properties. Thermal properties of the 13FOMA/ DMA random, gradient, and block copolymers were examined by differential scanning calorimetry (DSC) (Figure 5a). The block copolymer clearly showed two peaks originating from glass transition temperature (Tg) of poly(DMA) (−58 °C) and poly(13FOMA) (13 °C) segments, while the random copolymer exhibited a single Tg at −4 °C between the two Tg’s observed in the block copolymer. The gradient copolymer in turn showed a broad Tg signal at around −11 °C, where the

the hydrophobic DMA units in turn increase along a chain from the initiating terminal (α-end) to the growing Cl terminal (ωend). Importantly, Finst.DMA agreed well with DMA content in monomer as a function of total monomer conversion (red line in Figure 1d and Figure 1c). This means that the gradient monomer sequence is just determined by the instantaneous monomer composition of 13FOMA and DMA in copolymerization solutions. This is because both 13FOMA and DMA have close reactivity in copolymerization. Characterized by 1H NMR, the number-average molecular weight, total DP, 13FOMA/DMA (m/n) ratio, and Fcum.DMA were determined as follows: Mn(NMR, α) = 58 700; DP = 177; m/n = 76/101; Fcum.DMA = 57%. The introduction of perfluorinated pendants into the copolymer was also confirmed by 19F NMR. The monomer sequence of 13FOMA/DMA gradient copolymers can be further controlled by changing the concentration of Ti(Oi-Pr)4 (15, 30, and 40 mM) and/or 13FOMA (1000 or 2000 mM) (entries 1−4 in Table 1, Figure 2, and Figure S2). In all the cases, well-controlled gradient

Figure 2. Control of the gradient monomer sequence of 13FOMA/ DMA gradient copolymers by tuning the concentration of Ti catalyst and 13FOMA. (a−c) Total monomer conversion (black) and dodecyl methacrylate (DMA) content (blue) in monomer as a function of time: [13FOMA]0/[Ti(Oi-Pr)4]0 = (a) 1000/30 mM (entry 2 in Table 1), (b) 1000/40 mM (entry 3), and (c) 2000/30 mM (entry 4). (d) Instantaneous DMA content in products (Finst.DMA) as a function of normalized chain length: [13FOMA]0/[Ti(Oi-Pr)4]0 = 1000/15 mM (red), 1000/30 mM (green), 1000/40 mM (blue), 2000/30 mM (black).

copolymers with narrow molecular weight distribution were obtained (Mw/Mn = 1.2−1.3), whereas the gradient sequence was dependent on the conditions. In the case of 1000 mM 13FOMA coupled with 1500 mM 1-dodecanol, transesterification efficiently proceeded up to over 80% DMA content. The resulting gradient copolymers contained large DMA content (Fcum.DMA = 57−68%), where Fcum.DMA increased with increasing Ti concentration. The gradient slope (Finst.DMA) around the initiating terminal also turned steep with increasing Ti. The increase of DMA content is due to both the induction period of polymerization and promoted transesterification (Figures 1a and 2a,b). In contrast, 2000 mM 13FOMA condition made transesterification retarded at about 60% DMA D

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Figure 3. Synthesis of 13FOMA/RMA gradient copolymers via concurrent tandem living radical copolymerization of 13FOMA with ROH [(a, d) ethanol, (b, e) 1-octadecanol, (c, f) poly(ethylene glycol) methyl ether (PEGOH): Mn = 350]: [13FOMA]0/[ECPA]0/[Ru(Ind)Cl(PPh3)2]0/ [Ti(Oi-Pr)4]0 = (a, d) 2000/20/2/20 mM in toluene/ethanol (1/1, v/v, [ethanol]0 = 3300 mM), 1300/13/1.3/26 mM in toluene/1-octadecanol ([1-octadecanol]0 = 980 mM), and 1000/5/1/20 mM in toluene/PEGOH (1/1, v/v, [PEGOH]0 = 1000 mM) at 80 °C. (a−c) Total monomer conversion (black) and RMA content (blue) in monomer as a function of time. (d−f) Instantaneous RMA content in products (Finst.RMA) as a function of normalized chain length. RMA: (a, d) ethyl methacrylate (EMA), (b, e) octadecyl methacrylate (ODMA), and (c, f) poly(ethylene glycol) methyl ether methacrylate (PEGMA).

