Synthesis and characterization of pyrrole-containing microporous polymeric networks

Polymer 54 (2013) 3254e3260

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Synthesis and characterization of pyrrole-containing microporous polymeric networks Yanqin Yang a, b, Qiang Zhang a, Jifu Zheng a, Suobo Zhang a, * a b

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 January 2013 Received in revised form 2 April 2013 Accepted 17 April 2013 Available online 24 April 2013

Two types of microporous polymeric networks have been prepared from monomers containing N-tertbutoxycarbonyl-protected pyrrole by FeCl3-mediated oxidative coupling polymerization. These materials were predominantly microporous (with BET surface areas of 828 m2 g
1 and 1408 m2 g
1), exhibiting high CO2 uptake capacities (1.96 mmol g
1 and 2.69 mmol g
1 at 273 K, 1 bar). Novel microporous polymeric films (with BET surface areas of 570 m2 g
1 and 593 m2 g
1) were fabricated through in situ polymerization of monomers on a flat glass dish using a solegel process catalyzed by trifluoroacetic acid. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Microporous polypyrroles Solegel process Microporous film

1. Introduction A diverse range of microporous organic polymers (MOPs) with simple chemical routes for their synthesis and high intrinsic levels of porosity have recently spurred scientific interest in this class of materials. Depending on their design and properties, MOPs have been classified into covalent organic frameworks (COFs) [1e3], hyper-crosslinked polymers (HCPs) [4,5], polymers with intrinsic microporosity (PIMs) [6e8], conjugated microporous polymers (CMPs) [9,10], and porous aromatic frameworks (PAFs) [11,12]. They possess small pores (<2 nm) and very high surface areas and have been found to have potential applications particularly in molecular separations [13,14], heterogeneous catalysis [15,16], and gas storage [17,18]. MOPs possessing functional units have also been reported which fulfill certain tasks based on their functionalities [19e23]. Recently, some MOPs containing aromatic heterocyclic units have been synthesized by FeCl3-mediated oxidative coupling polymerization [22,23]. For instance, Thomas and coworkers have synthesized MOPs based on the poly(thienylene arylene) motif via oxidative coupling polymerization promoted by FeCl3 [22a]. The high density of thiophene functionalities in the networks makes the materials good candidates for encapsulation of metal catalyst. Han and coworkers have prepared a MOP named CPOP-1 via the oxidative polymerization of 1,3,5-tri(9-carbazolyl)-benzene [23].

* Corresponding author. Tel.: þ86 431 85262118; fax: þ86 431 85262117. E-mail address: [email protected] (S. Zhang). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.04.038

The incorporation of nitrogen-containing structures makes the network electron-rich, resulting in the materials possessing high CO2 uptake capacity. Additionally, FeCl3-mediated oxidative polymerization exhibits cost-effective advantages such as cheap catalyst, high yield, and room temperature reaction conditions which are essential for the large scale production of porous materials [22,23]. Polypyrrole as a conducting polymer has attracted considerable attention due to its excellent properties, such as appreciable environmental stability, biocompatibility, good optical and electrical properties and relatively low production cost [24e26]. Microporous polypyrrole is of particular interest since it possesses a conjugated backbone with NeH functional groups. Porosity is typically imparted to polypyrrole via a template process utilizing nanoporous carbons as porogen [27e29]. The porous structure may have an effect on some properties of electrode materials, such as mutual permeation with another substance at the interface, mobility and separation of charge, performance of sensor and so on [22a]. It has been reported that carbon nanotube and polypyrrole nanoporous composites are suited to energy storage applications such as supercapacitors and secondary batteries [29]. Searson and coworkers have constructed high charge density conducting polypyrrole/graphite fiber composite electrodes for battery applications [27]. More recently, Svec’s group have synthesized microporous polypyrroles by using N-alkylation with iodo compounds to hypercrosslink the nonporous precursor [30]. To our knowledge there are no literature reports on the synthesis of microporous

