Triblock Terpolymers Bearing a Redox-Cleavable Junction and a

Aug 1, 2014 - PEG-S2-PFOEMA-b-PCEMA dispersed in water and plasticized by a trace ... Yang Gao , Huibin Qiu , Hang Zhou , Xiaoyu Li , Robert Harniman ...
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Triblock Terpolymers Bearing a Redox-Cleavable Junction and a Photo-Cross-Linkable Block Muhammad Rabnawaz and Guojun Liu* Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6 S Supporting Information *

ABSTRACT: Two linear triblock terpolymers were synthesized and used to coat cotton textiles. These copolymers consist in sequence of a water-soluble poly(ethylene glycol) (PEG) block, a highly water- and oil-repellant poly[2-(perfluorooctyl)ethyl methacrylate] (PFOEMA) block, and a photo-cross-linkable poly(2-cinnamoyloxyethyl methacrylate) (PCEMA) block. The PEG block bonds to PFOEMA via a redox-cleavable disulfide junction (-S2-). To prepare the copolymers, monomethoxy PEG (PEG-OH) was derivatized to yield a PEG chain bearing one terminal thiol group (PEG-SH). Atom transfer radical polymerization and deprotection chemistry were used to prepare Py-S2-PFOEMA-b-PHEMA, where PHEMA and Py-S2- denote poly(2-hydroxyethyl methacrylate) and a terminal pyridin-2-yldisulfanyl group, respectively. Reacting PEG-SH with Py-S2PFOEMA-b-PHEMA via a thiol−disulfide exchange reaction yielded PEG-S2-PFOEMA-b-PHEMA, and the cinnamation of the PHEMA block produced PEG-S2-PFOEMA-b-PCEMA. PEG-S2-PFOEMA-b-PCEMA dispersed in water and plasticized by a trace amount of dimethyl phthalate was then used to coat cotton textiles, and this coating was secured by photo-cross-linking the PCEMA domains. Treating the coating with dithiothreitol cleft the disulfide junction and thus the PEG block, revealing the hidden PFOEMA block and its water- and oil-repellant properties. Aqueous solutions of these copolymers could thus be applied onto a substrate to provide amphiphobic coatings. much thicker than a layer afforded by Rf. A thick fluorinated layer reduces the opportunities for surface reconstruction to occur among the FL and GX blocks and maintains the longterm amphiphobicity of a coating. Furthermore, a thick fluorinated layer helps prevent the permeation of inorganic or hydrocarbon etchants and maintains the long-term stability of the anchoring layer and the coated substrate. A limitation of this diblock copolymer approach is that an organic solvent is normally required to prepare the amphiphobic coatings.2,4,6,7 We pondered on ways to eliminate the organic solvent and to prepare coatings from water. One possibility was to use a triblock copolymer that bore a terminal sacrificial water-soluble block. This block should help disperse the fluorinated diblock copolymer into water and thus allow coating to be prepared from aqueous media. In cases when only a monolayer coating was required on cotton fibers, the coating conditions could be optimized so that a triblock brush formed. In this brush layer, the water-soluble block would form the topmost sublayer (Scheme 1A). Beneath this layer would be a fluorinated FL sublayer and then an anchoring GX sublayer. After the GX layer was covalently grafted or cross-linked, the sacrificial surface block could be removed (Scheme 1B), exposing the fluorinated sublayer and revealing the latent amphiphobicity. This report describes the design and synthesis

I. INTRODUCTION Diblock copolymers FLn-b-GXm that comprise a fluorinated block of n FL units and a grafting/cross-linking block of m GX units can form a self-cleaning monolayer that coats a solid substrate.1,2 In this case the diblock copolymer forms a brush layer, where the grafting/cross-linking GX block anchors the copolymer onto the substrate, while the overlaying low-surfacetension FL block provides the desired repellency against water and oil. For example, FLn-b-GXm has been applied onto cotton fabrics to yield highly water- and oil-repellant (superamphiphobic) textiles that are breathable and repel most laboratory chemicals. Tailored into lab coats, these fabrics should greatly improve laboratory safety.1 FLn-b-GXm copolymers have also been used to coat silica particles, and the resultant silica particles have then been cast with or without a polymer binder onto various substrates to yield superamphiphobic particulate coatings.3−5 Since these coatings reject water- and oil-borne contaminants alike, they can help maintain not only aesthetical appeal but also long-term function of the devices or objects that they protect. As fluorinating agents, FLn-b-GXm copolymers have several obvious advantages when compared to traditional RfSi(OR)3 silane coupling agents, where Rf and Si(OR)3 denote a fluorinated alkyl group and a trialkoxysilane group, respectively. First, a GX block grafts more readily than a single Si(OR)3 group onto a substrate. Second, an anchored layer made of a GX block is more stable than a layer derived from Si(OR)3. Third, the fluorinated layer derived from a FL block can be © 2014 American Chemical Society

