Main-chain PPEGMEMA-b-PBTFVPP-b-PPEGMEMA perfluorocyclobutyl aryl ether-based amphiphilic ABA triblock copolymer: synthesis and self-assembly

Abstract

A series of perfluorocyclobutyl aryl ether-based amphiphilic ABA triblock copolymers consisting of hydrophilic poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMEMA) and hydrophobic poly(2,2′-bis(4-trifluorovinyl-oxyphenyl)propane) (PBTFVPP) blocks were synthesized via the site transformation strategy. A semi-fluorinated PBTFVPP segment was first prepared via thermal step-growth cycloaddition polymerization of BTFVPP trifluorovinyl aryl ether monomer followed by end functionalization to be transformed into Br–PBTFVBP–Br macroinitiator bearing one ATRP initiating group at each end. The target PPEGMEMA-b-PBTFVPP-b-PPEGMEMA triblock copolymers with relatively narrow molecular weight distributions (M_w/M_n ≤ 1.32) were synthesized via ATRP of PEGMEMA macromonomer initiated by Br–PBTFVBP–Br macroinitiator. A fluorescent probe technique was used to determine the critical micelle concentrations (cmc) of the obtained amphiphilic copolymers in aqueous solution. The morphologies of the micelles formed by the copolymers were investigated by transmission electron microscopy. It was shown that such triblock copolymers could self-assemble into ovals, well-defined cylinders, and spheres in aqueous solution with different initial water contents and compositions.

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Introduction

Poly(ethylene glycol) (PEG) is a kind of water-soluble, uncharged, and nontoxic polymer. In terms of its hydrophilicity, PEG typically increases the solubility of copolymers in aqueous media and yields improved circulation times in vivo when bound to other molecules,^1–3 thus, it has been extensively investigated for nano- and biotechnology applications.^4–6 It is generally accepted that the incorporation of fluorine atoms into the polymer film has been shown to be an effective method to reduce the surface energy and hence to produce non-wettable surfaces.^7,8 Therefore, it is interesting to analyze the properties of copolymers containing hydrophilic PEG segment and hydrophobic fluorinated segment.^9–14

The incorporation of fluorine into polymers with high performance involving a step-growth cycloaddition polymerization of trifluorovinyl aryl ether monomers provided a class of semi-fluorinated fluoropolymer containing perfluorocyclobutyl (PFCB) aryl ether linkage, which was first developed by Babb et al. of Dow Chemical Company.^15,16 Several studies reported new thermoplastic and thermoset PFCB aryl ether-based polymers by thermal chain extension of bis- and tri-functionalized trifluorovinyl aryl ether monomers.^17–19 For their low crystallinity, PFCB aryl ether-based polymers could improve the processability, which had solved the manufacturing problems of fluoropolymers to some extent compared to traditional fluoropolymers. Generally, the homopolymerization and random copolymerization of trifluorovinyl aryl ether (TFVE) monomer proceed thermal [2π + 2π] step-growth cyclopolymerization above 150 °C without any initiator and catalyst, either as a melt or in solution using solvent with high boiling point.^17,20 PFCB aryl ether-based polymers not only have the conventional properties of fluoropolymers, but possess many other advantages such as optical transparency and improved processability.^21,22

In order to enlarge application range of PFCB aryl ether-based polymers, it is necessary to combine the high performance of PFCB aryl ether-based polymer with other commercial polymers. To realize this idea, block copolymer with a stable covalent-bonded linkage between two different segments is a good and convenient choice, and among them, amphiphilic block copolymers have attracted much attention during the past two decades.^23,24 The thermodynamically incompatible segments in amphiphilic block copolymers may self-assemble into many different nanostructures with periodicity or compositional heterogeneity.^25,26 Block copolymers can be synthesized by sequential feeding of different monomers via living polymerization including anionic polymerization,²⁷ cationic polymerization,²⁸ group transfer polymerization,²⁹ and living radical polymerization³⁰ or the site transformation strategy using different polymerization approaches.^31,32 Among living radical polymerization, atom transfer radical polymerization (ATRP) has been a successful method used for the polymerization of a variety of common acrylic monomers and enabled them to form polymeric chains in a well-defined way with controllable molecular weights and polydispersities.³³

Amphiphilic block copolymers containing a fluorinated segment have unusual phase behaviors for their fluorophobic effect in comparison with the usual hydrocarbon-based block polymers.^34,35 The incorporation of fluorinated segments into amphiphilic block copolymers can result in interesting self-assembly characteristics for the combination of hydrophobicity and lipophobicity in fluorinated polymers.^36,37 In order to explore the properties of PFCB aryl ether-based amphiphilic block copolymers, our group has developed a new type of (meth)acrylate monomers with PFCB aryl ether-containing ester group,^38–40 which could be polymerized via living radical polymerization and enable us to obtain tailor-made PFCB aryl ether-based amphiphilic block copolymers with well-defined architecture. Previous studies showed that chain architecture of copolymers can dramatically affect their self-assembly behaviors, and thus vast number of work concerning self-assembly of non-linear copolymers have been done in order to elucidate this issue experimentally and theoretically.^41–46 Although self-assembly behaviors of amphiphilic block copolymers with PFCB aryl ether units as pendant groups have been studied in detail,^44–46 none has reported the amphiphilic block copolymers incorporating PFCB aryl ether units in the main chain. On one hand, it would be necessary to explore the influence of the difference in the location of PFCB aryl ether units on the self-assembly behaviors of PFCB aryl ether-containing copolymers, which should further deepen our understanding on the structure-property relationship of PFCB aryl ether-containing copolymers; on the other hand, Craig et al. have reported that polymers with PFCB units in the backbone can be mechanically degraded by pulsed ultrasound due to the chain scission of PFCB moiety and cyclo-reversion to TFVE and the degraded products can be re-amended at elevated temperature via [2π + 2π] cycloaddition of TFVE.⁴⁷ Compared to copolymers with PFCB units as pendant groups, the copolymers with PFCB groups along the backbone would be more feasible to prepare mechanically stimuli-responsive nanostructures by the virtue of special property of PFCB groups.

