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disulfide-containing methacrylate monomer: Synthesis, ATRP and nanocomposite
European Polymer Journal 119 (2019) 8–13
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A new ferrocene/disulfide-containing methacrylate monomer: Synthesis, ATRP and nanocomposite Wenhao Qiana, , Haiyan Zhanga, Tao Songa, Mao Yea, Chun Fengb, Guolin Lub, Xiaoyu Huangb, ⁎
T ⁎
a
Department of Stomatology, Shanghai Xuhui District Dental Center, 685 Zhaojiabang Road, Shanghai 200032, People’s Republic of China Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China
b
ARTICLE INFO
ABSTRACT
Keywords: Ferrocene Disulfide bond Methacrylate ATRP Grafting-from
A new methacrylate monomer containing ferrocene unit and disulfide bond, 2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate (FcMAss), was prepared by subsequent esterification reaction with bis(2hydroxyethyl) disulfide and methacryloyl chloride using ferrocenecarboxylic acid as starting material. A series of well-defined poly(2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate) (PFcMAss) homopolymers with narrow molecular weight distributions (Mw/Mn < 1.30) was then obtained by atom transfer radical polymerization (ATRP) of FcMAss monomer in toluene. Block copolymerization of FcMAss monomer was initiated by PEG-Br macroinitiator in THF to afford a poly(ethylene glycol)-b-poly(2-((2-(methacryloyloxy)ethyl) disulfanyl)ethyl ferrocenecarboxylate) (PEG-b-PFcMAss) diblock copolymer with a relatively low dispersity (Mw/Mn = 1.34). Graphene oxide (GO)-based nanocomposite, GO-PFcMAss, was prepared by surface-initiated ATRP of FcMAss via the grafting-from strategy, using GO-based macroinitiator obtained by treating tris(hydroxymethyl) aminomethane (TRIS)-modified GO with 2-bromo-2-methylpropionyl bromide. The nanocomposite was characterized by FT-IR and XPS, and showed excellent dispersibility and colloidal stability.
1. Introduction Graphene, a novel two-dimensional graphitic carbon system, has promptly inspired numerous interests across many fields, such as biosensors, electrochemical energy storage and electronics because of its specific properties of large surface area, good mechanical properties and extremely high electronic conductivity properties [1–6]. Graphene oxide (GO), the main derivative of graphene, affords potential for largescale manufacture of graphene and graphene-based materials. GO, prepared through cost-effective chemical approaches by using cheap graphite as raw material [7–10], not only can fabricate low-cost and large-scale graphene by chemical reduction, but also provides a robust platform for preparing functionalized graphene hybrid materials through the incorporation with various functional materials including inorganic nanostructures, polymers, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), biomaterials and carbon nanotubes (CNTs) due to the presence of oxygen-containing active groups of carboxyl, hydroxyl and epoxide groups, in its edge and surface [11–21]. Furthermore, the biocompatibility of GO renders it a good candidate for application in the biomedical field [12,19,22–24]. Ferrocene (Fc) is an interesting metal-containing compound
⁎
possessing unique redox, preceramic, etch-resistant and catalytic properties [25–27] so that it and corresponding derivatives are often employed as attractive building blocks for the preparation of functional materials. For instance, Fc has been extensively utilized to produce electrochemical biosensors because of its reversibility, regeneration at low potential and generation of stable redox states [25–29]. Nevertheless, it is not able to form stable adsorption layer on electrodes, therefore, an effective solution method is to chemically couple special ferrocene derivatives with carbon nanomaterials. Among carbon nanomaterials, GO is an excellent candidate since that the functional groups on its surface make it suitable for the construction of nano-hybrid materials. Although many GO/Fc-based nanomaterials have been prepared for the use in biosensors, supercapacitor electrodes, etc. [30–35]. However, only few reports were concerned with the chemical immobilization of ferrocene-containing polymeric chains on the surface of graphene oxide [36,37]. Moreover, there is no report on the preparation of GO/Fc-based nanocomposite by convenient and highly efficient atom transfer radical polymerization (ATRP) of ferrocene-containing methacrylate monomer via the common grafting-from strategy. Herein, we designed a new methacrylate monomer containing ferrocene unit and disulfide bond, 2-((2-(methacryloyloxy)ethyl)disulfanyl)
Corresponding authors. E-mail addresses: [email protected] (W. Qian), [email protected] (X. Huang).
https://doi.org/10.1016/j.eurpolymj.2019.07.009 Received 29 May 2019; Received in revised form 2 July 2019; Accepted 4 July 2019 Available online 04 July 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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Scheme 1. Preparation of FcMAss monomer and GO-PFcMAss nanocomposite.
ethyl ferrocenecarboxylate (FcMAss). Ferrocenecarboxylic acid was selected as starting material for the successive esterification reaction with bis (2-hydroxyethyl) disulfide and methacryloyl chloride to afford the target methacrylate monomer as shown in Scheme 1. Solution ATRP of FcMAss monomer was performed in toluene using ethyl α-bromoisobutyrate as initiator and CuBr/pentamethyldiethylenetriamine (PMDETA) as catalytic system, providing well-defined poly(2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocene- carboxylate) (PFcMAss) homopolymers. Subsequently, a poly(ethylene glycol)-b-poly(2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate) (PEG-b-PFcMAss) diblock copolymer was obtained through solution ATRP of FcMAss monomer initiated by a PEG-based macroinitiator. Finally, surface-initiated ATRP (SI-ATRP) of FcMAss monomer was initiated by a GO-based macroinitiator [38,39] to afford PFcMAss-functionalized GO sheets (GO-PFcMAss), showing improved dispersibility in various solvents.