Received: June 12, 2014 Revised: July 18, 2014 Published: August 1, 2014 5115

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monomethyl ether (Mn = 5000 g/mol, Aldrich), cinnamoyl chloride (98%, Aldrich), p-toluenesulphonyl chloride (99.0%, TCI), triethylamine (99.5+%, Sigma-Aldrich), α,α,α-trifluorotoluene (TFT, 99+%, Acros), anisole (99%, Sigma-Aldrich), CuBr (99.999%, Aldrich), CuBr2 (99.999%, Aldrich), bipyridine (99+%, Acros), pentafluoropyridine (C5F5N, 98.0%, TCI), hexafluorobenzene (C6F6, >99.0%, TCI), dithiothreitol (DTT, 98%, TCI), and potassium thioacetate (98%, Aldrich) were used as received. Characterization. Size exclusion chromatography (SEC) was performed at 70 °C using a Waters 515 system that was equipped with a Waters 2410 refractive index detector. The three columns were packed by American Polymer Standards Corporation with 5-μm AM 1000, 10 000, and 100 000 Å gels. The system was calibrated using monodisperse polystyrene (PS) standards. A solution of dimethylformamide (DMF) containing 2.5 g/L of tetrabutylammonium bromide was used as the eluent, and the flow rate was 0.900 mL/min. 1H NMR measurements were performed on Bruker Avance-300, Avance-400, or Avance-500 instruments using deuterated pyridine-d5, methanol-d4, or chloroform-d3 as the solvents and a 3 s relaxation delay. Synthesis of PEG113-OTs. PEG-OH (1.0 g, 2.0 × 10−4 mol) or PEG113-OH with 113 denoting the number of EG units per chain was dissolved in 10 mL of THF and cooled down to 5−7 °C in an ice bath. An aqueous solution of NaOH (2 mL, 0.3 M) was added to the above solution, and the mixture was stirred for 30 min at this temperature. A THF solution (2 mL) containing tosyl chloride (TsCl, 57 mg, 1.5 equiv) was added to the reaction mixture. The reaction mixture was allowed to warm to 15−18 °C and stirred for ∼7 h at this temperature. At regular intervals, 0.1 mL samples of the reaction mixture were collected. These samples were briefly extracted with chloroform before 1 H NMR analysis was performed in CDCl3. After the reaction was completed, it was quenched with 20 mL of cold water and was extracted with chloroform (4 × 20 mL). The combined organic layers were rinsed with a brine solution (10 mL) and dried over magnesium sulfate. The solution was subsequently concentrated via rotary evaporation, and the product was precipitated from diethyl ether (3 × 30 mL). The product (0.84 g) was obtained as a white precipitate in an 82% yield. 1H NMR (CDCl3): δ 7.88 (Ar−, 2H), 7.32 (Ar−, 2H), 3.9 (−CH2OTs, 2H), 3.4−3.8 (br, −OCH2CH2, 456H), 3.3 (−OCH3, 3H), 2.5 ppm (CH3, 3H). PEG113-SCOCH3. PEG113-OTs (600 mg, 1.15 × 10−4 mol) was dissolved into anhydrous DMF (in 3.0 mL) in a round-bottom flask before potassium ethanethioate (66 mg, 5 equiv) was added to this solution. Immediately after the addition of potassium ethanethioate, the reaction mixture became dark-brown. After 16 h, water (10 mL) was added to quench the reaction, and the reaction mixture was extracted with dichloromethane (3 × 20 mL). The combined organic layers were washed with distilled water (20 mL) before the organic solvent was rota-evaporated. The crude product was dissolved into THF and was added dropwise into diethyl ether (4 × 20 mL). The resultant white precipitate was dried under vacuum at room temperature, yielding 510 mg of the product in an 85% yield. 1H NMR (CDCl3 at 300 MHz): δ 3.9 (s, CH3OCH2−, 2H), 3.4−3.8 (m, −OCH2CH2, 456H), 3.03 (s, −OCH3, 3H), 3.03 (m, −CH2SAc, 2H), 2.23 ppm (s, SAc, 3H). PEG113-SH. PEG113-SCOCH3 was converted into PEG113-SH using a literature method that had been employed on organic molecules.31 First, PEG113-SCOCH3 (200 mg, 3.8 mmol) was dissolved in methanol (10 mL). Acetyl chloride (0.92 mL, 1.0 mmol) was subsequently added dropwise. The reaction mixture was stirred at room temperature for 12 h, and then water (10 mL) was added to quench the reaction. After 10.0 mL of saturated aqueous NaHCO3 was added, the mixture was extracted with dichloromethane (20 mL × 4). The combined organic layers were concentrated under reduced pressure to ∼5 mL and then added into diethyl ether to precipitate the polymer. After drying under vacuum, the polymer was obtained at an ∼88% yield. 1H NMR (CDCl3 at 300 MHz): δ 3.9 (CH3OCH2−, 2H), 3.4−3.8 (m, −OCH2CH2, 456H), 2.23 ppm (−CH2SH, 2H). 3-(Pyridin-2-yldisulfanyl)propyl 2-Bromo-2-methylpropanoate (Py-S2-Br). The initiator 3-(pyridin-2-yldisulfanyl)propyl 2bromo-2-methylpropanoate (Py-S2-Br) was synthesized through a two-