Therefore, we report the synthesis and self-assembly of ABA triblock copolymer, poly[poly(ethylene glycol) methyl ether methacrylate]-b-poly(2,2′-bis(4-trifluorovinyloxyphenyl)propane)-b-poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMEMA-b-PBTFVPP-b-PPEGMEMA). Due to the high polymerization temperature (>150 °C) and peculiar polymerization mechanism ([2π + 2π] cycloaddition) much different from the commonly used acrylic monomer, only the site transformation strategy can be adopted to synthesize amphiphilic block copolymers containing PFCB aryl ether units in the backbone. In 2005, our group has synthesized a new ATRP macroinitiator bearing PFCB aryl ether units in the backbone,^48,49 which could initiate ATRP of common vinyl monomers for forming hydrophobic block polymers. In the present work, we report the first synthesis of PFCB aryl ether-based (in the main chain) amphiphilic ABA triblock copolymer using 2,2-bis(4-hydroxyphenyl)propane as a starting material, which can make the fluorine-containing backbone more flexible than the similar polymers originating from 4,4′-biphenol.⁴⁸ The site transformation strategy was employed to synthesize poly[poly(ethylene glycol)methyl ether methacrylate]-b-poly(2,2′-bis(4-trifluorovinyloxyphenyl)propane)-b-poly[poly(ethylene glycol)methyl ether methacrylate] (PPEGMEMA-b-PBTFVPP-b-PPEGMEMA) triblock copolymers via the sequential thermal step-growth cyclopolymerization and ATRP as shown in Scheme 1. The properties of the resulting triblock copolymers were studied by measuring the critical micelle concentration (cmc). Transmission electron microscopy (TEM) was used to analyze the micellar morphologies with different initial water contents and compositions.

Scheme 1 Synthesis of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA block copolymer.

Experimental section

Materials

Poly(ethylene glycol) methyl ether methacrylate (PEGMEMA, M_n = 232 g mol⁻¹, Aldrich, 99%) was distilled in vacuo prior to use. Copper(I)bromide (CuBr, Aldrich, 99%) was purified by stirring overnight over CH₃COOH at room temperature followed by washing the solid with ethanol, acetone, and diethyl ether before drying in vacuo at 40 °C for 1 day. 1,2-Dibromotetrafluoroethane (BrCF₂CF₂Br) was prepared by condensing equimolar amounts of Br₂ and tetrafluoroethylene at −195 °C followed by warming up to 22 °C according to previous literature.⁵⁰ Granular zinc was activated by washing in 0.1 M HCl followed by drying in vacuo at 100 °C for 16 h. N-Phenyl-1-naphthylamine (PNA) (Alfa Aesar, 97%) was purified by recrystallization in ethanol three times. Diphenyl ether (Aldrich, 99%) were dried over CaH₂ and distilled under reduced pressure prior to use. Dichloromethane (CH₂Cl₂, Aldrich, 99.5%) and toluene (Aldrich, 99%) were dried over CaH₂ and distilled from sodium and benzophenone under N₂ prior to use. 2,2-Bis(4-hydroxyphenyl)propane (Aldrich, 99%), 4-hydroxybiphenyl (Aldrich, 99%), 2-bromopropionyl bromide (Alfa Aesar, 97%), aluminum(III) chloride (AlCl₃, TCI, 98%), N,N,N′,N′,N′′-pentamethyl-diethylenetriamine (PMDETA, Aldrich, 99%), potassium hydroxide (KOH, Aldrich, 90%), and dimethyl sulfoxide (DMSO, Aldrich, 99%) were used as received.

Measurements

FT-IR spectra were recorded on a Nicolet AVATAR-360 FT-IR spectrophotometer with a resolution of 4 cm⁻¹. All NMR analyses were performed on a Bruker Avance 500 spectrometer (500 MHz) using CDCl₃ as solvent; tetramethylsilane (¹H NMR) and CDCl₃ (¹³C NMR) were used as internal standards, and CF₃CO₂H was used as an external standard for ¹⁹F NMR. EI-MS was measured by an Agilent 5937N system. Relative molecular weights and molecular weight distributions were measured by a conventional gel permeation chromatography (GPC) system equipped with a Waters 515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (HR3 (500–30 000), HR4 (5000–600 000) and HR5 (50 000–4 000 000), 7.8 × 300 mm, particle size: 5 μm). GPC measurements were carried out at 35 °C using tetrahydrofuran (THF) as eluent with a flow rate of 1.0 mL min⁻¹. The system was calibrated with linear polystyrene standards (for PBTFVPP homopolymers and Br–PBTFVPP–Br macroinitiators) and poly(methyl methacrylate) standards (for PPEGMEMA-b-PBTFVPP-b-PPEGMEMA triblock copolymers). Steady-state fluorescence spectra were measured at 20 °C on a Hitachi F-2700 fluorescence spectrophotometer with the band width of 5 nm for excitation and emission, the emission intensity at 418 nm was recorded to determine the critical micelle concentration (cmc), where the excitation wavelength (λ_ex) was 340 nm. Hydrodynamic diameter (D_h) was measured by dynamic light scattering (DLS) with a Malvern Nano-ZS90 Zetasizer TEM images were obtained by a JEOL JEM-1230 instrument operated at 80 kV.

