Synthesis and complexation properties of polystyrene supported polymeric thiacrown ether

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Eur. Polym. J. Vol. 34, No. 5/6, pp. 761±766, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/98 $19.00 + 0.00 S0014-3057(97)00197-3

SYNTHESIS AND COMPLEXATION PROPERTIES OF POLYSTYRENE SUPPORTED POLYMERIC THIACROWN ETHER ZHENGANG ZONG,* SHIHUA DONG, YUNHUA HU, YUWU XU and WENLIN LIU Department of Chemistry, Wuhan University, Wuhan 430072, China (Received 5 December 1996; accepted in ®nal form 11 April 1997) AbstractÐA series of polystyrene supported thiacrown ethers were prepared from cross-linked chloromethylated polystyrene (CMPS). CMPS was etheri®ed with propenol to form poly (4-propenyloxy methyl styrene) (PA), followed by the bromoalkoxylation of PA with chloroethanol (CE) in the presence of N-bromosuccinimide, and the etheri®cation with w, w'-dimercaptopolyglycols. Their complexation capacity, selectivity and mechanism towards metal ions, especially precious metal ions, were also investigated. The polymeric thiacrown ethers possessed a high complexation capacity and selectivity for Ag+ and Au3+ then Pd2+, Pt4+, Hg2+, Cu2+, Zn2+, Mg2+ and K+. XPS revealed that both oxygen and sulfur atoms in the ring contributed to the coordination with metal ions. # 1998 Elsevier Science Ltd. All rights reserved

with propenol and then to poly[4-(2'-bromo-3'chloroethoxypropyl) oxymethyl styrene] (PB) by the bromoalkoxylation of PA with chloroethanol (CE) and N-bromosuccinimide (NBS). Polymeric thiacrown ethers (PC1±PC6) were synthesized by the cyclization of PB with w, w'-dimercaptopolyglycols. Their complexation capacity, selectivity and mechanism towards metal ions were also investigated.

INTRODUCTION

Polymeric crown ethers containing sulfur as donor atoms with special complexation properties towards transition metals, especially towards precious metals and toxic heavy metals, have potential applications to separation, enrichment, recovery, analysis and removal of such metal ions from various industrial wastewaters. Polymeric thiacrown ethers were usually prepared from thiacrown ether monomers containing functional groups via polyaddition, polycondensation or grafting reaction to polymer backbones [1, 2]. However, these monomers were somewhat malodorous, physicologically toxic, expensive and dicult to prepare, and thus limited the development and application of these types of polymeric thiacrown ethers. Recently, more convenient approaches to prepare polymeric thiacrown ethers via various types of reactions of on-crown ethers as starting materials attracted considerable attention [3±7]. We have reported the polymeric thiacrown ethers prepared via the intramolecular cyclization of poly (2'-chloroethyl-2, 3-epoxyproponyl ether) or poly (2'-chloroethyl-2, 3epithiopropanyl ether) with dimercaptans [8±10]. In this paper, we presented a new approach to prepare cross-linked polystyrene supported thiacrown ethers, as shown in Fig. 1. Chloromethylated cross-linked polystyrene (CMPS) was initially converted to poly (4-propenyloxymethyl styrene, PA) by etheri®cation of CMPS

EXPERIMENTAL

Reagents and instruments Chloromethylated crosslinked polystyrene (from Chemical Plant of Nankai University) was extracted by acetone and dried in vacuum, Cl% = 20.75, N-bromosuccinimide (NBS) was dried in vacuum. Propenol, 2-chloroethanol and other reagents were redistilled prior to use. Dioxane, THF and dimethyl diglycol ether were re¯uxed with metal sodium and then distilled. Elemental analysis was determined on Perkin-240B elemental analyser. IR spectra were recorded on Nicolet 170 SX FT-IR spectrometer. The concentrations of ions were determined on Jarrell-Ash Atom Scan-2000. X-ray electron energy spectra were performed on Kartos Xsam 800 instrument.

Preparation of w, w-dimercaptopolyglycols 1,2-Ethanedithiol [13], 3-oxapentane-1, 5-dithiol [14], 3thiapentane-1, 5-dithiol [15], 3,6-dioxaoctane-1, 8-dithiol [14], 3,6-dithiaoctane-1, 8-dithiol [16] and 3,6,9-trioxaundecne-1, 11-dithiol [16] were prepared according to the procedures in the literature, respectively.

*To whom all correspondence should be addressed. Current address: Marine Coatings Research Institute of The Ministry of Chemical Industry, 4 Jinhu Road, Qingdao 266071, People's Republic of China. 761

Preparation of poly(4-propenyloxymethyl styrene) (PA) To the stirred suspension of 247.2 g of propenol and 40.0 g of CMPS swollen in 300 mL of dioxane for 48 hr, 39 g of NaH was added batchwise under N2. The mixture

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Zhengang Zong et al.

