Letter pubs.acs.org/macroletters
Hyper-Cross-Linked Organic Microporous Polymers Based on Alternating Copolymerization of Bismaleimide Hui Gao,† Lei Ding,† Wenqing Li,† Guifeng Ma,‡ Hua Bai,† and Lei Li*,† †
College of Materials and ‡Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen, 361005, People’s Republic of China S Supporting Information *
ABSTRACT: A novel type of hyper-cross-linked organic microporous polymer (HOMP) has been successfully prepared based on the radical copolymerization of bismaleimides and divinylbenzene. In comparison with the HOMPs prepared with cross-linking techniques, the new radical strategy circumvents some intractable problems, such as low atom economy, structure irregularity and corrosive byproducts. The obtained HOMPs have defined molecular structures due to the intrinsic alternating copolymerization properties of the two monomers. A maximum BET surface area of 841 m2 g−1 and high gas capture capacity (CO2, 11.22 wt %, 273 K/1.0 bar; H2, 0.82 wt %, 77.3 K/1.0 bar; benzene, 545 mg g−1, room temperature/0.6 bar; and cyclohexane, 1736 mg g−1, room temperature/0.6 bar) were achieved. In addition, the polymers also displayed good chemical and thermal stability, which is critical for the practical application.
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found to exhibit an apparent BET surface area of >1000 m2 g−1.14 Another method involves the knitting of rigid, aromatic building blocks via Friedel−Crafts alkylation route external cross-linkers.15 Hyper-cross-linking reactions are difficult to control, and the structures of HOMPs are irregular. Friedel− Crafts alkylation, which is a widely used cross-linking reaction, requires overcatalytic concentrations of Lewis acid and is not atom economic.12b,16 Another associated problem is the negative environmental impact because the byproduct is usually corrosive.17 Therefore, new efficient synthetic approaches for the preparation of HOMPs with higher specific surface areas and novel functions are desired. Here, we report the preparation of a new type of HOMP based on alternating radical copolymerization of bismaleimides and divinylbenzene (DVB). This method has rarely been used to prepare HMOPs. No byproducts are formed during the polymerization; thus, this approach is highly atom economic. The polar structures of the two monomers result in an alternating copolymerization,18 endowing the HOMPs with predictable molecular structures, whereas the rigidity of the bismaleimides and high cross-linking degree ensure that the permanent microporosity is fixed between the molecular chains. As a consequence, large specific surface area and high gas-storage performance were obtained on these HOMPs.
he design and synthesis of microporous organic polymers (MOPs) with new building blocks has attracted significant interest due to their potential applications in the areas of molecular separation,1 heterogeneous catalysis,2 and gas storage.3 A wide range of synthesis methodologies has been adapted to the synthesis of MOPs, and there is enormous scope for polymer postmodification to introduce specific chemical functionalities.4 In the past decades, researchers have developed various new MOPs, including covalent organic frameworks (COFs),5 polymers of intrinsic microporosity (PIMs),6 and conjugated microporous polymers (CMPs).7 However, the monomers used in these reactions must bear halogen,8 ethynyl,9 or stereocontrolled structures,10 which require tedious synthesis. In addition, the successful preparation of the above MOPs requires expensive catalysts and high laboratory cost.11 Therefore, the unsustainable mass production and high cost limit the practical applications of MOPs prepared following the above-mentioned strategies. Hyper-cross-linked organic microporous polymers (HOMPs), referring to network polymers composed of rigid molecular linkers and exhibiting permanent microporosity in the dry state, have aroused increasing attention because of the simple chemical preparation and low cost.12 HOMPs can be produced in two ways: by intermolecular and intramolecular cross-linking of preformed polymer chains (either linear chains or lightly cross-linked gels) and by direct step growth polycondensation of suitable monomers. A representative example of HOMPs prepared by the first strategy is the Davankov resin.13 The linear polystyrene cross-linked with chloromethyl methyl ether or tris-chloromethyl mesitylene was © XXXX American Chemical Society
Received: January 7, 2016 Accepted: February 24, 2016
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DOI: 10.1021/acsmacrolett.6b00015 ACS Macro Lett. 2016, 5, 377−381
Letter
ACS Macro Letters The HOMPs were prepared through one-step solution polymerization (Scheme 1). Typically, N,N′-4,4′-diphenylScheme 1. Preparation of Bismaleimide-Based HOMPsa
a
Asterisks denote the connected sites with nitrogen atoms.