Preparation of 2,2-bis(4-bromotetrafluoroethoxyphenyl)propane

2,2-Bis(4-hydroxyphenyl)propane (34.20 g, 0.15 mol), KOH (18.70 g, 0.30 mol), DMSO (200 mL), and toluene (180 mL) were added to a 500 mL three-neck flask fitted with a Dean–Stark azeotropic distillation assembly. After pumping and back-filling with N₂ three times, the system was heated to 150 °C for 28 h during which H₂O was removed. The solution was cooled to 10 °C and BrCF₂CF₂Br (40.0 mL, 0.30 mol) was added slowly in 1 h with constant cooling to keep the temperature below 35 °C. The solution was allowed to stir at room temperature for 10 h and then at 60 °C for 10 h. The mixture was diluted with H₂O and extracted with n-hexane, and the organic phase was washed three times with brine and dried over MgSO₄ followed by rotary evaporation. The crude product was purified by column chromatography to give 2,2-bis(4-bromotetrafluoroethoxyphenyl)propane 1 (55.0 g, yield: 62.5%) as a colorless oil. ¹H NMR: δ (ppm): 1.68 (s, 6H, C(CH₃)₂), 7.14 (d, 4H, phenyl), 7.22 (d, 4H, phenyl). ¹⁹F NMR: δ (ppm): −68.5 (s, 4F), −86.4 (s, 4F).

Preparation of 2,2′-bis(4-trifluorovinyloxyphenyl)propane

Zn (19.5 g, 0.23 mol) and acetonitrile (200 mL) was first added to a dry 500 mL three-neck flask under N₂ and the mixture was heated to 90 °C. Next, 2,2-bis(4-bromo-tetrafluoroethoxyphenyl)propane 1 (35.0 g, 0.06 mol) in 100 mL of acetonitrile was added slowly and the mixture was refluxed for 13 h followed by rotary evaporation. The crude product was extracted with n-hexane and purified by column chromatography to provide 2,2′-bis(4-trifluorovinyloxyphenyl)propane 2 (BTFVPP, 18.05 g, yield: 77.9%) as a colorless oil. ¹H NMR: δ (ppm): 1.69 (s, 6H, C(CH₃)₂), 7.05 (d, 4H, phenyl), 7.22 (d, 4H, phenyl). ¹⁹F NMR: δ (ppm): −120.3 (dd, 2F), −126.9 (dd, 2F), −133.8 (dd, 2F). EI-MS: m/z 388.

Thermal cyclopolymerization of 2,2′-bis(4-trifluorovinyloxyphenyl)propane

In a typical procedure, BTFVBP 2 (3.02 g, 7.78 mmol) and diphenyl ether (6.0 mL) were added to a 25 mL Schlenk flask (flame-dried prior to use). The flask was degassed by three cycles of freezing-pumping-thawing followed by immersing the flask into an oil bath preset at 200 °C to start the polymerization. The polymerization lasted 2 h and was terminated by putting the flask into liquid N₂. The contents were dissolved in THF and precipitated into 100 mL of methanol twice. The obtained polymer, poly(2,2′-bis(4-trifluorovinyloxyphenyl)propane) (PBTFVPP) 3a, was dried in vacuo at room temperature until a constant weight and 1.90 g of PBTFVPP was obtained with a yield of 63%. GPC: M_n = 4500 g mol⁻¹, M_w/M_n = 1.29. FT-IR: ν (cm⁻¹): 2971, 2874, 1605, 1507, 1466, 1365, 1309, 1265, 1204, 1180, 1123, 1082, 1016, 962, 833, 742, 694. ¹H NMR: δ (ppm): 1.63 (6H, C(CH₃)₂), 6.96–7.35 (8H, phenyl). ¹⁹F NMR: δ (ppm): −120.1 (2F), −126.7 (2F), −128.1 to −132.6 (6F, cyclobutyl-F₆), −133.8 (2F). ¹³C NMR: δ (ppm): 30.6 (C(CH₃)₂), 42.1 (C(CH₃)₂), 104.5, 106.2 (4C, cyclobutyl), 114.6, 115.8, 117.6, 121.7, 127.9, 148.9 (8C, phenyl).

