Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Synthesis of Conjugated Microporous Polymers through Cationic Cyclization Polymerization Hao Zhou,† Bing Zhao,† Cheng Fu,‡ Ziqi Wu,† Chonggang Wang,† Yun Ding,† Bao-Hang Han,‡ and Aiguo Hu*,† †
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Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China S Supporting Information *
ABSTRACT: Conjugated microporous polymers (CMPs) are a kind of polymeric materials combining both high porosities and photoelectric functions and are applied in numerous fields. Herein, three CMPs are facilely synthesized through cationic cyclization polymerization of monomers with multiple enediyne moieties. The chemical structures of the CMPs were characterized with solid-state 13C cross-polarization/magic-angle spinning NMR spectroscopy and Fourier transform infrared spectroscopy. The obtained CMPs exhibit high Brunauer−Emmett−Teller specific surface areas (up to 780 m2 g−1) and excellent adsorption functionality. The highest uptake capacities are up to 12.7 wt % for carbon dioxide (1.0 bar, 273 K) and 246 wt % for iodine (353 K), which are superior to those of most porous organic polymers (POPs). We believe that this metal-free strategy will provide a novel approach toward the synthesis of CMPs and would further expand the application scope of POPs.
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coupling reactions such as Yamamoto,24 Sonogashira,25,26 and Suzuki cross-coupling reactions.27,28 For example, Cooper et al.29 applied the Yamamoto cross-coupling reaction to several kinds of multibromoarenes and produced CMPs of different emission colors in an anhydrous and anaerobic environment. Other kinds of reactions like the oxidative coupling reaction,30 Friedel−Crafts reaction,31 and light-induced tetrazole−alkene cycloaddition reaction32 have also been used to construct CMPs with special properties. Han et al. reported the synthesis of CMPs based on 1,3,5-tricarbazolyl-benzene through the oxidative coupling reaction.33 The resulting materials exhibited a high Brunauer−Emmett−Teller (BET) specific surface area of 2200 m2 g−1 and high hydrogen adsorption capacity of 2.80 wt % (1.0 bar, 77 K), superior to many other porous materials. In an interesting approach, electrochemical polymerization was used to produce electroactive CMP films with microporous properties and conjugated structures on electrodes.34 The thickness and shape of CMP films were controlled by adjusting the number of scanning cycles and the shape of electrodes. The research on CMPs has been going on for many years, and the synthetic methods have also been greatly developed. However, some problems in the preparation process of most CMPs are still inevitable. For example, noble-metal-based
INTRODUCTION Conjugated microporous polymers (CMPs) are one of the most valuable porous materials with conjugated organic frameworks and permanent micropores. Since Cooper et al. synthesized a series of conjugated microporous poly(aryleneethynylene) networks through the Sonogashira crosscoupling reaction in 2007,1 this kind of material has attracted considerable attention with a wide range of potential applications achieved in many areas such as heterogeneous catalysis,2,3 molecular separation,4−6 gas storage,7−12 electronic devices,13,14 and optical applications.15,16 Similar to other porous organic polymers (POPs), the structural features including specific surface area, pore size, and pore volume of CMPs are readily tuned by selecting monomers with different structural lengths17 or simply by varying the reaction solvent used.18−20 Moreover, the CMPs with a π-electron conjugated system have many remarkable photoelectric properties such as good nonlinear optical properties and semiconductor characteristics, greatly expanding the application scope of porous materials.21−23 CMPs have been prepared by self-condensation of single monomers or cross-coupling of two or more structural units. An ideal synthetic method should ensure both the simple operation process and high yield of CMPs. To achieve this goal, many synthetic strategies and a huge number of building blocks have been applied to produce CMPs. The most widely used preparation methods are transition-metal-catalyzed cross© XXXX American Chemical Society
Received: March 4, 2019 Revised: May 2, 2019
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DOI: 10.1021/acs.macromol.9b00437 Macromolecules XXXX, XXX, XXX−XXX