Scheme 1. (A) Structure of a Triblock Copolymer Brush Layer and (B) Structure of the Residual Diblock Brush on a Cotton Fiber after the Water-Soluble Topmost Sublayer Is Cleft

of a family of triblock terpolymers for the preparation of coatings through this aqueous dispersion strategy. We will also report results supporting the cleavage of the block junction under appropriate conditions and preliminary results suggesting our ability to prepare amphiphobic cotton fabrics from these polymers in water. The copolymer family that we targeted was poly(ethylene glycol)-disulf ide-poly[2-(perfluorooctyl)ethyl methacrylate]block-poly[2-(cinnamoyloxy)ethyl methacrylate] (PEG-S2PFOEMA-b-PCEMA) (Scheme 2). PEG was incorporated on Scheme 2. Chemical Structure of PEG-S2-PFOEMA-bPCEMA

account of its solubility in water, and the disulfide linkage was introduced because it could be cleft by a reducing agent, thus shedding off the sacrificial PEG block.8,9 PFOEMA was chosen for its low surface tension and high water and oil repellency.10,11 Meanwhile, PCEMA was selected because a PCEMA layer deposited around cotton fibers could be readily photo-cross-linked to yield a “permanent” network and thus provide a stable anchoring layer.12−14 In a broader context, PEG-S2-PFOEMA-b-PCEMA are doubly responsive triblock terpolymers. While -S2- undergoes cleavage in response to the addition of a reducing agent, the PCEMA block cross-links when irradiated by UV light. Stimuliresponsive block copolymers have been extensively studied during the past two decades for their various potential applications including control drug release,15−23 switching between hydrophilic or hydrophobic surfaces to remove stubborn stains,24−29 etc. The doubly responsive copolymers PEG-S2-PFOEMA-b-PCEMA reported here should provide a new addition to the repertoire of stimuli-responsive copolymers. Moreover, this study showcases a new application for stimuli-responsive block copolymers.

II. EXPERIMENTAL SECTION Materials. 2-Trimethylsiloxyethyl methacrylate (HEMA-TMS) was synthesized according to a literature method30 and was distilled over calcium hydride before it was stored in a refrigerator at 4 °C. Pyridine (ACS reagent, Fisher Scientific) was refluxed and distilled over CaH2 under nitrogen, while tetrahydrofuran (THF) was distilled over sodium and a small amount of benzophenone. Poly(ethylene glycol) 5116

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to dry in the air for ∼40 min. The swatches were subsequently heated for 25 min at 120 °C in an oven to evaporate any remaining water and to anneal the polymer coating. To cross-link the coating, each cotton swatch was irradiated for 60 min on each side with a focused UV beam from a 500 W mercury lamp that was mounted in an Oriel 6140 lamp housing and powered by an Oriel 6128 power supply. In order to cleave the PEG block, an irradiated swatch was stirred in water that was doped with DTT (∼1.5 mg, 6.4 mmol DTT solution) for 7 h. To fully remove the cleft PEG chains, the fabric was rinsed several times with THF. All samples were heated at 120 °C for 20−25 min before they were cooled to room temperature for contact angle measurements. Contact Angle Measurements. Prior to the contact angle measurements, the samples were flattened with a hydraulic unit (Carver hydraulic unit model # 3912). Droplets of water (Milli-Q with a surface tension of 72.8 mN/m at 20 °C) and diiodomethane (>99%, Sigma-Aldrich, with a surface tension of 50.8 mN/m at 20 °C) were added onto cotton samples as 5 μL droplets.33 Images of these droplets were recorded 3 min after the droplets had been dispensed. The reported contact angles represented the average values of three different measurements on same sample at different positions.