Preparation of 2-bromo-1-(4′-(1,2,2-trifluorovinyloxy)biphenyl-4-yl)propan-1-one

Preparation of compound 4 was similar to the reported procedure in previous literature.⁴⁸ 4-(1,2,2-Trifluorovinyloxy)biphenyl was first prepared by two steps similar to the preparation of compound 2 using 4-hydroxybiphenyl as starting material. 4-(1,2,2-Trifluorovinyloxy)biphenyl (8.01 g, 0.032 mol) and dichloromethane (80 mL) were added to a dry 250 mL three-neck flask under N₂. Next, 2-bromopropionyl bromide (8.28 g, 0.0385 mol) and AlCl₃ (5.545 g, 0.042 mol) were added to the solution and the mixture was heated to reflux for 8 h. The flask was cooled to 0 °C followed by adding granular ice for termination. 1 M HCl (50 mL) was added to the solution and the organic layer was washed by brine followed by drying over MgSO₄. A straw yellow solid, 2-bromo-1-(4′-(1,2,2-trifluorovinyloxy)biphenyl-4-yl)propan-1-one 4 (7.44 g, yield: 60.3%), was obtained by silica column chromatography. ¹H NMR: δ (ppm): 1.91 (d, 3H, COCH(CH₃)Br), 5.30 (q, 1H, COCH(CH₃)Br), 7.21 (d, 2H, phenyl), 7.61 (t, 4H, phenyl), 8.08 (d, 2H, phenyl). ¹⁹F NMR: δ (ppm): −119.8 (dd, 1F), −126.7 (dd, 1F), −134.6 (dd, 1F). EI-MS: m/z 384 ([M]⁺).

Preparation of Br–PBTFVPP–Br macroinitiator

In a typical procedure, PBTFVPP 3a (M_n,GPC = 4500 g mol⁻¹, M_w/M_n = 1.29, 1.885 g, 0.42 mmol) and 2-bromo-1-(4′-(1,2,2-trifluorovinyloxy)biphenyl-4-yl)propan-1-one 4 (1.805 g, 4.68 mmol) were first added to a 30 mL Schlenk flask (flame-dried prior to use) sealed with a rubber septum. The contents were purged with N₂ for 20 min to eliminate oxygen three times followed by adding diphenyl ether (6.0 mL) via a gastight syringe. The flask was degassed by three cycles of freezing-pumping-thawing followed by immersing the flask into an oil bath preset at 200 °C for 6 h. After cooling, the contents were diluted with THF followed by precipitation into excess methanol twice. The resulting product, Br–PBTFVPP–Br 5a, was dried in vacuo at room temperature until a constant weight. GPC: M_n = 5200 g mol⁻¹, M_w/M_n = 1.23. FT-IR: ν (cm⁻¹): 3043, 2972, 2875, 1688 (–C=O), 1606, 1507, 1435, 1399, 1309, 1265, 1204, 1122, 1016, 962, 897, 874, 833, 743. ¹H NMR: δ (ppm): 1.62 (6H, C(CH₃)₂), 1.93 (3H, COCH(CH₃)Br), 5.31 (1H, COCH(CH₃)Br), 6.95–7.26 (8H, phenyl), 7.65 (4H, phenyl), 8.11 (2H, phenyl). ¹⁹F NMR: δ (ppm): −128.1∼−132.6 (6F, cyclobutyl-F₆). ¹³C NMR: δ (ppm): 19.7 (COCH(CH₃)Br), 30.6 (C(CH₃)₂), 41.5 (COCH(CH₃)Br), 42.3 (C(CH₃)₂), 106.1, 108.8 (4C, cyclobutyl), 113.0, 115.4, 117.7, 128.1, 129.6, 147.2, 150.4 (8C, phenyl), 192.1 (COCH(CH₃)Br).

ATRP block copolymerization of PEGMEMA

In a typical procedure, Br–PBTFVPP–Br 5a (M_n,GPC = 5200 g mol⁻¹, M_n,NMR = 5900 g mol⁻¹, M_w/M_n = 1.23, 0.09 g, 0.030 mmol –CHBr initiating group) and CuBr (0.0090 g, 0.062 mmol) were first added to a 10 mL Schlenk flask (flame-dried under vacuum prior to use) sealed with a rubber septum for degassing and kept under N₂. Next, PEGMEMA (350 mg, 1.5 mmol), PMDETA (0.013 mL, 0.062 mmol), and 1,4-dioxane (1.0 mL) were introduced via a gastight syringe. The flask was degassed by three cycles of freezing-pumping-thawing and the mixture was stirred at room temperature for 20 min so that the mixture became homogeneous. The flask was immersed into an oil bath preset at 70 °C to start the polymerization. The polymerization lasted 8 h and was terminated by putting the flask into liquid N₂. The reaction mixture was diluted by THF and passed through an Al₂O₃ column to remove the residual copper catalyst. The solution was concentrated and precipitated into cold n-hexane. After repeated purification by dissolving in THF and precipitating in n-hexane, 310 mg of white viscous solid, PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6a triblock copolymer, was obtained after drying in vacuo overnight. GPC: M_n = 17 600 g mol⁻¹, M_w/M_n = 1.26. FT-IR: ν (cm⁻¹): 2875, 1731, 1606, 1507, 1456, 1306, 1266, 1203, 1120, 1017, 963, 834. ¹H NMR: δ (ppm): 0.86, 1.02 (3H, C(CH₃) of polymethacrylate), 1.62 (6H, C(CH₃)₂), 1,80, 1.93 (2H, CH₂ of polymethacrylate), 3.39 (3H, CH₂OCH₃), 3.56 (2H, −CH₂OCH₃), 3.67 (8H, OCH₂CH₂), 4.09 (2H, CO₂CH₂), 6.98, 7.08, 7.15 (8H, phenyl). ¹⁹F NMR: δ (ppm): −128.1∼−132.6 (6F, cyclobutyl-F₆). ¹³C NMR: δ (ppm): 16.6, 18.5 (C(CH₃) of polymethacrylate), 30.6 (C(CH₃)₂), 42.1 (C(CH₃)₂), 44.6 (C(CH₃) of polymethacrylate), 59.0 (CH₂OCH₃), 63.7 (CO₂CH₂CH₂), 68.3 (CO₂CH₂CH₂), 70.5 (OCH₂CH₂), 71.8 (CH₂OCH₃), 117.4, 128.0, 147.3, 150.4 (8C, phenyl), 177.2 (C=O).

