A pillar[5]arene-containing cross-linked polymer: synthesis, characterization and adsorption of dihaloalkanes and n-alkylene dinitriles

Abstract

A pillar[5]arene-containing cross-linked polymer was synthesized via Williamson reaction involving monohydroxy pillar[5]arene with chloromethylated polystyrene. Its adsorption behavior towards guests was investigated. It was found that the pillar[5]arene-containing polymer exhibited good adsorption capacities towards dihaloalkanes and dinitriles by host-guest complexation.

---

Pillararenes, first reported in 2008,¹ are a new family of macrocyclic hosts^2–6 besides crown ethers, cyclodextrins, calixarenes, and cucurbiturils. They consist of hydroquinone units linked by methylene bridges at para-positions and contain a rigid pillar-like electron-rich cavity. The unique structure and easy functionalization of pillararenes have given them the intriguing ability to selectively bind guests and provide a useful platform for the construction of abundant interesting supramolecular systems, including supramolecular polymers,^7–9 artificial transmembrane channels,^10–12 molecular machines,^13–16 drug delivery and controlled release systems,^17,18 functional vesicles,^19–24 nano materials,^25–28 chemical sensors,^29–33 and so forth. Although various applications of pillararenes have been reported, new applications in adsorption fields are still in their infancy.^34–38

The chemical contamination of water from a vast array of toxic derivatives, in particular organic compounds such as alkyl halides and alkyl nitriles, remains a serious environmental and public problem owing to their potential human toxicity. Moreover, chlorination is currently the most prevalent water disinfection treatment, which can ensure the biological security of drinking water. However, various types of disinfection by-products,^39,40 such as haloalkanes and alkyl nitriles which may lead to carcinogenesis, teratogenesis and mutagenesis,⁴¹ might be formed during the chlorination. Hence, it is essential to develop new technologies that can remove toxic pollutants found in wastewater and tap water. Membrane processes,^42–44 biological treatments,^45–47 chemical and electrochemical techniques,^48–50 advanced oxidation processes,^51–53 and adsorption procedures^54–57 are the most widely used to remove metals and organic compounds from the industrial effluents. Among all the treatment methods, adsorption is one of the most prevalent methods for the removal of pollutants. The rigid pillar-like structure and electron-rich cavity of pillararenes furnish prominent ability to complex different types of guests such as alkyl halides and alkyl nitriles.^58,59 It is of great significance to exploit pillararene-containing polymers to remove these toxic pollutants as novel adsorption materials.

For certain applications, such as adsorption toward organic compounds from aqueous media or fabrication of devices, it is crucial for a pillararene derivative to be available in solid form. However, there are only very few examples about immobilizing pillararenes onto polymers through covalent bond.^37,60–62 Merrifield resin, a divinylbenzene cross-linked copolymer of styrene and chloromethyl styrene, remains solid state in common organic solvents and aqueous media. Thus, it is predicted that immobilizing pillararene moieties onto Merrifield resin might endow the polymeric pillararenes with insolubility and the advantages of easy handling and adsorption ability toward organic compounds.

Herein, we report the synthesis of a cross-linked pillar[5]arene-containing polymer (PS-P5) (Scheme 1) for the first time. Its host–guest properties and adsorption behavior toward dihaloalkanes and n-alkylene dinitriles were investigated.

Scheme 1 Synthesis of pillar[5]arene-containing polymer PS-P5 (50 mol% functionalization) and structural formulas of the adsorbates.

As depicted in Scheme 1, monohydroxy pillar[5]arene (HOP5) was reacted with commercially available chloromethylated polystyrene (CMPS), namely Merrifield resin, in the presence of K₂CO₃ and KI in acetone under reflux for 5 days. Then, the polymer PS-P5 was obtained by filtration and washing with acetone repeatedly to remove unreacted HOP5. The amount of loaded pillar[5]arene on CMPS was evaluated by the difference of the weight before and after the coupling reaction with HOP5. The result showed that about 50% of the chlorine content was converted to pillar[5]arene moieties, which is in accordance with the calculations from the elemental analysis (Table S1†). Thus, there are 1.2 mmol pillar[5]arene units per gram of PS-P5. The incomplete conversion of chlorine may be mainly due to the unfavorable steric hindrance of pillar[5]arene.

