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Oct 8, 2018 - State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan,. Ningxia 75002...
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Letter Cite This: ACS Macro Lett. 2018, 7, 1283−1288

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Preparation of Microporous Organic Polymers via UV-Initiated Radical Copolymerization Tingting Zhu,†,§ Feifei Xie,†,§ Ting Huang,† Ke Tian,† Zhengchen Wu,† Haoqing Yang,† and Lei Li*,†,‡ †

College of Materials and Fujian Provincial Key Laboratory of Materials Genome, Xiamen University, Xiamen, 361005, P.R. China State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan, Ningxia 750021, P.R. China

‡

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S Supporting Information *

ABSTRACT: UV-initiated radical copolymerization has been successfully applied for the preparation of two kinds of microporous organic polymers (MOPs) based on divinylbenzene (DVB)/bismaleimide (BMI) monomers and DVB/ pentaerythritol tetraacrylate (PET4A). The obtained MOPs exhibit high BET surface areas and good CO2 storage performance. The maximum BET surface areas of DVB/ BMI and DVB/PET4A are 585 and 887 m2 g−1, respectively. Furthermore, the ester groups embedded on the DVB/PET4A copolymer skeleton can be easily converted into carboxylic groups by hydrolysis and show superior adsorption capacity of 485 mg g−1 toward methylene blue dye. The adoption of this highly efficient polymerization strategy offers the possibility for the economical and continuous preparation of MOPs and demonstrates great potential for industrial application.

M

electron-acceptor monomers (bismaleimides or fumaronitrile) and electron-donor monomer (divinylbenzene (DVB)). The resultant MOPs have defined molecular structure and high cross-linking degree which endow the copolymers with high specific surface area and excellent gas adsorption ability. Usually, radical polymerization in solution is carried out with a batch mode. The elongation of reaction time is to obtain high yield but not high polymerization degree. Comparably, photochemical polymerization exhibits fast rate, high yield and room operation characters due to the high initiator decomposition rate and the low activation energy.23,24 The added advantage of this technique is that continuous production in industrial practice can be realized.25,26 To the best of our knowledge, the UV-initiated polymerization has rarely been employed to prepare MOPs. Herein, we report the preparation of two kinds of MOPs using UV-initiated copolymerization based on DVB with three types of bismaleimide (BMI) monomers and DVB with pentaerythritol tetraacrylate (PET4A) (denoted as B-MOP and P-MOP, respectively). All the polymerization process can be completed within 1 h and the obtained MOPs demonstrate high surface areas and good CO2 adsorption capacities. Furthermore, the ester groups on DVB/PET4A copolymers are converted into carboxylic ones via sequent hydrolysis treatment, endowing them with an outstanding absorption capability of methylene blue (MB) dye.

icroporous organic polymers (MOPs) have attracted enormous attention in porous materials science due to their highly designable structure coupled with the diversity of synthesis methodologies. The progress of polymerization methodology highly promotes the development of novel MOPs and their practical applications.1-3 Usually, the interand intramolecular condensation reactions of Suzuki crosscoupling reaction,4 nitrile cyclotrimerization,5,6 Schiff-base reaction,7,8 boroxine− and boronate−ester formation,9 and Friedel−Crafts alkylation reaction,10,11 have been employed to construct the prevailing MOPs, such as covalent organic frameworks (COFs),12,13 polymers of intrinsic microporosity (PIMs),14,15 conjugated microporous polymers (CMPs),16,17 and hyper-cross-linked polymers (HCPs).18−20 To achieve the aforementioned condensation reactions, expensive noble metal-based catalysts, monomers with particular structures, high temperature, and overcatalytic concentration of Lewis acid are necessary, and corrosive byproducts sometimes are inevitable because of the inherent characteristics of condensation reactions. In addition, a long reaction time (>10 h) is necessary to achieve high cross-linking degree and surface area owing to the thermodynamic character of the condensation reaction. Therefore, the need of low-cost, timesaving and sustainable preparation strategies of MOPs motivates the pursuit of polymerization optimization protocols on high throughput. Radical polymerization is an alternative choice which fulfils atom-economic principle. Recently, bismaleimide and fumaronitrile-based MOPs have been designed and prepared in our group.21,22 The copolymerization was achieved through radical alternating polymerization in solution between © XXXX American Chemical Society