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quickly turned black, and precipitation was detected immediately. To ensure the complete network formation and consumption of the intermediates, the mixtures were further heated at 100 °C for 72 h, and the CMP products were obtained in high yields. In comparison, Bergman cyclization of monomers M1−M3 through the conventional thermal-induced condition produced almost no CMP network even at a very high temperature (260 °C), highlighting the advantages of this novel cationic cyclization polymerization. All of the prepared CMPs were insoluble in a full range of organic solvents including toluene, ethanol, dichloromethane, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF). The brown color of CMP1 and CMP3 as well as the black color of CMP2 suggested the presence of conjugated networks (Figure S1). Like many other microporous polymers, all of these CMPs show significant physical and chemical stability. Thermogravimetric analysis (TGA) results (Figure 1)
catalysts are always needed for transition-metal-catalyzed crosscoupling reactions, which increases the cost and complicates the postprocessing of materials. The intrinsic properties of CMPs could be affected by a very small amount of residual catalysts. In addition, some low-cost strategies have limited application scope and unsatisfactory yields. Therefore, it is essential to explore efficient, economical, and convenient polymerization processes to reveal new applications of CMPs.35 Bergman cyclization36,37 is an intramolecular reaction of enediyne compounds to generate reactive biradicals, which would abstract hydrogen atoms from the environment or trigger other monomers to form conjugated polymers.38,39 The metal-free and byproduct-free features of Bergman cyclization polymerization endow it with great potential in the preparation of conjugated polymers with various structural characteristics including CMPs.40 Recently, we developed a cationic cyclization polymerization of enediynes to achieve conjugated polymers with well-defined backbone structures under mild conditions.41 Herein, we report the synthesis of three CMPs through this novel cationic cyclization polymerization of enediyne compounds. The obtained CMPs exhibit high BET specific surface areas as well as high carbon dioxide and iodine uptake capacities.
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RESULTS AND DISCUSSION The Bergman cyclization of enediyne compounds is a facile approach to prepare conjugated polymers.42−44 It can be triggered by various reagents, such as nucleophilic reagents, electrophilic reagents, ultraviolet rays, metal catalysts, and redox reagents.45 The enediyne compounds have a very broad response spectrum to be used in polymer chemistry.46,47 We recently showed that the cationic cyclization polymerization of enediynes induced by CF3SO3H produced structurally welldefined conjugated polymers under much milder conditions compared with the traditional thermal Bergman cyclization.41 Taking advantage of this newly uncovered reaction, we designed three monomers (denoted M1−M3) to prepare CMPs (Scheme 1). All of them feature multiple enediyne
Figure 1. TGA analysis of CMPs.
confirmed that all of the CMPs have excellent thermal stability. The thermal decomposition temperature of the CMPs was as high as 510 °C (Figure S2), and the mass loss was only about 20 wt % when they were heated to 1000 °C. The remarkable thermal stability of the CMPs originates from their rigid structure and high degree of cross-linking. The mass loss at lower temperatures was attributed to the volatilization of the trapped solvents, which is commonly found in many porous organic polymers.48 Fourier transform infrared (FT-IR) spectroscopy is an important means of characterizing polymer structures (Figure S3). M1 was taken as an example to analyze the changes of infrared spectra before and after polymerization (Figure 2). The stretching vibration band at 2215 cm−1 was attributed to the triple bonds for enediyne moieties. After cationic
Scheme 1. Molecular Structures of Designed Monomers and Their Corresponding CMPs
moieties to ensure the formation of a cross-linked network and were synthesized by the conventional Sonogashira coupling reaction between 1-ethynyl-2-(phenylethynyl)benzene and multihalo substituted aromatics (Supporting Information). The cationic cyclization polymerization was performed by addition of CF3SO3H to the 1,2-dichloroethane solutions of these monomers at room temperature. The reaction mixture
Figure 2. FT-IR spectra of M1 and CMP1. B
DOI: 10.1021/acs.macromol.9b00437 Macromolecules XXXX, XXX, XXX−XXX
