Synthesis of a sun-shaped amphiphilic copolymer consisting of a cyclic perfluorocyclobutyl aryl ether-based backbone and lateral PMAA side chains

Abstract

A novel amphiphilic sun-shaped copolymer, c-PMBTFVB-g-PMAA (MBTFVB: 2-methyl-1,4-bistrifluorovinyloxybenzene, MAA: methacrylic acid) containing a cyclic perfluorocyclobutyl (PFCB) aryl ether-based backbone and PMAA lateral side chains with narrow molecular weight distribution (M_w/M_n ≤ 1.38), was synthesized via the site transformation strategy. First, a PMBTFVB linear precursor was prepared by thermal step-growth cycloaddition polymerization of MBTFVB trifluorovinyl monomer. After the end functionalization of the linear precursor with alkyne, Glaser coupling reaction was performed to produce c-PMBTFVB cyclic homopolymer. The pendant methyls on PFCB aryl ether-based backbone were then converted to Br-containing ATRP initiating groups by a mono-bromination reaction with N-bromosuccinimide (NBS) and benzoyl peroxide (BPO) to give c-PMBTFVB-Br cyclic macroinitiator without affecting the main chain, where the density of ATRP initiation groups could be tuned from 33% to 58% by varying the feeding ratio of NBS to the pendant methyl. Subsequently, c-PMBTFVB-g-PMAA sun-shaped amphiphilic copolymers with hydrophilic PMAA side chains were synthesized by ATRP of tert-butyl methacrylate (tBMA) initiated by c-PMBTFVB-Br, followed by the selective hydrolysis of hydrophobic PtBMA segment into hydrophilic PMAA segment using CF₃COOH. The obtained c-PMBTFVB cyclic homopolymer and its precursor were well characterized by GPC, NMR, and DSC and all the observations indicated a high efficiency of the intra-macromolecular cyclization via Glaser coupling reaction. The critical micelle concentrations of the obtained amphiphilic copolymers were determined by fluorescence probe technique and the morphologies of the formed micelles were investigated by TEM.

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Introduction

Perfluorocyclobutyl (PFCB) aryl ether-based polymers discovered by Babb et al. of Dow Chemical in the early 1990s,^1,2 represent a class of partially fluorinated polymers.^3–5 They have attracted much interest in many fields such as photonics,^6–8 polymer light-emitting diodes,^9,10 and proton exchange membranes for fuel cells.⁴ Since PFCB aryl ether-based polymers were synthesized via a relatively unusual thermal [2π + 2π] step-growth thermal cycloaddition polymerization of trifluorovinyl monomer at very high polymerization temperature (>150 °C), the majority of research on PFCB aryl ether-based polymers has focused on linear PFCB aryl ether-based homopolymers and copolymers with different functionalities.¹¹ It is well-known that physical properties of a certain polymer are inherently related to its architecture. Therefore, a range of non-linear PFCB aryl ether-based (co)polymer architectures, such as star, graft, cross-linked, and hyperbranched, have been prepared and investigated to address their potentially diverse applications.¹¹ Recently, cyclic polymers with “endless” and closed ring macromolecular topology have attracted considerable interest in fundamental polymer science because cyclic polymers possess a particularly unique set of physical properties compared to their linear analogues, including higher glass transition temperature, smaller hydrodynamic volume, lower intrinsic viscosity and so on.^12–18 Up to now, a variety of cyclic polymers, such as polystyrene (PS),^19–22 poly(N-isopropylacrylamide),^23,24 poly(α-peptiod),²⁵ oligonucleotide,²⁶ poly(2-vinylnaphthalene),²⁷ poly(ethylene oxide) (PEO),²⁸ poly(octene),²⁹ polyester,¹⁶ and their related copolymers,^30–32 have been prepared either by the combination of “living”/controlled polymerization with “click” chemistry through an end-to-end linking process, or ring-expansion metathesis polymerization. However, to our best knowledge, a cyclic PFCB aryl ether-based polymer has not been explored yet, possibly due to its unusual preparation approach of thermal [2π + 2π] step-growth thermal cycloaddition polymerization of trifluorovinyl monomer.

With the development of polymer chemistry and the desire for better understanding the fundamental structure–property relationships exhibited by novel and complicated polymer architectures, cyclic polymers were also employed as building blocks for the fabrication of copolymers with much more complex topological architectures, including 8-shaped,^33,34 P-shaped,³⁵ jellyfish-shaped,³⁶ tadpole-shaped,³⁷ flower-shaped,³⁸ sun-shaped^18,39–49 and so on.^50–57 Among them, sun-shaped copolymer, consisting of a cyclic backbone and lateral side chains connected to the backbone, was a kind of special graft copolymer. Therefore, sun-shaped copolymer is also endowed with fascinating properties such as wormlike conformation, compact molecular dimension, and notable chain end effects resulting from their confined and compact structure.⁵⁸ For examples, Deffieux et al. grafted polystyryllithium or both polystyryllithium and polyisoprenyllithium into cyclic poly(chloroethyl vinyl ether) to afford sun-shaped macrocyclic graft copolymers⁴⁰ and these cyclic graft copolymers could be visualized by atomic force microscopy (AFM), which was similar with previous reports on “linear” graft copolymers.^59,60 Interestingly, the obtained sun-shaped copolymer with both PS and polyisoprene side chains could self-assemble into rigid tubular objects up to 700 nm in heptane. Similarly, Frechet and Grubbs et al. utilized a direct route of ring-expansion metathesis polymerization of dendronized macromonomer to prepare sun-shaped copolymers and the individual copolymer ring, thus, was also able to be visualized by AFM.⁴¹ Tew et al. also employed ring-expansion metathesis polymerization to synthesize cyclic backbone and lateral side chains were connected to the backbone via metallo-supramolecular interactions instead of covalent bones.⁴² The obtained sun-shaped copolymers could be visualized by transmission electron microscopy (TEM). Besides these, some other groups also reported sun-shaped copolymers with different compositions. Kricheldorf et al. reported sun-shaped copolymer with cyclic poly(ether ketone)s backbone and hyperbranched side chains derived from a polycondensation of 3,5-bis(4-fluorobenzoyl)phenol.⁴³ Huang el al. demonstrated the synthesis of a series of amphiphilic sun-shaped copolymers of c-PEO-g-PS, c-PEO-g-poly(ε-caprolactone), and c-poly(2-hydroxylethyl methacrylate)-g-(PS-b-PEO) on the basis of anionic ring opening polymerization, atom transfer radical polymerization, nitroxide radical coupling reaction, and azide–alkyne “click” chemistry.^44–47 Experiments on the extraction for dyes of Cresol red, Thymol blue, Bromophenol blue, and Thymolphthalein by sun-shaped c-PEO-g-PS and comb-like PEO-g-PS showed that c-PEO-g-PS had a stronger conjugation ability with the dyes than that of comb-like PEO-g-PS.⁴⁷ Jerome and his co-workers reported the preparation of amphiphilic sun-shaped copolymers of c-PCL-g-PEO by the esterification of carboxylic acid terminated PEO with hydroxyls of cyclic PCL.⁴⁸ Grayson et al. prepared sun-shaped copolymers with cyclic PS backbone and dendritic polyester side chains up to 4^th generation by azide–alkyne “click” reaction of alkyne-functionalized PS cyclic backbone with azide-functionalized polyester dendrons.⁴⁹ Although significant advances have occurred in the synthesis of sun-shaped copolymers, a straight-forward and efficient synthetic method is still challenging for the preparation of sun-shaped copolymer with PFCB aryl ether-based segment considering the unusual approach for its preparation.

