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* E-mail: zhengqy@iccas.ac.cn; Tel.: 0086-010-62652811 Received February 26, 2013; accepted April 17, 2013; published online XXXX, 2013. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300138 or from the author. Chin. J. Chem. 2013, XX, 1—5 © 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1

DOI: 10.1002/cjoc.201300138

Synthesis and Properties of Porous Organic Polymers from a Rigid Macrocyclic Building Block

Jingru Song, Zhitang Huang, and Qiyu Zheng*

Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

Two rigid macrocyclic CTV-based porous organic polymers, Click-POP-1 and Click-POP-2, have been synthe-sized by Click reaction of a cyclotriveratrylene analogue with alkyne groups and aromatic azides in one pot. FTIR, solid 13C NMR and elemental analysis confirmed the formation of the polymers. Both of them possess high thermo-stability which is up to 410 and 900 ℃, respectively, and moderate hydrogen storage properties with 0.46 wt% at 77 K. Their nitrogen uptake showed type-I isotherm with BET surface areas up to 342 and 317 m2•g−1.

Keywords cyclotriveratrylene, porous organic polymer, hydrogen uptake, click reaction, amorphous

Introduction Porous organic polymers, with the high thermo-sta-

bility, low density, high surface area and adjustable pore size, are becoming new kind of promising porous mate-rials, which could be used in gas storage,[1] separation[2] and heterogeneous catalysis.[3] In the past years, nu-merous porous organic polymers have been synthesized including crystalline covalent organic frameworks (COFs)[4] based on reversible reactions and amorphous polymers[5] from irreversible reactions. The latter can be classified into some subsets including hyper-crosslinked polymers (HCPs),[6] polymers of intrinsic micropor-ousity (PIMs),[7] conjugated microporous polymers (CMPs),[8] benzimidazole-linked polymers (BILPs),[9] and porous polymer networks (PPNs).[10] Despite the loss of crystalline, these polymers have similar tunable pore sizes and surface areas to COFs, simultaneously, easy controllable reaction condition and scale-up prepa-ration which precede COFs.

Hydrogen, as a safe and clean energy source, is a promising fuel instead of petroleum. However, lack of a safe and efficient method for hydrogen storage and transportation limited its development.[11] Recently, zeo-lite-like organic porous materials have been investigated deeply in the area of gas storage such as hydrogen and CO2 due to their low weight and remarkable ability of gas uptake. Among them, amorphous porous organic polymers with high BET surface areas have been syn-thesized with hydrogen uptake up to 2.3 wt% (77 K, 1bar)[9b], 3.94 wt% (77 K, 10 bar)[7a], and 8.34 wt% (77 K, 55 bar).[10] The building blocks for these porous polymers can be planar or non-planar, which resulted in

the materials with different surface area, pore volume and gas storage abilities.[12] Click reaction is one kind of efficient and easily handled reactions, which is widely used in polymerization of alkyne including porous ma-terials.[13] Macrocycles with intrinsic cavity as building blocks have unique advantage in preparation of porous organic polymers due to inherent and post-synthetic pore parts. Cyclotriveratrylene (CTV) and its analogues are macrocyclic molecules with a rigid hydrophobic bowl-shaped cavity.[14] In 2006, McKeown group found that PIM from cyclotricatechylene (CTC) building block showed better hydrogen uptake capacity than the PIM from the planar units.[7d] Our group also synthesized CTC-based COFs and conjugated porous networks based on CTV exhibiting alike superior hydrogen stor-age.[15] Thus, we consider that the aromatic macrocycle would give a better gas storage ability. Keeping this consideration in mind, we synthesized two amorphous porous organic polymers based on “Click reaction” be-tween the linear azide and CTV-based alkyne to inves-tigate the advantage of macromolecule.

Experimental General method

Azide-1, azide-2 and CTV-alkyne were synthesized according to literature methods.[16-18] DMF was distilled by the standard method. All of the other reagents were used as received without further purification. Click- POP-1 and Click-POP-2 were synthesized according to the route in Scheme 1.

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Experimental procedure Click-POP-1 CTV-alkyne (120 mg, 0.278 mmol),

azide-1 (67 mg, 0.558 mmol), CuSO4•5H2O (21 mg, 0.084 mmol), sodium ascorbate (16.6 mg, 0.0694 mmol) and DMF (7 mL) were placed in a 50 mL flask, and heated to 100 ℃ for 48 h under nitrogen (Scheme 1). Upon completion of the reaction, the mixture was fil-trated and washed with water, methanol and DCM. The residue was stirred in water for another 5 h. Brown powder (154 mg, 82%) was obtained after filtration and drying at 100 ℃ in vacuum. Anal. calcd for (C13H10N3O)n: C 69.64, H 4.46, N 18.75; found C 64.39, H 4.83, N 17.05.

Click-POP-2 CTV-alkyne (88 mg, 0.203 mmol), azide-2 (73 mg, 0.309 mmol), CuSO4•5H2O (15.3 mg, 0.0612 mmol), sodium ascorbate (12.0 mg, 0.05 mmol) and DMF (7 mL) were placed in a 50 mL flask, and heated to 100 ℃ for 48 h under nitrogen (Scheme 1). Upon completion of the reaction, the mixture was fil-

trated and washed with water, methanol and DCM. The residue was stirred in water for another 5 h. Brown powder (136.7 mg, 85%) was obtained after filtration and drying at 100 ℃ in vacuum. Anal. calcd for (C16H12N3O)n: C 72.45, H 4.53, N 16.98; found C 70.33, H 4.76, N 13.97.