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(Figures 4a and S6). All isotherms exhibited a rapid gas uptake at relative pressures less than 0.1, which confirmed the microporosity in the obtained polymers. The Brunauer− Emmett−Teller (BET) specific surface areas of CMP1, CMP2, and CMP3 were 767, 624, and 780 m2 g−1, respectively, whereas the total pore volumes were 0.39, 0.38, and 0.48 cm3 g−1, respectively, at a relative pressure of P/P0 = 0.95 calculated based on N2 adsorption (Table 1). To further demonstrate the
cyclization polymerization, this peak disappeared completely, indicating the complete conversion of monomers. The stretching vibration peaks corresponding to the benzene ring skeleton migrated from 1594 and 1577 to 1610 cm−1 after polymerization, representing the change of the conjugated structure of the monomer before and after polymerization. The new peak at 1703 cm−1 was associated with carbonyl stretching vibrations. The formation of carbonyl groups was due to the hydrolysis of trifluoromethylsulfonate groups41 at the end of polymer chains, which was promoted by the trace amount of water in the solvent. This peak was observed due to the high infrared activity of carbonyl groups despite their low content in the CMPs. The generation of carbonyl groups in such kinds of reactions was described by Overman et al. as a “nucleophilepromoted electrophilic cyclization”.49 The chemical identity of CMPs was proven by 13C crosspolarization/magic-angle spinning (CP/MAS) solid-state NMR spectroscopy (Figure 3). The spectra of all polymer
Table 1. Porous Properties and CO2 and I2 Uptake of the CMPs CMP
SBETa (m2 g−1)
CMP1 CMP2 CMP3
767 624 780
SMicrob VTc VMicrob (m2 g−1) (cm3 g−1) (cm3 g−1) 481 302 134
0.39 0.38 0.48
0.19 0.12 0.02
CO2 uptake wt %
I2 uptake wt %
12.96 11.76 10.43
178 224 246
a
Surface area calculated from N2 adsorption isotherms using the BET equation. bCalculated using the t-plot method. cCalculated based on N2 adsorption at P/P0 = 0.95.
microporosity of the obtained polymers, the micropore specific surface areas and micropore volumes were calculated using the t-plot method. The micropore specific surface areas of the obtained polymers were up to 481, 302, and 134 m2 g−1, respectively, and micropore volumes were 0.19, 0.12, and 0.02 cm3 g−1, respectively (Table 1). Moreover, all three polymers showed that the desorption branch lies above the adsorption branch, and the isotherm is not completely closed. This phenomenon is commonly found for microporous polymers,50 and the cause is diverse. The expansion of polymer networks is one of the most important reasons, which can also explain the excellent I2 uptake capacity demonstrated next. The existence of mesopores is also part of the reason,51 as well as very small micropores that kinetically limit the access and exit of nitrogen.31 The pore size distribution profiles (PSDs) (Figure S6) show that both micropores and mesopores exist in CMP1 (1.5−2, 2−3.5 nm) and CMP3 (1.5, 2−8 nm). As for CMP2, the PSD is very narrow (1.5 nm), with few mesopores (2−4 nm). The molecular size of carbon dioxide (CO2) is only 0.36 nm, smaller than that of the nitrogen molecule, so it is much more sensitive to micropores than nitrogen.52 To further characterize the microporosity of the CMPs, carbon dioxide adsorption was measured at 273.15 K (Figure 4b). The amount of CO2 adsorption of the three CMPs increased with pressure continually and reached 12.70, 11.55, and 10.23 wt % at 1.0 bar (Table 1). The CO2 adsorption capacity of the three
Figure 3. Solid-state 13C CP/MAS NMR spectra of CMPs.
networks showed some general features: (1) a resonance at 130 ppm that could be referred to unsubstituted phenyl carbon atoms and (2) a broad signal in the region from 140 to 160 ppm owing to the resonance of substituted phenyl carbon atoms and carbon atoms of newly formed conjugated olefinic bonds. A signal around 69 ppm was observed in the 13C CP/ MAS NMR spectrum of CMP3, which could be attributed to the central sp3 carbon atoms of the 9,9′-spirobi[fluorene] structure. The powder X-ray diffraction patterns (Figure S4) showed that all three obtained CMPs are amorphous in nature, which is commonly observed for other CMPs. Scanning electron microscope (SEM) images showed that the CMPs were composed of aggregated nanoparticles, and no regular morphology was observed (Figure S5). Nitrogen adsorption−desorption experiments were performed to characterize the porous properties of the CMPs
Figure 4. (a) N2 adsorption−desorption isotherms and (b) CO2 adsorption isotherms of CMPs. C
DOI: 10.1021/acs.macromol.9b00437 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. (a) Gravimetric uptake of I2 as a function of time at 80 °C, (b) UV−vis/NIR spectra of CMP3 before and after uptake of I2, (c) release profile of I2 in the first 70 min from CMP3, and (d) release profile of I2 upon heating the loaded CMP3 at 398.15 K.
