Synthesis of Nitrogen-Rich Polymers by Click Polymerization Reaction and Gas Sorption Property

Microporous organic polymers (MOPs) are promising materials for gas sorption because of their intrinsic and permanent porosity, designable framework, and low density. The introduction of nitrogen-rich building block in MOPs will greatly enhance the gas sorption capacity. Here, we report the synthesis of MOPs from the 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine unit and aromatic azides linkers by click polymerization reaction. Fourier transform infrared (FTIR) and solid-state 13C CP-MAS (Cross Polarization-Magic Angle Spinning) NMR confirm the formation of the polymers. CMOP-1 and CMOP-2 exhibit microporous networks with a BET (Brunauer–Emmett–Teller) surface area of 431 m2·g−1 and 406 m2·g−1 and a narrow pore size distribution under 1.2 nm. Gas sorption isotherms including CO2 and H2 were measured. CMOP-1 stores a superior CO2 level of 1.85 mmol·g−1 at 273 K/1.0 bar, and an H2 uptake of up to 2.94 mmol·g−1 at 77 K/1.0 bar, while CMOP-2, with its smaller surface area, shows a lower CO2 adsorption capacity of 1.64 mmol·g−1 and an H2 uptake of 2.48 mmol·g−1. In addition, I2 vapor adsorption was tested at 353 K. CMOP-1 shows a higher gravimetric load of 160 wt%. Despite the moderate surface area, the CMOPs display excellent sorption ability for CO2 and I2 due to the nitrogen-rich content in the polymers.

The N-heterocycle derived from the efficient click reaction can further increase the nitrogen content for abundant sorption sites.Keeping this consideration in mind, we synthesized nitrogen-rich polymers named CMOPs from 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine (TEPT) and azides by click polymerization reaction (Scheme 1).The polymers were characterized by FTIR, solid state 13 C CP-MAS, NMR, and elemental analysis (EA).Thermogravimetric analysis (TGA) showed the high thermal stability.Powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) confirmed the amorphous but uniform phase.Nitrogen adsorption-desorption isotherms at 77 K were investigated to explore the surface area, porosity, and pore size.In addition, adsorption measurements of CO 2 , H 2 , and I 2 were researched to evaluate the gas sorption capacity.Scheme 1.The synthetic route to themicroporous organic polymers.

Synthesis and Character
The CMOPs were synthesized by click reaction in DMF solvent from TEPT and aromatic azides, according to the procedure in Scheme 1.The resulting brown powders were insoluble in common organic solvent such as tetrahydrofuran, dichloromethane, methanol, chloroform, acetone, and N,Ndimethylformamide.
In the Fourier transform infrared (FTIR) spectra of CMOPs (Figure S1), characteristic peaks for TEPT at 1505 cm −1 and 3252 cm −1 attenuated dramatically, while peaks at 2106 cm −1 and 2095 cm −1 for azide-1 and azide-2 disappeared, and new peaks at 1617, 2921 cm −1 for CMOP-1 and 1612, 2927 cm −1 for CMOP-2 could be observed, confirming the occurring of click reaction and the formation of a triazole ring.The solid state 13 C CP-MAS NMR measurements gave similar spectra for the two CMOPs (Figure S2).The peak at ~169 ppm could be attributed to the carbon atom in triazine ring, while the peak at ~ 146 ppm was ascribed to phenyl carbon, which was directly linked with the triazine ring.The main peaks at ~141 ppm and ~134 ppm originated from the two carbon atoms of the triazole ring.The broad signals at about ~ 128-110 ppm corresponded to the phenyl rings.
Thermogravimetric analysis in the N2 atmosphere showed that the polymers could keep thermal stability under 240 °C (CMOP-1) and 210 °C (CMOP-2).The first 5% weight loss was mainly due to the volatilization of solvent adsorbed in the polymers (Figure S3).Powder X-ray diffraction (PXRD) analysis in the angle range of 5° to 35° suggested the amorphous nature of the CMOPs (Figure S4).We considered that the relative strong peaks may originate the light copper complex due to the Scheme 1.The synthetic route to themicroporous organic polymers.