Here for the first time, we reported the synthesis of PFCB aryl ether-based cyclic homopolymer and sun-shaped amphiphilic copolymers with PFCB aryl ether-based cyclic backbone and PMAA side chains. PFCB aryl ether-based cyclic homopolymer was first prepared via [2π + 2π] step-growth thermal cycloaddition polymerization of 4-methoxytrifluorovinyloxybenzene and Glaser coupling reaction. Subsequently, pendant methyls on the backbone were transformed into ATRP initiating groups using NBS and BPO. Amphiphilic sun-shaped copolymers were synthesized by ATRP graft copolymerization of tBMA using brominated PFCB aryl ether-based cyclic polymer as macroinitiator, followed by the acidic selective hydrolysis of hydrophobic PtBMA side chains into hydrophilic PMAA side chains. In addition, critical micelle concentrations (cmc) of resulting amphiphilic sun-shaped copolymers were measured by fluorescence spectroscopy and their micellar morphologies were visualized by TEM. It should be noticed that with the view of high tolerance of Glaser coupling reaction, the versatility of thermal [2π + 2π] step-growth thermal cycloaddition polymerization on the synthesis of PFCB aryl ether-based polymers with diverse functional moieties, and ATRP on the preparation of (co)polymers with different functionalities, composition, and architectures, it is logical to assume that the approach addressed in this work can lead to a variety of PFCB aryl ether-based sun-shaped copolymers with desired features.

Experimental section

Materials

4-Methoxytrifluorovinyloxybenzene was prepared according to previous literatures.^1–5 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 the previous reports.^1–5 Granular zinc was activated by washing in 0.1 M HCl followed by drying at 100 °C in vacuo for 16 h. N-Bromosuccinimide (NBS, Aldrich, 99%) was recrystallized from water and dried in vacuo at 35 °C for 1 day. Benzoyl peroxide (BPO, Alfa Aesar, 97%) was purified by dissolving in acetone and precipitating in water followed by drying in vacuo at room temperature for 1 day. tert-Butyl methacrylate (tBMA, Aldrich, 99%) was distilled with CaH₂ under reduced pressure 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 at 40 °C in vacuo for 1 day. N-Phenyl-1-naphthylamine (PNA, Alfa Aesar, 97%) was purified by recrystallization in ethanol for three times. Diphenyl ether (Aldrich, ≥99%), acetonitrile (Aldrich, ≥99.5%), and 1,4-dioxane (Aldrich, ≥99%) were dried with CaH₂ and distilled under vacuum. 4-Methoxyphenol (Aldrich, 99%), 2-methylhydroquinone (Aldrich, 99%), N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), borontribromide (BBr₃, Aldrich, 99%), sodium hydride (NaH, Aldrich, 99%), and 3-bromo-1-propyne (Aldrich, 99%) were used as received. Other solvents were obtained from commercial sources and used as received.

Measurements

All NMR analyses were performed on a Bruker Avance 500 spectrometer (500 MHz). 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. FT-IR spectra were recorded on a Nicolet AVATAR-360 FT-IR spectrophotometer with a resolution of 4 cm⁻¹. Relative molecular weights and molecular weight distributions were measured by gel permeation chromatography (GPC) system equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector and a set of Waters Styragel columns (HR3, HR4 and HR5, 7.8 × 300 mm). GPC measurements were carried out at 35 °C using THF as eluent with a flow rate of 1.0 mL min⁻¹. The system was calibrated with linear polystyrene standards. Absolute molecular weights of the sun-shaped copolymers were determined by GPC equipped with a multiangle light scattering detector (GPC/MALS), THF was used as the eluent with a flow rate of 1.0 mL min⁻¹, detectors: Wyatt Optilab rEX refractive index detector and Wyatt DAWN HELEOS 18-angle light scattering detector with a 50 mW solid-state laser operating at 658 nm. Differential scanning calorimetry (DSC) measurements were run on a TA Q200 system under N₂ purge with a heating rate of 10 °C min⁻¹. The glass transition temperature (T_g) was record from the second heating process after a quick cooling from 200 °C, and the value was determined from the midpoint of C_p curve. Steady-state fluorescence spectra of PNA were measured on a Hitachi F-2700 spectrofluorometer with the bandwidth of 5 nm for excitation and emission; the emission intensity at 418 nm was recorded to determine the cmc with an excitation wavelength (λ_ex) of 340 nm. Hydrodynamic diameter (D_h) was measured by dynamic light scattering (DLS) with a Malvern Nano-ZS90 Zetasizer. Transmission Electron Microscope (TEM) images were obtained by a JEOL JEM-1230 instrument operated at 80 kV.

Synthesis of MBTFVB 1

MBTVFB 1 trifluorovinyl monomer was first prepared from 2-methylhydroquinone via standard fluoroalkylation followed by Zn-mediated dehalogenation¹ as shown in Scheme 1. The crude product was purified by silica column chromatography to afford MBTFVB 1 as clear oil. ¹H NMR (500 MHz, CDCl₃): δ (ppm): 2.32 (s, 3H, CH₃), 6.93 (m, 1H, phenyl), 6.97 (m, 1H, phenyl), 7.03 (m, 1H, phenyl). ¹⁹F NMR (470 MHz, CDCl₃): δ (ppm): −120.1, −127.2, −134.2. ¹³C NMR (125 MHz, CDCl₃): δ (ppm): 15.6 (CH₃), 114.3, 115.5, 119.0, 129.9, 132.7, 135.2, 143.9, 144.6, 146.5, 147.1, 149.3, 149.9 (OCF=CF₂), 150.5, 151.8 (phenyl). EI-MS: m/z: 284. FT-IR (KBr): ν (cm⁻¹): 3045, 2968, 1832, 1603, 1496, 1422, 1311, 1275, 1178, 1157, 1009, 945, 874, 808, 766.

Scheme 1 Synthesis of c-PMBTFVB via Glaser coupling reaction.

Thermal cycloaddition polymerization of MBTFVB 1

PMBTFVB 2 homopolymer was prepared according to previous report.¹ To a 30 mL Schlenk flask (flame-dried under vacuum prior to use) sealed with a rubber septum, MBTFVB 1 (1.42 g, 5.00 mmol) and diphenyl ether (5.0 mL) were first added under N₂. The flask was degassed by three cycles of freezing–pumping–thawing followed by immersing the flask into an oil bath set at 200 °C. 4-Methoxytrifluorovinyloxybenzene (0.612 g, 3.00 mmol) was introduced via a gastight syringe after 6 h for end-capping. The polymerization was terminated by putting the flask into liquid N₂ after another 6 h. THF (5.0 mL) was added to dilute the solution followed by precipitating into methanol. After repeated purification by dissolving in THF and precipitating in methanol, 0.91 g of MeO–PMBTFVB–OMe 2 as a white powder was obtained after drying in vacuo overnight. GPC: M_n = 6100 g mol⁻¹, M_w/M_n = 1.27. ¹H NMR (500 MHz, CDCl₃): δ (ppm): 2.07, 2.28 (3H, CH₃), 3.77 (3H, OCH₃), 6.81–7.10 (3H, phenyl). ¹⁹F NMR (470 MHz, CDCl₃): δ (ppm): −128.2 to −132.8 (cyclobutyl-F₆). ¹³C NMR (125 MHz, CDCl₃): δ (ppm): 16.0, 55.6 (OCH₃), 105.8, 109.2, 112.7, 116.6, 121.5, 131.0, 148.2. FT-IR (KBr): ν (cm⁻¹): 3052, 2933, 1599, 1498, 1314, 1267, 1201, 1128, 1006, 962, 925, 813, 741.

