Room Temperature One-Step Conversion from Elemental Sulfur to

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Room Temperature One-Step Conversion from Elemental Sulfur to Functional Polythioureas through Catalyst-Free Multicomponent Polymerizations Tian Tian, Rongrong Hu, and Ben Zhong Tang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018

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Journal of the American Chemical Society

Room Temperature One-Step Conversion from Elemental Sulfur to Functional Polythioureas through Catalyst-Free Multicomponent Polymerizations Tian Tian,† Rongrong Hu,*,† and Ben Zhong Tang*,†,‡ †

State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640, China.

‡

Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China.

ABSTRACT: The utilization of sulfur is a global concern, considering the abundant and cheap source of sulfur from nature and petroleum industry, its limited consumption and the safety/environmental problems caused during storage. The economic and efficient transformation of sulfur remains to be a great challenge for both academia and industry. Herein, a room temperature conversion from sulfur to functional polythioureas was reported through a catalyst-free multicomponent polymerization of sulfur, aliphatic diamines, and diisocyanides in air with 100% atom economy. The polymerization enjoys quick reaction and wide monomer scope, which affords 16 polythioureas with well-defined structures, high molecular weights (Mws up to 242 500 g/mol), and excellent yields (up to 95%). The polythioureas can be utilized to detect mercury pollution with high sensitivity (Ksv = 224 900 L/mol) and high selectivity, clean Hg2+ with high removal efficiency (> 99.99%) to achieve drinking water standard, and monitor the real-time removal process by fluorescence.

INTRODUCTION Sulfur, as the third most abundant element in the fossils, has become one of the major byproducts from worldwide petroleum industry. Compared with its large surplus from over 70 million tons annual production,1,2 the consumption of sulfur is quite small, which is mainly used in the production of sulfuric acid,3 resulting in rapidly growing mountains of sulfur on the oil sands. While the dispose of sulfur is costly, the storage of sulfur has also been proved to be a challenging issue, considering the flammable nature of sulfur and its slow transformation to SOx which might cause serious environmental problems such as acid rain.4 It is hence urgently demanded to develop economic and efficient method for the utilization of elemental sulfur in the preparation of functional materials. Sulfur-containing polymers have attracted much attention because of their outstanding characteristics such as high refractive indices,5,6 high transparency,7 metal coordination ability,8 self-healing capability,9,10 electrochemical properties,11,12 and photocatalytic activity.13 Massive effort has been devoted to investigate the direct utilization of elemental sulfur in the preparation of sulfurcontaining functional polymers such as vulcanized rubbers,14 polysulfides,15-17 and high sulfur-content copolymers,2 which generally involve high temperature, ambiguous structure, and poor solubility of products.18 Few example has been reported to directly use sulfur to produce

soluble polymers with well-defined structures.19,20 Among that, multicomponent polymerizations (MCPs) with fascinating features such as high efficiency, simple operation, high atom economy, great structural diversity, and mild conditions, provide great opportunity for the direct utilization of sulfur.21-25 For example, the MCP of diamine, dialdehyde and sulfur at 115 oC was reported to synthesis polythioamides.26,27 In these approaches, high temperature is generally required for the polymerization of sulfur. New methods with concerns of environmental friendliness and energy saving are desired. Recently, we have reported a catalyst-free MCP of sulfur, alkynes, and aliphatic amines to afford a series of polythioamides with well-defined structures, high yields, high molecular weights, and high refractive indices at 100 o 28 C. It can be expected that when alkynes are replaced by more reactive isocyanides, the polymerization reactivity could be enhanced, which might decrease the polymerization temperature. Indeed, an autocatalytic multicomponent reaction of isocyanides, aliphatic amines, and sulfur was reported to afford a library of asymmetric thioureas.29 In this reaction, cyclic S8 first reacts with an aliphatic amine to yield a zwitterionic ammonium polysulfide chain A. The negatively charged sulfur terminus of A then attacks the carbenoid carbon of the isocyanide to yield an isothiocyanate intermediate C, which then reacts with another amine molecule to produce a thiourea D (Scheme S1).29,30