Fig. 1. Synthetic route of polystyrene supported thiacrown ethers (PC). was re¯uxed for 72 hr and cooled. The crude product was ®ltered, extracted with THF on a Soxhlet for 24 hr, washed thoroughly with water and dried under vacuum (44.3 g) Pale yellow beads were obtained with 98.37% of yield. Elemental analysis: Cl% = 0, (IR, KBr pellets).

Preparation of poly[4-(2'-bromo-3'-chloroethoxypropyl) oxymethyl styrene] (PB) To the stirred suspension of 350 g chloroethanol and 30.0 g PA swollen in dioxane for 48 hr, 33.0 g of NBS was slowly added at 458C under N2 atmosphere. The mixture was stirred for additional 5 hr and cooled. The crude product was ®ltered, washed with acetone and water, and dried under vacuum. pale yellow beads 48.0 g were obtained with 82.6% of yield. Elemental analysis: X(Cl,Br) = 3.38 mmol/g.

Preparation of polymeric thiacrown ethers (PC) To the mixture of 7  10ÿ3 mol dithiol, 80 mL of diglycol dimethyl ether and 3.0 g of PB swollen in the same solvent for 48 hr, 1.6 g of NaH was gradually added under N2 atmosphere at 120258C. The reaction mixture was stirred for additional 60 hr and cooled. The mixture was poured into methanol, ®ltered, extracted with THF on Soxhlet, washed thoroughly with water and dried in vacuum. Light brown beads of PC was obtained with the yield of 83±90%.

Determination of complexation properties of PC General procedure: PC bead was suspended in aqueous solution of metal ions and the mixture was shaken at room temperature for 24 hr. After the bead was ®ltered,

Synthesis and complexation properties of thiacrown ether

763

Table 1. E€ects of reaction conditions on the conversion of PAa Weight of reactant (g) No. 1 2 3 4 5 6 7 8

CMPS

Propenol

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

0.75 5.12 5.12 5.12 7.0 12.80 12.80 12.80

Molar ratio NaH 0.42 0.70 1.40 2.50 1.40 2.50 2.50 2.50

0Cl:0OH:NaH

Solvent

T(8C)

1:1. 2:1. 2 1:7. 6:2 1:7. 6:4 1:7. 6:7 1:10:4 1:18:7 1:18:7 1:18:7

THF THF THF THF THF THF Dioxane Dioxane

60 60 60 60 60 60 80 100

Content of residual Cl (%)

Conversion b (%)

16.9 3.93 3.25 3.10 2.90 1.60 1.40 0.00

16.84 79.21 82.71 83.49 84.47 91.3 92.5 100

a

Volume of solvent: 60 ml; reaction time: 72 hr. Conversion (%) were calculated from the content of residual Cl

b

the ion concentrations in the ®ltrate were detected. The details are listed in the respective tables.

RESULTS AND DISCUSSION

Synthesis and characterization Preparation of PA. In order to ®nd the optimum reaction conditions in preparation of PA from CMPS by etheri®cation with propenol, we have investigated the e€ects of molar ratio reactants, temperature, solvents and bases. The data are listed in Table 1. The data show that the increase in the ratio of propenol to CMPS resulted in increase of

the substitution of Cl by the propenyloxy group; similar results were also observed when larger amounts of NaH were used. Solvent with lower boiling point limited the reaction temperature, thus dioxane is preferable to tetrahydrofuran, although they all swelled CMPS well. Elevating the temperature of etheri®cation led to a higher conversion percentage of the functional group. Stronger bases facilitated the reaction and the conversion percentage was much higher when NaH was substituted for NaOH. In summary, in the optimum reaction condition that dioxane was chosen as solvent, NaH as base, molar ratio of CMPS-Cl:base:propenol as 1:7:18, temperature at 1018C, the conversion of Cl

Fig. 2. IR spectra of CMPS, PA, PB and PC.

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Zhengang Zong et al. Table 2. E€ects of reaction condition on PB

No. 1 2 3

PA

Weight of reactant (g) NBS

1.0 2.0 30.0

1.0 2.0 33.0

CE 4.0 21.4 350

a

Molar ratio C1C:NBS:CE

Solvent

Yield (%)