Y. Yang et al. / Polymer 54 (2013) 3254e3260

polypyrrole networks directly from pyrrole-containing monomers using an oxidative polymerization process. Herein, we describe the synthesis of pyrrole-containing polymers by oxidative polymerization of two novel monomers, namely Tetrakis(4-(N-(tert-butoxycarbonyl)-pyrrole)phenyl)methane (TNPM) and 1,3,5-tri(4-(N(tert-butoxycarbonyl)-pyrrole)phenyl)benzene (TNPP). 2. Experimental 2.1. Materials Tetrakis(4-bromophenyl)methane (TPM) and 1,3,5-tri(4-bromophenyl)benzene were prepared according to reported methods [11,31]. N-(tert-butoxycarbonyl)-pyrrol-2-ylboronic acid and Pd(PPh3)4 were purchased from Sigma Aldrich and used as received. CH2Cl2 (DCM) was dried over CaH2 prior to use. Anhydrous potassium carbonate was finely powdered prior to use. Other chemicals were used as received. 2.2. Synthesis of monomers 2.2.1. Synthesis of tetrakis(4-(N-(tert-butoxycarbonyl)-pyrrole) phenyl)methane (TPNM) To a solution of tetrakis(4-bromophenyl)methane (0.6360 g, 1 mmol) and N-(tert-butoxycarbonyl)-pyrrol-2-ylboronic acid (1.0128 g, 4.8 mmol) in 30 mL of THF was added 10 mL of a saturated aqueous solution of potassium carbonate, and then degassed by bubbling nitrogen gas for 15 min. After adding Pd(PPh3)4 (57 mg, 0.05 mmol) under a nitrogen atmosphere, the reaction mixture was stirred at 80  C for 12 h, and then cooled to room temperature. The resulting mixture was extracted with ethyl acetate three times. The combined organic layer was washed with deionized water and dried over sodium sulfate. After concentration under reduced pressure, the residual oil was purified by column chromatography using n-hexane: ethyl acetate (30: 1) as the eluent to give the desired product as a light yellow solid in 81% yield. 1H NMR (400 MHz, DMSO-d6, d): 7.35 (t, J ¼ 2.0 Hz, 4 H), 7.30 (d, J ¼ 8.0 Hz,

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8 H), 7.26 (d, J ¼ 8.0 Hz, 8 H), 6.27 (s, 8 H), 1.29 (s, 36 H). 13C NMR (600 MHz, CDCl3, d): 149.92, 145.93, 134.96, 132.37, 130.58, 128.67, 123.22, 115.03, 110.98, 84.10, 64.85, 28.05. 2.2.2. Synthesis of 1,3,5-tri(4-(N-(tert-butoxycarbonyl)-pyrrole) phenyl)benzene (TNPP) TNPP was synthesized by the reaction of 1,3,5-tri(4-bromophenyl) benzene and N-(tert-butoxycarbonyl)-pyrrol-2-ylboronic acid according to the same method. 1H NMR (400 MHz, DMSO-d6, d): 7.90 (s, 3 H), 7.75 (d, J ¼ 8.0 Hz, 6 H), 7.53 (d, J ¼ 8.0 Hz, 6 H), 7.44 (s, 3 H), 6.32 (s, 6 H), 1.47 (s, 18 H). 13C NMR (600 MHz, CDCl3, d): 149.79, 142.52, 140.26, 135.31, 134.09, 126.92, 125.44, 123.19, 115.08, 111.10, 110.39, 84.14, 28.10. 2.3. Synthesis of microporous polypyrroles Microporous polypyrroles were synthesized by oxidative coupling polymerization of the tert-butoxycarbonyl-protected monomers promoted by anhydrous FeCl3. The polymers prepared from TNPM and TNPP are denoted as PTNPM and PTNPP, respectively. Polymeric films prepared from TNPM and TNPP are named as PTNPM-F and PTNPP-F, respectively. The representative preparations for PTNPM and PTNPM-F are given in detail as follows. 2.3.1. The preparation of PTNPM PTNPM was prepared by following an analogous procedure for the synthesis of CPOP-1 [23]. To the solution of TNPM (0.196 g, 0.2 mmol) in 20 mL of anhydrous CH2Cl2 was added anhydrous FeCl3 (156 mg, 0.96 mmol) under a nitrogen atmosphere. The solution mixture was stirred at room temperature for 48 h. Then 100 mL of methanol was added to the reaction mixture and the resulting mixture was stirred for 1 h. The brown precipitate obtained was filtered and successively washed with water, methanol, and CH2Cl2. After extraction in a Soxhlet extractor with methanol for 24 h, and then with THF for another 24 h, the desired polymer was collected and dried in a vacuum oven at 120  C for 24 h and 190  C for 1 h (83% yield).

Scheme 1. Synthesis of monomers and polymeric networks.