was measured by an Agilent FTMS-7.0 Fourier transformation mass spectrometer. Relative molecular weights and molecular weight distributions were measured by conventional 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, particle size: 5 μm). GPC measurements were carried out at 35 °C using THF as eluent with a flow rate of 1.0 mL/min. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI 5000c ESCA photoelectron spectrometer. 2.3. Synthesis of 2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate (FcMAss) Ferrocenecarboxylic acid (13.1525 g, 0.057 mol) and dry DCM (200 mL) were added into a 500 mL round bottom flask. This mixture was cooled to −5 °C in an ice bath and a solution of oxalyl chloride (32.9073 g, 0.26 mol) in DCM (20 mL) was added dropwise. The mixture was naturally warmed to room temperature and then stirred overnight at room temperature. The mixture was evaporated under reduced pressure to remove DCM and unreacted oxalyl chloride. The residue was treated with dry hexane at 50 °C and then filtrated. The filtrate was concentrated to get 12.3328 g (87%) of orange red solid, ferrocenoyl chloride (FcOCl). FcOCl (12.3328 g, 0.05 mol) was added in dry DCM (170 mL) and cooled to −5 °C in an ice bath. A DCM (70 mL) solution containing bis(2hydroxyethyl) disulfide (8.6374 g, 0.056 mol) and Et3N (7.8 mL, 0.056 mol) was added dropwise to the above mixture. After naturally warmed to room temperature and stirred overnight, the mixture was washed in succession with saturated sodium bicarbonate solution, water and saturated NaCl solution. The organic layer is dried over anhydrous magnesium sulfate and the solvent is removed by distillation. The resultant was directly dissolved in dry DCM (100 mL) and then Et3N (7.8 mL, 0.056 mol) was added. Subsequently, a solution of methacryloyl chloride (10.8 mL, 0.103 mol) in 20 mL of dry DCM was dropwise added to the above mixture at −5 °C. The resulting mixture was naturally warmed to room temperature and stirred overnight. The mixture was diluted with DCM and washed with saturated NaCl solution three times. The organic phase was dried over anhydrous magnesium sulfate and concentrated. The residue was purified by column chromatography (nhexane/ethyl acetate (v:v) = 10:1) to give 2-((2-(methacryloyloxy)ethyl) disulfanyl)ethyl ferrocenecarboxylate (FcMAss) as a wine red oil (12.5302 g, 58%). FI-IR (film): ν (cm−1): 2965, 2930, 2860, 1715, 1458, 1264, 1131, 822. 1H NMR (400 MHz, CDCl3): δ (ppm): 1.95 (3H, CH3C), 3.02 (4H, CO2CH2CH2S-SCH2CH2O2C), 4.21, 4.40, 4.81 (9H, ferrocenyl), 4.45 (4H, CO2CH2CH2S-SCH2CH2O2C), 5.59, 6.14 (2H, CH2 = C). 13C
2. Experimental 2.1. Materials Tetrahydrofuran (THF, Aldrich, 99%), dichloromethane (DCM, Aldrich, 99.5%) and toluene (Aldrich, 99%) were dried over CaH2 and distilled from sodium and benzophenone under N2 prior to use. Triethylamine (Et3N, Aldrich, 99.5%) was dried over KOH and distilled over CaH2 under N2 prior to use. Copper(I) bromide (CuBr, Aldrich, 98%) was purified by stirring overnight over CH3COOH at room temperature, followed by washing with ethanol, diethyl ether and acetone prior to drying at 40 °C in vacuo for one day. GO-based macroinitiator containing ATRP initiation group (eCOC(CH3)2Br), TRIS-GO-Ini, is obtained by treating tris(hydroxymethyl) aminomethane (TRIS)-modified GO with 2-bromo-2-methylpropionyl bromide according to our previous reports [38–40]. PEG-Br macroinitiator is prepared according to previous literatures [41]. Ferrocenecarboxylic acid (Alfa Aesar, 98%), oxalyl chloride (J&K, 98%), bis(2-hydroxyethyl) disulfide (Alfa Aesar, 90%), methacryloyl chloride (Aldrich, 90%), pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%) and ethyl α-bromoisobutyrate (EBiB, Aldrich, 98%), were used as received. 2.2. Measurements FT-IR spectra were recorded on a Nicolet AVATAR-360 FT-IR spectrophotometer with a 4 cm−1 resolution. 1H and 13C NMR measurements were performed on a JEOL resonance ECZ 400S (400 MHz) in CDCl3, TMS and CDCl3 was used as internal standards for 1H and 13C NMR, respectively. Electrospray ionization mass spectrometry (ESI-MS) 9
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NMR (101 MHz, CDCl3): δ (ppm): 18.3, 37.3, 37.6, 61.9, 62.5, 69.9, 70.3, 70.7, 71.6, 76.8, 77.2, 77.5, 126.0, 136.0, 167.1, 171.6. ESI-MS m/z: 434.0 (M+). HR-MS (MALDI) m/z: M+, calcd for C19H22O4S254Fe+1 432.0360, Found 432.03502.
30 min in a thermostatic water bath at 25 °C. After two cycles of freezing-pumping-thawing, PMDETA (4.3 μL, 0.02 mmol) and EBiB (3.3 μL, 0.02 mmol) were added and the flask was degassed by three cycles of freezing-pumping-thawing followed by immersing the flask into an oil bath set at 50 °C. The polymerization lasted 2 days and it was terminated by putting the flask into liquid N2. After diluting with toluene, the reaction mixture was filtered through a 0.22 μm filter and washed exhaustively with toluene and deionized water until the filtrate turned clear. The obtained black solid was dried overnight at 45 °C in vacuo so as to afford 55.2 mg of GO-PFcMAss. Meanwhile, the filtrate was passed through a short Al2O3 column to remove the residual copper complex and precipitated into cold n-hexane/ diethyl ether (v/v = 1:1) three times. Subsequently, free PFcMAss homopolymer was obtained by drying in vacuo overnight at 45 °C. GPC: Mn = 13,600 g/mol, Mw/ Mn = 1.48. GO-PFcMAss: XPS (molecular molar ratio): C, 72.71%; N, 1.31%; O, 23.26%, Fe, 1.01%, S, 1.60%. FT-IR: ν (cm−1): 2958, 2852, 1708, 1639, 1593, 1459, 1400, 1271, 1138, 817.
2.4. ATRP of FcMAss CuBr (7.2 mg, 0.05 mmol) and FcMAss (434.8 mg, 1.0 mmol) were first added to a 25 mL Schlenk flask (flame-dried under vacuum prior to use) sealed with a rubber septum. After evacuating and backfilling with Ar, PMDETA (20 μL, 0.10 mmol), EBiB (7.2 μL, 0.05 mmol) and dry toluene (3 mL) were introduced via a gas-tight syringe followed by three cycles of freezing-pumping-thawing. The flask was immersed into an oil bath set at 50 °C. After 48 h, the polymerization was terminated by immersing the flask into liquid N2. The reaction mixture was diluted by THF and filtered through a short Al2O3 column to remove the residue catalyst. THF was then evaporated under reduced pressure and the crude product was purified by dissolving in THF and precipitating in cold n-hexane/diethyl ether (v:v = 1:1) three times followed by drying in vacuo overnight to give 213.7 mg of orange-yellow powder, PFcMAss. GPC: Mn = 6800 g/mol, Mw/Mn = 1.28. FI-IR: ν (cm−1): 2980, 2871, 1718, 1458, 1362, 1228, 1076, 919, 824. 1H NMR (400 MHz, CDCl3): δ (ppm): 0.76–1.26 (12H, CH3C, CO2CH2CH3 and (CH3)2C), 1.75–2.11 (2H, CH2C), 2.85–3.12 (4H, CO2CH2CH2SSCH2CH2O2C), 3.75 (2H, CO2CH2CH3), 4.22, 4.42, 4.82 (9H, ferrocenyl), 4.48 (4H, CO2CH2CH2S-SCH2CH2O2C).
3. Results and discussion 3.1. Synthesis and ATRP of FcMAss monomer In the present work, a new methacrylate monomer of FcMAss, in which the ferrocene unit was connected to methacrylate segment by disulfide bond, was designed so as to construct GO/Fc-based nanocomposite via the grafting-from strategy by living/controlled radical polymerization. Herein, disulfide bond was introduced into FcMAss monomer so that the obtained GO/Fc-based nanocomposite can be used to deposit Au nanoparticles onto GO surface in the following work, which is inspired by Tong’s work that dialkyl disulfide can serve as anchoring group to stabilized AuNPs, similar to thiol [42]. FcMAss was synthesized by three steps starting from ferrocenecarboxylic acid (Scheme 1). Firstly, ferrocenecarboxylic acid was treated with oxalyl chloride to give ferrocenoyl chloride followed by reacting with bis(2hydroxyethyl) disulfide to provid the mono-esterification product. Finally, the remaining hydroxyl in the mono-esterification was esterified by methacryloyl chloride in the presence of Et3N to afford the target FcMAss methacrylate monomer. The chemical structure of FcMAss monomer was characterized by 1 H NMR, and 13C NMR, FT-IR and MS. The proton resonance signals of the double bond were found to be located at 6.14 and 5.59 ppm in 1H NMR spectrum as shown in Fig. 1A. Three peaks at 4.81, 4.40 and 4.21 ppm belonged to 9 protons of ferrocene unit. The characteristic signal of methylene unit linked to disulfide bond (4 protons) appeared at 3.02 ppm, while the signal of methylene unit connected to the ester group (4 protons) was located at 4.45 ppm. The existence of double bond was also verified by the characteristic peaks at 126.0 and 136.0 ppm in 13C NMR spectrum (Fig. 1B), while the peaks at 69.9, 70.3, 70.7 and 71.6 ppm corresponded to ferrocene unit. In FT-IR spectrum (blue line in Fig. 2), typical signals originating from ferrocene unit were found to appear at 2965, 1458 and 822 cm−1, also witnessing the presence of ferrocene unit. Besides, an even pseudmolecular ion peak at m/z 434.0 [M+] was found in ESI/MS spectrum and HRMALDI/MS data (m/z 432.03502 [M+]) gave rise to the molecular formula of C19H22O4S254Fe+1. All these results clearly confirmed the successfully synthesis of FcMAss monomer consisting of disulfide bond and ferrocene moiety.