step literature procedure.32 The overall yield of Py-S2-Br was 55%. 1H NMR (CDCl3, 500 MHz): δ 8.6 (m, 1H), 7.7 (m, 2H), 7.10 (m, 1H), 4.3 (t, J = 6.0 Hz, 2H), 2.9 (t, J = 7.12 Hz, 2H), 2.1 (m, 2H), 1.94 ppm (s, 6H). PEG-S2-PFOEMA-b-PHEMA. To prepare PEG-S2-PFOEMA12-bPHEMA60, Py-S2-PFOEMA12-b-PHEMA60 was first prepared by atom transfer radical polymerization as described in the Supporting Information. After dissolving Py-S2-PFOEMA12-b-PHEMA60 (50 mg, 0.0025 mmol) in anhydrous DMF (0.4 mL) within a 1.0 mL sealed vial, 10 μL of glacial acetic acid was added. This was followed by the addition of PEG-SH (36 mg, 0.0075 mmol) in 0.2 mL of DMF. The resultant mixture was stirred at room temperature, and samples (20 μL) were collected and diluted with DMF to 3.0 mL. The absorbance at 375 nm was recorded to monitor reaction progress. The first sample was collected 0.2 min after the mixing of the precursors. Absorbance monitoring was continued until the absorbance at 375 nm stopped increasing, indicating the completion of reaction. Once the reaction was completed after 3 h, DMF was rota-evaporated. The crude PEGS2-PFOEMA-b-PHEMA copolymer was washed with diethyl ether (3 × 2 mL) and subsequently dried under vacuum at room temperature for 24 h. The diethyl ether helped to remove 2-pyridithione, while the excess PEG-SH was removed after the cinnamation step. 1H NMR (in DMSO-d6:pentafluoropyridine (3:1 v/v), at 400 MHz): δ 4.8 (br, −OH), 4.3 (br, −COOCH2CH2CF2), 3.8 (br, −COOCH2), δ 3.7 (br, −COOCH2CH2OH), 2.6 (br, −CH2CF2), 1.8−2.2 (br, −CH2), 0.6− 1.1 ppm (br, −CH3). PEG13-S2-PFOEMA10-b-PHEMA20 was analogously synthesized except that Py-S2-PFOEMA10-b-PHEMA20 was used as the diblock copolymer precursor. In Situ Monitoring of the Reaction between PEG113-SH and Py-S2-PFOEMA12-b-PHEMA60. To monitor the progress of the reaction between PEG-SH and Py-S2-PFOEMA12-b-PHEMA60, Py-S2PFOEMA12-b-PHEMA60 (4.5 mg, ∼0.22 μmol) was stirred with a small magnetic bar at 300 rpm in anhydrous DMF (2.5 mL) within a sealed UV cuvette. Glacial acetic acid (3 μL) was then added. A 0.5 mL DMF solution of PEG-SH (10 mg, 0.0020 mmol) was subsequently added to the reaction mixture. UV−vis spectra were recorded at various intervals. PEG-S2-PFOEMA-b-PCEMA. To prepare PEG113-S2-PFOEMA12-bPCEMA60 (also abbreviated as P1), PEG113-S2-PFOEMA12-bPHEMA60 (62 mg of the copolymer containing 0.189 mmol of hydroxyl groups) was dissolved into dry pyridine (4 mL), and the solution was stirred for ∼30 min before cinnamoyl chloride (60 mg, ∼0.36 mmol, 1.9 equiv) was added. The reaction mixture was stirred overnight at room temperature in the dark. The pyridinium salt was removed via centrifugation at 3900 rpm (2600g) for 10 min. The supernatant was subsequently concentrated via rotary evaporation to 1.0 mL before it was added into 50 mL of diethyl ether to precipitate the polymer. The precipitate was again dissolved into 3 mL of THF and precipitated from diethyl ether (50 mL). The last procedure was repeated twice. Finally, the obtained precipitate was washed twice with 2 mL of methanol to remove PEG-SH. The resultant brown solid was dried at room temperature under vacuum for 24 h, yielding 66 mg of P1 in an 82% yield. 1H NMR (in CDCl3, at 500 MHz): δ 8.0−6.6 (aromatic and CC protons, 7H), 4.25 (br, −COOCH2CH2CF2, 24H), 4.2−4.0 (br, −COOCH2 and CH2CH2OOC, 240H), δ 3.5−3.6 (br, −CH2CH2O, 456H), 2.4 (br, −CH2CF2, 24 H), 1.8−2.2 (br, −CH2,144H), 0.8−1.4 ppm (br, −CH3, 216H). PEG113-S2-PFOEMA10-b-PCEMA20 (also denoted as P2) was synthesized analogously except that PEG 113 -S 2 -PFOEMA 10 -bPHEMA20 was used as the precursor. Cotton Coating. P1 (17.5 mg) was dissolved in 1.00 mL of THF. To this solution was then added water at 7−8 drops/min until the water volume fraction f H2O reached 75%. This solution was subsequently stirred at 40 °C for 2 h to evaporate THF until no THF could be detected by smell and the final P1 concentration reached ∼6 mg/mL. This was followed by the addition of the plasticizer dimethyl phthalate (∼1.7 mg, 10 wt % relative to P1). After the micelles were stirred for 2 h, cotton swatches were immersed in it for 30 min before they were removed from this solution and allowed

III. RESULTS AND DISCUSSION Two PEG-S2-PFOEMA-b-PCEMA samples (P1 and P2) were synthesized. The PEG block of both P1 and P2 was 113 units long. The FOEMA and PCEMA blocks of P1 were 12 and 60 units long, respectively, while the corresponding blocks were individually 10 and 20 units long for P2. To prepare the triblock copolymers, monomethoxy PEG or PEG-OH was initially derivatized to yield PEG-SH, which consisted of PEG bearing one terminal thiol group. Also, Py-S2-PFOEMA-bPHEMA samples with PHEMA denoting poly(2-hydroxyethyl methacrylate) and Py-S2- denoting a terminal pyridin-2yldisulfanyl group were synthesized using a functional initiator via atom transfer radical polymerization and polymer derivatization. Reacting PEG-SH with Py-S2-PFOEMA-bPHEMA via a thiol−disulfide exchange reaction yielded PEGS2-PFOEMA-b-PHEMA (Scheme 3) and the cinnamation of the PHEMA block produced Py-S2-PFOEMA-b-PCEMA. Scheme 3. Synthesis of PEG-S2-PFOEMA-b-PHEMA via the Thiol−Disulfide Exchange Reaction between PEG-SH and Py-S2-PFOEMA-b-PHEMA

PEG-SH. We attempted to prepare PEG-SH by reacting NaSH·H2O with PEG-OTs, where Ts denotes a tosyl group (Scheme S1 in the Supporting Information). However, the product of this reaction exhibited two SEC peaks. While one of these peaks was for PEG-SH, the other was more dominant and had a peak molecular weight that was roughly twice that of PEG-SH (Figure S2). We suspected that this signal corresponded to PEG-S2-PEG and attempted to reduce the disulfide bond using DTT to yield PEG-SH. Despite the use of excess DTT, the “dimer” peak decreased by only ∼50% in 5117