The FT-IR spectra of HOP5 (curve a), PS-P5 (curve b), and CMPS (curve c) were shown in Fig. 1. The peaks at 1450 cm⁻¹ and 1264 cm⁻¹ in the FT-IR spectrum of CMPS were assigned to the –CH₂ in-plane bending and out-of-plane bending, respectively. The peak at 759 cm⁻¹ could be the C–Cl stretching of –CH₂Cl. For HOP5, the peak at 3421 cm⁻¹ was assigned to the vibration of –OH stretching. The peaks at 2827 cm⁻¹ and 1400 cm⁻¹ were corresponded to the C–H symmetric stretching and symmetric bending of –OCH₃, respectively. The peak at 1047 cm⁻¹ could be assigned to the symmetric stretching of C–O–C in HOP5. As shown in Fig. 1 (curve b), we can see that (1) new peaks at 2827 cm⁻¹, 1400 cm⁻¹ and 1047 cm⁻¹ appeared; (2) the peak at 3421 cm⁻¹ disappeared, and (3) the transmittances of the peaks at 1450 cm⁻¹, 1264 cm⁻¹ and 759 cm⁻¹ decreased. These phenomena confirmed that pillar[5]arene modified Merrifield resin PS-P5 was successfully achieved.

Fig. 1 FT-IR spectra of (a) HOP5, (b) PS-P5 and (c) CMPS.

The attachment of pillar[5]arene units onto Merrifield resin CMPS was also supported by CP/MAS ¹³C solid-state NMR spectroscopy. As is shown in Fig. 2, the ¹³C solid-state NMR spectra of HOP5 (curve a), PS-P5 (curve b), and CMPS (curve c) were compared. The peaks observed at 29.39 ppm and 54.33 ppm in the HOP5 spectrum, unequivocally attributed to the carbon of methylene bridge and methoxy group, respectively, were also found in the PS-P5 spectrum (δ = 29.85 ppm and δ = 54.73 ppm) with small downshift, indicating that pillar[5]arene was successfully immobilized on Merrifield's resin CMPS. Moreover, the steps of weight loss for the hybrid material were shown in the curves of thermogravimetric analysis (Fig. S9†).

Fig. 2 CP/MAS ¹³C solid-state NMR spectra of (a) HOP5, (b) PS-P5, and (c) CMPS.

Based on the previous studies,^58,59 the electron rich cavity of the pillar[5]arene can complex dihaloalkanes and n-alkylene dinitriles via multiple CH/π interactions. Therefore we investigated the host–guest complexation and the adsorption properties of this new polymer-supported pillar[5]arene PS-P5 toward dichlorobutane (DCB), dichlorohexane (DCH), dibromobutane (DBB), dibromohexane (DBH), diiodobutane (DIB), diiodohexane (DIH), butanedinitrile (DNB) and hexanedinitrile (DNH). We firstly determined the relationship between the peak areas and the concentrations by Gas Chromatography (GC) to receive standard curves. The standard curves for eight adsorbates are given in Fig. S1–S8.† A typical procedure for the adsorption experiments as follows: PS-P5 (50 mg, i.e. 0.06 mmol pillar[5]arene units) was added into 20 mL of DBB solution in toluene (0.03 mmol DBB). The mixture was stirred at room temperature for 12 h. Then PS-P5 was removed by centrifugation. The filtrate was measured by GC to determine the concentration of the residual DBB. The amount of DBB adsorbed on PS-P5 and the adsorption ratio were calculated. The adsorption ratio (A%) was calculated as:

A% = (C₀ − C)/C₀ × 100% (1)

where C₀ and C are the initial and residual concentrations of DBB before and after the adsorption, respectively.

The adsorption ratios of all the adsorbates are summarized in Table 1. From the results, it is clear that PS-P5 exhibited stronger binding affinity toward DCB, DBB, DIB and DNB than the corresponding DCH, DBH, DIH and DNH. It is in good agreement with the results reported by Li et al.^58,59 that a symmetric guest containing a linear alkyl chain with four methylenes exhibits higher complexing ability with pillar[5]arene than corresponding compounds with longer or shorter alkyl chains. The association constants between pillar[5]arene and guests are shown in Table S2.† Furthermore, in terms of the adsorbates with the chain of four carbons, the adsorption ratios with PS-P5 increases in the order of DCB < DBB < DIB < DNB, which is also well consistent with previous results.^58,59 Interestingly, the adsorption ratio for DIB is slightly higher than that for DNH, which is opposite to the preceding work.^58,59 This may primarily be attributed to the hindrance of Merrifield resin CMPS units to the complete complexation between pillar[5]arene and DNH, leading to the consequence that only one cyano group locates at the host's cavity, resulting in weaker C–H⋯N interactions between the host's alkyls and the guest's nitrogen atoms, and weaker dipole–dipole interactions between the guest's cyano groups and the host's oxygen atoms.⁵⁸