Synthesis and Character
The CMOPs were synthesized by click reaction in DMF solvent from TEPT and aromatic azides, according to the procedure in Scheme 1.The resulting brown powders were insoluble in common organic solvent such as tetrahydrofuran, dichloromethane, methanol, chloroform, acetone, and N,N-dimethylformamide.
In the Fourier transform infrared (FTIR) spectra of CMOPs (Figure S1), characteristic peaks for TEPT at 1505 cm −1 and 3252 cm −1 attenuated dramatically, while peaks at 2106 cm −1 and 2095 cm −1 for azide-1 and azide-2 disappeared, and new peaks at 1617, 2921 cm −1 for CMOP-1 and 1612, 2927 cm −1 for CMOP-2 could be observed, confirming the occurring of click reaction and the formation of a triazole ring.The solid state 13 C CP-MAS NMR measurements gave similar spectra for the two CMOPs (Figure S2).The peak at ~169 ppm could be attributed to the carbon atom in triazine ring, while the peak at ~146 ppm was ascribed to phenyl carbon, which was directly linked with the triazine ring.
The main peaks at ~141 ppm and ~134 ppm originated from the two carbon atoms of the triazole ring.The broad signals at about ~128-110 ppm corresponded to the phenyl rings.
Thermogravimetric analysis in the N 2 atmosphere showed that the polymers could keep thermal stability under 240 • C (CMOP-1) and 210 • C (CMOP-2).The first 5% weight loss was mainly due to the volatilization of solvent adsorbed in the polymers (Figure S3).Powder X-ray diffraction (PXRD) analysis in the angle range of 5 • to 35 • suggested the amorphous nature of the CMOPs (Figure S4).We considered that the relative strong peaks may originate the light copper complex due to the interaction between Cu 2+ and nitrogen atoms.Scanning electron microscopy (SEM) images revealed the uniform morphology (Figure S5).

Porosity
The surface area and porosity of CMOPs was investigated by N 2 adsorption-desorption measurements at 77 K.The results were illustrated in Figure 1.CMOP-1 and CMOP-2 exhibited type-I isotherms that showed a sharp adsorption at relative low pressure (P/P o < 0.05), indicating a permanent microporous network.The Brunauer-Emmett-Teller (BET) surface areas were calculated to be 431 m 2 •g −1 and 406 m 2 •g −1 , while the microporous areas were 209 m 2 •g −1 and 188 m 2 •g −1 .The total pore volumes of CMOP-1 and CMOP-2 were 0.458 cm 3 •g −1 and 0.387 cm 3 •g −1 , with 0.09 cm 3 •g −1 and 0.08 cm 3 •g −1 microporous volume.The pore size distribution was estimated on the basis of the density functional theory (DFT) model.It revealed that the CMOPs exhibited two main distribution at 0.6 nm, 1.2 nm, and 0.7, 1.2 nm for CMOP-1 and CMOP-2, indicating the existence of ultramicroporosity (<1 nm), which was consistent with the microporous N 2 sorption isotherm (Figure S6).Although the azide-2 molecule is longer than that of azide-1, the pore sizes were almost the same due to the disordered overlap in the formation of the porous network.
interaction between Cu 2+ and nitrogen atoms.Scanning electron microscopy (SEM) images revealed the uniform morphology (Figure S5).

Porosity
The surface area and porosity of CMOPs was investigated by N2 adsorption-desorption measurements at 77 K.The results were illustrated in Figure 1.CMOP-1 and CMOP-2 exhibited type-I isotherms that showed a sharp adsorption at relative low pressure (P/Po < 0.05), indicating a permanent microporous network.The Brunauer-Emmett-Teller (BET) surface areas were calculated to be 431 m 2 •g −1 and 406 m 2 •g −1 , while the microporous areas were 209 m 2 •g −1 and 188 m 2 •g −1 .The total pore volumes of CMOP-1 and CMOP-2 were 0.458 cm 3 •g −1 and 0.387 cm 3 •g −1 , with 0.09 cm 3 •g −1 and 0.08 cm 3 •g −1 microporous volume.The pore size distribution was estimated on the basis of the density functional theory (DFT) model.It revealed that the CMOPs exhibited two main distribution at 0.6 nm, 1.2 nm, and 0.7, 1.2 nm for CMOP-1 and CMOP-2, indicating the existence of ultramicroporosity (<1 nm), which was consistent with the microporous N2 sorption isotherm (Figure S6).Although the azide-2 molecule is longer than that of azide-1, the pore sizes were almost the same due to the disordered overlap in the formation of the porous network.