Received: September 11, 2018 Accepted: October 8, 2018

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DOI: 10.1021/acsmacrolett.8b00688 ACS Macro Lett. 2018, 7, 1283−1288

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ACS Macro Letters Scheme 1. Schematic Diagram of the Preparation for B-MOPs and P-MOPs via UV-Initiated Copolymerizationa

a

Asterisks denote the connected sites with nitrogen atoms.

Table 1. Yields, BET Surface Areas, and Pore Properties of B-MOPs and P-MOPs samples

DVB content (mol %)

initiation method

yield (%)

SBETa (m2 g‑1)

SMicrob (m2 g‑1)

MPVc (cm3 g‑1)

PVd (cm3 g‑1)

B-MOP-1 B-MOP-2 B-MOP-3 polyPET4A P-MOP-1 P-MOP-2 P-MOP-3 P-MOP-4 P-MOP-5 P-MOP-6 P-MOP-7 P-MOP-8 HP-MOP-3

50 50 50 0 25.0 33.3 50.0 66.7 75.0 80.0 83.3 75.0 50.0

UV UV UV UV UV UV UV UV UV UV UV thermal -

91.35 91.48 90.51 95.84 91.11 94.02 92.98 94.94 90.41 87.21 85.15 34.02 -

501 585 517 542 554 575 712 817 887 822 780 385 419

257 258 315 224 187 216 249 264 267 269 177 193 132

0.21 0.24 0.22 0.22 0.22 0.23 0.29 0.33 0.36 0.33 0.30 0.16 0.17

0.47 0.43 0.40 0.52 0.61 0.60 0.92 0.99 1.09 1.05 1.27 0.25 0.43

a Surface area calculated from the nitrogen adsorption isotherms at 77.3 K using the BET equation. bMicropore surface area calculated from the nitrogen adsorption isotherms at 77.3 K using the t-plot equation. cMicropore volume calculated from the nitrogen isotherm at P/P0 = 0.15, 77.3 K using the t-plot equation. dPore volume calculated from the nitrogen isotherm at P/P0 = 0.99, 77.3 K.

(BMP, B-MOP-2), and N,N′-1,3-phenylenedimaleimide (mPDM, B-MOP-3), and PET4A (P-MOP-x, x represents the content of DVB, as shown in Table 1). Typically, BDM (0.3584 g, 1.0 mmol), DVB (0.1302 g, 1.0 mmol) and 2-

As depicted in Scheme 1, the preparation was achieved by UV-initiated copolymerization of DVB with three types of BMI monomers, N,N′-4,4′-diphenylmethanebismaleimide (BDM, B-MOP-1), N,N′-(4-methyl-1,3-phenylene)bismaleimide 1284

DOI: 10.1021/acsmacrolett.8b00688 ACS Macro Lett. 2018, 7, 1283−1288

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ACS Macro Letters

and methylene carbon,21 respectively. While the peak at 17 ppm, appearing in the spectrum of B-MOP-2 (Figure 1c), is assigned to the methyl carbon.30 In the spectra of P-MOPs (Figure 1d), the peak near 63 ppm belongs to the methylene carbon adjacent to the O−CO, originating from the PET4A.31 Additionally, the element contents of all the MOPs were measured and filed in Table S1. The measured contents of the B-MOPs are identical with the theoretical values, indicative of the alternating structure of the copolymers. Therefore, it can be concluded that the chemical structures of B-MOPs formed by UV-initiated polymerization are as same as those from thermal-initiated copolymerization.21 The PXRD patterns (Figure S2) demonstrate the amorphous structure of all the obtained MOPs. The SEM images reveal that all the MOPs are composed of loosely aggregated nanoparticles (Figure S3). As shown in Figure S4, the TEM images show that uniform worm-like nanochannels are throughout the whole networks. Moreover, TGA analyses indicate that all the polymers display an initial decomposition temperature above 370 °C under nitrogen atmosphere (Figure S5). Nitrogen adsorption−desorption measurement at 77 K was utilized to investigate the pore properties of the MOPs. Both the two kinds of polymers exhibit a combination of type I and IV nitrogen sorption isotherms according to the IUPAC classification,32 as shown in parts a and b of Figure 2,