Demethylation of MeO–PMBTFVB–OMe 2

To a 100 mL three-neck flask (flame-dried under vacuum prior to use), MeO–PMBTFVB–OMe 2 (M_n,GPC = 6100 g mol⁻¹, M_w/M_n = 1.27, 0.90 g, 0.25 mmol –OCH₃) was first added followed by three cycles of evacuating and backfilling with N₂. CH₂Cl₂ (30 mL) was introduced via a gastight syringe and the mixture was stirred at 0 °C. BBr₃ (0.20 mL, 2.12 mmol) in 5 mL of CH₂Cl₂ was added dropwise within 30 min, and the mixture was stirred for another 1 h. The reaction was terminated by adding 10 mL of methanol and the organic phase was washed twice with brine. The content was concentrated and precipitated into 20 mL of methanol twice. The obtained product, HO–PMBTFVB–OH 3, was dried in vacuo until a constant weight (0.823 g, 91.4%). GPC: M_n = 6000 g mol⁻¹, M_w/M_n = 1.25. ¹H NMR (500 MHz, CDCl₃): δ (ppm): 2.07, 2.28 (3H, CH₃), 6.80–7.11 (3H, phenyl). ¹⁹F NMR (470 MHz, CDCl₃): δ (ppm): −128.1 to −132.8 (cyclobutyl-F₆). ¹³C NMR (125 MHz, CDCl₃): δ (ppm): 16.0, 105.8, 109.2, 112.7, 116.6, 121.5, 131.0, 148.0. FT-IR (KBr): ν (cm⁻¹): 3051, 2933, 1596, 1314, 1267, 1125, 1007, 962, 925, 813, 742.

Propargylation of HO–PMBTFVB–OH 3

To a 100 mL three-neck flask, NaH (0.20 g, 8.33 mmol) was first added followed by three cycles of evacuating and backfilling with N₂. THF (20 mL) was introduced via a gastight syringe and the mixture was stirred for 30 min. HO–PMBTFVB–OH 3 (M_n,GPC = 6000 g mol⁻¹, M_w/M_n = 1.25, 0.50 g, 0.139 mmol –OH) in 10 mL of THF was added dropwise slowly, and the mixture was stirred for another 1 h. 3-bromo-1-propyne (0.991 g, 8.33 mmol) was introduced via a gastight syringe and the mixture was stirred overnight at room temperature. The organic phase was washed twice with brine and the content was concentrated and precipitated into 10 mL of methanol twice. The obtained product, ≡–PMBTFVB–≡ 4, was dried in vacuo until a constant weight (0.468 g, 93.6%). GPC: M_n = 6300 g mol⁻¹, M_w/M_n = 1.27. ¹H NMR (500 MHz, CDCl₃): δ (ppm): 2.07, 2.28 (3H, –CH₃), 2.49 (1H, CH), 4.65 (2H, OCH₂), 6.80–7.12 (3H, phenyl). ¹⁹F NMR (470 MHz, CDCl₃): δ (ppm): −128.0 to −132.8 (cyclobutyl-F₆). ¹³C NMR (125 MHz, acetone-d₆): δ (ppm): 16.0, 56.1, 76.3, 78.1, 105.8, 109.2, 112.7, 116.7, 121.3, 131.3, 148.2. FT-IR (KBr): ν (cm⁻¹): 2930, 1732, 1497, 1317, 1266, 1203, 1110, 1015, 962, 812, 743.

Glaser coupling of ≡–PMBTFVB–≡ 4

CuBr (0.318 g, 2.22 mmol), PMDETA (0.463 mL, 2.22 mmol), and pyridine (400 mL) were first added to a 1000 mL three-neck flask (flame-dried under vacuum prior to use) and the mixture was stirred for 1 h. ≡–PMBTFVB–≡ 4 (M_n,GPC = 6300 g mol⁻¹, M_w/M_n = 1.27, 0.4433 g, 0.123 mmol alkynyl) in 100 mL of pyridine was added dropwise slowly using a syringe pump (5.0 mL h⁻¹) within 20 h and the mixture was stirred for another 24 h. The content was concentrated and filtered through a short neutral Al₂O₃ column to remove the residual copper catalyst. The crude product was precipitated into methanol twice and dried in vacuo at room temperature to afford a yellow powder with a constant weight (0.425 g, 95.9%), c-PMBTFVB 5 cyclic homopolymer. GPC: M_n = 5600 g mol⁻¹, M_w/M_n = 1.26. ¹H NMR (500 MHz, CDCl₃): δ (ppm): 2.07, 2.28 (3H, CH₃), 4.65 (1H, OCH₂), 6.81–7.10 (3H, phenyl). ¹⁹F NMR (470 MHz, CDCl₃): δ (ppm): −127.8 to −132.5 (cyclobutyl-F₆). ¹³C NMR (125 MHz, acetone-d₆): δ (ppm): 16.1, 56.1, 70.2, 75.5, 105.8, 109.2, 112.5, 116.6, 121.5, 131.2, 148.1. FT-IR (KBr): ν (cm⁻¹): 3042, 2933, 2227, 1595, 1313, 1267, 1125, 1009, 961, 929, 813, 741.

Bromination of c-PMBTFVB 5

NBS and BPO were used to mono-brominate the pendant methyls of c-PMBTFVB 5 homopolymer according to our previous reports^61,62 as shown in Scheme 2. In a typical procedure, c-PMBTFVB 5 (M_n,GPC = 5600 g mol⁻¹, M_w/M_n = 1.26, 0.1260 g, 0.444 mmol –CH₃), NBS (37.5 mg, 0.21 mmol), and BPO (15.3 mg, 0.063 mmol) were first added to a 100 mL three-neck flask (flame-dried under vacuum prior to use) fitted with a reflux condenser and the solution was deoxygenated under N₂. Next, CCl₄ (30 mL) was introduced via a gastight syringe and the solution was refluxed at 80 °C for 12 h. After filtration, CCl₄ was removed from the filtrate by rotary evaporation. The obtained solid was dissolved in 20 mL of ethyl acetate and the resulting solution was washed with brine twice followed by drying over MgSO₄. The solution was concentrated and precipitated into methanol. After repeated purification by dissolving in THF and precipitating in methanol, 0.108 g of c-PMBTFVB-Br 6a macroinitiator as a yellow powder was obtained after drying in vacuo overnight. GPC: M_n = 5700 g mol⁻¹, M_w/M_n = 1.27. EA : Br%: 8.1%. ¹H NMR (500 MHz, CDCl₃): δ (ppm): 2.03, 2.26 (3H, CH₃), 4.27, 4.45 (2H, CH₂Br), 6.75–7.30 (3H, phenyl). ¹⁹F NMR (470 MHz, CDCl₃): δ (ppm): −127.6 to −132.5 (cyclobutyl-F₆). ¹³C NMR (125 MHz, acetone-d₆): δ (ppm): 15.5 (CH₃), 24.6 (CH₂Br), 105.0, 107.8, 111.7 (4C, PFCB), 116.5, 118.7, 120.9, 129.1, 148.6. FT-IR (KBr): ν (cm⁻¹): 3050, 2933, 1599, 1498, 1315, 1203, 1125, 1009, 962, 813, 741.

Scheme 2 Synthesis of c-PMBTFVB-g-PMAA.