1:1:8.5 1:1:22.7 1:1:25

THF Dioxane Dioxane

65.2 72.5 82.9

Content of halogen (mmol/g) 2.23 2.96 3.38

a

Reaction temperature: 458C, reaction time: 5 hr

in CMPS was almost quantitative. No chlorine was found in the resulted product. The expected structure of PA was also con®rmed by IR spectra. The characteristic absorption peaks of 0CH2Cl in CMPS, as shown in Fig. 2, were observed at 1269 (dCHC1) and at 672 cmÿ1 (gC0Cl) in IR spectra, but not in PA. Alternatively, new bands were found at 1645 (gC1C), 921 (g1CH2) and 1081 cmÿ1 (gC0O0C) in the IR spectrum of PA, which indicated the existence of the propenyloxy group. In addition, there was no absorption at 3200±3400 cmÿ1, assigned to 0OH, which might be generated by the hydrolysis during the etheri®cation reaction and treatments afterward. Preparation of PB. Okahara and his coworkers [11] reported the bromoalkoxylation of ole®n with 2-chloroethanol and NBS resulting in 2-bromoalkoxyl-2-chloroethyl ether structure from a double carbon bond, which could couple further with diols (dithiols) to form crown ethers (thiacrown ethers). The only disadvantage was that it produced a mixture of two possible isomers when asymmetrically substituted ole®n was used as the starting reactant. In this paper, we tried to apply the bromoalkoxylation to the macromolecule and succeeded in introducing halogens in pairs to the side group of PB. The experimental data are listed in Table 2. In the bromoalkoxylation, excess 2-chloroethanol vs PA led to a higher conversion percentage, since it might serve as both reactant and good solvent. Excess 2chloroethanol could be recovered by ®ltration and distillation and the recovery percentage was about 80%. The content of halogen (Br and Cl) in PB thus obtained was found 3.38 mol/g. By comparison of IR spectra of PA and PB in Fig. 2, we found that the characteristic absorption bands of C1C at 1654 (nC1C) and 921 cmÿ1 (n1CH2) disappeared, new absorption bands appeared at 1252 (dC0Cl), 665 (nC0Cl) and 554 cmÿ1 (nC0Br) assigned to C0Cl and C0Br bonds, respectively in PB, and a band at 1094 cmÿ1 (nC0O0C) was stronger and broader than that in PA. These ®ndings, combined with elemental analysis, supported the expected structure of PB.

Preparation of PC. Considering the extent of reaction of the cross-linked macromolecule with dithiols and the probability of the formation of noncyclized linear units, we chose a 1:1.35 molar ratio of halogen in PB: dithiol and NaH as base, expecting to obtain a higher content of macrocylic thiacrown ether units. The experimental data for PC are listed in Table 3. Elemental analysis data indicated that the content of residual halogen in PC were below 0.5 mmol/g, much lower than that of the initial halogen in PB (3.38 mmol/g). It is obvious that the sulfur contents in PC, although a little lower than the calculated values, increased in accordance with that of dithiols used. IR spectra of PC (Fig. 2) showed the absorption bands at 650± 590 cmÿ1 (nC0S), however, all bands assigned to C0X bands disappeared and no band was found near 2600 cmÿ1 (nS0B). All data above supported the expected cyclic polythiaether structures. Complexation properties of PC Thiacrown ethers, including polymeric thiacrown ethers, possess generally good complexation properties towards transition metal ions, especially towards precious metal ions and heavy metal ions. However, many papers in the previous literature reported the complexation behavior of thiacrown ethers only focusing on Hg2+, Cu2+ and Ag+. In this paper, we have investigated the complexation capacities, complexation selectivity and complexation mechanism of PC towards various types of metal ions, especially precious metal ions. Complexation capacities of PC Complexation capacities of PC at equilibrium absorption were determined and the data were listed in Table 4. As expected, PC1±PC6 had higher capacities for Ag+ and Au3+, fair capacities for Pd2+, Pt4+, Hg2+ and Cu2+, and poor capacities for Zn2+, Mg2+ and K+. All PC displayed similar complexation behavior to di€erent types of ions. In addition, it seems that more sulfur atoms replacing oxygen atoms in the crown ring led to a higher ca-

Table 3. Synthesis of polymeric thiacrown ethers Dithiol

Yield

Elemental analysis (%)

PC

z

m

(%)

C

PC1 PC2 PC3 PC4 PC5 PC6

S S S O O O

0 1 2 1 2 3

88.1 85.7 87.1 90.8 83.3 84.5

69.74 65.84 62.00 62.68 64.01 66.02

a

H 7.67 7.14 7.06 6.78 6.08 6.73

S 10.73 12.14 14.07 8.54 8.30 7.12

a

Content of residual halogen (mmol/g)

Conversion to the ring structure (%)

0.50 0.29 0.38 0.26 0.28 0.16

Molar ratio of (Br + Cl): SH = 1 : 1.35; solvent: glycol dimethyl ether; reaction temperature: 1208C; reaction time: 60 hr

85.21 91.42 88.76 92.31 91.72 95.27

Synthesis and complexation properties of thiacrown ether Table 4. Complexation capacities of PC for metal ions (mmol/g) PC PC1 PC2 PC3 PC4 PC5 PC6