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coupling. The 1H NMR spectrum of PEG-SH (Figure 1c) reveals that the signal at 2.3 ppm corresponding to SOCCH3 had disappeared, confirming the removal of the acetyl group. The SEC trace of PEG-SH was recorded using DMF as the mobile phase as shown in Figure 2. This SEC trace consisted of

intensity. Since we did not know the exact origin of the reduction-resistant dimer, we tried another approach to synthesize PEG-SH. In the second method (Scheme 4), PEG-OH was first reacted in THF/water at 15−18 °C with TsCl under basic Scheme 4. Synthetic Pathway toward PEG-SH

conditions to prepare α-methoxy-ω-tosyl-PEG (PEG-OTs). PEG-OTs was then reacted with potassium thioacetate under ambient conditions to yield PEG-SAc. The acetyl group was subsequently removed in step 3 to yield PEG-SH. PEG-OTs, PEG-SOCCH3, and PEG-SH were all characterized by 1H NMR spectroscopy in CDCl3, and their spectra along with peak assignments are shown in Figure 1.

Figure 2. SEC traces of PEG113-S2-PFOEMA12-b-PCEMA60 (P1), PyS2-PFOEMA12-b-PHEMA60, and PEG-SH. DMF was used as eluent at a flow rate of 0.90 mL/min.

a narrow main peak at ∼28.0 min with a polydispersity index of 1.05 based on PS standards and a small shoulder peak at 26.5 min. The main peak had a peak shape and retention time comparable to those of PEG-OH and thus is consistent with a trace that would be anticipated for PEG-SH. The shoulder probably arose from PEG-S2-PEG, which was expected to form due to the auto-oxidation of PEG-SH in air. Since the shoulder was small and PEG-S2-PEG would not introduce undesirable side effects, we did not further purify the crude PEG-SH sample before it was used in the subsequent reaction. 3-(Pyridin-2-yldisulfanyl)propyl 2-Bromo-2-methylpropanoate (Py-S2-Br). Py-S2-Br was synthesized according to a literature method that is summarized in Scheme S2 of the Supporting Information. During the first step, 3-(pyridine-2yldisulfanyl)propan-1-ol was synthesized by the acetic-acidcatalyzed thiol−disulfide exchange reaction between 2,2dithiopyridine and mercaptopropanol. Subsequently, 3-(pyridine-2-yldisulfanyl)propan-1-ol was reacted with 2-bromo-2methylpropionic acid using N,N′-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP) as coupling agents to yield Py-S2-Br. Figure S3 in the Supporting Information shows the 1H NMR spectra of the products obtained from steps 1 and 2 along with their signal assignments. Meanwhile, Figure 3a shows a 1H NMR spectrum of Py-S2-Br along with its peak assignments. This spectrum confirms the successful synthesis of Py-S2-Br. Py-S2-PFOEMA-b-PHEMA. The diblock copolymer Py-S2PFOEMA-b-PHEMA was synthesized by sequential ATRP of FOEMA and HEMA-TMS using Py-S2-Br as the initiator and subsequently removing the TMS protecting groups from the PHEMA block (Scheme 5). To optimize the FOEMA polymerization, FOEMA conversions were monitored via 1H NMR spectroscopy by collecting samples at various polymerization times and at different temperatures. This study established that the optimal conditions were obtained by polymerizing FOEMA in TFT/anisole (v/v = 9/1) at 85 °C for 70 min. The number of repeating units was confirmed from the

Figure 1. 1H NMR spectra and signal assignments for (a) PEG-OTs, (b) PEG-SAc, and (c) PEG-SH. All spectra were recorded in CDCl3.

Comparison of the integral of the PEG-OTs signal at 7.3 ppm (corresponding to the aromatic protons of the tosyl group) with that at 3.6 ppm (corresponding to the main PEG chain, Figure 1A) suggests that 96% or 100% (within experimental error) of the PEG-OH chains had been endcapped with a tosyl group. In these calculations, the molar mass of PEG-OH was assumed to be 5.0 × 103 g/mol as quoted by the supplier. Upon comparison of the spectra of PEG-SAc (Figure 1b) and PEG-OTs (Figure 1a), we noted that the signals at 7.3 and 7.9 ppm had disappeared while a new peak at 2.3 ppm had emerged. This new signal corresponded to SOCCH3, thus confirming that the OTs end group had been substituted by this functional group. The acetyl group of PEG-SAc was removed by hydrolysis at room temperature in methanol. The HCl catalyst required for this reaction was generated in situ by adding acetyl chloride at 26 mol % relative to −SAc. The mild reaction conditions suppressed undesired side reactions such as thiol−thiol 5118

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Figure 3. 1H NMR spectra of (a) the initiator Py-S2-Br (recorded in CDCl3) and (b) the diblock copolymer Py-S2-PFOEMA12-b-PHEMA60 (recorded in DMSO-d6/C5F5N at v/v = 3/1). The peaks marked with stars arose from NMR solvent used. Meanwhile, small peaks between 7.0 and 8.5 ppm belongs to C5F5N (98% purity along with some nonfluorinated impurities), which was used to solubilize the PFOEMA block.