methyl-4′-(methylthio)-2-morpholinopropiophenone (Irgucure 907) (0.0024 g, 0.5 wt % with respect to the monomers mass) were dissolved in 1.5 mL of N,N-dimethylformamide (DMF). The solution was degassed by three consecutive freeze−pump−thaw cycles. Then, the solution was poured into a glass Petri dish covered with a piece of quartz glass. The polymerization was initiated under UV irradiation with a wavelength of 365 nm and an intensity of irradiation 100 W cm−1 and lasted for 1 h. The formed copolymer was separated by filter-suction and purified by Soxhlet extraction with tetrahydrofuran (THF) overnight. After vacuum drying, the polymerized B-MOP-1 network was obtained as yellow powder (Yield: 91.35%). The yields of all the obtained MOPs are in the range of 80%−95%. The correlation between the yield and the feed ratio of DVB and PET4A is found to be unidentified in this work (as explained in the Supporting Information). All the products are insoluble in common organic solvents, such as dimethyl sulfoxide, THF, and DMF. The successful formation of B-MOPs and P-MOPs was proven by Fourier transform infrared (FTIR) and solid state 13 C cross-polarization magic angle spinning nuclear magnetic resonance (13CP/MAS NMR) measurements. The FTIR spectra of all the B-MOPs (Figure 1a) exhibit CO stretching

Figure 1. FTIR spectra of B-MOPs (a) and P-MOPs (b). Solid state 13 C cross-polarization magic-angle spinning (CP/MAS) NMR spectra of B-MOPs (c) and P-MOPs (d). Asterisks denote spinning sidebands.

Figure 2. Nitrogen adsorption−desorption isotherms measured at 77.3 K for B-MOPs (a) and P-MOPs (b). The isotherms of MOPs are shifted vertically for better visibility. Pore size distributions based on NLDET calculation for B-MOPs (c) and P-MOPs (d). For clarity, the curves of MOPs are shifted vertically.

vibrations at 1710 cm−1, as well as the asymmetry stretching vibrations of C−N near 1364 cm−1, which originate from the bismaleimide monomers.27 The intense carbonyl (CO) peak at 1740 cm−1 from PET4A appears in all P-MOPs spectra (Figure 1b).28 Compared with the FTIR spectra of DVB, PET4A and BMI monomers (Figure S1), the characteristic band around 3110 cm−1 associated with the stretching vibrations of C−H becomes weak in the spectra of the polymers. The stretching vibrations of CC at 1621 cm−1 in the polymers also disappear or weaken. Meanwhile, a new band appears at 2927 cm−1, proving the formation of the saturated aliphatic linkages.29 In the solid-state 13C CP/MAS NMR spectra (Figure 1c and Figure 1d), all the products show four resonance peaks around 176, 139, 128, and 43 ppm, which are ascribed to the carbonyl carbon, the substituted aromatic carbon, the nonsubstituted aromatic carbon, and the methyne

respectively. All the samples demonstrate a steep uptake at a very low relative pressure (P/P0 < 0.01), proving that abundant micropores are dominant in the materials. Whereas the hysteresis loop at the medium pressure region (0.4 < P/P0 < 0.7) indicates the existence of mesopores.33 B-MOP-1, BMOP-3 and P-MOPs show H4-type hysteresis loops, while BMOP-2 exhibit a characteristic H2-type hysteresis loop, suggesting the different pore shapes of the MOPs.34,35 The pore size distributions calculated by nonlocal density functional theory (NLDFT) model (parts c and d of Figure 2) also confirm the existence of micropores and mesopores. The detailed porosity parameters are summarized in Table 1. The apparent BET surface areas of B-MOP-1, B-MOP-2, and B1285

