Economic Sulfur Conversion to Functional Polythioamides through Catalyst-Free Multicomponent Polymerizations of Sulfur, Acids, and Amines

Article abstract of DOI:10.1021/jacs.9b11066

Sulfur utilization is a global concern because of its abundant nature sources and the safety or environmental problems caused by its burning or oxidation during storage, while sulfur-containing polymers are popular materials in virtue of their fascinating

Full text of DOI:10.1021/jacs.9b11066

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Article
Economic Sulfur Conversion to Functional Polythioamides Through
Catalyst-free Multicomponent Polymerizations of Sulfur, Acids, and Amines
Wenxia Cao, Fengying Dai, Rongrong Hu, and Ben Zhong Tang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b11066 • Publication Date (Web): 16 Dec 2019
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Economic Sulfur Conversion to Functional
Polythioamides Through Catalyst-free
Multicomponent Polymerizations of Sulfur,
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Acids, and Amines
Wenxia Cao,† Fengying Dai,†, 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.
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ABSTRACT
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Sulfur utilization is a global concern because of its abundant nature sources and the
safety or environmental problems caused by its burning or oxidation during storage,
while sulfur-containing polymers are popular materials in virtue of their fascinating
properties such as metal coordination ability, high refractive indices, and
semiconducting property. The synthesis of sulfur-containing polymers is challenging,
especially directly from elemental sulfur. Herein, catalyst-free and scalable
multicomponent polymerizations of all commercially available elemental sulfur,
dicarboxylic acids, and diamines were reported to facilely construct 12 polythioamides
with diverse and well-defined structures, high molecular weights (Mws up to 86 200
g/mol), and excellent yields (up to 99%) from elemental sulfur. Besides commonly used
aliphatic diamines, aromatic diamine monomers are also applicable to these MCPs,
affording polythioamides with unique rigid structures and improved functionality than
the previously reported polythioamides. These polythioamides can be applied in gold
recovery which could extract trace amount of Au3+ from practical acidic leaching
solution of discarded electronic waste selectively, rapidly (1 min), sensitively (10 ppb),
and efficiently (>99.99%) with high extraction capacity up to 0.60 g Au3+/g to directly
afford high purity elemental gold after pyrolysis. The MCPs could make use of both
abundantly existed sulfur waste and trace amount of precious gold residue in electronic
wastes, demonstrating its great potential in resource utilization.
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INTRODUCTION
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The utilization of elemental sulfur is a global concern because of the large sulfur
production about 80 million tons per year from the worldwide petroleum refine
industry,1 and its limited application such as the production of H2SO4. The storage of
sulfur is difficult due to its flammable nature and gradual oxidation in air, which can
cause serious safety issue and environmental problems. On the other hand, sulfur-
containing polymers are a group of attractive modern materials, owing to their
fascinating properties such as metal coordination ability,2-5 high refractive indices,6-8
self-healing capability,9 semiconducting property,10 etc, whose preparation generally
involve instable and smelly sulfur-containing reagents, tedious procedures, and
structural limitations. It is hence a great choice to use elemental sulfur as the economic
source for the preparation of sulfur-containing polymer materials.11-15 However, the
transformation of sulfur is challenging because there are barely proper solvent and
transition metal catalyst for sulfur-based reactions due to the poor solubility of sulfur
in organic solvents and its toxicity to transition metal catalysts or free radical
initiators.16 Much effort has been made to utilize sulfur in the preparation of sulfur-
containing polymers such as vulcanized rubbers,17,18 high sulfur-content copolymers,19-
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and polythiophenes,22 which generally involve high temperature, irregular chaotic
structures, and poor solubility of products.
Of all the sulfur-containing functional polymers, polythioamide is an unique type
of material with metal coordination ability, high refractivity, and luminescence
behavior.2-4,6,7 The synthesis of polythioamide is demanding, for example, dithioesters
were reported to react with diamines to produce polythioamides, accompanying with
the releasing of harmful methanethiol, and the reactant is also difficult to prepare;23,24
Lawesson reagent was also used to convert polyamide to polythioamide, but hydrolytic
degradation and incomplete conversion were generally involved;25 the reaction of
dithiocarboxylic acid-O,O-diethyl esters and diamines for up to 54 days were reported
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in 1971 to synthesize polythioamide;26 polymerization of diamine and sulfur was also
reported to generate polythioamide with limited and irregular structures.7
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Recently, taking advantage of the unique chemical reactivity of sulfur,
multicomponent polymerizations (MCPs) have proved to be a powerful tool to convert
sulfur to sulfur-containing polymers with high atom economy, high efficiency, mild
condition, simple operation, and great structural diversity.27,28 For example, the MCPs
of sulfur, aromatic dialdehydes and aliphatic diamines are reported to synthesize