Scheme 2. Synthesis of 13FOMA/DMA (a) Random or (b) Block Copolymers via Ru-Catalyzed Living Radical Polymerization

Figure 4. (a) Instantaneous DMA content and (b) SEC curves of 13FOMA/DMA random (green), gradient (red), and block (blue) copolymers used for the characterization of physical properties.

similar thermal stability. The three samples showed 1% weight loss at similar temperature (random: 218 °C; gradient: 216 °C; block: 218 °C), independent of the sequence distribution of the fluorinated monomer. Phase Separation. Owing to the unclear boundary, i.e., gradient distribution, of the segregating monomer pairs, the 13FOMA/DMA gradient copolymer is expected to show phase separation behavior different from corresponding random or block counterparts.26,28,29 13FOMA/DMA random, gradient, and block copolymers were thus analyzed by small-angle X-ray scattering (SAXS) (Figure 6). SAXS profiles of the three samples were clearly different. The gradient copolymer showed a single scattering intensity maximum at 0.26 nm−1, while the

temperature was close to that of the random counterpart. To reveal the difference of their heat flow curves, the first derivative of the heat flow curves as a function of temperature was further evaluated (Figure 5b). This definitely indicates the broad Tg of the gradient copolymer spreading from −30 to 30 °C compared with the random counterpart. Such weak Tg signal and/or broad temperature range are characteristic of gradient copolymers; this tendency has been often reported in other gradient copolymers and relating previous researches.24,25,27,32 Confirmed by thermogravimetric analysis (TGA), the 13FOMA/DMA random, gradient, and block copolymers had E

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Figure 5. DSC measurement of 13FOMA/DMA block (blue), gradient (red), and random (green) copolymers: (a) heat flow and (b) first-derivative heat flow; heating rate = 1 °C/min. Figure 7. Surface tension of the toluene solutions of 13FOMA/DMA block (blue), gradient (red), and random (green) copolymers: [polymer] = 0.005−5 mg/mL in toluene at 25 °C. Dashed line: surface tension of toluene (28.4 mN/m).

to that of the block counterpart in the wide range of concentration, while it was clearly smaller than that of the random counterpart. Typically, 5 mg/mL toluene solutions of the copolymers had the following surface tension: 22.8 (random), 19.6 (gradient), and 19.2 mN/m (block). These results indicated that the gradient copolymer containing a poly(13FOMA)-rich segment is accumulated and arranged at the solution/air interface, similar to the block counterpart.22 This is due to the strong segregation effect of the locally condensed poly(13FOMA) segment. The surface tension of the polymer solutions turned almost constant around 5 mg/mL. This suggests that the concentration is over critical micelle concentration; the copolymers form micelles or aggregates comprising fluorinated cores in toluene. Thus, aggregation properties of 13FOMA/DMA random, block, and gradient copolymers were evaluated by dynamic light scattering in toluene at 25 °C at 5 mg/mL concentration (Figure S5). The gradient copolymer induced intermolecular self-assembly to form micelles with bimodal size distribution (Rh = 14 and 99 nm). In contrast, the block copolymer produced uniform micelles with narrow size distribution (Rh = 23 nm), while the random copolymer formed unimer or multichain micelles (Rh = 6 nm) smaller than the gradient and block counterparts. The small micelles would be formed by self-folding of the random copolymer in toluene, similar to those obtained with amphiphilic/fluorous random copolymers in water.18 These results indicate that the sequence distribution of perfluorinated units apparently affect selfassembly behavior of the 13FOMA/DMA copolymers. The small part of the gradient copolymer micelles (Rh = 14) has size between the random copolymer micelles and the block counterparts. This suggests that the small gradient copolymer micelles have aggregation number smaller than the block copolymer micelles owing to the gradient sequence distribution of fluorous units. Contact Angle. Water/oil repellency was one of the most intriguing properties of fluorinated polymers.9−12 Thus, the contact angle of water or n-hexadecane (n-HD) on the films of 13FOMA/DMA random, gradient, and block copolymers cast

Figure 6. SAXS profiles of 13FOMA/DMA block (blue), gradient (red), and random (green) copolymers.