adsorption capacity to that of CMP3 and much higher than that of CMP1. This phenomenon is probably due to the strong interactions of iodine and the swelling of the adsorbents.57 While CMP1 exhibits a high BET specific surface area, its high structural rigidity resulting from its shortest structure limits the network expansion and rearrangement, leading to the relatively low iodine adsorption capacity. The UV−vis result of CMP3 showed that after I2 uptake, two peaks at 300 and 390 nm appeared over the otherwise broad peak of CMP3 (Figure 5b). These peaks were attributed to the formation of a charge transfer complex between conjugated backbones of CMPs and iodine,58,59 corroborating the strong interactions between CMPs and iodine. Iodine release was also studied to achieve reversible adsorption of iodine. I2 release occurred when iodine-loaded polymers were heated or placed in alcohol. The I2-loaded CMP3 was placed in ethanol, and the color of the solution darkened with time, indicating the gradual release of iodine (Figure S9). The amount of iodine released increased linearly with time, indicating that the I2 release is governed by the host−guest interaction (Figure 5).60,61 From the perspective of simple operation, loaded CMP3 was also heated at 398.15 K, and iodine release efficiency was up to 91% after about 1 h. These results show that it is feasible to use the CMPs prepared from cationic cyclization polymerization for efficient reversible I2 capture. We also performed desorption experiments under high vacuum at 85 °C. Even after 2 h treatment, about 6% iodine residue was still present in the CMPs, showing that the residual iodine may be bonded to the CMP framework chemically.
polymers decreased slightly with the decrease of the micropore specific surface area, CMP1 > CMP2 > CMP3. This order is commonly observed in other microporous polymers.52 Moreover, the highest carbon dioxide uptake capacity was superior to that of many other POPs (Table S1). Since the heteroatoms that are generally acknowledged to enhance the CO 2 adsorption capacity such as nitrogen were not introduced into these CMP series, we believe that the high adsorption entirely originated from the high microporosity of the polymers generated through the cationic cyclization polymerization. X-ray photoelectron spectroscopy showed that only a negligible amount of heteroatoms like sulfur was introduced during the polymerization process (Figure S7 and Table S2). The radioactive iodine isotopes are a kind of dangerous substance in nuclear waste because of their long radioactive half-life and ability to be absorbed by the human body.53−55 It has been proven that the phenyl ring is one effective adsorption site for adsorbing iodine.55,56 Considering that the obtained CMPs were electron-rich aromatic networks with a great number of aromatic rings and excellent porosity, they were expected to exhibit high iodine capture performance. The CMPs were exposed to iodine vapor at 353 K and 1.0 bar, and nonradioactive iodine was used for demonstration. Significant color changes from brown to black of CMP1 and CMP3 were observed during the experiment (Figure S8). The gravimetric method was used to measure the amount of iodine captured. After about 60 min, the mass of the iodine-loaded adsorbents no longer increased, indicating that the adsorption has reached equilibrium. The equilibrium I2 uptakes were as high as 178, 224, and 246 wt % (Figure 5). To the best of our knowledge, the values for CMP2 and CMP3 were some of the highest reported to date (Table S3). CMP3, with the highest BET specific surface area, had the highest iodine adsorption capacity. However, the BET specific surface area is not the decisive factor affecting iodine adsorption. CMP2, with the lowest BET specific surface area, had a comparable iodine