DOI: 10.1021/acsmacrolett.8b00688 ACS Macro Lett. 2018, 7, 1283−1288

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ACS Macro Letters MOP-3 are found to be 501, 585, and 517 m2 g−1, respectively, which are only slightly lower than those of the corresponding samples prepared by thermal-initiated polymerization for 24 h in our previous work.21 These results evidently indicate that the UV-initiated polymerization demonstrates faster reaction kinetic. The reason can be elucidated according to radical polymerization kinetic equation.36 Upon exposed to intense UV irradiation, photo initiator decomposes rapidly and generates large amount of free radicals in short time thus a higher polymerization rate is achieved compared with thermal polymerization. The dependence of the surface area of PMOPs on the molar ratio between DVB and PET4A is plotted in Figure S6 and explained as follows.37 The product stemming from PET4A homopolymer (polyPET4A) has the minimum BET surface area of 542 m2 g−1. With the increasing of DVB content, the BET surface area gradually increases, and a maximum BET surface area of 887 m2 g−1 is achieved when the content of DVB further increases up to 75 mol %. Here, the reactivity ratios are roughly estimated to be 0.75 and 0.18 for DVB and PET4A, respectively, based on the monomer reactivity ratios between styrene (St) and methyl acrylate (MA).38 Therefore, the DVB and PET4A system is coincident with a typical nonideal azeotropic copolymerization, with an azeotrope point at 76 mol % DVB content.39 When the content of DVB is lower than 76 mol %, DVB will be enriched in the macromolecular structure with the elongation of polymerization, resulting in the formation of PET4A homopolymer after the consumption of DVB. The flexible units of PET4A facilitate conformational rearrangements in the final network and lead to the lower BET surface area. While the content of DVB exceeds 76 mol %, the existence of large amounts of DVB units leads to the early phase separation even at low conversion, thus impeding the formation of an extensive network and resulting in lower BET surface area. Eventually, PMOP-5, prepared with the monomers molar ratio close to the value of azeotrope point, achieves the optimal structure and reaches a maximum BET surface area. In a control experiment, P-MOP with DVB content of 75 mol % was prepared via thermal-initiated copolymerization with the same reaction time and concentration at 80 °C (denoted as P-MOP-8). The yield of P-MOP-8 (34 wt %) is found to be much lower than that of P-MOP-5 (90 wt %). Moreover, P-MOP-8 only exhibits a BET surface area of 385 m2 g−1 (Figure S7 and Table 1), which is dramatically lower than that of P-MOP-5. Thus, this novel method for the preparation for MOPs is a highly efficient and economical polymerization strategy in comparison with thermal polymerization. The CO2 adsorption properties of the polymers were investigated at 273 and 298 K under 1.0 bar (Figure S8). Owing to the large surface areas and high heteroatoms contents, the MOPs display CO2 uptake ranging from 5.4 to 9.1 wt % at 273 K and 1.0 bar (Table S2). As the obtained P-MOPs possess abundant ester groups which can be easily converted into carboxylic groups, it is expected that the hydrolyzed P-MOPs (HP-MOPs) can be utilized as an adsorbent to remove cationic dyes from water. Therefore, P-MOP-3 was employed as the precursor to hydrolyze with 1 wt % NaOH solution due to its high surface area and PET4A content (denoted as HP-MOP-3). FTIR spectra (Figure S9) provide direct evidence of its hydrolyzation, in which a broad absorbance assigned to the carboxylic acid at about 3400 cm−1 becomes intensive, and the carbonyl stretching shifts from 1743 cm−1 for P-MOP-3 to 1732 cm−1 for HP-MOP-3.40 Besides, the BET surface area of