2.5. ATRP block copolymerization of FcMAss As shown in Scheme 2, FcMAss (435.3 mg, 1.0 mmol), PEG-Br (Mn,NMR = 2135 g/mol, 40.0 mg, 0.020 mmol ATRP initiating group) and CuBr (2.8 mg, 0.02 mmol) were first added to a 25 mL Schlenk flask (flame-dried under vacuum prior to use) sealed with a rubber septum for degassing and kept under N2. PMDETA (6 μL, 0.06 mmol) and dry THF (1.0 mL) were introduced via a gastight syringe. The polymerization system was degassed by three cycles of freezing-pumping-thawing followed by immersing the flask into an oil bath set at 50 °C. After 24 h, the polymerization was terminated by immersing the flask into liquid N2. The reaction mixture was diluted by THF and the residual copper complex was removed by a short Al2O3 column. THF was then evaporated under reduced pressure and the crude product was purified by dissolving in THF and precipitating in cold n-hexane/diethyl ether (v:v = 1:1) three times followed by drying in vacuo overnight to give 143 mg of colorless oil, PEG-b-PFcMAss. GPC: Mn = 10,800 g/mol, Mw/ Mn = 1.34. 1H NMR (400 MHz, CDCl3): δ (ppm): 0.80–1.30 (12H, CH3C, CO2CH2CH3 and (CH3)2C), 1.75–2.11 (2H, CH2C), 2.85–3.12 (4H, CO2CH2CH2S-SCH2CH2O2C), 3.65 (4H, OCH2CH2 of PEG segment), 4.22 (5H, ferrocenyl), 4.42 (2H, ferrocenyl, 4H, CO2CH2CH2SSCH2CH2O2C and 2H, CO2CH2CH2O), 4.82 (2H, ferrocenyl). 2.6. Preparation of PFcMAss-grafted graphene sheets (GO-PFcMAss) CuBr (3.0 mg, 0.02 mmol), TRIS-GO-Ini (65.8 mg) and FcMAss (625.2 mg, 1 mmol) were added to a 25 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, 5 mL of dried toluene was charged via a gastight syringe and the mixture was sonicated for
Scheme 2. Preparation of PEG-b-PFcMAss diblock copolymer. 10
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Table 1 ATRP Homopolymerization of FcMAss.a Sample
[FcMAss]: [EBiB]
Conv.b (%)
Mn,GPCc (g/ mol)
Mw/Mnc
Mn,NMRb (g/ mol)
PFcMAss-a PFcMAss-b PFcMAss-c
20:1 50:1 100:1
74% 61% 51%
6800 12,500 20,600
1.26 1.24 1.28
6400 11,600 21,700
a Polymerization temperature: 50 °C, polymerization time: 24 h, [EBiB]: [CuBr]: [PMDETA] = 1:1:2. b Obtained from 1H NMR. c Measured by GPC in THF at 35 °C.
Fig. 1. 1H (A) and
13
C (B) NMR spectra of FcMAss monomer in CDCl3.
Fig. 3. GPC curves of PFcMAss homopolymers in THF.
Fig. 2) exhibited three absorptions at 2980, 1458 and 824 cm−1, indicative of the presence of ferrocene unit; meanwhile, the stretching vibration of carbonyl appeared at 1718 cm−1. 1H NMR spectrum after polymerization (Fig. 4) showed the disappearance of proton resonance signal of double bond and the characteristic peaks concerned with polymethacrylate backbone appeared at 0.76–1.26 and 1.75–2.11 ppm. The proton resonance signals of ferrocene unit still appeared at 4.22 (‘i’), 4.42 (‘h’) and 4.82 (‘g’) ppm, while that of two methylene groups connected to disulfide bond appeared at 2.85–3.12 ppm, which demonstrated that disulfide bond and ferrocene group are stable during the ATRP polymerization process. In addition, a minor peak at 3.75 ppm (‘l’) which is assigned to two protons of methylene group in the initiating group (eCO2CH2CH3) was found, which illustrated that PFcMAss homopolymer was indeed synthesized via ATRP of FcMAss monomer initiated by EBiB. Therefore, ‘real’ molecular weight of PFcMAss (Mn,NMR) can also be evaluated according to Eqs. (1) and (2)
Fig. 2. FT-IR spectra of FcMAss monomer and PFcMAss homopolymer.
ATRP is considered as the most versatile process among different reversible- deactivation radical polymerization (RDRP) techniques because of its mild polymerization conditions and good tolerance of monomer functionalities so that it has been utilized in preparation of well-defined polymers with controlled topologies including linear, graft, star, cyclic, network and (hyper)branched [43–54]. In the current case, we conceived the construction of GO/Fc-based nanocomposite via the grafting-from strategy by RDRP. Therefore, we need to make sure whether FcMAss is suitable for ATRP. Solution ATRP of FcMAss was then conducted in toluene at 50 ℃ using EBiB as initiator and CuBr/PMDETA as catalytic system to provide the corresponding PFcMAss homopolymer. A series of well-defined PFcMAss homopolymers was obtained by varying the feeding ratio of FcMAss monomer to EBiB as summarized in Table 1. GPC curves (Fig. 3) of all three homopolymers showed unimodal and symmetric elution peaks with narrow molecular weight distributions (Mw/Mn < 1.30), and their molecular weights increased linearly with the ascending of the feeding ratio of FcMAss monomer to the initiator of EBiB, which is the characteristic of RDRP [55]. The structure of PFcMAss homopolymer was examined by FT-IR and 1 H NMR. FT-IR spectrum of PFcMAss homopolymer (magenta line in
Fig. 4. 1H NMR spectrum of PFcMAss homopolymer in CDCl3. 11
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Fig. 5. GPC curves of PEG-b-PFcMAss diblock copolymer and PEG-Br in THF.
(NFcMAss is the number of FcMAss repeated unit per PFcMAss chain; Sg and Sl are the integration area of peak ‘g’ at 4.82 ppm and peak ‘l’ at 3.75 ppm in Fig. 4; 434 and 195 are the molar weights of FcMAss monomer and EBiB initiator, respectively). The values are listed in Table 1 and these values are comparable with those determined by conventional GPC. Thus, it is clear that FcMAss monomer is suitable for ATRP so that it can be employed in the grafting-from strategy for the construction of GO/Fc-based nanocomposite.