random copolymer showed no peak and the block copolymer in turn exhibited four peaks of integer order (1:2:3:4). This result importantly indicates that the gradient copolymer induces microphase separation in contrast to the random counterpart. The phase separation of the gradient copolymer comprises the broad interface between the poly(13FOMA)-rich segment and the poly(DMA)-rich counterpart without a longrange ordered structure, though the block counterpart forms long-range ordered lamellar structure (domain spacing: d = 34 nm). The phase separation structure with broad interface in the gradient copolymer is fully consistent with the broad range of the glass transition temperature. These results suggest that the gradient sequence distribution of segregating monomers is one option to create unique microstructures in solid copolymers. Surface Tension and Self-Assembly. To investigate the surface activities of 13FOMA/DMA random, gradient, and block copolymers in organic media, surface tension of the toluene solutions was measured by changing polymer concentration at 25 °C (Figure 7).17 The gradient copolymer gradually and effectively reduced surface tension with increasing polymer concentration from 0.005 to 5 mg/mL. The surface tension of the gradient copolymer solution was almost identical F

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on silicon wafers was evaluated (Table 2). The gradient copolymer film showed contact angles against both water and

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.T.).

Table 2. Contact Angles (deg) of 13FOMA/DMA Copolymersa

ORCID

sample

immediateb (H2O)

after 30 sb (H2O)

immediateb (nHD)

after 30 sb (nHD)

random gradient block

111 116 117

103 114 107

67 70 72

54 70 70

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

The authors declare no competing financial interest.

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a

Samples correspond to the copolymers shown in Figure 4. bContact angle of H2O or n-hexadecane (nHD) on the films of 13FOMA/DMA random, gradient, and block copolymers cast on silicon wafers from the HFC225/CHCl3 (75/25, v/v) solutions (10 mg/mL). Contact angle was determined immediately or 30 s later after dropping H2O or nHD solutions on the polymer films.

ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants JP24245026, JP26410134, JP17H03066, and JP17K19159, by Mizuho Foundation for the Promotion of Sciences, by Sekisui Chemical through “Innovations Inspired by Nature” Research Support Program, by The Mazda Foundation, and by The Sumitomo Electric Group Social Contribution Foundation. The SAXS measurements were performed at BL45XU in SPring-8 with the approval of RIKEN (Proposal No. 20150023). We also thank Mr. Ikuo Yamamoto (Daikin Industries, Ltd.) for thermogravimetric analysis and surface tension and contact angle measurements.

n-HD almost identical to the block counterpart; the contact angles against water and n-HD were slightly larger than those with the random copolymer film. Thus, it was revealed that the gradient copolymer had water/oil repellency similar to the block counterpart.

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CONCLUSION Fluorous gradient copolymers bearing perfluorinated units were successfully synthesized via the concurrent tandem catalysis of ruthenium-mediated LRP and Ti-mediated transesterification of a perfluoroalkyl methacrylate (13FOMA) in the presence of alcohols. The physical properties of fluorous gradient copolymers were also investigated compared with those of corresponding random and block counterparts. Because of the electron-withdrawing perfluoroalkyl segment, 13FOMA was efficiently transesterified with various alcohols including 1dodecanol, 1-octadecanol, ethanol, and poly(ethylene glycol) methyl ether. As a result, the tandem catalysis efficiently produced well-controlled fluorous/hydrophobic or fluorous/ hydrophilic gradient copolymers; the gradient sequence was controlled by changing the concentration of 13FOMA and Ti catalyst. Thus, the tandem catalytic system developed herein is one of the most efficient and versatile strategies to produce fluorous gradient copolymers. A fluorous/hydrophobic 13FOMA/DMA gradient copolymer further exhibited unique physical properties dependent on the sequence distribution of perfluorinated units. Typically, the gradient copolymer showed broad range of Tg and phase separation different from the corresponding random or block copolymers. The gradient copolymer effectively reduced the surface tension of the toluene solution as well as the block copolymer. Therefore, the design of fluorous gradient copolymers would be a new option to create fluorous polymeric materials with intriguing physical properties distinct from conventional random or block copolymers.

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REFERENCES

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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.7b02512. Experimental details, SEC, 1H and 19F NMR, and DLS of polymers (PDF) G

DOI: 10.1021/acs.macromol.7b02512 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b02512 Macromolecules XXXX, XXX, XXX−XXX