ACS Paragon Plus Environment

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Inspired by the advantages of this MCR such as low reaction temperature, catalyst-free, high atom economy, and procedure simplicity, in this work, the polymerization of elemental sulfur, aliphatic diamines, and diisocyanides was investigated to produce polythioureas. Among the sulfur-containing polymers, polythioureas possess great potential in self-healing materials,9 heat-resistant materials,31 extraction of heavy metal ions,32 and dielectric materials,33 which are usually prepared by polycondensation of toxic monomers such as carbon disulfide, isothiocyanates, or thiophosgene.34-37 Herein, this MCP can be carried out smoothly at room temperature under nitrogen or in air, affording 16 polythioureas with great structural diversity, high molecular weights in high yields. The analysis of in situ IR spectra suggests that the MCP can be

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finished rapidly in air in 2 h at room temperature or 10 min at 100 oC, respectively, suggesting its fast polymerization speed and high efficiency of sulfur conversion. To the best of our knowledge, this is the first example demonstrating room temperature polymerization with elemental sulfur. Moreover, the thiourea groups endow the polymers sensitive and selective coordination with Hg2+ with their potential applications as fluorescent chemosensors and removal adsorbents for mercury.

RESULTS AND DISCUSSION Catalyst-Free MCPs of Sulfur, Aliphatic Diamines, and Diisocyanides. To develop multicomponent polymerizations of sulfur, aliphatic diamines, and diisocyanides, sublimed sulfur 1 was selected because of its low

Figure 1. (A) Catalyst-free multicomponent polymerization of sulfur 1, diamines 2a-d, and diisocyanides 3a-d. (B) Chemical structures and polymerization results of P1/2a-d/3a-d. Mws are determined by GPC in DMF based on polystyrene standard samples.


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cost among the various existence forms of elemental sulfur, and commercially available 1,4phenylenedimethanamine 2a as well as 1,4bis(isocyanomethyl)benzene 3a prepared from 2a are selected as the representative monomers (Scheme S2).38 The polymerization of 1, 2a, and 3a was first conducted in toluene at 50 oC for 4 h under nitrogen without catalyst, giving an insoluble product in toluene with a molecular weight (Mw) of 17 400 g/mol in 72% yield (Figure 1A). Polar solvents such as DMF, DMSO, and toluene/DMF mixed solvents were investigated and the best polymerization result was obtained in toluene/DMF (v/v, 1/2), furnishing polymer with an improved Mw of 37 900 g/mol in 82% yield (Table S1). The potential hydrogen bonds between thiourea groups and polar solvent molecules might have facilitated the polymer dissolution process. When the temperature was optimized from room temperature to 100 oC, the yield was increased from 53% to 85%, and the Mw of product was increased from 27 000 to 73 900 g/mol (Table S2). The monomer loading ratio and concentration are also crucial for the MCP. When strict theoretical monomer loading ratio of [1]: [2a]: [3a] = 2: 1: 1 was adopted, polymer with a Mw of 42 200 g/mol was obtained in 75% yield. Increasing the amount of sulfur benefits both yields and Mws (Table S3). Based on the optimized ratio of [1]: [2a]: [3a] = 4: 1: 1, the concentrations of 2a and 3a were then increased from 0.1 M to 1.0 M. The Mw was increased from 28 700 to 77 100 g/mol, while the yield was not affected much. Further increase of the monomer concentration led to highly viscous polymerization solution, which was terminated in 1 h to afford similar result (Table S4). Interestingly, neat polymerization can be realized at 100 oC, which proceeds smoothly and provides satisfactory results in 1 h with the Mw of 22 100 g/mol, indicating potential economic impact of this catalyst- and solvent-free approach. The study of polymerization time from 1 to 8 h does not show obvious influence on yield and Mw of the product, indicating that this MCP is quite fast and efficient (Table S5). To explore the monomer scope of this catalyst-free MCP and enrich the structural diversity of products, a