NFcMAss = Sg/ Sl
(1)
Mn,NMR = 195 + 434NFcMAss
(2)
Fig. 6. 1H NMR spectrum of PEG-b-PFcMAss diblock copolymer in CDCl3.
Moreover, we selected PEG as the hydrophilic segment to construct the diblock copolymer bearing PFcMAss segment. The PEG-based macroinitiator (PEG-Br) was first prepared via the esterification between 2-bromopropionyl chloride and PEG-OH (Mn = 2000 g/mol) following a commonly used procedure [41]. PEG-b-PFcMAss diblock copolymer was then obtained by ATRP of FcMAss initiated by PEG-Br at 50 °C in THF with CuBr/PMDETA as catalytic system. GPC curve of the resulting PEG-b-PFcMAss copolymer (Fig. 5) shows only a unimodal and symmetrical eluent peak with a relatively narrow molecular weight distribution (Mw/Mn = 1.34), which proved the “living” nature of ATRP of FcMAss. The relative molar weight of PEG-b-PFcMAss diblock copolymer (Mn,GPC = 10,800 g/mol) is much higher than that of PEGBr, indicative of the extension of PFcMAss chain from PEG-Br. The chemical structure of PEG-b-PFcMAss diblock copolymer was confirmed by 1H NMR. Typical proton resonance signal originating from PEG block appeared at 3.65 ppm (‘a’) in 1H NMR spectrum after ATRP of FcMAss initiated by PEG-Br (Fig. 6). The resonance signals located at 4.22 (‘k’), 4.42 (‘j’) and 4.82 (‘i’) ppm corresponded to 9 protons of ferrocene moiety in FcMAss repeated unit, whereas the peaks ‘e’, ‘f’, ‘g’ and ‘h’ are attributed to methylene groups. On the basis of integrations of peak ‘a’ and ‘f’, ‘g’, ‘real’ molecular weight of PEG-bPFcMAss diblock copolymer (Mn,NMR) was estimated to be 11,900 g/ mol by Eq. (3) (Sa and Sf,g are the integration area of peak ‘a’ at 3.65 ppm and peak ‘f’ and ‘g’ at 2.85–3.12 ppm in Fig. 6, 434 and Mn,PEG-Br (2135 g/mol) are the molecular weights of FcMAss monomer and PEG-Br macroinitiator, respectively), which is similar to the value acquired from conventional GPC (Mn,GPC = 10,800 g/mol).
Mn,NMR = Mn,PEG
Br
+ 434*(43.5Sf,g/ Sa )
Fig. 7. FT-IR spectra of GO, PFcMAss and GO-PFcMAss.
(3)
3.2. Preparation and characterization of GO-PFcMAss nanocomposite In our previous work [38,39], GO could be modified by tris(hydroxymethyl) aminomethane (TRIS) to increase the amount of reactive sites on the surface of GO and subsequent introduction of ATRP initiating group (COC(CH3)2Br) could give a GO-base macroinitiator, TRIS-GO-Ini, which can initiate RDRP of functional monomer to prepare versatile polymer-functionalized graphene-based materials. Herein, surface-initiated ATRP of FcMAss was conducted at 50 °C in toluene with TRIS-GO-Ini as initiator and CuBr/PMDETA as catalytic system. EBiB was also added as a sacrificial initiator for subsequent
Fig. 8. XPS spectra of TRIS-GO-Ini and GO-PFcMAss and digital photo (inset) of GO-PFcMAss in organic solvents and water.
determination of molecular weight of PFcMAss chain grafted from GO since that it has been reported that free polymers formed in the solution have the same molecular weight as those formed on the surface assuming fast exchange between the two populations of polymers in the same system [56–58]. A unimodal and symmetrical elution peak in GPC 12
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Planning (SHXH201706).
Table 2 Elemental analysis of TRIS-GO-Ini and GO-PFcMAss.a Sample
C% (mol %)
N% (mol %)
O% (mol %)
Fe% (mol %)
S% (mol %)
TRIS-GO-Ini GO-PFcMAss
79.51 72.71
1.83 1.31
17.48 23.26
– 1.01
– 1.60
a
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Obtained from XPS.
curve of free PFcMAss polymer gave the molecular weight of ~13,600 g/mol, thus the molecular weight of PFcMAss chains grafted from GO was also ~13,600 g/mol. The polydispersity (Mw/Mn = 1.48) increased compared to that obtained by solution ATRP in toluene ((Mw/ Mn < 1.30), suggesting the presence of termination processes because of the steric effect of GO surface. The resulting PFcMAss-functionalized GO, GO-PFcMAss, was characterized by FT-IR spectroscopy as shown in Fig. 7. In comparison with FTIR spectrum of GO (olive line in Fig. 7), it can be seen in FT-IR spectrum after surface-initiated ATRP (blue line in Fig. 7) that typical peaks of ferrocene group appeared at 2958, 1459 and 817 cm−1 and a new peak located at 2852 cm−1 was attributed to the stretching vibration of methylene group, similar with that of PFcMAss homopolymer (red line in Fig. 7). In addition, the intensity of the characteristic peak of carbonyl at 1708 cm−1 increased obviously, also verifying the introduction of PFcMAss polymeric chains. The detailed compositional analysis of GOPFcMAss nanocomposite was performed by XPS (Fig. 8). Compared to XPS spectrum of TRIS-GO-Ini (black line in Fig. 8), the Fe2p and Fe3p3 peaks at a binding energy value of 709 and 71 eV were observed in XPS spectrum after surface-initiated ATRP (blue line in Fig. 8), suggesting the presence of ferrocene unit on the surface of GO, while the S2s and S2p3 peaks appeared at 229 and 165 eV, which were originated from the disulfide bond in PFcMAss polymeric chain. Furthermore, the element contents of TRISGO-Ini and GO-PFcMAss obtained from XPS results are outlined in Table 2, indicating the presence of C, N, O, Fe and S elements in GO-PFcMAss. From these values of element content, the weight content of PFcMAss in GO-PFcMAss nanocomposite can be evaluated to be ~44.2 wt%. From all the aforementioned evidences, it can be concluded that PFcMAss polymeric chains were successfully grafted on the surface of GO via SI-ATRP, affording GO-PFcMAss nanocomposite. The as-prepared GO-PFcMAss nanocomposite show good dispersible properties in organic solvents such as acetone, chloroform, toluene and DMF (inset of Fig. 8). However, this nanocomposite cannot well disperse in water and methanol which are poor solvents for PFcMAss polymer. This phenomenon also further indicated that GO is indeed covered with PFcMAss polymeric chains. 4. Conclusion In summary, we synthesized a new methacrylate monomer consisting of ferrocene unit and disulfide bond, FcMAss, which is suitable for ATRP to provide well-defined PFcMAss homopolymer with narrow molecular weight distribution. Furthermore, PEG-b-PFcMAss diblock copolymer was prepared by ATRP of FcMAss initiated by PEG-Br macroinitiator. Surface-initiated ATRP of FcMAss initiated by GO-based macroinitiator provided PFcMAss-functionalized GO, GO-PFcMAss. This nanocomposite shows excellent dispersibility in organic solvents. Acknowledgement The authors thank the financial support from Shanghai Scientific and Technological Innovation Project (17ZR1426700 and 18ZR1448600), Scientific Research Project of Science and Technology Commission of Xuhui Municipality (SHXH201613) and Scientific Research Project of Xuhui Provincial Commission of Health and Family
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
A new ferrocene/disulfide-containing methacrylate monomer: Synthesis, ATRP and nanocomposite Wenhao Qiana, , Haiyan Zhanga, Tao Songa, Mao Yea, Chun Fengb, Guolin Lub, Xiaoyu Huangb, ⁎
T ⁎
a
Department of Stomatology, Shanghai Xuhui District Dental Center, 685 Zhaojiabang Road, Shanghai 200032, People’s Republic of China Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China
b
ARTICLE INFO
ABSTRACT
Keywords: Ferrocene Disulfide bond Methacrylate ATRP Grafting-from
A new methacrylate monomer containing ferrocene unit and disulfide bond, 2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate (FcMAss), was prepared by subsequent esterification reaction with bis(2hydroxyethyl) disulfide and methacryloyl chloride using ferrocenecarboxylic acid as starting material. A series of well-defined poly(2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate) (PFcMAss) homopolymers with narrow molecular weight distributions (Mw/Mn < 1.30) was then obtained by atom transfer radical polymerization (ATRP) of FcMAss monomer in toluene. Block copolymerization of FcMAss monomer was initiated by PEG-Br macroinitiator in THF to afford a poly(ethylene glycol)-b-poly(2-((2-(methacryloyloxy)ethyl) disulfanyl)ethyl ferrocenecarboxylate) (PEG-b-PFcMAss) diblock copolymer with a relatively low dispersity (Mw/Mn = 1.34). Graphene oxide (GO)-based nanocomposite, GO-PFcMAss, was prepared by surface-initiated ATRP of FcMAss via the grafting-from strategy, using GO-based macroinitiator obtained by treating tris(hydroxymethyl) aminomethane (TRIS)-modified GO with 2-bromo-2-methylpropionyl bromide. The nanocomposite was characterized by FT-IR and XPS, and showed excellent dispersibility and colloidal stability.