Determination of cmc

PNA was used as fluorescence probe to measure the cmc of PPEGMEMA-b- PBTFVPP-b-PPEGMEMA 6 amphiphilic triblock copolymer. Acetone solution of PNA (1 mg mL⁻¹) was added to a large amount of water until the concentration of PNA reached 0.001 mg mL⁻¹. Next, different amounts of THF solution of copolymer 6 (1, 0.1, or 0.01 mg mL⁻¹) were added to water containing PNA ([PNA] = 0.001 mg mL⁻¹).

Micellar morphology

THF solution of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 triblock copolymer ([copolymer] = 0.20 mg mL⁻¹) was first filtered through a membrane with a nominal pore size of 0.45 μm. Next, a certain amount of deionized water was added slowly (0.36 mL h⁻¹) to 1.00 g of THF solution of copolymer 6 by a microsyringe until the preset water content (10 wt%, 30 wt%, and 50 wt%, respectively) was reached. Subsequently, the solution was dialyzed against deionized water with slow stirring for 6 days to remove THF completely, and deionized water was changed twice a day to obtain aqueous micellar solution. For TEM measurement, 10 μL of micellar solution was deposited on an electron microscopy copper grid coated with carbon film and the water evaporated at room temperature. TEM images were obtained by a JEOL JEM-1230 instrument operated at 80 kV.

Results and discussion

Synthesis of Br–PBTFVPP–Br macroinitiator

For the target PPEGMEMA-b-PBTFVPP-b-PPEGMEMA triblock copolymer, PBTFVPP block can only be obtained from thermal step-growth cyclopolymerization of BTFVPP 2 monomer, which is not suitable for PEGMEMA monomer at all; and PPEGMEMA segment can be constructed by ATRP of PEGMEMA macromonomer. Therefore, the site transformation strategy should be employed for the synthesis of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA triblock copolymer, requiring a PBTFVPP-based macroinitiator bearing ATRP initiating groups at both ends.

BTFVPP 2 bistrifluorovinyl aryl ether monomer was first synthesized according to standard procedures²⁰ of fluoroalkylation with BrCF₂CF₂Br and Zn-mediated elimination in two steps with a total yield of 48.7%, using commercially available 2,2-bis(4-hydroxyphenyl)propane as starting material. PBTFVPP 3 homopolymer was then obtained via thermal [2π + 2π] step-growth thermal cycloaddition polymerization of BTFVPP 2 monomer in diphenyl ether at 200 °C. Two PBTFVPP 3 homopolymers with different molecular weights were obtained by varying the polymerization time (Table 1). Obviously, the molecular weights and molecular weight distributions of PBTFVPP 3 homopolymer increased with the prolongation of polymerization time.

Table 1 Preparation of PBTFVPP 3 Homopolymer^a

Entry	Time (h)	Yield^b (%)	Mn^c (g mol^−1)	Mw/Mn^c
a Polymerization temperature: 200 °C.b Measured by weighting after purification.c Measured by GPC in THF at 35 °C.
3a	2.0	63	4500	1.29
3b	12.0	75	10 800	1.43

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The chemical structure of PBTFVPP 3 homopolymer was examined by ¹H and ¹⁹F NMR. The resonance signal at 1.63 ppm in ¹H NMR spectrum after thermal homopolymerization (Fig. 1A) belonged to 6 protons of C(CH₃)₂ group while the multiplets between 6.96 ppm and 7.35 ppm corresponded to 8 protons of phenyl. The multiplets in the range of −128.1 ppm to −132.6 ppm in ¹⁹F NMR spectrum (Fig. 1B) were attributed to 6 fluorine atoms in PFCB ring and other three peaks at −120.1, −126.7, and −133.8 ppm were originated from 3 fluorine atoms in trifluorovinyl groups at both ends, verifying the successful preparation of PBTFVPP 3 homopolymer bearing trifluorovinyl groups at both ends.

Fig. 1 ¹H (A) and ¹⁹F (B) NMR spectra of PBTFVPP 3 homopolymer in CDCl₃.

The bifunctional compound 4 containing trifluorovinyl and –COCH(CH₃)Br ATRP initiating group was first prepared from 4-(1,2,2-trifluorovinyloxy)biphenyl and 2-bromopropionyl bromide using the similar procedure described in previous report.⁴⁸ The ATRP initiating group, –COCH(CH₃)Br, has been proved to be stable at 200 °C in previous report⁴⁴ and the trifluorovinyl in compound 4 was easy to form PFCB ring via thermal intermolecular [2π + 2π] cycloaddition reaction. PBTFVPP 3 homopolymer was then end-capped by compound 4, giving Br–PBTFVPP–Br 5 macroinitiator as listed in Table 2.