Scheme 5. Synthetic Pathway toward Py-S2-PFOEMA12-b-PHEMA60

Table 1. Properties of P1 and P2 as Well as Their Precursors sample

SEC Mw (kg/mol)

PEG113-SH

14

Py-S2-PFOEMA12-b-PHEMA60-Br Py-S2-PFOEMA12-b-PCEMA60-Br P1

25 30 41

Py-S2-PFOEMA10-b-PHEMA20-Br Py-S2-PFOEMA10-b-PCEMA20-Br P2

10b 13b 25

SEC Mw/Mna

NMR m/n or l/m/n

NMR Mn (kg/mol)

m

n

1.05 P1 and its precursors 1.20 1.15 P2 and its precursors

1.20

12/60c 12/60 113/12/60

13 21 27

12 12 12

60 60 60

10/20 10/20 113/10/20

7.8 10.5 15.5

10 10 10

20 20 20

a

SEC analysis performed in DMF. The Mw and Mw/Mn values are based on PS standards. bThey were the peak molecular weights. The molecular weights of these samples were too low for our instrument to provide Mw and Mw/Mn values. cThis ratio was calculated by 1H NMR, while, PEG-5K was taken as 113 units based on the information provided by supplier.

difficult to find a common solvent for both HEMA and Py-S2PFOEMA-Br. Our optimization experiments concluded that ∼80% HEMA-TMS was converted in ∼5.5 h at 82 °C in a TFT/anisole solvent mixture at v/v = 10/1. The crude diblock copolymer Py-S2-PFOEMA-b-P(HEMA-TMS) was freed of the ligated copper by passage through an alumina column.

peak ratios corresponding to aromatic region of the initiator at 8.4 ppm to the polymer peak at 4.5 ppm (Figure S4 in the Supporting Information). At the next stage, Py-S2-PFOEMA-Br was used as the macroinitiator for the polymerization of HEMA-TMS. HEMATMS rather than HEMA was polymerized because it was 5119

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The absorbance of this signal increased rapidly initially with reaction time and then leveled off after ∼40 min as depicted in Figure 5b, suggesting that the coupling reaction had completed at this point. For larger-scale preparations, much longer reaction times were always used, such as 4 h.

Subsequently, the TMS groups were cleft from Py-S2PFOEMA-b-P(HEMA-TMS) by stirring the copolymer in a THF/methanol/water solvent mixture. Py-S2-PFOEMA12-b-PHEMA60, where the subscripts denote the repeat unit numbers, was analyzed in a DMSO-d6/C5F5N solvent mixture at v/v = 3/1. The resultant spectrum (Figure 3b) clearly exhibited peaks corresponding to both PFOEMA and PHEMA, confirming the structure of the targeted diblock copolymer. However, the integrations of the signals corresponding to the PFOEMA block were unduly low. We later realized that this phenomenon was due to the reduced solubility of this block in the deuterated solvent mixture and our difficulty in identifying a solvent mixture that suitably solubilized both PHEMA and PFOEMA. Consequently, we confirmed the block ratio of PFOEMA to PCEMA (HEMA was cinnamated to CEMA) in the PEG113-S2-PFOEMA12-PCEMA60 triblock copolymer using CDCl3/C6F6 (v/v = 3/1) as solvent (Figure S5). In this case, the integration ratio between the signal at 2.5 ppm for PFOEMA and the signal at 6.6 ppm for PCEMA was 1.0/5.1. This value was consistent with the respective repeat unit numbers of 12 and 60 for the PFOEMA and PHEMA blocks that were calculated based on the monomer conversions and the molar feed ratios used for the monomers and initiators during ATRP. Py-S2-PFOEMA12-b-PHEMA60 was also analyzed via SEC using DMF as the eluent. Figure 2 shows a SEC trace for this sample. The polydispersity index Mw/Mn of this sample in terms of PS standards was 1.20 (Table 1). Py-S2-PFOEMA10-b-PHEMA20 was analogously analyzed. The molecular characteristics of this copolymer are given in Table 1. PEG-S2-PFOEMA-b-PHEMA. PEG-SH and Py-S2-PFOEMA-b-PHEMA were coupled together via a thiol−disulfide exchange reaction to yield PEG-S2-PFOEMA-b-PHEMA.34 Since PEG-SH undergoes auto-oxidation in air, freshly prepared PEG-SH was always used.8 To maximize Py-S2-PFOEMA-bPHEMA production, an excess of PEG-SH was used. The forward reaction in Scheme 3 was also likely facilitated by the tautomerization of the formed thiol, 2-mecaptopyridine, into 2pyrithione. 2-Pyrithione absorbs light at ∼375 nm.8 Therefore, the increase in the absorbance at 375 nm was used to monitor the progress of the coupling reaction. Figure 4a shows a set of absorption spectra that were recorded at different times after the mixing of PEG-SH with Py-S2-PFOEMA12-b-PHEMA60 in a sealed cuvette. A new peak with maximum absorbance at 375 nm appeared immediately after the mixing of the two reactants.

Figure 5. SEC traces for P2, PEG-SH, and Py-S2-PFOEMA10-bPCEMA20. DMF was used as the eluent at a flow rate of 0.90 mL/min.