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CONCLUSIONS In summary, a novel and facile approach to construct conjugated microporous polymers has been reported through cationic cyclization polymerization. This strategy shows several advantages like metal-free, simple postprocessing, high yield, D
DOI: 10.1021/acs.macromol.9b00437 Macromolecules XXXX, XXX, XXX−XXX
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collected, washed with saturated NaCl solution three times, dried and concentrated under reduced pressure, and purified through silicagel column chromatography (petroleum ether/dichloromethane= 5:1) to give the product as a pale yellow powder (98 mg, 72%). 1H NMR (400 MHz, chloroform-d) δ 7.82 (d, J = 7.9 Hz, 4H), 7.64 (dd, J = 7.9, 1.4 Hz, 4H), 7.53−7.50 (m, 4H), 7.45 (m, 12H), 7.29−7.24 (m, 20H), 7.02 (d, J = 1.4 Hz, 4H). 13C NMR (101 MHz, chloroform-d) δ 148.4, 141.4, 132.3, 131.9, 131.8, 131.6, 128.5, 128.5, 128.1, 128.1, 127.1, 125.8, 125.8, 123.3, 123.2, 120.6, 93.8, 93.8, 89.4, 88.4, 65.5. MS (MALDI-TOF) [C89H48Ag]: m/z (M + Ag)+: calculated: 1225.3, found: 1225.2. Preparation of CMPs. Under a nitrogen environment, M1 (260 mg, 0.38 mmol) was dissolved in 1,2-dichloroethane (3 mL), and then trifluoromethanesulfonic acid (TfOH, 0.57 g, 3.83 mmol) was added dropwise at room temperature. The reaction flask was sealed, and the mixture was stirred at 100 °C for 72 h. After being cooled to room temperature, the resulting polymer was filtered and washed with water and dichloromethane. The solid was further purified using Soxhlet extraction with THF and dichloromethane successively. The resulting product was dried under vacuum to afford brown polymers (224 mg, 86%, named CMP1). CMP2 (black polymers, obtained from M2) and CMP3 (brown polymers, obtained from M3) were prepared and isolated through a similar procedure as described for CMP1. Instrumental Characterization. The 1H NMR and 13C CP/ MAS NMR spectra were recorded on a Bruker Avance III 400 NMR spectrometer. Fourier transform infrared (FT-IR) spectroscopy was performed in KBr pellets on Spectrum One (PerkinElmer Instruments Co. Ltd). Thermal gravimetric analysis (TGA) was peformed on a Pyris Diamond thermogravimetric/differential thermal analyzer by heating the samples from room temperature to 1000 °C at a heating rate of 10 °C min−1 in a nitrogen atmosphere. Ultraviolet−visible adsorption (UV−vis) spectra were obtained using a Lambda 950 spectrometer (PerkinElmer Instruments Co. Ltd). Field emission scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 microscope (Hitachi Ltd, Japan) at an acceleration voltage of 6.0 kV. Nitrogen adsorption isotherms and carbon dioxide uptake experiments were carried out using a TriStar II 3020 surface area and a porosity analyzer (Micromeritics) at 273 and 77 K, respectively, with the samples degassed at 120 °C for 12 h.
and mild operating conditions. The obtained CMPs exhibit high BET specific surface areas and excellent performance in gas adsorption. The highest uptake capacities are up to 12.7 wt % for carbon dioxide and 246 wt % for iodine, which are superior to those of most organic porous polymers. As the enediyne chemistry has been well developed in recent years, this strategy would also open a wide synthetic and application scope of CMPs.