ATRP graft copolymerization of tBMA

ATRP bulk graft copolymerization of tBMA was performed using c-PMBTFVB-Br 6 as macroinitiator and CuBr/PMDETA as catalytic system. ATRP macroinitiator 6a (M_n,GPC = 5700 g mol⁻¹, M_w/M_n = 1.27, 0.02 g, 0.02 mmol –CH₂Br) and CuBr (2.9 mg, 0.02 mmol) were first added to a 10 mL Schlenk flask (flame-dried under vacuum prior to use) sealed with a rubber septum under N₂. After three cycles of evacuating and purging with N₂, tBMA (0.65 mL, 4 mmol), PMDETA (0.004 mL, 0.02 mmol), and 1,4-dioxane (0.5 mL) were charged via a gastight syringe. The flask was degassed by three cycles of freezing–pumping–thawing followed by immersing the flask into an oil bath set at 70 °C. The polymerization was terminated by immersing the flask into liquid N₂ after 12 h. The reaction mixture was diluted with THF and passed through a neutral Al₂O₃ column to remove the residual copper catalyst. The solution was concentrated and precipitated into methanol. After repeated purification by dissolving in THF and precipitating in methanol, c-PMBTFVB-g-PtBMA 7a cyclic copolymer as a white powder was obtained after drying in vacuo overnight. GPC: M_n = 11 300 g mol⁻¹, M_w/M_n = 1.36. ¹H NMR (500 MHz, CDCl₃): δ (ppm): 1.01, 1.21 (3H, CCH₃ of polymethacrylate), 1.36 (9H, C(CH₃)₃), 1.81, 2.03 (2H, CH₂ of polymethacrylate), 6.75–7.30 (3H, phenyl). ¹⁹F NMR (470 MHz, CDCl₃): δ (ppm): −127.2 to −132.3 (m, cyclobutyl-F₆). ¹³C NMR (125 MHz, CDCl₃): δ (ppm): 18.8, 19.0, 27.9, 46.3, 80.7, 119.5, 128.1, 176.2. FT-IR (KBr): ν (cm⁻¹): 3051, 2963, 1733, 1558, 1540, 1497, 1456, 1435, 1398, 1317, 1263, 1205, 1123, 1017, 963, 898, 801, 741.

Acidolysis of c-PMBTFVB-g-PtBMA 7

In a typical procedure, c-PMBTFVB-g-PtBMA 7a (M_n,GPC = 11 300 g mol⁻¹, M_w/M_n = 1.36, 30 mg) was first dissolved in 2 mL of dichloromethane at 0 °C and 2 mL of CF₃COOH was added via a syringe. The mixture was stirred at room temperature for 24 h followed by concentration. The solution was precipitated into cold n-hexane followed by drying in vacuo at room temperature for 1 day to provide 15.6 mg of white powder, c-PMBTFVB-g-PMAA 8a. GPC: M_n = 11 500 g mol⁻¹, M_w/M_n = 1.35. ¹H NMR (500 MHz, DMSO-d₆): δ (ppm): 0.95 (3H, CCH₃), 1.60–1.83, 2.23 (2H, CH₂), 6.81–7.23 (3H, phenyl), 12.30 (1H, COOH). ¹⁹F NMR (470 MHz, acetone-d₆): δ (ppm): −127.5 to −133.2 (cyclobutyl-F₆). ¹³C NMR (125 MHz, CDCl₃): δ (ppm): 16.2, 18.0, 27.9, 46.3, 108.2, 111.5, 129.3, 176.5. FT-IR (KBr): ν (cm⁻¹): 3322, 2931, 1736, 1483, 1447, 1407, 1372, 1255, 1151, 1009, 966, 842, 813, 745.

Determination of critical micelle concentration

PNA was used as fluorescence probe to measure the cmc of c-PMBTFVB-g-PMAA 8. 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 solutions of c-PMBTFVB-g-PMAA 8 (1, 0.1, or 0.01 mg mL⁻¹) were added to water containing PNA ([PNA] = 0.001 mg mL⁻¹). All fluorescence spectra were recorded at 20 °C.

Micellar morphology

THF solution of c-PMBTFVB-g-PMAA 8 ([copolymer] = 0.2 mg mL⁻¹) was first filtered through a membrane with a 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 by a micro-syringe, and the solution was dialyzed against water with stirring for 5 days. For TEM measurement, a drop of micelle solution (10 μL) was deposited on an electron microscopy copper grid coated with carbon film, and the water was evaporated at room temperature for 1 day prior to measurement.

Results and discussion

End capping of PMBTFVB

MBTVFB 1 trifluorovinyl monomer was first synthesized according to standard procedures of fluoroalkylation with BrCF₂CF₂Br and Zn-mediated elimination in two steps using commercially available 2-methylhydroquinone as starting material.¹ PMBTFVB 2 homopolymer was then obtained via thermal [2π + 2π] step-growth thermal cycloaddition polymerization of bifunctional monomer 1 in diphenyl ether at 200 °C and both ends of the homopolymer were capped by mono-functional 4-methoxytrifluoro-vinyloxybenzene according to our previous reports.^61–64 It should be point out that because during the cyclization reaction, only chain ends took part in the reaction and ¹H and ¹³C NMR analyses were employed to determine the transformation of end groups, thus PFCB aryl ether-based backbone with a relative low molecular weight was prepared on purpose for getting better ¹H and ¹³C NMR spectra with high resolution. Fig. 1A shows ¹H NMR spectrum of the homopolymer end-capped with methoxyl. The signals at 2.07 and 2.28 ppm (peak “a”) corresponded to 3 protons of the pendant methyls on the benzene ring and the broad peaks between 6.81 ppm and 7.10 ppm were attributed to 3 protons of benzene ring. The signal originating from 3 protons of terminal methoxyl was found to clearly appear at 3.77 ppm (peak “b”), which confirmed the effective end-capping with 4-methoxytrifluorovinyloxybenzene.

Fig. 1 ¹H NMR spectra of linear MeO–PMBTFVB–OMe 2 (A) and HO–PMBTFVB–OH 3 (B) in CDCl₃.

The relative molecular weight of MeO–PMBTFVB–OMe 2 (M_n = 6100 g mol⁻¹) was obtained by conventional GPC which utilized linear polystyrene for calibration, therefore, ¹H NMR was employed to determine the “absolute” molecular weight of the polymer according to eqn (1) (S_a and S_b are the integration area of peaks “a” at 2.07 and 2.28 ppm, and peak “b” at 3.77 ppm in Fig. 1A, respectively; 284 and 204 are the molecular weights of MBTFVB 1 monomer and 4-methoxytrifluorovinyloxybenzene) assuming the complete end-capping with 4-methoxytrifluorovinyloxybenzene. The molecular weight was calculated to be 7200 g mol⁻¹ and this value indicated that every PMBTFVB chain possessed 24.0 MBTFVB repeated units.

M_n,NMR = 284 × (2S_a/S_b) + 204 × 2 (1)

Preparation of linear precursor for Glaser coupling

Glaser coupling reaction between two terminal alkynyls has shown to be a powerful tool for the formation of 1,3-diyne with a high yield and excellent functionality tolerance, which has been employed in a variety of fields.⁶⁵ In 2010, Huang et al. reported the utilization of this reaction for the preparation of cyclic polymer under pseudo-ultradilute conditions with a nearly quantitative yield.²⁸ Compared to widely used azide–alkyne and thiol–ene “click” reactions between two different and complementary groups, Glaser coupling reaction is a homo-coupling reaction taking place between two same terminal alkynyls while it is more convenient and easier to introduce two same end alkynyls into a polymeric chain. Since the mechanism of step-growth thermal cycloaddition polymerization made the obtained PMBTFVB homopolymer possess two same end groups, Glaser coupling reaction was naturally chosen for synthesizing cyclic polymer considering the ease in the functionalization of two same chain ends of PMBTFVB.