series of commercially available primary and secondary aliphatic diamines 2a-d, aliphatic diisocyanides 3a-b, and facilely available aromatic diisocyanides 3c-d are selected as monomers (Figure 1A).39 The MCPs of all the 16 combinations of four diamines and four diisocyanides were carried out in toluene/DMF (v/v, 1/2) at 100 oC under nitrogen. All the polymerizations proceeded smoothly and rapidly to produce soluble polymers in 1 h with satisfactory yields of 64-95% and high Mws of up to 242 500 g/mol, demonstrating general monomer applicability and high efficiency (Figure 1B). In particular, primary amines 2a-c can afford better polymerization results compared with secondary amine 2d, owing to their higher reactivity. Among all the combinations, aromatic diisocyanide 3c with good solubility affords the highest Mw upon polymerization with sulfur and 2c, despite of its steric hindrance. Room Temperature MCPs of Sulfur, Aliphatic Diamines, and Diisocyanides in Air. To further enhance the potential of this MCP, the polymerizations were optimized at room temperature (Table 1). The MCPs of eight monomer combinations among primary/secondary diamines 2a-d, and aliphatic/aromatic diisocyanides 3a-c were conducted as examples at room temperature for 1 h under nitrogen, generating satisfactory results with up to 79% yields and Mws of up to 44 500 g/mol. Compared with the above-mentioned optimal condition at 100 oC, the reaction time was then prolonged to 4 h and the concentration of diamine was increased to 1.1 M, considering that amine was less reactive at room temperature than at 100 oC. Eight polymers were obtained with good to excellent yields of 67-94% and high Mws of 30 500-116 100 g/mol, demonstrating excellent polymerization results at room temperature. Furthermore, the room temperature MCP was investigated in air, considering that no air-sensitive monomer or catalyst was involved. All the eight MCPs provide satisfactory results in air with Mws ranging from 21 300 to 99 000 g/mol in 56-87% yields, proving that the MCP of sulfur, aliphatic diamines and diisocyanides is quite robust. The mild condition at room temperature in air, general ap-

Table 1. Polymerization Results of Room Temperature MCPs of 1, 2a-d, and 3a-c.a b

c

1 h, N2 polymer yield (%)

P1/2a/3a P1/2b/3a P1/2c/3a P1/2d/3a P1/2a/3b P1/2a/3c P1/2b/3c P1/2c/3c

79 55 78 trace 79 33 35 44

4 h, N2

Mw (g/mol) 24 500 24 900 30 800 16 500 30 800 41 600 36 400 44 500

c

PDI

yield (%)

Mw d (g/mol)

PDI

yield (%)

Mw (g/mol)

1.34 1.54 1.51 1.87 1.45 1.64 1.38 1.40

94 79 87 67 87 76 89 87

35 200 40 500 33 700 30 500 32 200 61 400 48 600 116 100

1.68 1.51 1.67 1.34 1.64 1.69 1.41 1.84

81 72 87 57 60 71 56 82

26 700 21 300 37 400 23 400 25 700 60 600 33 800 99 000

d

d

4 h, air d

a

d

d

PDI

1.41 1.37 1.69 1.27 1.57 1.69 1.32 1.53

Carried out at room temperature in toluene/DMF (v/v, 1/2). b[1] = 4.0 M, [2a-d] = [3a-c] = 1.0 M. c[1] = 4.0 M, [2a-d] = 1.1 M, [3a-c] = 1.0 M. dMws are determined by GPC in DMF based on polystyrene standard samples.


Journal of the American Chemical Society plicability, high efficiency, high convenience, and low cost of this catalyst-free MCP may bring it great potential in industrial development. Kinetic Study of the MCP by In Situ IR Spectrometry. To reveal the kinetic process of this MCP, in situ IR spectrometry was used to monitor the formation of polythiourea. Model compound 9 was synthesized to assist the characterization of the polymer structure (Scheme S2). The regular IR spectra of 2a, 3a, 9, and P1/2a/3a were compared in Figure S1. The -N≡C stretching vibration of 3a at 2149 cm-1 was disappeared, and the -NH2 stretching vibration of 2a at 3312 cm-1 was converted to -NH- stretching vibration at 3250 and 3267 cm-1 in the spectra of 9 and P1/2a/3a, respectively. Most importantly, new strong peaks emerged at 1563 and 1544 cm-1 in the spectra of 9 and P1/2a/3a, respectively, which were in accordance with the stretching vibration of the newly formed C=S bonds, confirming the formation of thiourea moieties. Similarly,

C-N

o

B

100 C r.t.

Absorbance (a.u.)