1. Introduction Graphene, a novel two-dimensional graphitic carbon system, has promptly inspired numerous interests across many fields, such as biosensors, electrochemical energy storage and electronics because of its specific properties of large surface area, good mechanical properties and extremely high electronic conductivity properties [1–6]. Graphene oxide (GO), the main derivative of graphene, affords potential for largescale manufacture of graphene and graphene-based materials. GO, prepared through cost-effective chemical approaches by using cheap graphite as raw material [7–10], not only can fabricate low-cost and large-scale graphene by chemical reduction, but also provides a robust platform for preparing functionalized graphene hybrid materials through the incorporation with various functional materials including inorganic nanostructures, polymers, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), biomaterials and carbon nanotubes (CNTs) due to the presence of oxygen-containing active groups of carboxyl, hydroxyl and epoxide groups, in its edge and surface [11–21]. Furthermore, the biocompatibility of GO renders it a good candidate for application in the biomedical field [12,19,22–24]. Ferrocene (Fc) is an interesting metal-containing compound
⁎
possessing unique redox, preceramic, etch-resistant and catalytic properties [25–27] so that it and corresponding derivatives are often employed as attractive building blocks for the preparation of functional materials. For instance, Fc has been extensively utilized to produce electrochemical biosensors because of its reversibility, regeneration at low potential and generation of stable redox states [25–29]. Nevertheless, it is not able to form stable adsorption layer on electrodes, therefore, an effective solution method is to chemically couple special ferrocene derivatives with carbon nanomaterials. Among carbon nanomaterials, GO is an excellent candidate since that the functional groups on its surface make it suitable for the construction of nano-hybrid materials. Although many GO/Fc-based nanomaterials have been prepared for the use in biosensors, supercapacitor electrodes, etc. [30–35]. However, only few reports were concerned with the chemical immobilization of ferrocene-containing polymeric chains on the surface of graphene oxide [36,37]. Moreover, there is no report on the preparation of GO/Fc-based nanocomposite by convenient and highly efficient atom transfer radical polymerization (ATRP) of ferrocene-containing methacrylate monomer via the common grafting-from strategy. Herein, we designed a new methacrylate monomer containing ferrocene unit and disulfide bond, 2-((2-(methacryloyloxy)ethyl)disulfanyl)
Corresponding authors. E-mail addresses: [email protected] (W. Qian), [email protected] (X. Huang).
https://doi.org/10.1016/j.eurpolymj.2019.07.009 Received 29 May 2019; Received in revised form 2 July 2019; Accepted 4 July 2019 Available online 04 July 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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Scheme 1. Preparation of FcMAss monomer and GO-PFcMAss nanocomposite.
ethyl ferrocenecarboxylate (FcMAss). Ferrocenecarboxylic acid was selected as starting material for the successive esterification reaction with bis (2-hydroxyethyl) disulfide and methacryloyl chloride to afford the target methacrylate monomer as shown in Scheme 1. Solution ATRP of FcMAss monomer was performed in toluene using ethyl α-bromoisobutyrate as initiator and CuBr/pentamethyldiethylenetriamine (PMDETA) as catalytic system, providing well-defined poly(2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocene- carboxylate) (PFcMAss) homopolymers. Subsequently, a poly(ethylene glycol)-b-poly(2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate) (PEG-b-PFcMAss) diblock copolymer was obtained through solution ATRP of FcMAss monomer initiated by a PEG-based macroinitiator. Finally, surface-initiated ATRP (SI-ATRP) of FcMAss monomer was initiated by a GO-based macroinitiator [38,39] to afford PFcMAss-functionalized GO sheets (GO-PFcMAss), showing improved dispersibility in various solvents.