Firstly, methoxyl-terminated PMBTFVB was converted to hydroxyl-terminated PMBTFVB via demethylation of MeO–PMBTFVB–OMe 2 using BBr₃. As shown in Fig. 1B, the signal of 3 protons of terminal methoxyl at 3.77 ppm (peak “b” in Fig. 1A) completely disappeared after the demethylation while keeping the signals of pendant methyls and benzene ring. Moreover, the signal of carbon of terminal methoxyl at 55.6 ppm (peak “a” in Fig. S1A†) also could not be detected after the demethylation (Fig. S1B†). We need to point out that GPC curves of polymers 2 and 3 (Fig. S2†) could be overlapped very well and no peak attributed to the product with larger and smaller molecular weights were observed. These results verified the efficient demethylation of MeO–PMBTFVB–OMe 2 without crosslinking or degradation of PMBTFVB backbone.

Next, two terminal hydroxyls PMBTFVB were transformed into alkynyls using 3-bromo-1-propyne for providing Glaser coupling reaction precursor, ≡–PMBTFVB–≡ 4. The chemical structure of ≡–PMBTFVB–≡ 4 was characterized by ¹H and ¹³C NMR. Typical proton resonance signal of alkynyl was located at 2.49 ppm (peak “a” in Fig. 2A) and the peak at 4.65 ppm in Fig. 2A was attributed to 2 protons of methylene adjacent to alkynyl. The resonance signals of 2 carbons of alkynyl were found to be located at 76.3 (peak “a”) and 78.1 (peak “a”) ppm in Fig. 3 and the peak at 56.1 ppm (peak “c”) was ascribed to the carbon of methylene. These results clearly evidenced the effective propargylation of HO–PMBTFVB–OH 3 by 3-bromo-1-propyne.

Fig. 2 ¹H NMR spectra of ≡–PMBTFVB–≡ 4 (A) and c-PMBTFVB 5 (B) in CDCl₃.

Fig. 3 ¹³C NMR spectra of ≡–PMBTFVB–≡ 4 (A) and c-PMBTFVB 5 (B) in acetone-d₆.

Preparation of c-PMBTFVB via Glaser coupling

The intramolecular cyclization of ≡–PMBTFVB–≡ 4 was achieved under high dilution conditions by the end-to-end Glaser coupling reaction between terminal alkynyls. The coupling reaction was carried out using CuBr/PMDETA as catalytic system in pyridine according to previous report.²⁸ Utilizing a syringe pump, 100 mL pyridine solution of linear precursor 4 with a concentration of 7.04 × 10⁻⁴ mol L⁻¹ was added dropwise into 400 mL pyridine solution containing catalysts at the speed of 5.0 mL h^{−1 1}H and ¹³C NMR were employed to examine the obtained product. The peak at 4.65 ppm in Fig. 2B belonged to 2 protons of methylene adjacent to alkynyl while the signal of alkynyl at 2.49 ppm in Fig. 2A was found to vanish completely in Fig. 2B. Furthermore, the resonance signals of 2 carbons of alkynyl were shifted to 70.2 and 75.5 ppm after the cyclization via Glaser coupling reaction as shown in Fig. 3B, which was consistent with previous report about chemical shift of 1,3-diyne.⁶⁶ The complete disappearance of ¹H NMR signal of one proton of terminal alkynyl and the obvious shift of ¹³C NMR signals originating from two carbons of alkynyl after Glaser reaction distinctly suggested the efficient occurrence of cyclization of PMBTFVB polymeric chain.

GPC profiles of linear ≡–PMBTFVB–≡ 4 and cyclic c-PMBTFVB 5 were presented in Fig. 4A. One can see a unimodal GPC profile of c-PMBTFVB 5 with a relatively narrow molecular weight distribution (M_w/M_n = 1.26) and a clear shift of the elution peak of c-PMBTFVB 5 to the lower molecular weight side as compared to its linear precursor 4. This observation indicated that cyclic c-PMBTFVB 5 had a smaller hydrodynamic volume than that of linear precursor 4, which was consistent with previous studies on cyclic polymer.^23,24 It is worthy of noting that there is no peak or shoulder at the higher molecular weight side, which meant that possible intermolecular coupling reaction was effectively suppressed under current high dilution conditions. In addition, glass transition temperatures (T_g) of the obtained cyclic c-PMBTFVB 5 and linear precursor 4 were measured by DSC. As shown in Fig. 4B, T_g of c-PMBTFVB 5 was 62.9 °C, which was 2.2 °C higher than that of linear precursor 4. Previous studies showed that T_gs of cyclic polymers with relative low molecular weights were higher than those of corresponding linear counterparts because fewer degrees of freedom needed to be frozen out.^16,67 Therefore, the shift of elution peak and higher T_g of cyclic polymers compared to those of linear analogues were regarded as invaluable tools for differentiating cyclic polymers from linear analogs, and in the current case, all aforementioned results proved the efficient intramolecular cyclization of ≡–PMBTFVB–≡ 4 to afford cyclic c-PMBTFVB 5.

Fig. 4 GPC (A) and DSC (B) curves of ≡–PMBTFVB–≡ 4 (A) and c-PMBTFVB 5.

Preparation of PMBTFVB-Br macroinitiator

Mono-bromination of pendant methyls of c-PMBTFVB 5 for introducing Br-containing ATRP initiating sites into cyclic backbone was conducted in CCl₄ by reacting with NBS and BPO according to our previous reports^61,62 so that c-PMBTFVB 5 was transformed into c-PMBTFVB-Br 6 macroinitiator. The degree of bromination was tuned by the feeding ratio of NBS to methyl as listed in Table 1. It can be seen from Table 1 that 0.5 equivalent of NBS to methyls of c-PMBTFVB 5 gave a bromination content of 8.1% for c-PMBTFVB-Br 6a. With increasing the amount of NBS to 0.8 and 1.0 equivalent, the bromination extent was raised to 10.8% and 13.5%, respectively.

Table 1 Preparation of c-PMBTFVB-Br 6 macroinitiator

	NBS (eq.)	Br%^a	Mn,GPC^b (g mol^−1)	Mw/Mn^b	DBr^c	NBr^d
a Determined by the titration with Hg(NO3)2.b Measured by GPC in THF at 35 °C.c Grafting density of –CH2Br ATRP initiating group obtained from element analysis.d The number of –CH2Br ATRP initiating group per chain obtained from element analysis.
6a	0.5	8.1%	5700	1.27	33%	8
6b	0.8	10.8%	5800	1.28	46%	11
6c	1.0	13.5%	6100	1.28	58%	14

---

Fig. 5 shows ¹H NMR spectrum of c-PMBTFVB-Br 6. It is clear that two new peaks at 4.27 and 4.45 ppm (peak “a”) appeared in Fig. 5, which corresponded to 2 protons of newly formed –CH₂Br group. The signals at 2.03 and 2.26 ppm (peak “b”) belonged to 3 protons of the methyls which were not brominated. In addition, Fig. S3† shows a series of peaks between −127.6 and −132.5 ppm attributed to PFCB linkage in ¹⁹F NMR spectrum of c-PMBTFVB-Br 6 (Fig. S3A†) and a unimodal and symmetric elution peak without tail or shoulder at the lower or higher molecular weight sides in GPC trace (Fig. S3B†), which illustrated that the architecture of cyclic polymeric chain was not destroyed during the bromination. Overall, it can be concluded from the above-mentioned results that Br-containing ATRP initiating sites were successfully introduced by the mono-bromination of methyls of c-PMBTFVB 5 without affecting the polymeric backbone.

Fig. 5 ¹H NMR spectrum of c-PMBTFVB-Br 6 macroinitiator in CDCl₃.