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EXPERIMENTAL SECTION
Materials. Tetrahydrofuran (THF), N,N-dimethylformamide (DMF), triethylamine (Et3N), and diphenyl ether were dried before use. 1,3,5-Tris(4-iodophenyl)benzene, 2,2′,7,7′-tetraiodo-9,9′-spirobi[fluorene], and 1-ethynyl-2-(2-phenylethynyl)benzene (1c) were prepared according to literature procedures with minor modifications.62−64 Other chemical reagents were of commercial grade and used as received. Synthesis of 1,3,5-Tris((2-(phenylethynyl)phenyl)ethynyl)benzene (M1). To a sealed tube (10 mL) were successively added bis(triphenylphosphine)palladium(II) chloride (0.046 g, 0.066 mmol), cuprous iodide (0.025 g, 0.132 mmol), N,N-diisopropylethylamine (0.128 g, 0.99 mmol), 1,3,5-triiodobenzene (0.1 g, 0.22 mmol), 1c (0.2 g, 0.99 mmol), and DMF (2 mL) in a nitrogen environment. The mixture was degassed with a freeze−pump−thaw cycle three times and then sealed. After being stirred at 60 °C for 72 h, the reaction was suspended by saturated NH4Cl aqueous solution. The mixture was diluted with dichloromethane and then the organic layer was collected, washed with saturated NaCl solution three times, dried and concentrated under reduced pressure, and purified through silicagel column chromatography (petroleum ether/dichloromethane = 5:1) to give the product as a pale yellow powder (113 mg, 76%). 1H NMR (400 MHz, chloroform-d) δ 7.65 (s, 3H), 7.51−7.42 (m, 12H), 7.27−7.14 (m, 15H). 13C NMR (101 MHz, chloroform-d) δ 134.4, 131.9, 131.9, 131.8, 128.6, 128.6, 128.5, 128.1, 126.3, 125.5, 124.3, 123.1, 94.1, 92.0, 89.7, 88.2. Mass spectra (MS) (matrix-assisted laser desorption ionization time-of-flight, MALDI-TOF) [C54H30Ag]: m/z (M + Ag)+: calculated: 787.1, found: 787.1. Synthesis of 4,4″-Bis((2-(phenylethynyl)phenyl)ethynyl)-5′(4-((2-(phenylethynyl)phenyl)ethynyl)phenyl)-1,1′:3′,1″-terphenyl (M2). To a sealed tube (10 mL) were successively added 1,3,5-tris(4-iodophenyl)benzene (0.1 g, 0.146 mmol), cuprous iodide (0.017 g, 0.088 mmol), bis(triphenylphosphine)palladium(II) chloride (0.031 g, 0.044 mmol), N,N-diisopropylethylamine (0.085 g, 0.658 mmol), 1c (0.133 g, 0.658 mmol), and DMF (2 mL) in a nitrogen environment. The mixture was degassed with a freeze− pump−thaw cycle three times and then sealed. After being stirred at 60 °C for 72 h, the reaction was suspended by saturated NH4Cl aqueous solution. The mixture was diluted with dichloromethane and then the organic layer was collected, washed with saturated NaCl solution three times, dried and concentrated under reduced pressure, and purified through silica-gel column chromatography (petroleum ether/dichloromethane = 5:1) to give the product as a pale yellow powder (95 mg, 72%). 1H NMR (400 MHz, chloroform-d) δ 7.82 (s, 3H), 7.71 (s, 12H), 7.65−7.58 (m, 12H), 7.40−7.32 (m, 15H). 13C NMR (101 MHz, chloroform-d) δ 141.8, 140.8, 132.3, 132.0, 131.9, 131.8, 128.6, 128.6, 128.2, 128.2, 127.4, 126.0, 125.9, 125.3, 123.4, 122.8, 93.8, 93.6, 89.5, 88.5. MS (MALDI-TOF) [C72H42Ag]: m/z (M + Ag)+: calculated: 1015.2, found: 1015.2. Synthesis of 2,2′,7,7′-Tetrakis((2-(phenylethynyl)phenyl)ethynyl)-9,9′-spirobi[fluorene] (M3). To a sealed tube (10 mL) were successively added 2,2′,7,7′-tetraiodo-9,9′-spirobi[fluorene] (0.1 g, 0.122 mmol), cuprous iodide (0.009 g, 0.049 mmol), bis(triphenylphosphine)palladium(II) chloride (0.017 g, 0.024 mmol), N,N-diisopropylethylamine (0.110 g, 0.854 mmol), 1c (0.148 g, 0.732 mmol), and DMF (2 mL) in a nitrogen environment. The mixture was degassed with a freeze−pump−thaw cycle three times and then sealed. After being stirred at 60 °C for 72 h, the reaction was suspended by saturated NH4Cl aqueous solution. The mixture was diluted with dichloromethane and then the organic layer was
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00437. Detailed synthesis of precursors of enediyne compounds; characterization spectra of CMPs; photographs of CMP1, CMP2, and CMP3; DTG analysis of CMPs; XRD spectra of CMPs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +86-021-64253037. ORCID
Bao-Hang Han: 0000-0003-1116-1259 Aiguo Hu: 0000-0003-0456-7269 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. E
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21674035, 21474027), the Fundamental Research Funds for the Central Universities (22221818014), and Shanghai Leading Academic Discipline Project (B502). A.H. thanks the “Eastern Scholar Professorship” support from Shanghai local government.
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