was measured by an Agilent FTMS-7.0 Fourier transformation mass spectrometer. Relative molecular weights and molecular weight distributions were measured by conventional 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, particle size: 5 μm). GPC measurements were carried out at 35 °C using THF as eluent with a flow rate of 1.0 mL/min. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI 5000c ESCA photoelectron spectrometer. 2.3. Synthesis of 2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocenecarboxylate (FcMAss) Ferrocenecarboxylic acid (13.1525 g, 0.057 mol) and dry DCM (200 mL) were added into a 500 mL round bottom flask. This mixture was cooled to −5 °C in an ice bath and a solution of oxalyl chloride (32.9073 g, 0.26 mol) in DCM (20 mL) was added dropwise. The mixture was naturally warmed to room temperature and then stirred overnight at room temperature. The mixture was evaporated under reduced pressure to remove DCM and unreacted oxalyl chloride. The residue was treated with dry hexane at 50 °C and then filtrated. The filtrate was concentrated to get 12.3328 g (87%) of orange red solid, ferrocenoyl chloride (FcOCl). FcOCl (12.3328 g, 0.05 mol) was added in dry DCM (170 mL) and cooled to −5 °C in an ice bath. A DCM (70 mL) solution containing bis(2hydroxyethyl) disulfide (8.6374 g, 0.056 mol) and Et3N (7.8 mL, 0.056 mol) was added dropwise to the above mixture. After naturally warmed to room temperature and stirred overnight, the mixture was washed in succession with saturated sodium bicarbonate solution, water and saturated NaCl solution. The organic layer is dried over anhydrous magnesium sulfate and the solvent is removed by distillation. The resultant was directly dissolved in dry DCM (100 mL) and then Et3N (7.8 mL, 0.056 mol) was added. Subsequently, a solution of methacryloyl chloride (10.8 mL, 0.103 mol) in 20 mL of dry DCM was dropwise added to the above mixture at −5 °C. The resulting mixture was naturally warmed to room temperature and stirred overnight. The mixture was diluted with DCM and washed with saturated NaCl solution three times. The organic phase was dried over anhydrous magnesium sulfate and concentrated. The residue was purified by column chromatography (nhexane/ethyl acetate (v:v) = 10:1) to give 2-((2-(methacryloyloxy)ethyl) disulfanyl)ethyl ferrocenecarboxylate (FcMAss) as a wine red oil (12.5302 g, 58%). FI-IR (film): ν (cm−1): 2965, 2930, 2860, 1715, 1458, 1264, 1131, 822. 1H NMR (400 MHz, CDCl3): δ (ppm): 1.95 (3H, CH3C), 3.02 (4H, CO2CH2CH2S-SCH2CH2O2C), 4.21, 4.40, 4.81 (9H, ferrocenyl), 4.45 (4H, CO2CH2CH2S-SCH2CH2O2C), 5.59, 6.14 (2H, CH2 = C). 13C
2. Experimental 2.1. Materials Tetrahydrofuran (THF, Aldrich, 99%), dichloromethane (DCM, Aldrich, 99.5%) and toluene (Aldrich, 99%) were dried over CaH2 and distilled from sodium and benzophenone under N2 prior to use. Triethylamine (Et3N, Aldrich, 99.5%) was dried over KOH and distilled over CaH2 under N2 prior to use. Copper(I) bromide (CuBr, Aldrich, 98%) was purified by stirring overnight over CH3COOH at room temperature, followed by washing with ethanol, diethyl ether and acetone prior to drying at 40 °C in vacuo for one day. GO-based macroinitiator containing ATRP initiation group (eCOC(CH3)2Br), TRIS-GO-Ini, is obtained by treating tris(hydroxymethyl) aminomethane (TRIS)-modified GO with 2-bromo-2-methylpropionyl bromide according to our previous reports [38–40]. PEG-Br macroinitiator is prepared according to previous literatures [41]. Ferrocenecarboxylic acid (Alfa Aesar, 98%), oxalyl chloride (J&K, 98%), bis(2-hydroxyethyl) disulfide (Alfa Aesar, 90%), methacryloyl chloride (Aldrich, 90%), pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%) and ethyl α-bromoisobutyrate (EBiB, Aldrich, 98%), were used as received. 2.2. Measurements FT-IR spectra were recorded on a Nicolet AVATAR-360 FT-IR spectrophotometer with a 4 cm−1 resolution. 1H and 13C NMR measurements were performed on a JEOL resonance ECZ 400S (400 MHz) in CDCl3, TMS and CDCl3 was used as internal standards for 1H and 13C NMR, respectively. Electrospray ionization mass spectrometry (ESI-MS) 9
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NMR (101 MHz, CDCl3): δ (ppm): 18.3, 37.3, 37.6, 61.9, 62.5, 69.9, 70.3, 70.7, 71.6, 76.8, 77.2, 77.5, 126.0, 136.0, 167.1, 171.6. ESI-MS m/z: 434.0 (M+). HR-MS (MALDI) m/z: M+, calcd for C19H22O4S254Fe+1 432.0360, Found 432.03502.
30 min in a thermostatic water bath at 25 °C. After two cycles of freezing-pumping-thawing, PMDETA (4.3 μL, 0.02 mmol) and EBiB (3.3 μL, 0.02 mmol) were added and the flask was degassed by three cycles of freezing-pumping-thawing followed by immersing the flask into an oil bath set at 50 °C. The polymerization lasted 2 days and it was terminated by putting the flask into liquid N2. After diluting with toluene, the reaction mixture was filtered through a 0.22 μm filter and washed exhaustively with toluene and deionized water until the filtrate turned clear. The obtained black solid was dried overnight at 45 °C in vacuo so as to afford 55.2 mg of GO-PFcMAss. Meanwhile, the filtrate was passed through a short Al2O3 column to remove the residual copper complex and precipitated into cold n-hexane/ diethyl ether (v/v = 1:1) three times. Subsequently, free PFcMAss homopolymer was obtained by drying in vacuo overnight at 45 °C. GPC: Mn = 13,600 g/mol, Mw/ Mn = 1.48. GO-PFcMAss: XPS (molecular molar ratio): C, 72.71%; N, 1.31%; O, 23.26%, Fe, 1.01%, S, 1.60%. FT-IR: ν (cm−1): 2958, 2852, 1708, 1639, 1593, 1459, 1400, 1271, 1138, 817.
2.4. ATRP of FcMAss CuBr (7.2 mg, 0.05 mmol) and FcMAss (434.8 mg, 1.0 mmol) were first added to a 25 mL Schlenk flask (flame-dried under vacuum prior to use) sealed with a rubber septum. After evacuating and backfilling with Ar, PMDETA (20 μL, 0.10 mmol), EBiB (7.2 μL, 0.05 mmol) and dry toluene (3 mL) were introduced via a gas-tight syringe followed by three cycles of freezing-pumping-thawing. The flask was immersed into an oil bath set at 50 °C. After 48 h, the polymerization was terminated by immersing the flask into liquid N2. The reaction mixture was diluted by THF and filtered through a short Al2O3 column to remove the residue catalyst. THF was then evaporated under reduced pressure and the crude product was purified by dissolving in THF and precipitating in cold n-hexane/diethyl ether (v:v = 1:1) three times followed by drying in vacuo overnight to give 213.7 mg of orange-yellow powder, PFcMAss. GPC: Mn = 6800 g/mol, Mw/Mn = 1.28. FI-IR: ν (cm−1): 2980, 2871, 1718, 1458, 1362, 1228, 1076, 919, 824. 1H NMR (400 MHz, CDCl3): δ (ppm): 0.76–1.26 (12H, CH3C, CO2CH2CH3 and (CH3)2C), 1.75–2.11 (2H, CH2C), 2.85–3.12 (4H, CO2CH2CH2SSCH2CH2O2C), 3.75 (2H, CO2CH2CH3), 4.22, 4.42, 4.82 (9H, ferrocenyl), 4.48 (4H, CO2CH2CH2S-SCH2CH2O2C).
3. Results and discussion 3.1. Synthesis and ATRP of FcMAss monomer In the present work, a new methacrylate monomer of FcMAss, in which the ferrocene unit was connected to methacrylate segment by disulfide bond, was designed so as to construct GO/Fc-based nanocomposite via the grafting-from strategy by living/controlled radical polymerization. Herein, disulfide bond was introduced into FcMAss monomer so that the obtained GO/Fc-based nanocomposite can be used to deposit Au nanoparticles onto GO surface in the following work, which is inspired by Tong’s work that dialkyl disulfide can serve as anchoring group to stabilized AuNPs, similar to thiol [42]. FcMAss was synthesized by three steps starting from ferrocenecarboxylic acid (Scheme 1). Firstly, ferrocenecarboxylic acid was treated with oxalyl chloride to give ferrocenoyl chloride followed by reacting with bis(2hydroxyethyl) disulfide to provid the mono-esterification product. Finally, the remaining hydroxyl in the mono-esterification was esterified by methacryloyl chloride in the presence of Et3N to afford the target FcMAss methacrylate monomer. The chemical structure of FcMAss monomer was characterized by 1 H NMR, and 13C NMR, FT-IR and MS. The proton resonance signals of the double bond were found to be located at 6.14 and 5.59 ppm in 1H NMR spectrum as shown in Fig. 1A. Three peaks at 4.81, 4.40 and 4.21 ppm belonged to 9 protons of ferrocene unit. The characteristic signal of methylene unit linked to disulfide bond (4 protons) appeared at 3.02 ppm, while the signal of methylene unit connected to the ester group (4 protons) was located at 4.45 ppm. The existence of double bond was also verified by the characteristic peaks at 126.0 and 136.0 ppm in 13C NMR spectrum (Fig. 1B), while the peaks at 69.9, 70.3, 70.7 and 71.6 ppm corresponded to ferrocene unit. In FT-IR spectrum (blue line in Fig. 2), typical signals originating from ferrocene unit were found to appear at 2965, 1458 and 822 cm−1, also witnessing the presence of ferrocene unit. Besides, an even pseudmolecular ion peak at m/z 434.0 [M+] was found in ESI/MS spectrum and HRMALDI/MS data (m/z 432.03502 [M+]) gave rise to the molecular formula of C19H22O4S254Fe+1. All these results clearly confirmed the successfully synthesis of FcMAss monomer consisting of disulfide bond and ferrocene moiety.