The number of –CH₂Br ATRP initiating group per cyclic chain (N_Br) and grafting density of –CH₂Br ATRP initiating group (D_Br) could be estimated via the data of Br% (Table 1) according to eqn (2) and (3) (24.0 is the total number of repeated unit with pendant –CH₂Br or –CH₃ group in c-PMBTFVB-Br cyclic chain, 454 and 80 are the molecular weights of connection group and bromine atom, 363 and 284 are the molecular weights of repeated units with –CH₂Br or –CH₃ group). The values of N_Br for c-PMBTFVB-Br 6a, 6b, and 6c were calculated to be 8, 11, and 14, respectively. Thus, the grafting densities of –CH₂Br ATRP initiating group were 33%, 46%, and 58% for macroinitiators 6a, 6b, and 6c (Table 1), respectively.

80N_Br/(363N_Br + 284(24 − N_Br) + 454) = Br% (2)

D_Br = N_Br/24.0 (3)

Construction of sun-shaped c-PMBTFVB-g-PMAA amphiphilic copolymer

ATRP of tBMA was initiated by c-PMBTFVB-Br 6 macroinitiator at 70 °C using CuBr/PMDETA as catalytic system to yield two sun-shaped c-PMBTFVB-g-PtBMA 7 copolymers through the grafting-from strategy and the results are summarized in Table 2. One can see that both copolymers have relatively narrow molecular weight distributions (M_w/M_n ≤ 1.38) and the molecular weights were much higher than that of macroinitiator. These observations demonstrated that ATRP of tBMA was successfully performed and intermolecular coupling reactions were suppressed using a high feeding ratio of tBMA to –CH₂Br initiating group (200 : 1).^68,69

Table 2 Synthesis of c-PMBTFVB-g-PtBMA 7 copolymer^a

	[tBMA] : [6]	Mn^d (g mol^−1)	Mw/Mn^d	Mn,GPC/MALS^e (g mol^−1)	NtBMA^f	ntBMA^g
a Polymerization temperature: 70 °C, time: 12 h.b Initiated by 6a (Mn = 5700 g mol^−1, Mw/Mn = 1.27).c Initiated by 6b (Mn = 5800 g mol^−1, Mw/Mn = 1.28).d Measured by GPC in THF at 35 °C.e Obtained by GPC equipped with a multiangle light scattering detector (GPC/MALS) in THF with a flow rate of 1.0 mL min^−1.f Total number of tBMA repeated unit obtained from GPC/MALS.g The number of tBMA repeated unit per PtBMA side chain obtained from GPC/MALS.
7a^b	200 : 1	11 300	1.36	21 600	96.5	12.1
7b^c	200 : 1	12 100	1.38	23 200	106.1	9.6

---

¹H NMR spectrum of c-PMBTFVB-g-PtBMA 7 copolymer is shown in Fig. 6A. It was found that the signal of 2 protons of –CH₂Br initiating group connected to the benzene ring located at 4.27 and 4.45 ppm (peak “a” in Fig. 5) disappeared, indicative of the complete involvement of ATRP initiating groups in graft copolymerization of tBMA. Moreover, one also can see the signal of 9 protons of tert-butyl of PtBMA side chains located at 1.36 ppm (peak “c”) and the broad peak between 6.75 ppm and 7.30 ppm attributed to phenyl of cyclic PMBTFVB backbone. These evidences confirmed the structure of c-PMBTFVB-g-PtBMA 7 sun-shaped copolymer.

Fig. 6 ¹H NMR spectra of (A) c-PMBTFVB-g-PtBMA 7 in CDCl₃ and (B) c-PMBTFVB-g-PMAA 8 in DMSO-d₆.

Since the molecular weight of graft copolymer measured by GPC is much lower than the “real” value,^44,68 absolute molecular weights of c-PMBTFVB-g-PtBMA 7 sun-shaped copolymers were measured by GPC/MALS in THF. As listed in Table 2, the molecular weights obtained from GPC/MALS were much higher than those measured by GPC as expected. From the data of absolute molecular weights of copolymers (M_n,GPC/MALS in Table 2), the total number of tBMA repeated unit (N_tBMA) and the number of tBMA repeated unit per PtBMA side chain (n_tBMA) were calculated according to eqn (4) and (5) (N_Br is the number of –CH₂Br ATRP initiating group per chain as listed in Table 1, 24 is the total number of repeated unit with pendant –CH₂Br or –CH₃ group in the cyclic backbone, 454, 363, 284, and 142 are the molecular weights of connection group, repeated units with –CH₂Br or –CH₃ group, and tBMA monomer, respectively) and the results are summarized in Table 2. The molecular weights of copolymers 7a and 7b were also calculated on the basis of ¹H NMR results, which were 20 200 and 21 600 g mol⁻¹, respectively. These values were very close to those obtained by GPC/MALS.

N_tBMA = [M_n,GPC/MALS − 363N_Br − 284(24 − N_Br) − 454]/142 (4)

n_tBMA = N_tBMA/N_Br (5)

Thus, we can conclude that c-PMBTFVB-g-PtBMA 7 sun-shaped copolymers consisting of a PFCB aryl ether-based backbone with 24 repeated units and PtBMA side chains (7a: 8 PtBMA side chains with 12.1 tBMA repeated units per side chain; 7b: 11 PtBMA side chains with 9.6 tBMA repeated units per side chain) were synthesized by ATRP of tBMA initiated by c-PMBTFVB-Br 6 macroinitiator.

Selective acidic hydrolysis of tert-butoxycarbonyls of PtBMA side chain was achieved by treating c-PMBTFVB-g-PtBMA 7 copolymer with anhydrous CF₃COOH in CH₂Cl₂ at room temperature for 1 day according to previous reports,^70–72 to give c-PMBTFVB-g-PMAA 8 copolymer. Fig. 6B shows ¹H NMR spectrum of the hydrolyzed product. It is clear that the signal at 1.36 ppm originating from 9 protons of tert-butyl of PtBMA side chain (peak “c” in Fig. 6A) completely disappeared with the appearance of a new peak at 12.30 ppm attributed to 1 proton of newly formed carboxyl (peak “c” in Fig. 6B). Form FT-IR spectra before and after the hydrolysis, we can also notice that a much stronger peak at 3322 cm⁻¹ attributed to the newly formed carboxyl appeared in FT-IR spectrum after the hydrolysis in comparison with that before the hydrolysis. Furthermore, it was found from ¹³C NMR spectra of c-PMBTFVB-g-PtBMA 7 and c-PMBTFVB-g-PMAA 8 (Fig. 7) that typical peaks at 80.7 ppm (peak “a” in Fig. 7A) and 27.9 ppm (peak “b” in Fig. 7A) corresponding to tert-butyl of PtBMA side chain completely disappeared after the hydrolysis (Fig. 7B), indicative of quantitative hydrolysis of tert-butoxycarbonyl into carboxyl. It should be noted that typical signals of other groups in the sun-shape copolymer were found to remain in ¹H (Fig. 6B) and ¹³C (Fig. 7B) NMR spectra after the hydrolysis. This observation suggested that other groups in the sun-shaped copolymer kept intact during the hydrolysis, consistent with previous results demonstrating the selectively hydrolysis of tert-butoxycarbonyl in CF₃COOH.^70–72 All these results evidenced the efficient and selective hydrolysis of tert-butoxycarbonyl in c-PMBTFVB-g-PtBMA 7 so that hydrophobic c-PMBTFVB-g-PtBMA 7 was converted to amphiphilic c-PMBTFVB-g-PMAA 8 with hydrophilic PMAA side chains.

Fig. 7 ¹³C NMR spectra of c-PMBTFVB-g-PtBMA 7 (A) and c-PMBTFVB-g-PMAA 8 (B) in CDCl₃.