Table 1 Adsorption ratios for PS-P5 and CMPS toward eight adsorbates at 298 K

Adsorbate	Adsorbent	Adsorption ratio (%)
DCB	CMPS	0.03
	PS-P5	53.2
DCH	CMPS	0.62
	PS-P5	21.8
DBB	CMPS	0.24
	PS-P5	56.1
DBH	CMPS	0.25
	PS-P5	38.1
DIB	CMPS	1.85
	PS-P5	60.9
DIH	CMPS	1.60
	PS-P5	22.1
DNB	CMPS	0.02
	PS-P5	67.1
DNH	CMPS	0.16
	PS-P5	58.0

---

In order to confirm that this adsorption property is actually due to the pillar[5]arene cavities rather than Merrifield resin CMPS itself, a control experiment was carried out to test the adsorption capability of free Merrifield resin for adsorbates. The results (Table 1) showed that the background adsorption amount toward adsorbates onto CMPS without pillar[5]arene cavities was less than 2%, indicating that the adsorption was primarily attributed to the complex formation between pillar[5]arene and the adsorbates.

Since the adsorption material PS-P5 exhibits good adsorption capacities toward the adsorbates with a chain of four carbons, the saturated adsorption amounts of PS-P5 toward DCB, DBB, DIB and DNB were analyzed. In order to reach the saturated absorption, we increased the stirring time (24 h) and the quantities of adsorbates up to 0.06 mmol which is two times of that of pillar[5]arene in PS-P5 (25 mg, 0.03 mmol). Thus, the saturated adsorption amount toward DBB was 2.682 mg per 25 mg of PS-P5, i.e., 107.3 mg per gram of PS-P5. The saturated adsorption amounts toward DCB, DBB, DIB and DNB were 76.12, 107.30, 125.90 and 86.88 mg per gram of PS-P5, respectively.

To simulate the process of column chromatography, a short glass column was packed with 50 mg of PS-P5. Then, DBB solution in toluene (120 mL, 107.28 μg mL⁻¹) was passed through the column at a flow rate of 1 mL min⁻¹. Every 10 mL volume of the effluent was measured by GC to determine the DBB concentration, and the amount of DBB adsorbed on PS-P5 was calculated. As is depicted in Fig. 3, the DBB adsorption quantity increased rapidly with time during the first 40 min, and after 100 min the adsorption equilibrium was gradually reached. It is noted that the total adsorbed amount of DBB on column was less than the maximum adsorption amount mentioned above, which may attribute to the incomplete adsorption in column for DBB and PS-P5 within the much short time. Furthermore, the relationship between the adsorbed amount of DBB and time meets exponential function (Fig. S10†).

Fig. 3 Relationship between the amount m (μg) adsorbed on the PS-P5 column and the elution time t (min).

To explore the potential application in removing toxic pollutants from wastewater and tap water, the adsorption material PS-P5 was utilized to adsorb DBB in water. To a mixture of DBB (10 mg) in 20 mL of water was added PS-P5 (50 mg). The mixture was stirred for 24 h at room temperature and then PS-P5 was removed by centrifugation. The filtrate was extracted with toluene (40 mL) three times. Finally the organic layer was dried over anhydrous Na₂SO₄ and measured by GC to determine the residual concentration of DBB. The results show that the amount of DBB adsorbed on PS-P5 is 4.82 mg, leading to the adsorption ratio of 48.2%, which indicates that PS-P5 is expected to be a promising adsorption material in adsorbing toxic pollutants from wastewater and tap water. Moreover, the reversibility of PS-P5 was tested. PS-P5 was repeated for three times and the results were shown in Table S3.†

In conclusion, a polymeric material with pillar[5]arene cavities PS-P5 was prepared. The results showed that PS-P5 exhibited good adsorption affinity for dihaloalkanes and dinitriles based on the host–guest inclusion complexation. The process of column chromatography using PS-P5 as adsorption material was simulated. Futhermore, the potential application in removing adsorbates from aqueous phase was also explored.

Acknowledgements

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (21572069, 21072064), the National Basic Research Program of China (2012CB720801) and the Natural Science Foundation of Guangdong Province (2015A030313209, 2016A030311034).