C=S

in the IR spectra of the other 15 polymers, strong -NHstretching vibration peaks at 3174-3276 cm-1 and C=S stretching vibration peaks at 1502-1553 cm-1 emerged, confirming their expected thiourea structures (Figure S2-5). The reaction progress of the MCP of 1, 2a, and 3a was then monitored as an example in air at both 100 oC and room temperature. The in situ IR spectra of the toluene/DMF (v/v, 1/2) solutions of 2a, 3a, P1/2a/3a, and the polymerization solutions after reaction at 100 oC for 4 h or at room temperature for 8 h, respectively, were compared in Figure 2A. It is clear that two new peaks emerged at ~1540 cm-1 and 1340 cm-1 in the spectra of both polymerization solutions, which can be assigned as the stretching vibrations of C=S and C-N bonds of the thiourea moiety, respectively (Figure S6).40 The time-dependent peak intensity and the kinetic variation of the 3D-FTIR profiles at ~1540 cm-1 were recorded to follow the track of the generation of thiourea moieties at both 100 oC and room tem-

Absorbance (a.u.)

2a 3a P1/2a/3a o 1+2a+3a, 100 C 1+2a+3a, r.t.

A

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

1

2

3

4

5

Reaction time (min)

1600

1450

1300

1150

1000 0

40

-1

C

80

120

240

Reaction time (min)

Wavenumber (cm )

D

Figure 2. (A) The in situ IR spectra of 2a, 3a, and P1/2a/3a in toluene/DMF (v/v, 1/2), and the polymerization solutions of 1, o 2a, and 3a after reaction at 100 C for 4 h or at room temperature for 8 h, respectively. (B) The time-dependent peak intensi-1 o -1 ty at 1540 cm (100 C) or 1544 cm (room temperature). Three-dimensional Fourier transform IR profiles of the peaks at ~ -1 o 1540 cm for the MCP (C) at 100 C and (D) at room temperature, respectively.


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perature, which clearly suggested the quick initiation and completion of the MCP (Figure 2B-D). The peak intensity at ~1540 cm-1 increased rapidly to reach saturation after 5 min at 100 oC, or after 1 h at room temperature, suggesting the fast rate and high efficiency of this MCP. Under the guidance of the in situ IR analysis, the time of the MCPs of 1, 2a, and 3a can be further optimized. The polymerizations at 100 oC were conducted in air for 3, 5, 10, and 60 min. The yields have already reached 81% in 3 min, while the Mws of the polymer are gradually increased from 30 200 to 47 600 g/mol in 10 min, which do not change much afterwards (Table S6). For room temperature polymerization, both yields and Mws of product were increased upon elongation of the reaction time, and polythiourea with a Mw of 24 400 g/mol can be obtained in 82% yield in 2 h. Characterization of the Polythioureas. The welldefined structure of the polythioureas are confirmed by their 1H and 13C NMR spectra. In the 1H NMR spectra of 9 and P1/2a/3a, the resonance of -NH2 of 2a at δ 1.68 has disappeared; the -CH2- peaks of 2a and 3a at δ 3.67 and

4.88, respectively, have combined into a single peak at δ 4.64-4.67, owing to the similar chemical environment of these two -CH2- groups in 9 and P1/2a/3a. Most importantly, new peaks emerged at δ 7.92 and 7.89 in the spectra of 9 and P1/2a/3a, respectively, corresponding to the -NH- proton from the newly formed thiourea group (Figure 3A-D). Similarly, in the 13C NMR spectra of 9 and P1/2a/3a, the isocyanide peak of 3a at δ 156.7 has disappeared and new thiourea peaks emerge at δ 183.0 and 183.5, respectively. Meanwhile, the -CH2- peaks of 2a at δ 45.5 and 3a at δ 44.5 have combined into a single peak at δ 47.4 in the spectrum of P1/2a/3a (Figure 3E-H). For all the 16 polythioureas, the characteristic -NH- peaks at δ 9.55-7.05 all emerged in their 1H NMR spectra, and the characteristic C=S peaks at δ 179.6-183.6 all emerged in their 13C NMR spectra, confirming their polythiourea structures (Figure S7-14). Solubility and Thermal Stability. The polythioureas possess excellent solubility in polar solvents such as DMF and DMSO, while they generally show poor solubility in THF, chloroform and dichloromethane. Thermogravimet-

Hb Ha

H2 N

NH2

Hb Ha

A

E

Hc C

Hc

N N

C

C1

B

He

S

Hd

He'

H N

Hd + Hd'

C

F

C1

N H

H N

N H

H N

Hf

1

C3

Hf N H

H N

Hg

S

Hg 7.6

G

C2 S

8.4

C2 N H

S

He + He'

D

Hd'

n

H

C3 4.9 4.2 3.5 2.0 Chemical Shift (ppm)