Gas Sorption Capacity
The microporous framework with a narrow pore size distribution and nitrogen-rich character inspired us to explore CO 2 adsorption capacity.As shown in Figure 2, CMOP-1 exhibited a higher CO 2 adsorption ability of 1.85 mmol• g −1 (8.2 wt%) at 273 K and 0.80 mmol• g −1 (3.5 wt%) at 298 K, while CMOP-2, with its smaller surface area, showed a slightly lower uptake to 1.64 mmol• g −1 (7.3 wt%) and 0.57 mmol• g −1 (2.5 wt%) at 1.0 bar, which was superior to many heteroatom-rich polymers such as FPOP-1 (S BET : 538 m 2 •g −1 , 6.3 wt%), NOP-55 (S BET : 526 m 2 •g −1 , 8.6 wt%), CPOP-22 (S BET : 440 m 2 •g −1 , 6.9 wt%), and CuPor-BPDC (S BET : 442 m 2 •g −1 , 5.5 wt%) [37][38][39][40], but lower than other triazine-based polymers [26][27][28][29][30][31].In addition, H2 uptake was also investigated at 77 K (Figure 3).The adsorption capacity was increasing along with the pressure, and then increased up to saturation.The maximum uptakes were 2.94 mmol•g −1 (0.6 wt%) and 2.48 mmol•g −1 (0.5 wt%) for CMOP-1 and CMOP-2.The moderate capacity originated from the low surface area.It was found that nitrogen-rich polymer favored the sorption toward iodine, owing to the stronger interaction between nitrogen atom and electron-deficient I2 [4,32,33].The images in Figure S7 showed the color change of I2-cyclohexane (1 mg•ml −1 ) solution after adding 25 mg of CMOP-1 and CMOP-2.As time passed, the pink color of I2-cyclohexane solution faded clearly, indicating the good removal efficiency.To evaluate the I2 uptake ability of CMOPs, the sorption of I2 vapor was investigated at 353 K.The solid powder was placed in an open bottle that was exposed to vapor of I2 in a sealed container.As shown in Figure 4, the gravimetric uptake was gradually increasing, and then reached a plateau after about 24 h.The sorption ability was 160 wt% and 93 wt% for CMOP-1 and CMOP-2.The larger capacity of CMOP-1 was mainly attributed to the larger surface area and greater amount of nitrogen atoms in the surface.In addition, H 2 uptake was also investigated at 77 K (Figure 3).The adsorption capacity was increasing along with the pressure, and then increased up to saturation.The maximum uptakes were 2.94 mmol•g −1 (0.6 wt%) and 2.48 mmol•g −1 (0.5 wt%) for CMOP-1 and CMOP-2.The moderate capacity originated from the low surface area.In addition, H2 uptake was also investigated at 77 K (Figure 3).The adsorption capacity was increasing along with the pressure, and then increased up to saturation.The maximum uptakes were 2.94 mmol•g −1 (0.6 wt%) and 2.48 mmol•g −1 (0.5 wt%) for CMOP-1 and CMOP-2.The moderate capacity originated from the low surface area.It was found that nitrogen-rich polymer favored the sorption toward iodine, owing to the stronger interaction between nitrogen atom and electron-deficient I2 [4,32,33].The images in Figure S7 showed the color change of I2-cyclohexane (1 mg•ml −1 ) solution after adding 25 mg of CMOP-1 and CMOP-2.As time passed, the pink color of I2-cyclohexane solution faded clearly, indicating the good removal efficiency.To evaluate the I2 uptake ability of CMOPs, the sorption of I2 vapor was investigated at 353 K.The solid powder was placed in an open bottle that was exposed to vapor of I2 in a sealed container.As shown in Figure 4, the gravimetric uptake was gradually increasing, and then reached a plateau after about 24 h.The sorption ability was 160 wt% and 93 wt% for CMOP-1 and CMOP-2.The larger capacity of CMOP-1 was mainly attributed to the larger surface area and greater amount of nitrogen atoms in the surface.It was found that nitrogen-rich polymer favored the sorption toward iodine, owing to the stronger interaction between nitrogen atom and electron-deficient I 2 [4,32,33].The images in Figure S7 showed the color change of I 2 -cyclohexane (1 mg•mL −1 ) solution after adding 25 mg of CMOP-1 and CMOP-2.As time passed, the pink color of I 2 -cyclohexane solution faded clearly, indicating the good removal efficiency.To evaluate the I 2 uptake ability of CMOPs, the sorption of I 2 vapor was investigated at 353 K.The solid powder was placed in an open bottle that was exposed to vapor of I 2 in a sealed container.As shown in Figure 4, the gravimetric uptake was gradually increasing, and then reached a plateau after about 24 h.The sorption ability was 160 wt% and 93 wt% for CMOP-1 and CMOP-2.The larger capacity of CMOP-1 was mainly attributed to the larger surface area and greater amount of nitrogen atoms in the surface.