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2.3.2. The preparation of PTNPM-F 5 mL of CF3COOH was added dropwise to the solution of TNPM (0.196 g, 0.2 mmol) in 5 mL of anhydrous CH2Cl2 at 0  C under nitrogen in about 2 min. The solution mixture was stirred for 1 h and then transferred to a preheated glass dish where the polymerization occurred, followed by solvent evaporation yielding a black, thin film. The film was dipped in 0.5 M NaOH solution for 24 h to remove the excess CF3COOH. After the film was washed with water and methanol several times, it was dried in a vacuum oven at 120  C for 24 h (93% yield).

Japan) at an accelerating voltage of 6.0 kV and equipped with a Horiba energy dispersive X-ray spectrometer. Volumetric gas adsorption was performed on an Autosorb-1 instrument (Quantachrome). Nitrogen adsorption isotherms at 77 K were measured in liquid-nitrogen baths. P and P0 are the vapor pressure of the adsorbate in adsorption measurements and the saturated vapor pressure of the adsorbate, respectively, and P/P0 denotes the relative pressure of the adsorbate.

2.4. Characterization

3.1. Synthesis and characterization

Solution-state 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 400 MHz and 600 MHz spectrometer, respectively. Chemical shifts are reported in ppm downfield from SiMe4. Solidstate NMR spectra were obtained on a Chemagnetics CMS-400 spectrometer. Powder X-ray diffraction data were collected using a Bruker D8-ADVANCE qe2q diffractometer. The FT-IR spectra were obtained using a Bruker Vertex 70 spectrometer at a nominal resolution of 2 cm
1. TGA was performed on a PU 4K (Rigaku) at a heating rate of 10  C min
1 with N2 as carrier gas. Size and morphology of the samples were observed under a field-emission scanning electron microscope (HitachiS-4800). SEM observations were carried out using a Hitachi S-4800 microscope (Hitachi Ltd.,

The monomers were synthesized from the Suzuki reaction of the corresponding poly bromo aryl hydrocarbons and N-(tertbutoxycarbonyl)-pyrrol-2-ylboronic acid, as shown in Scheme 1. The yield of the reaction was about 80%, and the proposed structure was confirmed from 1H NMR, 13C NMR and FT-IR spectra (Fig. 1). The FT-IR spectra showed a disappearance of the CeBr vibration of tetrakis(4-bromophenyl)methane at 511 cm
1 and the appearance of the characteristic absorption bands of pyrrole in the 700e1600 cm
1 range (i.e., 1500, 1370, 1250 and 875 cm
1 were

Fig. 1. FT-IR spectra of TPM, TNPM and PTNPM (top) and TNPP, PTNPP and PTNPP-F (bottom).

3. Results and discussion

Fig. 2. Solid-state

13

C NMR spectrum for PTNPM (top) and PTNPP (bottom).

Y. Yang et al. / Polymer 54 (2013) 3254e3260

considered to be the result of pyrrole ring stretching vibration, CeN stretching vibration and ]CeH in-plane deformation vibration, respectively) [32]. In addition, the appearance of a typical absorption band for e(C]O)e absorption at around 1750 cm
1, indicated the successful cross-linking of the two reagents. The molecular weight measured by MS matched with the proposed structure, which also confirmed the successful synthesis of the monomers. The microporous polymeric networks were synthesized by oxidative coupling polymerization promoted by anhydrous FeCl3. This reaction produced brown powdery compounds, which had a low density and were not soluble in any of the common organic solvents. For purification, the samples were successively washed with water, methanol, and CH2Cl2 to remove solvent residues and starting materials, and were finally heated at 120  C for 24 h under vacuum to remove volatile entities. PTNPM and PTNPP were also heated at 190  C for 1 h to thermally deprotect the pyrrole units [33]. The polymers were amorphous as evidenced by powder X-ray diffraction analysis (Supporting Information, Fig. S1). They were also characterized from their FT-IR spectra (Fig. 1). The characteristic

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absorption bands of pyrrole in the 700e1600 cm
1 range were found, which meant the pyrrole ring remained unaffected during the polymerization. The almost disappearance of the absorption band for e(C]O)e absorption at 1750 cm
1 and appearance of the broad peak for NeH absorptions at 3400 cm
1 indicated the deprotection of the pyrrole moieties in the polymer. The structure of the networks has been characterized at the molecular level by 13C solid-state NMR and the signal assignments for the spectra are displayed in Fig. 2. The 13 C solid-state NMR confirmed the presence of sp2 carbons from the pyrrole and benzene as well as sp3 carbon from methyl in tetrakisphenylmethane containing network. The scanning electron micrograph showed that the polymeric networks consisted of fused spheres with diameters of 100 nm for PTNPM (Fig. 3a) and 50 nm for PTNPP (Fig. 3b). Thermo gravimetric analysis (TGA) indicated that the polymer networks remained stable upto 300  C (Fig. 4). The polymers were also stable towards water, base and acid. For instance, there was negligible change in the FT-IR spectra of PTNPM after dipping in 1 M HCl for 2 days or 1 M NaOH for 30 days at room temperature (Fig. S2).