HP-MOP-3 decreases to 419 m2 g−1 (Table 1 and Figure S10), which may be attributed to the collapse of the network structure during the hydrolysis and the hydrogen-bonding interactions between carboxylic acids.41,42 Methylene blue (MB) was chosen as a representative cationic dye for the dye adsorption studies. The time-dependent experiment indicates that HP-MOP-3 has a rapid adsorption rate of MB at room temperature and the adsorption attains an equilibrium approximately 90 min (Figure 3a). The digital photos (the

Figure 3. (a) Plot of contact time on the adsorption of MB (inset: photographs of MB solution after stirring with HP-MOP-3 for different contact time). (b) Langmuir isothermal model fitting for methylene blue. (c) Freundlich isothermal model fitting for methylene blue. (d) UV−vis absorbance spectra with corresponding photo images of MB solution mixing with P-MOP-3 and HP-MOP-3, respectively. The initial concentration of MB solution is 200 mg L−1.

inset in Figure 3a) represent the color change at various contact time at the initial concentration of 200 mg L−1 of MB solution, indicative of the high efficiency for the removing the MB from aqueous solution. To further investigate the adsorption behavior of HP-MOP-3, two isotherm equations, Langmuir isotherm model and Freundlich isotherm model, were used to fit the equilibrium data of dye adsorption (Figure 3, parts b and c) and the simulation results were listed in Table S3. The correlation coefficient of Langmuir isotherm model (RL2 = 0.994) is higher than the correlation coefficient of Freundlich isotherm model (RF2 = 0.976), suggesting that the adsorption of MB can be well described by the Langmuir model. According to this model, the saturated adsorption capacity (qmax) of the HP-MOP-3 for MB is calculated as 485 mg g−1, which is higher than that of most porous materials (Table S4). The dye adsorption capacity of microporous materials is determined by various factors, including pore structure, interaction between dye molecules, and adsorbent. Compared with P-MOP-3, HP-MOP-3 exhibits much higher adsorption capacity toward MB dye (Figure 3d) though its BET surface area is lower. We speculate that the MB molecule, with a molecule diameter of 14 Å,43 may easily pass in and out of mesopores because of small spatial hindrance,44 resulting in lower adsorption capacity for P-MOP-3. In addition, the electrostatic interaction between the carboxyl groups in HPMOP-3 and the MB molecules also plays more important roles for the MB dye adsorption process.45,46 Thus, the synergistic effect of the physical adsorption of the porous network 1286

DOI: 10.1021/acsmacrolett.8b00688 ACS Macro Lett. 2018, 7, 1283−1288

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ACS Macro Letters

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structure and the chemical adsorption of the carboxyl groups endow HP-MOP-3 with enhanced removal properties of MB from water, demonstrating its potential use in wastewater treatment. In conclusion, we have successfully prepared a series of MOPs based on the UV-initiated radical copolymerization. The obtained MOPs show high surface areas and excellent thermal stability. Moreover, the ester groups embedded on the skeleton of DVB/PET4A copolymers can be hydrolyzed into carboxylic ones and the MB dye adsorption capacity achieves up to 485 mg g−1. The UV-initiated polymerization strategy exhibits the advantages, including faster reaction rate, polymerization at room temperature, as well as continuous operation in industry, in comparison with conventional thermal-initiated manner. We expect that this strategy can be utilized for the continuous and mass production of MOPs and promote their practical applications.

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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/acsmacrolett.8b00688. Experimental section, charaterization data, FTIR spectra, X-ray diffraction spectra, SEM images, TEM images, TGA curves, BET surface areas, and adsorption− desorption isotherms (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(L.L.) E-mail: [email protected]. ORCID

Lei Li: 0000-0003-2732-9116 Author Contributions §

T.Z. and F.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Nos. 51373143 and 21674087). REFERENCES

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DOI: 10.1021/acsmacrolett.8b00688 ACS Macro Lett. 2018, 7, 1283−1288

Letter

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DOI: 10.1021/acsmacrolett.8b00688 ACS Macro Lett. 2018, 7, 1283−1288