1.5

180

160

140

120

50

45

Chemical Shift (ppm) 13

Figure 3. H NMR spectra of (A) 2a, (B) 3a, (C) 9, and (D) P1/2a/3a. C NMR spectra of (E) 2a, (F) 3a, (G) 9, and (H) P1/2a/3a in DMSO-d6.



can reach 224 900 L/mol and the detection limit of Hg2+ can reach 0.1 ppm, demonstrating high sensitivity of mercury detection (Figure 4 inset). Moreover, a large variety of metal ions including Na+, K+, Mg2+, Ca2+, Al3+, Pb2+, Mn2+, Fe2+, Fe3+, Ru3+, Co2+, Ir3+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Ce3+, and Sm3+ were tested with the same aqueous mixture of P1/2b/3d. While negligible change on PL intensity and fluorescence image was observed for the other metal ions, only Hg2+ exhibits dramatic fluorescence quenching, demonstrating high specificity of mercury detection (Figure 4). It is observed that when HgCl2 solution and DMF solution of polythiourea were mixed, white precipitate was formed due to the poor solubility of the polymer-Hg2+ complex. The polythioureas were hence utilized for the removal of Hg2+ from aqueous solutions (Figure 5A,B). For example, the DMF solution or solid powder of P1/2a/3a were added into 2 mL of HgCl2 solution and stirred at room temperature for 1 h or 40 min, respectively. After the precipitate was removed by centrifugation, the remaining [Hg2+] in the filtrate was measured by F732-VJ Cold-atomic Absorption Spectroscopy. The DMF solution of P1/2a/3a can reduce [Hg2+] sharply from 60 mg/L to 20 μg/L using 9.6 mg polymer, which is lower than the standard of the discharge limit of the industrial waste (50 μg/L);50 or to 1.6 μg/L with 99.99% removal efficiency using 24 mg polythiourea, reaching the standard limit for drinking water (2 μg/L).45 Three precipitation-filtration cycles with 2.4 mg/mL polymer can also clean the aqueous solution to drinking water standard (Figure 5C). The solid powder of P1/2a/3a showed better performance with the standard limit for drinking water achieved when 12 mg polymer was used to decrease the [Hg2+] to 0.9 μg/L. The high removal efficiency of 99.99% also applies to various initial [Hg2+]0 such as 10 mg/L and 0.05 mg/L.

ric analysis suggests that these polythioureas enjoy good thermal resistance with their decomposition temperatures at 5 wt% weight loss ranging from 209 to 279 oC, owing to the intramolecular and intermolecular hydrogen bonds among the abundant thiourea moieties (Figure S15). Application of Polythioureas in Mercury Detection and Removal. Thiourea compounds are well-known ligands for heavy metal ions with strong binding to mercury ion.41 The detection and removal of Hg2+ are highly desired considering that Hg2+ is a commonly found toxic heavy metal ion extensively distributed in water and soil, causing lethal effects on living organisms and neurological systems.42 Currently, clays and zeolites, activated carbon, biomaterials, nanoparticles, and covalent organic frameworks are utilized as mercury adsorbents,43-35 with their performance regarding to removal efficiency, affinity and capacity, stability, or availability to be further improved. The polythioureas prepared from this economic and facile MCP of sulfur are hence ideal candidates as mercury sensors and adsorbents. Of all the 16 polythioureas, P1/2a-d/3d prepared from tetraphenylethene-containing diisocyanide 3d enjoy aggregation-induced emission characteristics.46,47 Their DMF solutions absorb at 328-336 nm and emit weakly at 492-498 nm (Figure S16 and S17). Their photoluminescence (PL) spectra suggest that in DMF/water mixtures with more than 40 vol% water as poor solvent, the polymers form nanoaggregates to emit brightly. The DMF/water mixture of P1/2b/3d with 50 vol% water fraction was selected to detect Hg2+, owing to its high fluorescence quantum efficiency of 13.3% (Table S7). Upon addition of Hg2+ from 0 to 10 μM, the emission of the aqueous suspension of P1/2b/3d gradually decreased, attributing to the formation of polythiourea-Hg2+ complexes (Figure S18).48,49 The Stern-Volmer plot of the relative PL intensity (I0/I) versus [Hg2+] suggests that the quenching constant 30

12

Ksv,I = 11 500 L/mol Ksv,II = 57 300 L/mol

H N 18

9

Ksv,III = 224 900 L/mol III

S

6

I0/I

24

I0/I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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S 12