2.5. ATRP block copolymerization of FcMAss As shown in Scheme 2, FcMAss (435.3 mg, 1.0 mmol), PEG-Br (Mn,NMR = 2135 g/mol, 40.0 mg, 0.020 mmol ATRP initiating group) and CuBr (2.8 mg, 0.02 mmol) were first added to a 25 mL Schlenk flask (flame-dried under vacuum prior to use) sealed with a rubber septum for degassing and kept under N2. PMDETA (6 μL, 0.06 mmol) and dry THF (1.0 mL) were introduced via a gastight syringe. The polymerization system was degassed by three cycles of freezing-pumping-thawing followed by immersing the flask into an oil bath set at 50 °C. After 24 h, the polymerization was terminated by immersing the flask into liquid N2. The reaction mixture was diluted by THF and the residual copper complex was removed by a short Al2O3 column. THF was then evaporated under reduced pressure and the crude product was purified by dissolving in THF and precipitating in cold n-hexane/diethyl ether (v:v = 1:1) three times followed by drying in vacuo overnight to give 143 mg of colorless oil, PEG-b-PFcMAss. GPC: Mn = 10,800 g/mol, Mw/ Mn = 1.34. 1H NMR (400 MHz, CDCl3): δ (ppm): 0.80–1.30 (12H, CH3C, CO2CH2CH3 and (CH3)2C), 1.75–2.11 (2H, CH2C), 2.85–3.12 (4H, CO2CH2CH2S-SCH2CH2O2C), 3.65 (4H, OCH2CH2 of PEG segment), 4.22 (5H, ferrocenyl), 4.42 (2H, ferrocenyl, 4H, CO2CH2CH2SSCH2CH2O2C and 2H, CO2CH2CH2O), 4.82 (2H, ferrocenyl). 2.6. Preparation of PFcMAss-grafted graphene sheets (GO-PFcMAss) CuBr (3.0 mg, 0.02 mmol), TRIS-GO-Ini (65.8 mg) and FcMAss (625.2 mg, 1 mmol) were added to a 25 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, 5 mL of dried toluene was charged via a gastight syringe and the mixture was sonicated for
Scheme 2. Preparation of PEG-b-PFcMAss diblock copolymer. 10
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Table 1 ATRP Homopolymerization of FcMAss.a Sample
[FcMAss]: [EBiB]
Conv.b (%)
Mn,GPCc (g/ mol)
Mw/Mnc
Mn,NMRb (g/ mol)
PFcMAss-a PFcMAss-b PFcMAss-c
20:1 50:1 100:1
74% 61% 51%
6800 12,500 20,600
1.26 1.24 1.28
6400 11,600 21,700
a Polymerization temperature: 50 °C, polymerization time: 24 h, [EBiB]: [CuBr]: [PMDETA] = 1:1:2. b Obtained from 1H NMR. c Measured by GPC in THF at 35 °C.
Fig. 1. 1H (A) and
13
C (B) NMR spectra of FcMAss monomer in CDCl3.
Fig. 3. GPC curves of PFcMAss homopolymers in THF.
Fig. 2) exhibited three absorptions at 2980, 1458 and 824 cm−1, indicative of the presence of ferrocene unit; meanwhile, the stretching vibration of carbonyl appeared at 1718 cm−1. 1H NMR spectrum after polymerization (Fig. 4) showed the disappearance of proton resonance signal of double bond and the characteristic peaks concerned with polymethacrylate backbone appeared at 0.76–1.26 and 1.75–2.11 ppm. The proton resonance signals of ferrocene unit still appeared at 4.22 (‘i’), 4.42 (‘h’) and 4.82 (‘g’) ppm, while that of two methylene groups connected to disulfide bond appeared at 2.85–3.12 ppm, which demonstrated that disulfide bond and ferrocene group are stable during the ATRP polymerization process. In addition, a minor peak at 3.75 ppm (‘l’) which is assigned to two protons of methylene group in the initiating group (eCO2CH2CH3) was found, which illustrated that PFcMAss homopolymer was indeed synthesized via ATRP of FcMAss monomer initiated by EBiB. Therefore, ‘real’ molecular weight of PFcMAss (Mn,NMR) can also be evaluated according to Eqs. (1) and (2)
Fig. 2. FT-IR spectra of FcMAss monomer and PFcMAss homopolymer.
ATRP is considered as the most versatile process among different reversible- deactivation radical polymerization (RDRP) techniques because of its mild polymerization conditions and good tolerance of monomer functionalities so that it has been utilized in preparation of well-defined polymers with controlled topologies including linear, graft, star, cyclic, network and (hyper)branched [43–54]. In the current case, we conceived the construction of GO/Fc-based nanocomposite via the grafting-from strategy by RDRP. Therefore, we need to make sure whether FcMAss is suitable for ATRP. Solution ATRP of FcMAss was then conducted in toluene at 50 ℃ using EBiB as initiator and CuBr/PMDETA as catalytic system to provide the corresponding PFcMAss homopolymer. A series of well-defined PFcMAss homopolymers was obtained by varying the feeding ratio of FcMAss monomer to EBiB as summarized in Table 1. GPC curves (Fig. 3) of all three homopolymers showed unimodal and symmetric elution peaks with narrow molecular weight distributions (Mw/Mn < 1.30), and their molecular weights increased linearly with the ascending of the feeding ratio of FcMAss monomer to the initiator of EBiB, which is the characteristic of RDRP [55]. The structure of PFcMAss homopolymer was examined by FT-IR and 1 H NMR. FT-IR spectrum of PFcMAss homopolymer (magenta line in
Fig. 4. 1H NMR spectrum of PFcMAss homopolymer in CDCl3. 11
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Fig. 5. GPC curves of PEG-b-PFcMAss diblock copolymer and PEG-Br in THF.
(NFcMAss is the number of FcMAss repeated unit per PFcMAss chain; Sg and Sl are the integration area of peak ‘g’ at 4.82 ppm and peak ‘l’ at 3.75 ppm in Fig. 4; 434 and 195 are the molar weights of FcMAss monomer and EBiB initiator, respectively). The values are listed in Table 1 and these values are comparable with those determined by conventional GPC. Thus, it is clear that FcMAss monomer is suitable for ATRP so that it can be employed in the grafting-from strategy for the construction of GO/Fc-based nanocomposite.