Table 2 Preparation of Br–PBTFVPP–Br 5 Macroinitiator^a

Entry	Mn^b (g mol^−1)	Mw/Mn^b	NBTFVPP^c	Mn,NMR^d (g mol^−1)
a Reaction temperature: 200 °C, reaction time: 6 h.b Measured by GPC in THF at 35 °C.c The number of BTFVPP repeated unit per PBTFVPP chain obtained from ^1H NMR.d Obtained from ^1H NMR.
5a	5200	1.23	13.2	5900
5b	11 800	1.39	33.6	13 800

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Br–PBTFVPP–Br 5 macroinitiator was characterized by ¹H, ¹³C, and ¹⁹F NMR. Fig. 2A shows ¹H NMR spectrum of macroinitiator 5 and we can find the characteristic proton resonance signals of –C₆H₅COCH(CH₃)Br initiating group at 1.93 (peak ‘b’, COCH(CH₃)Br), 5.31 (peak ‘c’, COCH(CH₃)Br), and 8.11 (peak ‘g’, phenyl) ppm, which were similar to those of compound 4. The strong peak at 1.62 ppm (peak ‘a’) corresponded to 6 protons of C(CH₃)₂ group. This asset allowed for the exact determination of the number of BTFVPP repeated unit per BTFVPP chain (N_BTFVPP), i.e. the degree of polymerization, and the number-average molecular weight (M_n,NMR) by quantitative ¹H NMR end group analysis. They can be evaluated according to eqn (1) and (2) (S_a and S_b are the integration area of peak ‘a’ at 1.62 ppm and peak ‘b’ at 1.93 ppm in Fig. 2A, 388 and 385 are the molecular weights of BTFVPP 2 monomer and compound 4, respectively) and the results were summarized in Table 2. The ‘absolute’ molecular weights of Br–PBTFVPP–Br 5 macroinitiator determined by ¹H NMR were indeed a little higher than those relative molecular weights obtained from GPC in THF.

N_BTFVPP = S_a/S_b (1)

M_n,NMR = 388N_BTFVPP + 385 × 2 (2)

Fig. 2 ¹H (A), ¹³C (B), and ¹⁹F (C) NMR spectra of Br–PBTFVPP–Br 5 macroinitiator in CDCl₃.

¹³C NMR spectrum of macroinitiator 5 is shown in Fig. 2B and the signals attributed to the carbons in –C₆H₅COCH(CH₃)Br initiating group appeared at 19.7 (peak ‘a’, COCH(CH₃)Br), 41.5 (peak ‘c’, COCH(CH₃)Br), and 192.6 (peak ‘e’, COCH(CH₃)Br) ppm, respectively. The peaks at 106.1 and 108.8 ppm belonged to the carbons in PFCB ring. Fig. 2C depicts ¹⁹F NMR spectrum of macroinitiator 5 and the typical multiplets originating from PFCB ring still appeared in the range from −128.1 ppm to −132.6 ppm. In comparison with ¹⁹F NMR spectrum of homopolymer 3 in Fig. 1B, the resonance signals of trifluorovinyl disappeared after treating homopolymer 3 with compound 4, demonstrating the complete conversion of trifluorovinyl to –COCH(CH₃)Br initiating group. Thus, all aforementioned results illustrated the successful preparation of macroinitiator 5 with –COCH(CH₃)Br initiating group at both ends. Moreover, this kind of macroinitiator could dissolve in most common organic solvents including THF, CH₂Cl₂, chloroform, toluene, acetone, ethyl acetate, and DMF; however, it was insoluble in water and methanol.

Synthesis of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA triblock copolymer

Since PFCB aryl ether-based macroinitiator 5 was successfully synthesized with two ATRP initiating groups at both ends, we could use it to synthesize PFCB aryl ether-based ABA triblock copolymer. In the present work, PEGMEMA macromonomer was selected as the hydrophilic monomer for constructing PFCB aryl ether-based ABA amphiphilic triblock copolymer. ATRP of PEGMEMA was initiated by Br–PBTFVPP–Br 5 macroinitiator in 1,4-dioxane at 70 °C using CuBr/PMDETA as catalytic system and the results are summarized in Table 3. Different feeding ratios of PEGMEMA macromonomer to –C₆H₅COCH(CH₃)Br initiating group (50 : 1, 100 : 1, and 150 : 1) were employed to tune the block length of PPEGMEMA segment and two series of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA copolymer with the constant length of PBTFVPP hydrophobic segment and the different length of PPEGMEMA hydrophilic block were obtained and well characterized by GPC and NMR.

Table 3 Synthesis of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA block copolymer^a

Entry	[Br group] : [PEGMEMA]	Conv.^d (%)	Mn^e (g mol^−1)	Mw/Mn^e	m–n–m^f	Mn,NMR^g (g mol^−1)	cmc^h (g mL^−1)
a Polymerization temperature: 70 °C, polymerization time: 8 h, [Br group] : [CuBr] : [PMDETA] = 1 : 2 : 2.b Initiated by Br–PBTFVPP–Br 5a.c Initiated by Br–PBTFVPP–Br 5b.d Measured by ^1H NMR.e Measured by GPC in THF at 35 °C.f The composition of the block copolymer obtained from ^1H NMR.g Obtained from ^1H NMR.h Determined by fluorescence spectroscopy using PNA as probe.
6a^b	1 : 50	61	17 600	1.26	27–13–27	18 400	5.11 × 10^−6
6b^b	1 : 100	43	21 900	1.26	38–13–38	23 500	6.03 × 10^−6
6c^b	1 : 150	37	23 400	1.28	50–13–50	29 100	6.85 × 10^−6
6d^c	1 : 50	42	18 400	1.32	17–34–17	21 700	4.31 × 10^−6
6e^c	1 : 100	31	21 800	1.28	26–34–26	25 900	4.86 × 10^−6
6f^c	1 : 150	29	23 000	1.28	37–34–37	31 000	5.76 × 10^−6

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Moreover, polymerization kinetics was examined by employing Br–PBTFVPP–Br 5a as macroinitiator with a 1 : 2 : 2 : 100 feeding ratio of [Br] : [CuBr] : [PMDETA] : [PEGMEMA] at 70 °C. One can see from Fig. 3 that the polymerization showed a linear dependence of ln([M]₀/[M]) on the time and molecular weights increased linearly with the conversions of PEGMEMA macromonomer. The molecular weight distributions kept narrow throughout the polymerization. These results witnessed the “living”/controlled nature of ATRP of PEGMEMA.^51–54

Fig. 3 (A) Kinetics plot for solution ATRP of PEGMEMA initiated by Br–PBTFVPP–Br 5a. (B) Dependence of molecular weight (M_n) on the conversion of PEGMEMA macromonomer.