Fig. 3. SEM image showing fused spheres in the PTNPM (a) and PTNPP (b); (c) Photograph of PTNPM-F; (d) SEM image showing the smooth surface of PTNPM-F; (e and f) Crosssectional morphologies of PTNPM-F.

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Fig. 4. TGA curves of microporous polymer networks.

3.2. Gas adsorption studies The pore structures of the networks were evaluated by nitrogen sorption isotherms (Fig. 5) measured at 77 K, and the two networks exhibited a combination of type I and II reversible sorption

isotherms according to the IUPAC classification [34]. The high uptake at very low pressures indicated high microporosity. With the increase of pressure, the nitrogen uptake increased, which indicated a high external surface area due to the presence of very small particles [35]. Listed in Table 1 are the key structural properties derived from the sorption isotherm data, such as BET and Langmuir specific surface area, micropore specific area, total volume, and micropore volume. The BET surface area of PTNPM (1408 m2 g
1) was higher than that of PTNPP (828 m2 g
1). This can be attributed to the default diamondoid framework topology imposed by the tetrahedral monomer, which provides wide openings and interconnected pores to efficiently eliminate “dead space” [36,37]. The pore size calculated from nonlinear density functional theory (NLDFT) gave a value of about 1.7 nm for PTNPM and 1.4 nm for PTNPP (Fig. 5). The high surface area and the microporous nature of the polymers prompted us to study their CO2 sorption properties. Fig. 6 shows the CO2 isotherms of the networks measured at 273 K. The CO2 sorption was completely reversible and no significant hysteresis was observed. At 1 bar, the CO2 uptake of PTNPM and PTNPP reached values of 2.69 and 1.96 mmol g
1, respectively. As a comparison of the adsorption efficiency, BPL carbon (SBET ¼ 1150 m2 g
1; a common reference material for CO2 uptake) exhibited an uptake of 1.9 mmol g
1 at 1 bar and 273 K [38,39]. The covalent organic framework, COF-102 (SBET ¼ 3620 m2 g
1) showed a CO2 uptake of 1.56 mmol g
1 at 1 bar and 273 K [40]. As an additional reference, PAF-1 (SBET ¼ 4077 m2 g
1) adsorbed 2.65 mmol g
1 under the same conditions. The correlation between CO2 uptake capacities and the surface areas is not proportional, and chemical functionality also plays a significant role [17,41]. For our system, the high density of pyrrole functionalities makes the polymers electron-rich, which enhances the interaction between CO2 and the polymers [23], resulting in their relatively high CO2 uptake capacities. 3.3. Preparation and properties of microporous films

Fig. 5. Nitrogen adsorptionedesorption isotherms of microporous polymeric networks measured at 77 K (top); Pore size distribution of microporous polymeric networks (bottom). (The adsorption and desorption branches are labeled with filled and open symbols, respectively.)

The crucial factor that determines the performance of photoelectric devices is the quality of thin films. However, most of the microporous polymeric networks synthesized till date are intractable solids and processability of the networks into thin films is difficult [42]. In this context, the production of microporous network membranes using traditional methods which requires the formation of a polymer solution and subsequent solvent evaporation is limited [43,44]. Microporous network films can be synthesized by applying techniques similar to solegel processes that entail a polymerization and a concurrent solvent evaporation process [43]. Recently, Dai and coworkers have developed a facile synthesis of triazine-framework-based porous membranes (TFMs) for CO2 separation through superacid-catalyzed cross-linking reactions [43]. However till date, no electroactive microporous polymer prepared by this method has been reported. As 1H-pyrrole can self-polymerize in the presence of acid, we have prepared the microporous network films (PTNPM-F and PTNPP-F) through an acid catalyzed solegel process. Scheme 2 shows the synthetic protocol for the formation of PTNPM-F film. Trifluoroacetic acid (CF3COOH) was added to the solution of TNPM in DCM under a nitrogen atmosphere. After stirring at 0  C for 1 h, the transparent solution became viscous and was transferred to the glass dish which had been preheated to 40  C. At this stage, selfpolymerization of the deprotected TNPM had started. The wet film on the glass dish was heated at 80  C for 40 min for further polymerization and a concurrent solvent evaporation process, yielding the film PTNPM-F. The film could automatically peel off from the glass dish, to become a free-standing film as shown in Fig. 3c for PTNPM-F.