3

I 0

6

0

II

Nn H

N H

N H

0

2+

+

+

2+

2+

3+

2+

2+

2+

3+

blank Hg Na K Mg Ca Al Pb Mn Fe Fe

3+

2

2+

Ru Co

4 2+ 6 [Hg ] (µM) 3+

Ir

2+

2+

8

10

+

2+

2+

3+

3+

Ni Cu Ag Zn Cd Ce Sm

Figure 4. Fluorescence detection of mercury ion with polythiourea. Relative intensity (I0/I) at 493 nm of P1/2b/3d in DMF/H2O mixture (v/v, 1/1, 10 μM) in the presence of different metal ions (10 μM) and the corresponding fluorescence photos taken under UV irradiation. I0 = fluorescence intensity in the absence of metal ions. Inset: Stern-Volmer plot of relative intensity (I0/I) of 2+ 2+ P1/2b/3d in DMF/H2O mixtures (v/v, 1/1, 10 μM) versus [Hg ]. I0 = PL intensity in the absence of Hg . Excitation wavelength: ACS Paragon Plus Environment 336 nm.

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A

non-emissive, insoluble

emissive, soluble

B

C

D 15

15.0 µg/L 99.85%

E 1.0

2+

[Hg ]0 0 10 20 40 60 80 100 (mg/L) 2+ [Hg ] 0 0.6 4.8 16 33 190 270 (µg/L)

H N

S

0.8 S

5

0.6

I/I0

6.6 µg/L 99.93%

Nn H

N H

N H

10

2+

[Hg ] (µg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.0 µg/L > 99.99% 1.6 µg/L > 99.99%

0.4

0.8 µg/L > 99.99%

0.2

0 0.09

0.18

0.27

0.36

0.45

0

10

20

30

180

270

2+

mP1/2b/3d (mg)

[Hg ] (µg/L)

Figure 5. (A) Schematic diagram of facile mercury removal process. (B) The proposed mechanism for mercury removal with polythioureas. (C) The mercury removal efficiencies of both DMF solution and solid powder of P1/2a/3a with 2 mL of 2+ † 2+ Hg solution (60 mg/L). 20 mL of Hg solution was used for three precipitation-filtration cycles. n.d. = not detectable. 2+ 2+ (D) The mercury removal efficiencies of different amount of P1/2b/3d in DMF solution with 2 mL of Hg solution ([Hg ]0 2+ = 10 mg/L). (E) The plot of relative emission intensity (I/I0) versus [Hg ] when DMF solution of P1/2b/3d (0.27 mg/mL) 2+ 2+ was mixed with HgCl2 solutions with [Hg ]0 ranging from 10 to 100 mg/L. I0 = PL intensity in the absence of Hg . Excitation wavelength: 336 nm. Inset: fluorescence photographs of the resultant solutions taken upon UV irradiation.



The mercury sensing and removal functionalities can be further combined in fluorescent polythioureas which serve both as fluorescent indicators and removal adsorbents. In fact, fluorescent P1/2b/3d prepared from aromatic isocyanide possesses better performance than P1/2a/3a. When DMF solutions of 0.09-0.45 mg P1/2b/3d were added to 2 mL of HgCl2 solution with the [Hg2+]0 of 10 mg/L, the remaining [Hg2+] is gradually decreased to 0.8 μg/L below the standard limit of drinking water, with the removal efficiency higher than 99.99% (Figure 5D). Alternatively, when polymer solution was added into HgCl2 solutions with [Hg2+]0 ranging from 10 to 100 mg/L, the remaining [Hg2+] lay between 0.6 to 270 μg/L, accompanying with gradual fluorescence decrease (Figure 5E and Figure S19). The [Hg2+] can hence be correlated with the fluorescence intensity, enabling real time monitoring of removal process of mercury contaminants.