GPC traces of Br–PBTFVPP–Br 5 macroinitiator and PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 triblock copolymer are shown in Fig. 4. All triblock copolymers showed unimodal and symmetrical elution peaks with relatively narrow molecular weight distributions (M_w/M_n ≤ 1.32), which was characteristic of ATRP⁵¹ and also demonstrated that intermolecular coupling could be neglected.⁵² The molecular weights of all copolymers were much higher than that of macroinitiator, which illustrated that Br–PBTFVPP–Br 5 could act as macroinitiator to initiate ATRP of PEGMEMA macromonomer.

Fig. 4 GPC curves of Br–PBTFVPP–Br 5 macroinitiator and PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 triblock copolymer in THF.

Fig. 5A shows ¹H NMR spectrum of triblock copolymer 6 and we can find all corresponding proton resonance signals of both PBTFVPP and PPEGMEMA segments in the spectrum. The multiplets in the range of 6.98 ppm to 7.15 ppm were attributed to the protons of phenyl of aryl ether moiety and the signal at 1.62 ppm (peak ‘b’) belonged to 6 protons of C(CH₃)₂ spacer between two benzene ring. The peaks at 0.86 and 1.02 ppm (peak ‘a’) corresponded to 3 protons of methyl in polymethacrylate backbone of PPEGMEMA block while the signals at 1.86 and 1.93 ppm (peak ‘c’) were originated from 2 methylene protons in polymethacrylate backbone. The proton resonance signals of PEG side group were located at 3.39 (peak ‘d’), 3.56 (peak ‘e’), 3.67 (peak ‘f’), and 4.09 (peak ‘g’) ppm. This asset also allowed for the exact determination of the composition of the block copolymer and the number-average molecular weight (M_n,NMR) by quantitative ¹H NMR analysis. They can be evaluated according to eqn (3) and (4) (S_g and S_b are the integration area of peak ‘g’ at 4.09 ppm and peak ‘b’ at 1.62 ppm in Fig. 5A, 232 and M_n,NMR,5 are the molecular weights of PEGMEMA macromonomer and macroinitiator 5, respectively) as summarized in Table 3. It should be pointed out that the “absolute” molecular weights of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 triblock copolymer determined by ¹H NMR were obviously higher than those relative molecular weights obtained from GPC since that GPC was calibrated with linear poly(methyl methacrylate) standards.

N_PEGMEMA = 3(S_g/S_b) × N_BTFVPP (3)

M_n,NMR = 232N_PEGMEMA + M_n,NMR,5 (4)

Fig. 5 ¹H (A) and ¹⁹F (B) NMR spectra of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 triblock copolymer in CDCl₃.

¹⁹F NMR spectrum of triblock copolymer 6 is shown in Fig. 5B and the typical multiplets ranging from −128.1 ppm to −132.6 ppm clearly evidenced the preservation of PFCB linkage in the block copolymer. Thus, all above-mentioned results confirmed the successful synthesis of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA ABA triblock copolymers via the site transformation strategy using Br–PBTFVPP–Br as ATRP macroinitiator.

Self-assembly of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA in aqueous media

The different two segments in the as-prepared triblock copolymer 6 had the distinct solubility in water, PPEGMEMA was very easy to dissolve in water while PBTFVPP was insoluble in water; this kind of amphiphilicity endowed the block copolymer with capability to form nanostructure in aqueous media via self-assembly. Herein, the properties of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 triblock copolymer in aqueous solution were first analyzed by fluorescence spectroscopy. Fluorescence probe technique with PNA as probe was used to determine the cmc of triblock copolymer 6. The fluorescence spectrum of PNA is well-known to be very sensitive to the environment and the polarity of the surrounding, and PNA displays higher fluorescence activity in apolar surroundings, which can be easily quenched by polar solvents such as water.⁵⁵ If micelles are formed in aqueous media, PNA is solubilized within the interior of the hydrophobic part of micelles and its fluorescence intensity will be dependent on the concentration of the copolymer. The relationship of the fluorescence intensity ratio (I/I₀) as a function of the concentration of triblock copolymer 6a at 20 °C is plotted in Fig. 6, from which we could clearly find that I/I₀ increased sharply when the concentration of triblock copolymer 6a exceeded a certain value, indicative of the incorporation of PNA probe into the hydrophobic region of micelles. Thus, the intersection of two straight lines with a value of 5.11 × 10⁻⁶ g mL⁻¹ was determined to be the cmc of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6a triblock copolymer. The cmc of copolymers 6b–6f were also measured as summarized in Table 3 and these values were comparable with those of common surfactants or polymeric amphiphiles.^56,57 The cmc values of triblock copolymer 6 synthesized from the same Br–PBTFVPP–Br macroinitiator (6a–6c from 5a and 6d–6e from 5b) increased with the elevating of the molecular weight, i.e. the rising of the content of hydrophilic PPEGMEMA block.