Self-assembly of c-PMBTFVB-g-PMAA in aqueous media

Similar to small molecule surfactant, amphiphilic copolymer also has critical micellization concentration (cmc), which is an important indicator reflecting the solubility (dispersibility) or amphiphilicity of an amphiphilic copolymer. The cmc value of c-PMBTFVB-g-PMAA 8 sun-shaped copolymer was analyzed by fluorescence spectroscopy using PNA as probe. As it is well-known, fluorescence spectrum of PNA is sensitively to the environment with a high fluorescence activity in nonpolar surroundings, and a rather low fluorescence in polar solvents such as water.⁷³ Fig. 8A shows fluorescent spectra of aqueous solutions containing PNA with a constant content of 2.0 × 10⁻⁶ g mL⁻¹ and c-PMBTFVB-g-PMAA 8a with different concentrations. We can see that the fluorescent intensity rose with the increasing of the concentration of copolymer. We plotted the fluorescence intensity ratios of I/I₀ in Fig. 8B (I₀ and I are the fluorescent intensities at 418 nm for the aqueous solutions without and with the copolymer, respectively) against the concentration of copolymer. It can be seen from the figure that I/I₀ increased sharply when the concentration of c-PMBTFVB-g-PMAA 8a exceeded a certain value, which meant PNA probe was incorporated into the hydrophobic region of micelles. Therefore, the intersection of two straight lines with a value of 1.12 × 10⁻⁵ g mL⁻¹ was determined to be the cmc of c-PMBTFVB-g-PMAA 8a and the cmc of c-PMBTFVB-g-PMAA 8b was 9.91 × 10⁻⁶ g mL⁻¹ obtained by similar method. The morphologies of the micelles formed by c-PMBTFVB-g-PMAA 8a and 8b in aqueous solution with a content of 0.2 mg mL⁻¹, much higher than their cmc, were examined by TEM. For c-PMBTFVB-g-PMAA 8a, both hollow and solid spherical micelles with an average diameter of 102 nm obtained by DLS (Fig. S4A†) were observed as shown in Fig. 8C, while only solid spherical micelles with an average size of 125 nm determined by DLS (Fig. S4B†) were formed by c-PMBTFVB-g-PMAA 8b (Fig. 8D). Thus, these results illustrated the amphiphilicity of c-PMBTFVB-g-PMAA 8 sun-shaped copolymer.

Fig. 8 (A) Fluorescence emission spectra of PNA (2.0 × 10⁻⁶ g mL⁻¹) in aqueous solutions of c-PMBTFVB-g-PMAA 8a with different concentrations, λ_ex = 340 nm. (B) Dependence of fluorescence intensity ratios of PNA emission at 418 nm on the concentration of c-PMBTFVB-g-PMAA 8a. TEM images of micelles formed by c-PMBTFVB-g-PMAA 8a (C) and 8b (D) with a content of 0.2 mg mL⁻¹.

Conclusions

In summary, we have demonstrated a very feasible approach for the synthesis of novel sun-shaped amphiphilic copolymer consisting of perfluorocyclobutyl aryl ether-based cyclic backbone and PMAA lateral side chains with relatively narrow molecular weight distributions (M_w/M_n ≤ 1.38) by the combination of Glaser-coupling reaction, thermal step-growth cycloaddition polymerization, and ATRP. The perfluorocyclobutyl aryl ether-based cyclic polymer was prepared via thermal step-growth cycloaddition polymerization of 2-methyl-1,4-bistrifluorovinyloxybenzene, followed by efficient chain end functionalization and Glaser-coupling reaction. After the introduction of ATRP initiating sites by controllable mono-bromination of pendant methyls of cyclic polymer backbone to produce cyclic macroinitiators with tunable densities of ATRP initiating sites, ATRP of tBMA initiated by the cyclic macroinitiator was performed to prepare sun-shaped copolymers of c-PMBTFVB-g-PtBMA with different grafting densities and lengths of PtBMA side chain. Through the selective hydrolysis of tert-butoxycarbonyls of PtBMA side chains, the target c-PMBTFVB-g-PMAA amphiphilic sun-shaped copolymers were obtained. Since the thermal step-growth cycloaddition polymerization can be used to prepare a variety of perfluorocyclobutyl aryl ether-based polymers with different functionalities and ATRP also can be utilized to synthesize a large library of polymer chains with diverse functionalities, compositions, and architectures, we believe that the strategy outlined in this paper could be an versatile approach for the synthesis of desired sun-shaped copolymers with PFCB aryl ether-based cyclic backbone.

Acknowledgements

The authors thank the financial supports from National Basic Research Program of China (2015CB931900), National Natural Science Foundation of China (21274162, 51373196, and 21474127), and Shanghai Scientific and Technological Innovation Project (11ZR1445900, 12JC1410500, 13ZR1464800, 14QA1404500, 14JC1493400, and 14520720100).