methane-bismaleimide (0.7167 g, 2 mmol), DVB (0.2604 g, 2 mmol), and 2,2′-azobis(isobutyronitrile) (0.0038 g, 0.02 mmol) were dissolved in 10 mL of anhydrous N,N-dimethylformamide. The solution was stirred at room temperature, was deoxygenated by argon bubbling for 0.5 h, and then was heated at 80 °C for 24 h. The solid was collected by filtration, washed three times with tetrahydrofuran (THF), then extracted by Soxhlet with THF, and was finally dried under reduced pressure. Four different types of bismaleimides, N,N′-4,4′diphenylmethane-bismaleimide (BDM), N,N′-(4-methyl-1,3phenylene) bismaleimide (BMP), N,N′-1,3-phenylenedimaleimide (m-PDM), and N,N′-1,4-phenylenedimaleimide (p-PDM) were copolymerized with DVB in our study (Table S1). The produced HOMPs were denoted as BDM-DVB, BMP-DVB, mPDM-DVB and p-PDM-DVB. All of the samples are yellowish powders. The surface morphologies of the HOMPs were characterized by field emission scanning electron microscopy. Loose agglomerates of tiny particles with an irregular shape were observed (Figure S1). As shown in Figure S2, a wide size distribution in the range from 142 to 342 nm was found. The products were first investigated by Fourier transform infrared (FTIR) measurement. The characteristic band at 3098 cm−1 is assigned to C−H stretching vibrations of C−H, which is weakened significantly in the spectra of the products (Figure 1a). The band at 2929 cm−1, which appears in the spectra of the copolymers but is absent in that of bismaleimides (Figure S3), is attributed to the C−H stretching in −CH2−, originating from the DVB.19 The stretching vibrations of CC at 1584 cm−1 in the polymers also disappear or weaken. The CO stretching vibration at 1712 cm−1 and the C−N−C asymmetry stretching vibration at 1389 cm−1 are found in all spectra.20 The results reveal that both the bismaleimides and DVB are connected through polymerization. Solid state 13C cross-polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectrum was also employed to characterize the polymerization products (Figure 1b). All four products have four types of carbon signals with chemical shifts of 43, 128, 138, and 177 ppm. The broad peak at 43 ppm is attributed to the overlap of the peaks of methyne and methylene produced by the breaking of the double bond of the monomers. The other three peaks at 128, 138, and 177 ppm can be assigned to the nonsubstituted aromatic carbon,
Figure 1. (a) FTIR spectra of bismaleimide-based HOMPs. (b) Solid state 13C cross-polarization magic-angle spinning (CP/MAS) NMR spectra of bismaleimide-based HOMPs. Asterisks denote spinning sidebands.
the substituted aromatic carbon, and the imide carbon, respectively. These FTIR and 13C CP/MAS NMR results confirm the successful copolymerization of bismaleimides with DVB. Moreover, the electron-rich monomer DVB preferably polymerizes with electron-poor monomers of bismaleimides in an alternating mode; thus, the mole ratio of DVB to bismaleimide in the corresponding copolymer should be 1:1. This ratio is confirmed by the elemental analysis data shown in Table S2. The experimental elemental contents are in general agreement with the theoretical values, confirming the alternating structure of the copolymers. The BET surface areas and porosity parameters of the bismaleimide-based HOMPs were studied by nitrogen adsorption. Figure 2 presents the nitrogen adsorption isotherms and pore distribution curves of the four HOMPs prepared with different bismaleimide monomers. As shown in Figure 2a, all HOMPs exhibit type I isotherms according to the IUPAC classification. The high gas uptake at low pressures (P/P0 ≈ 0.1) indicates that these polymers possess abundant micropores.21 The hysteresis loop at the medium pressure region (P/ P0 = 0.4−0.7) reveals that a large number of mesopores exist in the materials.21b,22 Trace amount of macropores (>10 μm), which are not detectable by N2 adsorption method, are found in the products, but they have very little contribution to the specific surface areas (Figure S4). The pore size distribution (Figure 2b, calculated using DFT methods, slit pore models) also confirms the presence of micropores and mesopores. The detailed BET surface areas, pore volumes, and pore sizes of the HOMPs are listed in Table 1. The main reasons for the 378
DOI: 10.1021/acsmacrolett.6b00015 ACS Macro Lett. 2016, 5, 377−381
Letter
ACS Macro Letters
Figure 3. BET surface areas of BDM, BMP, m-PDM, and p-PDM series vs DVB contents.
content of DVB further increases to 50 mol %, the BET surface area of the synthesized p-PDM-DVB reaches 841 m2 g−1. A similar trend in the BET surface areas is also observed in the other three series of HOMPs. The increasing cross-linking degree at higher DVB content can generate more pores and enhance the rigidity of the polymers, thus increasing the specific surface areas of the HOMPs. The chemical and thermal stability of a porous material is important in practical applications; therefore, we examined the chemical and thermal stability of BDM-DVB. After the material was soaked in 1 M hydrochloric acid solution overnight, no obvious decay of the adsorption properties was observed after acid solution soaking, suggesting high resistance to acid solution (Figure S5). However, no surface area was detected in the HOMPs after treating them with 1 M sodium hydroxide solution because of the hydrolysis of imide groups and the resultant collapse of the network structures. The HOMPs exhibit high thermal stability, as demonstrated by the thermogravimetric analysis under N2 atmosphere. The plateau of thermal decomposition has no obvious decline until 450 °C (Figure S6), indicating excellent thermal stability. Additionally, a 47% weight loss in a narrow temperature window (between 450 and 500 °C) confirms the uniform chemical structure of the HOMP created by alternating copolymerization.23 The slight initial weight loss at