PEG-S2-PFOEMA-b-PCEMA. P1 and P2 were obtained by cinnamating the corresponding PEG-S2-PFOEMA-b-PHEMA samples with cinnamoyl chloride. Both PFOEMA and PCEMA were insoluble in methanol, but PEG was soluble. Therefore, the excess PEG-SH that had been used for the preparation of this triblock copolymer preparation was readily removed by rinsing the resultant P1 or P2 copolymers with methanol. In principle, P1 or P2 could have been obtained by directly coupling Py-S2-PFOEMA-b-PCEMA with PEG-SH. In practice, this approach did not work. When we attempted to couple PyS2-PFOEMA-b-PCEMA with PEG-SH directly, no signal appeared for 2-pyrithione absorption at 375 nm. Additionally, our SEC analysis failed to detect a peak expected for the targeted triblock copolymer. P1 and P2 were characterized by SEC and 1H NMR. Figure 2 compares the SEC traces of P1, PEG113-SH, and Py-S2PFOEMA12-b-PHEMA60. The fact that the P1 peak appeared at a substantially shorter elution time than the PEG-SH and Py-S2PFOEMA12-b-PHEMA60 peaks suggested the occurrence of the desired reaction. Indeed, cinnamation of Py-S2-PFOEMA12-bPHEMA60 could have also caused a shift toward a shorter elution time. Thus, Figure 5 compares the SEC traces of P2, PEG113-SH, and Py-S2-PFOEMA10-b-PCEMA20. The product peak in this case had still shifted significantly, unambiguously supporting the occurrence of the desired reaction. Figure 6 shows the 1H NMR spectrum of PEG-S2PFOEMA12-b-PCEMA60 along with its signal assignments. The observed signals were consistent with those that would be anticipated for the targeted copolymer. Based on the integration ratios of their corresponding signals, the repeat unit ratios of the PEG, PFOEMA, and PCEMA blocks were 113/12/60. P2 was characterized similarly and its detailed properties are given in Table 1. Stimuli-Responsiveness of P1. To cleave the disulfide linkage of P1 and thus liberate the PEG chains, ∼80 mol equiv of DTT was stirred with P1 in DMF for 3 h. After this reduction treatment, the reaction mixture was analyzed by SEC.

Figure 4. (a) UV−vis absorption spectra recorded at different times after the mixing of PEG-SH and Py-S2-PFOEMA12-b-PHEMA60 in a cuvette and (b) variation in the absorbance at 375 nm for the mixture vs the reaction time. 5120

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Figure 6. 1H NMR spectrum of PEG113-S2-PFOEMA12-b-PCEMA60 (P1) recorded in CDCl3/C6F6 at v/v = 3/1. The peaks asterisked at 7.27 and 7.35 ppm arise from hydrogenated impurities in solvents CDCl3 and C6F6..

SMA,1 and PFHEA-b-PGMA2 have been used by our group to coat cotton textiles. Here PDMS, PCEA, PIPSMA, PFHEA, and PGMA denote poly(dimethylsiloxane), poly(2-cinnamoyloxyethyl acrylate), poly[3-tri(isopropyloxy)silylpropyl methacrylate], poly(perfluorohexylethyl acrylate), and poly(glycidyl methacrylate), respectively. To coat cotton textiles with PDMSb-PCEA,14 for example, we first prepared a micellar solution of the copolymer with PDMS as the corona. Cotton swatches were then equilibrated with the micellar solution, withdrawn from this coating solution, dried in the air, heated at 120 °C, and eventually irradiated with UV light. Our scanning electron microscopy and X-ray photoelectron spectroscopy study and liquid contact angle analyses on the coated cotton fabrics at different stages suggested the following picture. During equilibration between cotton and the coating solution, micelles infiltrated the cotton matrix and some of these micelles became deformed and adsorbed onto the cotton fibers via their insoluble PCEA block. During solvent evaporation, some of the polymer would have deposited as micelles while some of the polymer would also have become deposited via the adsorption of the PCEA layer. It was only during the heating or annealing stage that more micelles dissociated and a dense brush layer with or without extraneous surface micelles was formed.14 Evidently, extraneous surface micelles existed only if the concentration of the polymer coating solution was high and more polymer than what was required for a dense brush layer was initially deposited onto the fibers after solvent evaporation. We have used a protocol similar to the one described above to prepare presumably P1 brush layers on cotton fibers using mostly water as the coating medium. This mission involved first preparing in water a P1 dispersion. To this dispersion consisting presumably of P1 micelles was added dimethyl phthalate at a low weight ratio of 1/10 relative to P1 to plasticize the PCEMA cores. The cotton swatches were then equilibrated with the P1 dispersion at ∼6 mg/mL for 30 min and then removed before they were dried in air. These coatings were further dried by annealing the coated samples at 120 °C for 20 min. The anchored PCEMA layer was subsequently cross-linked by UV irradiation. To cleave the PEG chains, the irradiated fabrics were treated with an aqueous DTT solution. Possible residual PEG chains were removed from the surfaces of the coatings by rinsing the fabrics with fresh THF.

Figure 7 compares the SEC traces of P1 before and after it had been reacted with DTT. After the DTT treatment, the peak at

Figure 7. SEC traces of P1 before (dotted line) and after (solid line) treatment with DTT.