General Information
TEPT and azides were synthesized according to the methods in the literature [41][42][43].All of the other reagents were purchased from J&K (J&K, ShangHai, China) and used without further purification.
Before all of the measurements, polymers were dried under vacuum at 120 °C overnight.FTIR spectra were recorded on a Thermo-Nicolet iS10 spectrometer using KBr discs (Thermo Scientific Co., Ltd., Madison, WI, USA).Solid state 13 C CP-MAS NMR spectra data were collected on a BRUKER AVANCE III HD 400MHz NMR spectrometer (Bruker, Leipzig, Germany).Elemental analysis for C, H, and N were carried out with Elementar VarioMICRO Cube analyzer (Elementar, Frankfurt, Germany).Thermogravimetric analysis was performed on SDT Q600 V20.9 Build 20 (TA instrument, New Castle, USA) with a temperature ramping rate of 10 °C min -1 from 30 °C to 850 °C.Scanning electron microscopy (SEM) was performed on ZEISS Evo 18 scanning electron microscope at 20.0 KV (Zeiss, Oberkochen, Germany).Powder X-ray diffraction (PXRD) data were recorded on Bruker D8 Advance (Bruker, Leipzig, Germany), from 5° up to 35° with 4°/min increment.Nitrogen adsorptiondesorption isotherms were carried out with a Demo 2020 instrument at 77 K (Micromeritics Instrument Corp., Norcross, GA, USA).Carbon dioxide adsorption isotherms were measured at 273 K and 298 K on 3Flex Version 4.02 (Micromeritics Instrument Corp., Norcross, GA, USA).Hydrogen adsorption isotherms were conducted at 77 K on Quadrasorb SI (Quantachrome, FL, USA).

General Information
TEPT and azides were synthesized according to the methods in the literature [41][42][43].All of the other reagents were purchased from J&K (J&K, ShangHai, China) and used without further purification.
Before all of the measurements, polymers were dried under vacuum at 120 • C overnight.FTIR spectra were recorded on a Thermo-Nicolet iS10 spectrometer using KBr discs (Thermo Scientific Co., Ltd., Madison, WI, USA).Solid state 13 C CP-MAS NMR spectra data were collected on a BRUKER AVANCE III HD 400MHz NMR spectrometer (Bruker, Leipzig, Germany).Elemental analysis for C, H, and N were carried out with Elementar VarioMICRO Cube analyzer (Elementar, Frankfurt, Germany).Thermogravimetric analysis was performed on SDT Q600 V20.9 Build 20 (TA Instrument, New Castle, DE, USA) with a temperature ramping rate of 10 • C•min −1 from 30 • C to 850 • C. Scanning electron microscopy (SEM) was performed on ZEISS Evo 18 scanning electron microscope at 20.0 KV (Zeiss, Oberkochen, Germany).Powder X-ray diffraction (PXRD) data were recorded on Bruker D8 Advance (Bruker, Leipzig, Germany), from 5 • up to 35 • with 4 • /min increment.Nitrogen adsorption-desorption isotherms were carried out with a Demo 2020 instrument at 77 K (Micromeritics Instrument Corp., Norcross, GA, USA).Carbon dioxide adsorption isotherms were measured at 273 K and 298 K on 3Flex Version 4.02 (Micromeritics Instrument Corp., Norcross, GA, USA).Hydrogen adsorption isotherms were conducted at 77 K on Quadrasorb SI (Quantachrome, FL, USA).

The Gravimetric Sorption of I 2 Vapor
10 mg of polymer was placed in an open bottle (10 mL) and the total weight was weighed (W 0 , mg).The open bottle was then putted into a sealed vessel (50 mL) that contained 200 mg of iodine at 353 K in the oil bath.The open bottle was taken out and weighed after 1 h, 4 h, 8 h, 12 h, 24 h, and 48 h (W i , mg), respectively.The gravimetric sorption (wt%) was calculated using the following formula: wt% = (W i − W 0 )/10 × 100%

Conclusions
In conclusion, two nitrogen-rich triazine-based microporous organic polymers were designed and synthesized by the condensation of a 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine building block and aromatic azides linkers for the first time.The resulting polymers possessed an ultramicroporosity network, good thermal stability, and high nitrogen content (CMOP-1: 18.17 wt%; CMOP-2: 15.49 wt%, from EA).Despite the moderate surface areas of 431 m 2 •g −1 and 406 m 2 •g −1 based on nitrogen adsorption, CMOPs exhibited relatively high CO 2 sorption and moderate H 2 sorption capacity that was comparable to or superior than many POPs with similar surface area.We ascribed the excellent CO 2 sorption capacity to nitrogen-rich content, which improved the dipole-quadruple interaction between CO 2 and polymers.Furthermore, the CMOPs displayed significant gravimetric uptake ability toward I 2 vapor up to 160 wt%.The successfully constructed networks confirmed the advantage of nitrogen atoms for gas capture, and inspired our great interest to explore more functional MOPs.Supplementary Materials: Supplementary materials are available online. Figure S1.FTIR spectra of TEPT, azides and CMOPs.Figure S2.13C CP-MAS NMR spectra of CMOP-1 and CMOP-2.Figure S3.Thermogravimetric analysis (TGA) curves of CMOP-1 and CMOP-2.

Figure 4 .
Figure 4.The uptake of I2 vapor at 353 K.

Figure 4 .
Figure 4.The uptake of I 2 vapor at 353 K.