Fig. 6 Dependence of fluorescence intensity ratio of PNA emission band at 418 nm on the concentration of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6a triblock copolymer in pure water, [PNA] = 0.001 mg mL⁻¹, excitation wavelength: 340 nm, band width: 5 nm for excitation and emission.

The self-assembly of PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 amphiphilic triblock copolymer in aqueous media was investigated by TEM and the micellar solution was prepared by dialysis strategy, which is a common method to prepare micelles formed by amphiphilic block copolymers.^58,59 The influence of initial water content on self-assembly of copolymer 6b was firstly examined. One can see from Fig. 7A that oval-shaped micelles with an average length of 145 ± 20 nm and an average central width of 71 ± 14 nm obtained from TEM image were formed with a 10 wt% initial water content. The average diameter of micelles was 167 nm obtained by DLS, which was larger than the apparent size obtained by TEM probably due to the hydration of corona of PPEGMEMA layer. as the water content increased to 30 wt%, the average length and central width of formed oval-shaped micelles were 232 ± 40 nm and 120 ± 25 nm obtained from TEM image, respectively (Fig. 7B), and the average size of micelles was 273 nm obtained from DLS; as the water content increased further to 50 wt%, connected spheres were observed with an average diameter of 75 nm obtained from DLS (Fig. 7C). Since the formed micelles were normally kinetically stable, not thermodynamically stable, the micelles were frozen or trapped in a certain stage.^60,61 The procedure of micelle preparation is one important factor affecting final micellar morphology, especially initial solvent quality. Our previous reports^62–64 and others' reports^65,66 indicated that an increase in the initial content of selective solvent for core-formed segment would lead to a short time for core-formed block to adjust its conformation towards its thermodynamic state. A higher initial content of selective solvent facilitate the formation of micelles with a larger interfacial area. Thus, micellar morphologies transformed from oval-shaped to sphere as the initial content of water increased from 10 wt% to 50 wt% due a higher interfacial area of sphere than that of oval.^66,67

Fig. 7 TEM images of micelles formed by PPEGMEMA-b-PBTFVPP-b-PPEGMEMA 6 triblock copolymer with different water contents, (A) 10 wt% formed by 6b, (B) 30 wt% formed by 6b, (C) 50 wt% formed by 6b, and 10 wt% formed by (D) 6d, (E) 6e, and (F) 6f, original concentration of triblock copolymer: 0.2 mg mL⁻¹.

It is necessary to point out that well-defined cylinders with an average length of 98 ± 30 nm and an average width of 27 ± 6 nm obtained from TEM were formed for copolymer 6d with a longer PBTFVPP chain (Fig. 7D), while the average size was 112 nm measured by DLS. The facet was straight, not curve as oval shaped micelles formed by copolymer 6b. Although this kind of aggregates had been reported, they were usually formed by copolymers with semi-crystalline core-formed segments, such as polythiophene, PFS, and PPV.^68–71 We speculated that PBTFVPP segment was rigid and rigidity of PBTFVPP block facilitated the formation of well-defined cylinders. For copolymers 6e and 6f with longer PPEGMEMA chains, spheres with average diameters of 93 and 106 nm obtained from DLS, respectively, were formed under the same conditions (Fig. 7E and F). These observations indicated that a longer hydrophilic corona-formed segment of PPEGMEMA facilitated the formation of micelles with a larger interfacial area, which was consistent with previous reports.^23,24 Similarly, the numbers of PEGMEMA repeated unit were 38 and 17 for copolymers 6b and 6d. Thus, the repulsion of PPEGMEMA corona segment for the micelle formed by copolymer 6b should be much higher than that of copolymer 6d. Therefore, compared to copolymer 6d, copolymer 6b tended to aggregate into micelles with a higher interfacial area, which might be the reason that copolymer 6b formed oval-shaped micelles and copolymer 6d aggregated into well-defined cylindrical micelles.^23,24

Conclusions

In summary, a novel PFCB aryl ether-based amphiphilic ABA triblock copolymer, PPEGMEMA-b-PBTFVPP-b-PPEGMEMA, was synthesized via the site transformation strategy. PBTFVPP homopolymer was converted to Br–PBTFVPP–Br macroinitiator by capping ATRP initiating groups at both ends. The block copolymers were synthesized via ATRP block copolymerization of PEGMEMA macromonomer initiated by Br–PBTFVBP–Br using CuBr/PMDETA as catalytic system. Fluorescence spectroscopy and TEM were used to study the self-assembly behavior of the copolymer in aqueous media. The cmc values and the diameters of the micelles increased with the increasing of the length of hydrophilic PPEGMEMA segment. Initial water content, lengths of PBTFVPP and PPEGMEMA segments played an important role on determining the morphology of micelles. The copolymers self-assembled into oval, cylinder, and sphere in aqueous solution with different initial water contents and compositions. These findings certainly could enrich the self-assembly studies of semi-fluorinated amphiphilic block copolymer.

Acknowledgements

The authors thank the financial support from National Basic Research Program of China (2015CB931900), National Natural Science Foundation of China (21174158, 21274162, and 21474127), and Shanghai Scientific and Technological Innovation Project (12JC1410500, 14JC1493400, 14QA1404500, and 14520720100).

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Footnote

† Both authors contributed equally to this work.

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