Notes and references

1. T. Ogoshi, S. Kanai, S. Fujinami, T.-a. Yamagishi and Y. Nakamoto, J. Am. Chem. Soc., 2008, 130, 5022–5023.
2. D. Cao, Y. Kou, J. Liang, Z. Chen, L. Wang and H. Meier, Angew. Chem., Int. Ed., 2009, 48, 9721–9723.
3. P. J. Cragg and K. Sharma, Chem. Soc. Rev., 2012, 41, 597–607.
4. M. Xue, Y. Yang, X. Chi, Z. Zhang and F. Huang, Acc. Chem. Res., 2012, 45, 1294–1308.
5. T. Ogoshi and T.-a. Yamagishi, Eur. J. Org. Chem., 2013, 2013, 2961–2975.
6. D. Cao and H. Meier, Asian J. Org. Chem., 2014, 3, 244–262.
7. Z. Zhang, Y. Luo, J. Chen, S. Dong, Y. Yu, Z. Ma and F. Huang, Angew. Chem., Int. Ed., 2011, 50, 1397–1401.
8. K. Wang, C. Y. Wang, Y. Wang, H. Li, C. Y. Bao, J. Y. Liu, S. X. Zhang and Y. W. Yang, Chem. Commun., 2013, 49, 10528–10530.
9. S. Dong, J. Yuan and F. Huang, Chem. Sci., 2014, 5, 247–252.
10. W. Si, L. Chen, X. B. Hu, G. Tang, Z. Chen, J. L. Hou and Z. T. Li, Angew. Chem., Int. Ed., 2011, 50, 12564–12568.
11. L. Chen, W. Si, L. Zhang, G. Tang, Z. T. Li and J. L. Hou, J. Am. Chem. Soc., 2013, 135, 2152–2155.
12. X. B. Hu, Z. Chen, G. Tang, J. L. Hou and Z. T. Li, J. Am. Chem. Soc., 2012, 134, 8384–8387.
13. T. Ogoshi, D. Yamafuji, T. Aoki and T. A. Yamagishi, J. Org. Chem., 2011, 76, 9497–9503.
14. Z. Zhang, C. Han, G. Yu and F. Huang, Chem. Sci., 2012, 3, 3026–3031.
15. Y. L. Sun, Y. W. Yang, D. X. Chen, G. Wang, Y. Zhou, C. Y. Wang and J. F. Stoddart, Small, 2013, 9, 3224–3229.
16. X. Hou, C. Ke, C. Cheng, N. Song, A. K. Blackburn, A. A. Sarjeant, Y. Y. Botros, Y. W. Yang and J. F. Stoddart, Chem. Commun., 2014, 50, 6196–6199.
17. Y. Zhou, L. L. Tan, Q. L. Li, X. L. Qiu, A. D. Qi, Y. Tao and Y. W. Yang, Chem.–Eur. J., 2014, 20, 2998–3004.
18. H. Zhang, X. Ma, K. T. Nguyen and Y. Zhao, ACS Nano, 2013, 7, 7853–7863.
19. Q. Duan, Y. Cao, Y. Li, X. Hu, T. Xiao, C. Lin, Y. Pan and L. Wang, J. Am. Chem. Soc., 2013, 135, 10542–10549.
20. Y. Yao, M. Xue, J. Chen, M. Zhang and F. Huang, J. Am. Chem. Soc., 2012, 134, 15712–15715.
21. G. Yu, C. Han, Z. Zhang, J. Chen, X. Yan, B. Zheng, S. Liu and F. Huang, J. Am. Chem. Soc., 2012, 134, 8711–8717.
22. G. Yu, Y. Ma, C. Han, Y. Yao, G. Tang, Z. Mao, C. Gao and F. Huang, J. Am. Chem. Soc., 2013, 135, 10310–10313.
23. G. Yu, M. Xue, Z. Zhang, J. Li, C. Han and F. Huang, J. Am. Chem. Soc., 2012, 134, 13248–13251.
24. G. Yu, X. Zhou, Z. Zhang, C. Han, Z. Mao, C. Gao and F. Huang, J. Am. Chem. Soc., 2012, 134, 19489–19497.
25. D.-X. Chen, Y.-L. Sun, Y. Zhang, J.-Y. Cui, F.-Z. Shen and Y.-W. Yang, RSC Adv., 2013, 3, 5765–5768.
26. H. Li, D. X. Chen, Y. L. Sun, Y. B. Zheng, L. L. Tan, P. S. Weiss and Y. W. Yang, J. Am. Chem. Soc., 2013, 135, 1570–1576.
27. M.-m. Tian, D.-X. Chen, Y.-L. Sun, Y.-W. Yang and Q. Jia, RSC Adv., 2013, 3, 22111–22119.
28. Y. Yao, M. Xue, X. Chi, Y. Ma, J. He, Z. Abliz and F. Huang, Chem. Commun., 2012, 48, 6505–6507.
29. G. Yu, Z. Zhang, C. Han, M. Xue, Q. Zhou and F. Huang, Chem. Commun., 2012, 48, 2958–2960.
30. T. Adiri, D. Marciano and Y. Cohen, Chem. Commun., 2013, 49, 7082–7084.