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Table 1 Porosity properties of microporous polymers. Network

SBETa(m2 g
1)

SLanga(m2 g
1)

Smicroa(m2 g
1)

Vporea(cm3 g
1)

SBETb(m2 g
1)

SLangb(m2 g
1)

Vporeb(cm3 g
1)

CO2 uptakeb(mmol g
1)

PTNPM PTNPP PTNPM-F PTNPP-F

1408 828 29 4

2109 1248 65 9

807 542 0 0

1.58 0.63 0.058 0.013

678 474 570 593

765 538 679 770

0.081 0.054 0.051 0.050

2.69 1.96 1.60 1.17

a b

Data calculated from N2 sorption isotherms at 77 K. Data calculated from CO2 sorption isotherms at 273 K.

Fig. 6. CO2 adsorption (closed)/desorption (open) isotherms of powders (top) and films (bottom) at 273 K.

As shown in Fig. 1, PTNPP and PTNPP-F possess similar FT-IR spectra, which indicate that they have almost identical chemical structures even though they were synthesized by different methodologies. However, the films contained a small residue of trifluoroacetate anions in their networks even though they were dipped in 0.5 M NaOH for 24 h and washed thoroughly with water and methanol (Fig. S3). The surface scanning electron micrographs showed that the films had smooth and uniform surfaces (Fig. 3d). In order to further investigate the morphology of the films, crosssectional SEM images of PTNPM-F was taken, as shown in Fig. 2e and f. PTNPM-F, with a thickness of 16 mm, consists of compact nano-size granules without any obvious inter-particulate porosity. Unfortunately, the films were effectively nonporous to nitrogen at 77 K (with the BET surface areas of 29 m2 g
1 for PTNPM-F and 4 m2 g
1 for PTNPP-F), suggesting limited N2 uptake at that temperature. This can be attributed either to the absence of interparticle porosity used to transport N2 molecules into the films [45] or the restricted access of N2 molecules within the narrow microporosity (<0.7 nm) [46]. In contrast, the adsorption of CO2 at 273 K offers a valuable complementary technique to analyze the microporosity, in particular the narrow microporosity [47,48]. Therefore, the permanent porosity of the films was measured by CO2 sorption at 273 K (Fig. 6). As summarized in Table 1, the BET surface areas calculated from CO2 sorption isotherms were 570 m2 g
1 for PTNPM-F and 593 m2 g
1 for PTNPP-F, which were similar to that of PTNPM (678 m2 g
1) and PTNPP (474 m2 g
1), even though the latter pair had much higher BET surface areas calculated from N2 sorption isotherms. In addition, the CO2 sorption isotherms of the films had obvious hysteresis, while their “powdery” counterparts had reversible sorption isotherms. All these results could be explained by the narrower microporosity in the films. The CO2 uptake capacities of films (1.60 mmol g
1 for PTNPMF and 1.17 mmol g
1 for PTNPP-F) were lower than that of their “powdery” counterparts (2.69 mmol g
1 for PTNPM and 1.96 mmol g
1 for PTNPP). This can be attributed to the presence of residual trifluoroacetate anions in the films, which in a way, led to the decrease of free NeH’s in the structure and the crowding of the pores.

Scheme 2. Schematic diagram for the synthesis of PTNPM-F by the solegel process.

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4. Conclusion Two types of microporous polymeric networks have been prepared from pyrrole-containing monomers by FeCl3-mediated oxidative coupling polymerization. The polymeric networks remained stable up to 300  C and were also stable towards water, base and acid. PTNPM and PTNPP exhibited BET surface areas of 828 and 1408 m2 g
1, with CO2 uptake capacities of 1.96 and 2.69 mmol g
1 at 273 K and 1 bar, respectively. Microporous polymeric films, with BET surface areas of 570 and 593 m2 g
1, were constructed through in situ deprotection and polymerization of monomers using a solegel process on a flat glass dish catalyzed by trifluoroacetic acid. Further applications in the area of energy storage and CO2 separation for the microporous films are currently being explored in our lab. Acknowledgments This research was financially supported by the National Basic Research Program of China (No. 2012CB932802), the National Science Foundation of China (No. 51133008, 51021003), and the National high Technology Research and Development Program of China (No. 2012AA03A601).

[14] [15] [16] [17] [18] [19]

[20] [21] [22]

[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

Appendix A. Supplementary data [34]

Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.polymer.2013.04.038.

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