Results and Discussion The FTIR spectra of two polymers are shown in

Figures 1 and 2. The absorption peaks for CTV-alkyne at 2102 and 3281 cm−1 disappear, while new peaks at 1615 cm−1 (Click-POP-1), 1611 cm−1 (Click-POP-2) for N＝N stretch and weak 2922 cm−1 (Click-POP-1), 2923 cm−1 (Click-POP-2) for C＝CH band can be observed, conforming the formation of triazole ring. Peak intensity at 2109 cm−1 (Click-POP-1) and 2136, 2099 cm−1 (Click-POP-2) for azide-1 and azide-2 is strongly at-tenuated, suggesting that the reactions smoothly occur with somewhat incompleteness.

Scheme 1 The synthetic route to the polymers

OMe

OMe

MeO

OMe

OMe

MeONNN

NN

N

NN

N

OMe

OMe

MeO

OMe

OMe

MeONNN

NN

N

NN

N

N3

N3

N3

N3

CTV-alkyne

+

CuSO4.5H2O

Na ascorbate

DMF, 100 oC, 48 h

CTV-alkyne

+

CuSO4.5H2O

Na ascorbate

azide-1

azide-2

Click-POP-1

Click-POP-2

(

)

(

)

)

)

DMF, 100 oC, 48 h

n

n

n

n

n

n

n

Synthesis and Properties of Porous Organic Polymers from a Rigid Macrocyclic Building Block

Chin. J. Chem. 2013, XX, 1—5 © 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 3

Figure 1 The FTIR spectra of (a) CTV-alkyne, (b) azide-1, and (c) Click-POP-1.

Figure 2 The FTIR spectra of (a) CTV-alkyne, (b) azide-2, and (c) Click-POP-2.

The solid state 13C CPMAS NMR spectra of the two polymers give similar result with the characteristic CTV peaks at δ 36.507, 36.667 (CH2) and 55.396, 55.314 (OCH3) respectively for POP-1 and POP-2. Aromatic region shows us board peaks for the benzene and tria-zole carbones (Figure S4).

Thermogravimetric analysis (Figure S2) of dried samples shows that these polymers are highly thermal stable. Click-POP-1 loses only 5 wt% below 274 ℃ and 17 wt% below 410 ℃, while Click-POP-2 loses 5 wt% till 900 ℃. These polymers are insoluble in com-mon solvents such as tetrahydrofuran, N,N-dimethyl- formamide, dichloromethane, methanol, and acetone. All above are common properties for amorphous poly-mers. Scanning electron microscopy (SEM) shows uni-form morphology for the two polymers (Figure S3).

The porosity of the two polymers was investigated by nitrogen uptake measurements. Fully activated and evacuated samples of polymers were used for the meas-urement of gas adsorption isotherms at 77 K. Figure 3 shows the adsorption and desorption isotherms. The

isotherms are type-I indicating a permanent micropor-ous nature. The observed hysteresis is attributed to the restricted access of adsorbate to the pores blocked by narrow openings in flexible microporous polymer framework. Applying the Brunauer-Emmett-Teller (BET) model within the pressure range of p/po＝0.06－0.20 resulted in surface areas of 342, 317 m2•g−1 for Click-POP-1 and Click-POP-2 respectively. Estimation of the pore size from a density functional theory (DFT) model shows a mainly pore width of 1.27 and 1.35 nm with a little mesoporous pore. We also tested the hydro-gen uptake at low pressure (0－823 mmHg) at 77 K (Figure 4). The maximum uptakes both are 0.46 wt% at 398 mmHg and 451 mmHg. The moderate uptake is consisting with the relative low surface area.

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350

.

(a)

Vol

ume

adso

rbed

/(cm

3 g-1)

Relative pressure (p/po)

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

.

(b)

Volu

me

adso

rbed

/(cm

3 g-1)

Relative pressure (p/po)

Figure 3 N2 uptake isotherms of the polymers (a) Click-POP-1, (b) Click-POP-2 (filled cycles indicate adsorption and open cycles indicate desorption).

Conclusions In conclusion, using the rigid macrocycle cyclo-

triveratrylene analogue CTV-alkyne as the building unit, two amorphous porous organic polymers were prepared by Click reaction. Compared to other synthetic methods, this route to porous polymers was easy to operate and

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0 200 400 600 800

0

10

20

30

40

50

60

.

(a)

Qua

ntity

ads

orbe

d/(c

m3 g

-1)

Absolute pressure/mmHg

0 200 400 600 800

0

10

20

30

40

50

60

.

(b)

Qua

ntity

ads

orbe

d/(c

m3 g

-1)

Absolute pressure/mmHg

Figure 4 H2 uptake isotherms of the polymers (a) Click-POP-1, (b) Click-POP-2.

the purification procedure was simple to achieve. Thermo-stability of two polymers could be up to 410 and 900 ℃, respectively. Type-I nitrogen uptake iso-therm confirmed the microporous character of these polymers, exhibiting pore size at 1.27 and 1.35 nm for Click-POP-1 and Click-POP-2, respectively. The BET surface areas were 342 and 317 m2•g−1. Although the surface area is lower, gravimetric hydrogen adsorption isotherms still show the moderate adsorption capacity 0.46 wt% at 77 K at low pressure.

Acknowledgement This work was supported by the National Natural

Science Foundation of China (No. 20932004), and the Major State Basic Research Development Program of China (No. 2011CB932501).

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