31. P. Wang, Y. Yao and M. Xue, Chem. Commun., 2014, 50, 5064–5067.
32. Y. Fang, C. Li, L. Wu, B. Bai, X. Li, Y. Jia, W. Feng and L. Yuan, Dalton Trans., 2015, 14584–14588.
33. A. Sengupta, Y. Fang, X. Yuan and L. Yuan, J. Lumin., 2015, 166, 187–194.
34. T. Ogoshi, R. Sueto, K. Yoshikoshi and T.-a. Yamagishi, Chem. Commun., 2014, 50, 15209–15211.
35. L. L. Tan, H. Li, Y. Tao, S. X. Zhang, B. Wang and Y. W. Yang, Adv. Mater., 2014, 26, 7027–7031.
36. Z. Zhang, Q. Zhao, J. Yuan, M. Antonietti and F. Huang, Chem. Commun., 2014, 50, 2595–2597.
37. T. Zhou, N. Song, H. Yu and Y. W. Yang, Langmuir, 2015, 31, 1454–1461.
38. W. Cheng, H. Tang, R. Wang, L. Wang, H. Meier and D. Cao, Chem. Commun., 2016, 52, 8075–8078.
39. S. D. Richardson, TrAC, Trends Anal. Chem., 2003, 22, 666–684.
40. M. G. Muellner, E. D. Wagner, K. McCalla, S. D. Richardson, Y.-T. Woo and M. J. Plewa, Environ. Sci. Technol., 2007, 41, 645–651.
41. S. D. Richardson, M. J. Plewa, E. D. Wagner, R. Schoeny and D. M. DeMarini, Mutat. Res., 2007, 636, 178–242.
42. V. V. Goncharuk, D. D. Kucheruk, V. M. Kochkodan and V. P. Badekha, Desalination, 2002, 143, 45–51.
43. B. Van der Bruggen and C. Vandecasteele, Environ. Pollut., 2003, 122, 435–445.
44. A. Cassano, R. Molinari, M. Romano and E. Drioli, J. Membr. Sci., 2001, 181, 111–126.
45. Y. Fu and T. Viraraghavan, Bioresour. Technol., 2001, 79, 251–262.
46. C. I. Pearce, J. R. Lloyd and J. T. Guthrie, Dyes Pigm., 2003, 58, 179–196.
47. R. Khan, P. Bhawana and M. H. Fulekar, Rev. Environ. Sci. Biotechnol., 2012, 12, 75–97.
48. X. Chen, G. Chen and P. L. Yue, Environ. Sci. Technol., 2002, 36, 778–783.
49. C. Y. Hu, S. L. Lo and W. H. Kuan, Water Res., 2003, 37, 4513–4523.
50. U. von Gunten, Water Res., 2003, 37, 1443–1467.
51. T. Kurbus, Y. M. Slokar, A. M. Le Marechal and D. B. Vončina, Dyes Pigm., 2003, 58, 171–178.
52. J.-M. Lee, M.-S. Kim, B. Hwang, W. Bae and B.-W. Kim, Dyes Pigm., 2003, 56, 59–67.
53. F. Torrades, M. Pérez, H. D. Mansilla and J. Peral, Chemosphere, 2003, 53, 1211–1220.
54. N. Calace, E. Nardi, B. M. Petronio and M. Pietroletti, Environ. Pollut., 2002, 118, 315–319.
55. T. Robinson, B. Chandran and P. Nigam, Water Res., 2002, 36, 2824–2830.
56. S. Al-Asheh, F. Banat and L. Abu-Aitah, Sep. Purif. Technol., 2003, 33, 1–10.
57. V. K. Garg, R. Gupta, A. B. Yadav and R. Kumar, Bioresour. Technol., 2003, 89, 121–124.
58. X. Shu, S. Chen, J. Li, Z. Chen, L. Weng, X. Jia and C. Li, Chem. Commun., 2012, 48, 2967–2969.
59. X. Shu, J. Fan, J. Li, X. Wang, W. Chen, X. Jia and C. Li, Org. Biomol. Chem., 2012, 10, 3393–3397.
60. S. Sun, X.-Y. Hu, D. Chen, J. Shi, Y. Dong, C. Lin, Y. Pan and L. Wang, Polym. Chem., 2013, 4, 2224–2229.
61. W. Xia, M. Ni, C. Yao, X. Wang, D. Chen, C. Lin, X.-Y. Hu and L. Wang, Macromolecules, 2015, 48, 4403–4409.
62. X. Liao, L. Guo, J. Chang, S. Liu, M. Xie and G. Chen, Macromol. Rapid Commun., 2015, 36, 1492–1497.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15728a

---