Ag+

b

Ag+

1.77 1.99 2.35 1.89 2.23 2.15

c

Au3+ d

3.66 3.89 4.38 3.87 3.93 3.84

Pd2+

1.61 1.42 1.69 1.17 1.59 1.30

a

Pt4+

0.53 0.27 0.38 0.04 0.19 0.16

e

Hg2+

0.04 0.04 0.08 0.02 0.07 0.10

f

765

a

Cu2+ g

Zn2+ g

0.17 0.20 0.09 0.09 0.12 0.08

0.14 0 0 0 0 0

0.91 0.91 1.56 0.57 0.80 0.38

Mg2+

b

0 0 0 0 0 0

K+b 0 0 0 0 0 0

a

Weight of PC bead: 50 mg; concentration of ions: 0.025 mol/L; volume of solution: 25 mL; detection method: Volhard titration for Ag+ and K+; EDTA titration for Hg2+, Cu2+, Zn2+ and Mg2+; I.C.P. method for Au3+, Pd2+ and Pt4+. b pH 7.0. c 2.0 mol L HNO3. d 2.0 mol/L Hcl. e 0.2 mol/L HCl. f 1.0 mol/L HNO3. g pH 5.6 Clark-Labs bu€er

Table 5. Complexation capacities (mmol/g) of PC from mixed solutions of Ag+, Cu2+ and Zn2+ a PC

Ag+

Cu2+

Zn2+

1.86 1.38

0.34 0.38

0.04 0.09

PC3 PC5

might be concluded as Au3+>Pd2+>Pt4+. The results from mixed ions solutions were well in accordance with those from single ion solutions. Complexation mechanism of PC

a

Weight of PC bead: 50 mg; ion concentration: 0.025 mol/L for each ion; medium: pH 7; volume of solution 25 mL; detection method: I.C.P. method

pacity for Ag+. The results were in accordance with those described in the literature [12]. Complexation selectivities of PC PC3 and PC5 were chosen for the investigation of the complexation selectivity. Several mixed solutions of metal ions were tested through competitive absorption at equilibrium condition. The experimental data are given in Tables 5 and 6. Table 5 showed that PC3 and PC5 absorbed more Ag+ than Cu2+ and Zn2+ from the mixed solution of Ag+, Cu2+ and Zu2+. On the other hand, Table 6 showed that PC3 and PC5 preferably absorbed Au3+ (almost quantitative) to Pd2+ and Pt4+ from dilute solution of Au3+ ±Pd2+, Au3+± Pt4+, Pd2+±Pt4+ and Au3+±Pd2+±Pt4+. The selectivity order of PC for these precious metal ions

In order to understand the complexation mechanism of PC to metal ions, binding energies of valence electrons were analysed by X-ray photoelectron spectroscopy for PC3 and its complexes with Au3+ and Pd2+. The data are listed in Table 7. By comparison of the binding energies of di€erent electrons in PC3
Au correspondingly with those in original PC3 and HAuCl4, we found that after the complex formed, the binding energy of the O1s electron increased about 0.5 eV and that of the S2p electron not only changed more remarkably, but also split into two bands (at 164.3 and 168.0 eV). On the other hand, binding energies of Au4f7/2, Au4f5/2 and Cl2p electrons shifted down by 2.28, 1.59 and 1.99 eV, respectively. Similar results were also observed from PC3
Pd. The data clearly indicated that charge transfer occurred from oxygen and sulfur atoms in PC3 to Au3+ (or Pd2+) and successively to the chloride atom. Therefore, it might be concluded that the coordinating bonds were formed between donor atoms (both 0 and S) to acceptors (Au3+ and Pd2+) in the complexes.

Table 6. Complexation eciency (%) of PC from mixed solution Au3+±Pd2+

Au3+±Pt4+

a

Pd2+±Pt4+

Au3+±Pd2+±Pt4+

PC

Au3+

Pd2+

Au3+

Pt4+

Pd2+

Pt4+

Au3+

Pd2+

Pt4+

PC3 PC5

99.8 99.8

72.5 45.0

99.8 99.8

67.5 27.5

80.4 37.5

20.8 5.7

99.8 99.8

65.1 48.8

17.9 5.1

a

Weight of PC bead: 100 mg; ion concentration: 50 mg/L for each ion; medium: 2.0 mol/L HCl; volume of solution: 25 mL; detection method: I.C.P. method

Table 7. Binding energies (eV) of valence electrons in PC3, PC3
Au and PC3
Pd Sample PC3 HAuCl4 PC3
Au PdCl2 Pd3
Pd

C1s

O1s

S2p

285.0

532.10

163.44

285.0

532.63 (168.0)

164.3

285.0

532.68

164.26

Au4f7/2

Au4f5/2

87.6 85.32

90.4 88.81

Pd3d5/2

Pd3d3/2

Cl2p 200.5 198.51

339.8 337.89

345.6 343.17

200.7 198.47

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Zhengang Zong et al. REFERENCES

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