EXPERIMENTAL SECTION General procedure of the multicomponent polymerization at 100 oC. The typical procedure for the MCP of 1, 2a, and 3a at 100 oC was given below as an example. Elemental sulfur 1 (77 mg, 2.40 mmol), 1,4benzenedimethanamine 2a (82 mg, 0.60 mmol), and 1,4benzenedimethanisocyanide 3a (94 mg, 0.60 mmol) were added into a 10 mL Schlenk tube equipped with a magnetic stir bar under nitrogen or in air. 0.4 mL of DMF and 0.2 mL of toluene were then injected into the tube and stirred at 100 oC for 1 h. The polymerization solution was then cooled to room temperature and diluted with 4.0 mL of DMF, which was then precipitated by adding the mixture dropwise into 80 mL of methanol through a cotton filter. The precipitates were filtrated and washed with methanol for three times (3 × 20 mL), and dried under vacuum to a constant weight to afford polymer P1/2a/3a as a white solid in 88% yield. Mw = 73 800 g/mol, Mw/Mn = 2.52. IR (KBr disk), ν (cm-1): 3267, 3053, 2918, 1544 (C=S), 1423, 1375, 1341, 1280, 1223. 1H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 7.89 (s, 4H), 7.23 (s, 8H), 4.64 (s, 8H). 13C NMR (125 MHz, DMSO-d6), δ (TMS, ppm): 183.5 (C=S), 138.3, 127.7, 47.4 (CH2). General procedure of the room temperature multicomponent polymerization. The typical procedure for the MCP of 1, 2a, and 3a at room temperature was given below as an example. Elemental sulfur 1 (77 mg, 2.4 mmol), 1,4-benzenedimethanamine 2a (90 mg, 0.66 mmol), and 1,4-benzenedimethanisocyanide 3a (94 mg, 0.60 mmol) were added into a 10 mL Schlenk tube equipped with a magnetic stir bar under nitrogen or in air. 0.4 mL of DMF and 0.2 mL of toluene were then injected into the tube and stirred at room temperature for 4 h. The polymerization solution was then diluted with 4.0 mL of DMF, which was then precipitated by adding the mixture dropwise into 80 mL of methanol through a cotton filter. The precipitates were filtered and washed with methanol for three times (3 × 20 mL), and dried under vacuum to a constant weight to afford polymer P1/2a/3a as a white solid in 94% yield. Mw = 35 200 g/mol, Mw/Mn = 1.68.

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CONCLUSIONS In this work, the first example of room temperature onestep conversion from elemental sulfur to functional polythioureas is demonstrated through a catalyst-free multicomponent polymerization of sulfur, dialiphatic amines, and diisocyanides. This MCP enjoys unique advantages of mild condition at room temperature under nitrogen or in air, cheap monomers and no catalyst, high efficiency and convenience, high atom economy and wide monomer scope, which can be applied to versatile primary amines, secondary amines, aliphatic isocyanides, and aromatic isocyanides, producing soluble polythioureas with high yields, large Mws, and great diversity of well-defined polymer structures. The real-time kinetic process of this MCP suggests rapid completion of the polymerization in 10 min at 100 oC or in 2 h at room temperature. The polythioureas can serve both as mercury sensors with high sensitivity and high selectivity, and efficient mercury removal adsorbents for polluted water with the cleaning process monitored simultaneously by fluorescence change. It is anticipated that the MCP can provide an economic, convenient, and efficient tool for the direct conversion of elemental sulfur to functional materials, solving problems regarding to sulfur hoarding and sulfur/mercury pollution, which kills two birds with one stone and may eventually benefit industry and environment.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials/instrumentation, synthetic procedures, in situ IR and mercury removal procedures, and characterization data; effect of solvent, reaction temperature, loading ratio of sulfur, monomer concentration, and time course on the polymerization; stacked in situ o 1 IR profiles at 100 C and room temperature; IR spectra, H 13 NMR and C NMR spectra of the model compound 9 and polythioureas P1/2a-d/3a-d; all the original HRMS spectra, 1 13 H NMR, and C NMR spectra; TGA spectra of the polymers P1/2a-d/3a-d; UV-vis and PL spectra of the polythioureas P1/2a-d/3a-d; and PL spectra of P1/2b/3d in DMF and H2O mixture with mercury ion.

AUTHOR INFORMATION Corresponding Author *(R. H.) [email protected] *(B. Z. T.) [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the National Science Foundation of China (21774034, 21490573, 21490574, and 21788102), the Young Elite Scientist Sponsorship Program of the China Association of Science and Technology (2015QNRC001), the Natural Science Foundation of Guangdong Province (2016A030306045 and 2016A030312002), the Innovation and Technology Commission of Hong Kong (ITCCNERC14SC01).


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10 ACS Paragon Plus Environment

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Room Temperature One-Step Conversion from Elemental Sulfur to

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