Notes and references

1. D. A. Babb, B. R. Ezzell, K. S. Clement, W. F. Richey and A. P. Kennedy, J. Polym. Sci., Part A: Polym. Chem., 1993, 31, 3465–3477.
2. A. P. Kennedy, D. A. Babb, J. N. Bermmer and A. J. Pasztor, J. Polym. Sci., Part A: Polym. Chem., 1995, 33, 1859–1865.
3. B. K. Spraul, S. Suresh, S. Glaser, D. Perahia, J. Ballato and D. W. Smith, J. Am. Chem. Soc., 2004, 126, 12772–12773.
4. L. A. Ford, D. D. DesMarteau and D. W. Smith, J. Fluorine Chem., 2005, 126, 653–660.
5. B. K. Spraul, S. Suresh, J. Y. Jin and D. W. Smith, J. Am. Chem. Soc., 2006, 128, 7055–7064.
6. G. Fischbeck, R. Moosburger, C. Kostrzema, A. Achen and K. Petermann, Electron. Lett., 1997, 33, 518–519.
7. D. W. Smith, S. Chen, S. Kumar, J. Ballato, H. Shah, C. Topping and S. Foulger, Adv. Mater., 2002, 14, 1585–1589.
8. J. Y. Jin, D. W. Smith, C. Topping, S. Suresh, S. Chen, S. Foulger, N. Rice and B. Mojazza, Macromolecules, 2003, 36, 9000–9004.
9. X. Jiang, S. Liu, M. S. Liu, P. Herguth, A. K. Y. Jen, H. Fong and M. Sarikaya, Adv. Funct. Mater., 2002, 12, 745–751.
10. X. Gong, D. Moses, A. J. Heeger, S. Liu and A. K. Y. Jen, Appl. Phys. Lett., 2003, 83, 183–185.
11. S. T. Iacono, S. M. Budy, J. Y. Jin and D. W. Smith, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 5705–5721.
12. K. Endo, Adv. Polym. Sci., 2008, 217, 121–183.
13. H. R. J. Kricheldorf, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 251–284.
14. E. Minatti, R. Borsali, M. Schappacher, A. Deffieux, V. Soldi, T. Narayanan and J. L. Putaux, Macromol. Rapid Commun., 2002, 23, 978–982.
15. M. Schappacher and A. Deffieux, J. Am. Chem. Soc., 2008, 130, 14684–14689.
16. J. N. Hoskins and S. M. Grayson, Polym. Chem., 2011, 2, 289–299.
17. B. A. Laurent and S. M. Grayson, Chem. Soc. Rev., 2009, 38, 2202–2213.
18. N. Nasongkla, B. Chen, N. Macaraeg, M. E. Fox, J. M. J. Frechet and F. C. Szoka, J. Am. Chem. Soc., 2009, 131, 3842–3843.
19. M. Kubo, T. Hibino, M. Tamura, T. Uno and T. Itoh, Macromolecules, 2002, 35, 5816–5820.
20. A. F. Voter and E. S. Tillman, Macromolecules, 2010, 43, 10304–10310.
21. S. S. Wang, K. Zhang, Y. M. Chen and F. Xi, Macromolecules, 2014, 47, 1993–1998.
22. B. A. Laurent and S. M. Grayson, J. Am. Chem. Soc., 2006, 128, 4238–4239.
23. J. Xu, J. Ye and S. Y. Liu, Macromolecules, 2007, 40, 9103–9110.
24. X. P. Qiu, F. Tanaka and F. M. Winnik, Macromolecules, 2007, 40, 7069–7071.
25. L. Guo and D. H. Zhang, J. Am. Chem. Soc., 2009, 131, 18072–18074.
26. R. Kumar, A. El-Sagheer, J. Tumpane, P. Lincoln, L. M. Wilhelmsson and T. Brown, J. Am. Chem. Soc., 2007, 129, 6859–6864.
27. G. G. Nossarev, J. Johnson, S. E. Bradforth and T. E. Hogen-Esch, J. Phys. Chem. C, 2013, 117, 10244–10256.
28. Y. N. Zhang, G. W. Wang and J. L. Huang, Macromolecules, 2010, 43, 10343–10347.
29. C. W. Bielawski, D. Benitez and R. H. Grubbs, Science, 2002, 297, 2041–2042.
30. S. Csihony, D. A. Culkin, A. C. Sentman, A. P. Dove, R. M. Waymouth and J. L. Hedrick, J. Am. Chem. Soc., 2005, 127, 9079–9084.
31. S. Honda, T. Yamamoto and Y. Tezuka, J. Am. Chem. Soc., 2010, 132, 10251–10253.
32. K. Adachi, S. Honda, S. Hayashi and Y. Tezuka, Macromolecules, 2008, 41, 7898–7903.
33. Y. Tezuka, R. Komiya and M. Washizuka, Macromolecules, 2002, 36, 12–17.
34. X. S. Fan, B. Huang, G. W. Wang and J. L. Huang, Macromolecules, 2012, 45, 3779–3786.
35. B. V. K. J. Schmidt, N. Fechler, J. Falkenhagen and J. F. Lutz, Nat. Chem., 2011, 3, 234–238.
36. T. Cai, W. J. Yang, K.-G. Neoh and E. T. Kang, Polym. Chem., 2012, 3, 1061–1068.
37. X. Wan, T. Liu and S. Liu, Biomacromolecules, 2011, 12, 1146–1154.
38. Z. Ge, Y. Zhou, J. Xu, H. Liu, D. Chen and S. Liu, J. Am. Chem. Soc., 2009, 131, 1628–1629.
39. S. M. Grayson, Nat. Chem., 2009, 1, 178–179.
40. M. Schappacher and A. Deffieux, Science, 2008, 319, 1512–1515.
41. A. J. Boydston, T. W. Holcombe, D. A. Unruh, J. M. J. Frechet and R. H. Grubbs, J. Am. Chem. Soc., 2009, 131, 5388–5389.
42. K. Zhang, Y. P. Zha, B. Peng, Y. M. Chen and G. N. Tew, J. Am. Chem. Soc., 2013, 135, 13994–13997.
43. H. R. Kricheldorf, L. Vakhtangishvili, G. Schwarz and R. P. Kruger, Macromolecules, 2003, 36, 5551–5558.
44. Z. F. Jia, Q. Fu and J. L. Huang, Macromolecules, 2006, 39, 5190–5193.
45. X. C. Pang, R. K. Jing and J. L. Huang, Polymer, 2008, 49, 893–900.
46. X. S. Fan, G. W. Wang, J. L. Wang and J. L. Huang, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 1361–1369.
47. X. C. Pang, G. W. Wang, Z. F. Jia, C. Liu and J. L. Huang, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 5824–5837.
48. H. Y. Li, R. Jerome and P. Lecomte, Macromolecules, 2008, 41, 650–654.
49. B. A. Laurent and S. M. Grayson, J. Am. Chem. Soc., 2011, 133, 13421–13429.
50. Y. Tezuka and K. Fujiyama, J. Am. Chem. Soc., 2005, 127, 6266–6270.
51. N. Sugai, H. Heguri, T. Yamamoto and Y. Tezuka, J. Am. Chem. Soc., 2011, 133, 19694–19697.
52. T. Suzuki, T. Yamamoto and Y. Tezuka, J. Am. Chem. Soc., 2014, 136, 10148–10155.
53. Z. Ge, D. Wang, Y. Zhou, H. Liu and S. Liu, Macromolecules, 2009, 42, 2903–2910.
54. Z. F. Jia, D. E. Lonsdale, J. Kulis and M. J. Monteiro, ACS Macro Lett., 2012, 1, 780–783.
55. A. Bunha, M. C. Tria and R. Advincula, Chem. Commun., 2011, 47, 9173–9175.
56. P. G. Clark, E. N. Guidry, W. Y. Chan, W. E. Steinmetz and R. H. Grubbs, J. Am. Chem. Soc., 2010, 132, 3405–3412.
57. K. Zhang, M. A. Lackey, J. Cui and G. N. Tew, J. Am. Chem. Soc., 2011, 133, 4140–4148.
58. C. Feng, Y. J. Li, D. Yang, J. H. Hu, X. X. Zhang and X. Y. Huang, Chem. Soc. Rev., 2011, 40, 1282–1295.
59. S. S. Sheiko, F. C. Sun, A. Randall, D. Shirvaniants, M. Rubinstein, H. I. Lee and K. Matyjaszewski, Nature, 2006, 440, 191–194.
60. I. Park, S. S. Sheiko, A. Nese and K. Matyjaszewski, Macromolecules, 2009, 42, 1805–1807.
61. H. Liu, Y. J. Li, S. Zhang, D. Yang, J. H. Hu and X. Y. Huang, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 11–22.
62. H. Liu, S. Zhang, Y. J. Li, D. Yang, J. H. Hu and X. Y. Huang, Polymer, 2010, 51, 5198–5206.
63. X. Y. Huang, G. L. Lu, D. Peng, S. Zhang and F. L. Qing, Macromolecules, 2005, 38, 7299–7305.
64. G. L. Lu, S. Zhang and X. Y. Huang, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 5438–5444.
65. P. Siemsen, R. C. Livingston and F. Diederich, Angew. Chem., Int. Ed., 2000, 39, 2633–2657.
66. W. Y. Yin, C. He, M. Chen, H. Zhang and A. W. Lei, Org. Lett., 2009, 11, 709–712.
67. P. G. Santangelo, C. M. Roland, T. Y. Chang, D. H. Cho and J. Roovers, Macromolecules, 2001, 34, 9002–9005.
68. L. N. Gu, Z. Shen, S. Zhang, G. L. Lu, X. H. Zhang and X. Y. Huang, Macromolecules, 2007, 40, 4486–4493.
69. G. L. Cheng, A. Boker, M. F. Zhang, G. Krausch and A. H. E. Muller, Macromolecules, 2001, 34, 6883–6888.
70. C. Feng, G. L. Lu, Y. J. Li and X. Y. Huang, Langmuir, 2013, 29, 10922–10931.
71. P. P. Li, Z. Y. Li and J. L. Huang, Macromolecules, 2007, 40, 491–498.
72. R. K. O'Reilly, M. J. Joralemon, K. L. Wooley and C. J. Hawker, Chem. Mater., 2005, 17, 5976–5988.
73. P. S. Xu, H. D. Tang, S. Y. Li, J. Ren, E. Van Kirk, W. J. Murdoch, M. Radosz and Y. Q. Shen, Biomacromolecules, 2004, 5, 1736–1744.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11630e

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