25.5 min was replaced by a new peak at 26.1 min bearing a shoulder that eluted at 28.0 min. Since 26.1 and 28.0 min corresponded to the peak maxima of Py-S2-PFOEMA12-bPCEMA60 and PEG113-SH, respectively, the SEC characterization suggested that the disulfide junction of P1 had been successfully cleaved under our experimental conditions. Similarly, P2 was also successfully cleaved in a DTT-doped DMF solution as shown in Figure S6 of the Supporting Information. We did not perform further experiments to show the photocross-linking of PCEMA under appropriate conditions because this [2 + 2]-cyclization chemistry of CEMA groups of different PCEMA chains has been extensively investigated by our group in the past.35 For example, the photo-cross-linking of diblock copolymer cylindrical micelles bearing a PCEMA core or diblock copolymer thin films containing cylindrical domains of PCEMA have been used by us to prepare diblock copolymer nanofibers.36−38 Coating of Cotton Fabrics with P1. Various diblock copolymers including PDMS-b-PCEA,14 PFOEMA-b-PIP5121

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Figure 8. Photographs of different cotton fabrics 3 min after water was dispensed onto the sample: (a) uncoated cotton, (b) cotton that was coated by P1 but was neither photolyzed nor rinsed with THF, (c) coated cotton that was subsequently photolyzed and then rinsed with THF, (d) coated cotton that was photolyzed and then treated by DTT, and (e) DTT-treated cotton that was further rinsed with THF. The absence of a water droplet after 3 min indicated that the water had been absorbed by the fabric. Arrows in the inset images for (a)−(c) mark the regions that were wetted by the absorbed water.

chemistry. PEG-S2-PFOEMA-b-PHEMA was prepared via a thiol−disulfide exchange reaction between Py-S2-PFOEMA12-bPHEMA60 and PEG-SH. P1 and P2 were finally obtained at relatively low polydispersities of 1.15 and 1.20, respectively, by cinnamating the resultant PEG-S2-PFOEMA-b-PHEMA samples. The methods described in this report should be useful for the preparation of other related block copolymers. P1 and P2 could be dispersed into water. An aqueous dispersion of P1 containing a trace amount of the plasticizer dimethyl phthalate has been used to coat cotton fabrics. To coat cotton, cotton swatches were equilibrated with the coating solution, dried in air, and heated at 120 °C to anneal the coating. Stable coatings that resisted extraction by THF were obtained after the fabrics had been photolyzed with UV light to cross-link the PCEMA sublayer. The PFOEMA wetting properties were revealed after the coatings were treated with DTT to cleave the disulfide bonds and thus shed off the PEG chains.39 Therefore, P1 enabled the preparation of amphiphobic cotton fabrics through the use of an aqueous coating solution.

While we reserve discussion of the optimization of P1 coating to the future, we compare in Figure 8 the wetting properties of uncoated cotton and cotton samples that were removed at different stages during the P1 coating process. After water droplets were dispensed onto fabrics that were either uncoated or coated by P1 but not treated, water was absorbed (Figures 8a and 8b, respectively). This also occurred on cotton fabrics that were coated by P1, photolyzed, and then rinsed with THF (Figure 8c). However, the situation changed after the coated fabrics were treated by DTT in water and then dried at 120 °C for 25 min. Water droplets did not become absorbed in this case. Rather, water and diiodomethane formed stable droplets with contact angles of 134 ± 2° (Figure 8d) and 115 ± 3°, respectively. The water and diiodomethane contact angles increased further to 150 ± 2° (Figure 8e) and 140 ± 2°, respectively, after the cleft PEG chains were extracted from the surfaces of the DTT-treated coatings with THF and the sample was subsequently dried at 120 °C for 20−25 min. The increase in the water and diiodomethane contact angles after the fabrics were treated with DTT suggested that the PEG block was indeed removed and that the PFOEMA block was exposed in accordance with our experimental design. The large water and oil (diiodomethane) contact angles suggested that the resultant coating were highly amphiphobic. The fact that the coating was not be removed by THF rinsing suggested that the PCEMA layer was cross-linked around the cotton fibers and that the coatings were stable.

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ASSOCIATED CONTENT

S Supporting Information *

ATRP protocols, a description of the attempted synthesis of PEG-SH by reacting NaSH·H2O with PEG-OTs, additional 1H NMR spectra, and additional SEC traces. This material is available free of charge via the Internet at http://pubs.acs.org.

IV. CONCLUSIONS Two novel triblock terpolymers, P1 and P2, have been synthesized. Of the precursors, PEG113-SH was synthesized by reacting PEG113-OH with TsCl to yield PEG-OTs and then removing the acetyl group. These reactions were performed under mild conditions. Simultaneously, Py-S2-PFOEMA12-bPHEMA60 and Py-S2-PFOEMA10-b-PHEMA20 were synthesized via ATRP using a functional initiator and deprotection

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (G.L.). Notes

The authors declare no competing financial interest. 5122

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(38) Guo, A.; Liu, G.; Tao, J. Macromolecules 1996, 29, 2487. (39) Asakawa, T.; Shimizu, Y.; Ozawa, T.; Ohta, A.; Miyagishi, S. J. Oleo Sci. 2007, 57, 243.

ACKNOWLEDGMENTS NSERC of Canada is gratefully acknowledged for financially sponsoring this research. Guojun Liu thanks NSERC of Canada for a Canada Research Chair position in Materials Science.

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REFERENCES

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