NFcMAss = Sg/ Sl
(1)
Mn,NMR = 195 + 434NFcMAss
(2)
Fig. 6. 1H NMR spectrum of PEG-b-PFcMAss diblock copolymer in CDCl3.
Moreover, we selected PEG as the hydrophilic segment to construct the diblock copolymer bearing PFcMAss segment. The PEG-based macroinitiator (PEG-Br) was first prepared via the esterification between 2-bromopropionyl chloride and PEG-OH (Mn = 2000 g/mol) following a commonly used procedure [41]. PEG-b-PFcMAss diblock copolymer was then obtained by ATRP of FcMAss initiated by PEG-Br at 50 °C in THF with CuBr/PMDETA as catalytic system. GPC curve of the resulting PEG-b-PFcMAss copolymer (Fig. 5) shows only a unimodal and symmetrical eluent peak with a relatively narrow molecular weight distribution (Mw/Mn = 1.34), which proved the “living” nature of ATRP of FcMAss. The relative molar weight of PEG-b-PFcMAss diblock copolymer (Mn,GPC = 10,800 g/mol) is much higher than that of PEGBr, indicative of the extension of PFcMAss chain from PEG-Br. The chemical structure of PEG-b-PFcMAss diblock copolymer was confirmed by 1H NMR. Typical proton resonance signal originating from PEG block appeared at 3.65 ppm (‘a’) in 1H NMR spectrum after ATRP of FcMAss initiated by PEG-Br (Fig. 6). The resonance signals located at 4.22 (‘k’), 4.42 (‘j’) and 4.82 (‘i’) ppm corresponded to 9 protons of ferrocene moiety in FcMAss repeated unit, whereas the peaks ‘e’, ‘f’, ‘g’ and ‘h’ are attributed to methylene groups. On the basis of integrations of peak ‘a’ and ‘f’, ‘g’, ‘real’ molecular weight of PEG-bPFcMAss diblock copolymer (Mn,NMR) was estimated to be 11,900 g/ mol by Eq. (3) (Sa and Sf,g are the integration area of peak ‘a’ at 3.65 ppm and peak ‘f’ and ‘g’ at 2.85–3.12 ppm in Fig. 6, 434 and Mn,PEG-Br (2135 g/mol) are the molecular weights of FcMAss monomer and PEG-Br macroinitiator, respectively), which is similar to the value acquired from conventional GPC (Mn,GPC = 10,800 g/mol).
Mn,NMR = Mn,PEG
Br
+ 434*(43.5Sf,g/ Sa )
Fig. 7. FT-IR spectra of GO, PFcMAss and GO-PFcMAss.
(3)
3.2. Preparation and characterization of GO-PFcMAss nanocomposite In our previous work [38,39], GO could be modified by tris(hydroxymethyl) aminomethane (TRIS) to increase the amount of reactive sites on the surface of GO and subsequent introduction of ATRP initiating group (COC(CH3)2Br) could give a GO-base macroinitiator, TRIS-GO-Ini, which can initiate RDRP of functional monomer to prepare versatile polymer-functionalized graphene-based materials. Herein, surface-initiated ATRP of FcMAss was conducted at 50 °C in toluene with TRIS-GO-Ini as initiator and CuBr/PMDETA as catalytic system. EBiB was also added as a sacrificial initiator for subsequent
Fig. 8. XPS spectra of TRIS-GO-Ini and GO-PFcMAss and digital photo (inset) of GO-PFcMAss in organic solvents and water.
determination of molecular weight of PFcMAss chain grafted from GO since that it has been reported that free polymers formed in the solution have the same molecular weight as those formed on the surface assuming fast exchange between the two populations of polymers in the same system [56–58]. A unimodal and symmetrical elution peak in GPC 12
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Planning (SHXH201706).
Table 2 Elemental analysis of TRIS-GO-Ini and GO-PFcMAss.a Sample
C% (mol %)
N% (mol %)
O% (mol %)
Fe% (mol %)
S% (mol %)
TRIS-GO-Ini GO-PFcMAss
79.51 72.71
1.83 1.31
17.48 23.26
– 1.01
– 1.60
a
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Obtained from XPS.
curve of free PFcMAss polymer gave the molecular weight of ~13,600 g/mol, thus the molecular weight of PFcMAss chains grafted from GO was also ~13,600 g/mol. The polydispersity (Mw/Mn = 1.48) increased compared to that obtained by solution ATRP in toluene ((Mw/ Mn < 1.30), suggesting the presence of termination processes because of the steric effect of GO surface. The resulting PFcMAss-functionalized GO, GO-PFcMAss, was characterized by FT-IR spectroscopy as shown in Fig. 7. In comparison with FTIR spectrum of GO (olive line in Fig. 7), it can be seen in FT-IR spectrum after surface-initiated ATRP (blue line in Fig. 7) that typical peaks of ferrocene group appeared at 2958, 1459 and 817 cm−1 and a new peak located at 2852 cm−1 was attributed to the stretching vibration of methylene group, similar with that of PFcMAss homopolymer (red line in Fig. 7). In addition, the intensity of the characteristic peak of carbonyl at 1708 cm−1 increased obviously, also verifying the introduction of PFcMAss polymeric chains. The detailed compositional analysis of GOPFcMAss nanocomposite was performed by XPS (Fig. 8). Compared to XPS spectrum of TRIS-GO-Ini (black line in Fig. 8), the Fe2p and Fe3p3 peaks at a binding energy value of 709 and 71 eV were observed in XPS spectrum after surface-initiated ATRP (blue line in Fig. 8), suggesting the presence of ferrocene unit on the surface of GO, while the S2s and S2p3 peaks appeared at 229 and 165 eV, which were originated from the disulfide bond in PFcMAss polymeric chain. Furthermore, the element contents of TRISGO-Ini and GO-PFcMAss obtained from XPS results are outlined in Table 2, indicating the presence of C, N, O, Fe and S elements in GO-PFcMAss. From these values of element content, the weight content of PFcMAss in GO-PFcMAss nanocomposite can be evaluated to be ~44.2 wt%. From all the aforementioned evidences, it can be concluded that PFcMAss polymeric chains were successfully grafted on the surface of GO via SI-ATRP, affording GO-PFcMAss nanocomposite. The as-prepared GO-PFcMAss nanocomposite show good dispersible properties in organic solvents such as acetone, chloroform, toluene and DMF (inset of Fig. 8). However, this nanocomposite cannot well disperse in water and methanol which are poor solvents for PFcMAss polymer. This phenomenon also further indicated that GO is indeed covered with PFcMAss polymeric chains. 4. Conclusion In summary, we synthesized a new methacrylate monomer consisting of ferrocene unit and disulfide bond, FcMAss, which is suitable for ATRP to provide well-defined PFcMAss homopolymer with narrow molecular weight distribution. Furthermore, PEG-b-PFcMAss diblock copolymer was prepared by ATRP of FcMAss initiated by PEG-Br macroinitiator. Surface-initiated ATRP of FcMAss initiated by GO-based macroinitiator provided PFcMAss-functionalized GO, GO-PFcMAss. This nanocomposite shows excellent dispersibility in organic solvents. Acknowledgement The authors thank the financial support from Shanghai Scientific and Technological Innovation Project (17ZR1426700 and 18ZR1448600), Scientific Research Project of Science and Technology Commission of Xuhui Municipality (SHXH201613) and Scientific Research Project of Xuhui Provincial Commission of Health and Family
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