polythioamides at 115 oC.29,30 In 2015, we have also reported the catalyst-free MCPs of
sulfur, aromatic diynes, and aliphatic diamines at 100 oC to afford polythioamides with
well-defined structures, high molecular weights (Mws) in high yields.6 Later, room
temperature conversion of sulfur has been realized by the catalyst-free MCPs of sulfur,
diisocyanides, and aliphatic diamines, producing polythioureas with well-defined
structures, high Mws, high yields, and potential application in mercury removal.31
However, to realize sulfur conversion at industrial scale, these diynes and
diisocyanides monomers used in the above-mentioned MCPs should be replaced by
readily available raw materials. Carboxylic acids hence can be considered as ideal
monomer candidates because they are commercially available, inexpensive, stable, eco-
friendly, and many of them can even be obtained from nature sources.32,33 Besides, the
rich chemical properties of carboxylic acids endow them numerous opportunities in
organic and polymer synthesis.34-38 For example, a multicomponent reaction of sulfur,
carboxylic acids, and amines can produce thioamides with a decarboxylative reaction
involved.39
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In this work, the MCPs of elemental sulfur, dicarboxylic acids, and diamines are
developed to produce polythioamide materials efficiently and economically. All the
three monomers of the MCPs are commercially available, and no transition metal
catalyst is involved in the polymerization, enabling scalable preparation of
polythioamide materials with a great diversity of well-defined structures, high Mws, and
high yields. Different from the previous reports of polythioamide synthesis, aromatic
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diamines can be adopted in this MCP, furnishing polythioamide structures that could
not be accessed by other elemental sulfur-based synthetic methods. Furthermore, with
the strong coordination between thioamide groups and Au3+, the polythioamides were
able to extract Au3+ under practical conditions with high sensitivity, high selectivity,
fast speed, high efficiency, and large capacity, recover elemental gold by simply
pyrolyzing the polymer-Au3+ complexes, demonstrating the potential application of
these polythioamide materials.
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RESULTS AND DISCUSSION
MCPs of Sulfur, Diacids and Diamines. To develop multicomponent polymerizations
of sulfur, diacids and diamines, commercially available sublimed sulfur 1 (S8), 1,4-
phenylenediacetic acid 2a and hexamethylenediamine 3a, were selected as the three
monomers. The MCP of 1, 2a, and 3a was first carried out in DMSO at 100 oC under
nitrogen for 10 h. With the theoretical monomer loading ratio of 1/8[S8]: [2a]: [3a] =
2: 1: 1 and a high monomer concentration of [3a] = 0.5 M, polymer product with a Mw
of 22 500 g/mol can be obtained. The monomer loading ratio 1/8[S8]: [3a] and [2a]:
[3a] were investigated, respectively, to optimize the MCP. Excess amount of sulfur is
beneficial to the polymerization as the other sulfur-based MCPs.6,7,29-31 Surprisingly,
unlike other polymerizations, the excess amount of dicarboxylic acid could
significantly improve the yield and Mw of the product. The best result was obtained
with the monomer feeding ratio of 1/8[S8]: [2a]: [3a] = 3.0: 1.5: 1.0 and polymer with
a Mw of 52 300 g/mol was furnished in 81% yield (Table S1). To reveal the role of the
excess amount of acids, benzoic acid was added with the ratio 1/8[S8]: [2a]: [3a]:
[benzoic acid] = 4: 1: 1: 1, polymer product with a Mw of 29 700 g/mol was obtained in
58% yield, which is improved compared with the result obtained without excess amount
of acid, suggesting that the carboxylic acid can effectively facilitate the MCP.
Increasing monomer concentration is generally beneficial to the polymerization until it
is increased beyond 0.6 M (Table S2). Because at such high monomer concentration,
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the diacid 2a and diamine 3a may react to form large amount of insoluble ammonium
salt, forming heterogeneous reaction system and inhibiting complete conversion.
Elongation of the polymerization time can dramatically increase the yield and Mw and
after 15 h, polymer with a Mw of 54 600 g/mol could be obtained in 96% yield (Table
S3). The polymerization temperature is crucial to this MCP. Below 100 oC, both yield
and Mw of the product is only 1/3 of that obtained at 100 oC, probably due to the poor
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o
solubility of the ammonium salt; above 100 C, the Mw of the product was decreased
o
by half compared with that obtained at 100 C while the yield does not show much
influence (Table S4).
H
N
H
N
HO
OH
DMSO
OH
S8
1
H2N
NH2
n
O
O
S
S
2a-c
3a-h
1-12
P
HO
O
O
OH
O
O
O
O
HO
OH
HO
2a
2b
2c
NH2
H2N
NH2
O
NH2
H2N
H2N
O
H2N
NH2
3a
3b
3c
3d
H2N
NH2
H2N
NH2
H2N
NH2
H2N
NH2
S
O
S
O
O
O
3e
3f
3g
3h
Figure 1. Multicomponent polymerizations of sulfur 1, dicarboxylic acids 2a-c and
o
diamines 3a−h. Carried out at 100 C under nitrogen atmosphere in DMSO for 15 h.
[3a-h] = 0.5 M.
The MCPs of sulfur, diacids, and diamines are widely applicable to many different
acid and amine monomers, which can afford a great structural diversity of
polythioamides. A series of commercially available dicarboxylic acids 2a-b, aliphatic
primary diamines 3a-c and aromatic diamines 3d-h were designed as the monomers to
test the universality of this MCP. MCPs with 12 different monomer combinations were
carried out under the optimal condition in DMSO at 100 oC under nitrogen (Figure 1,
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S
S
S
H
H
S
S
N
O
N
H
N
N
O
N
H
N
H
H
H
N
n
n
N
H
n
n
S
S
S
1
2
3
4
P
P
P
P
8
yield = 96%
yield = 39%
yield = 78%
yield = 85%
Mw = 54 600 g/mol
Mn = 26 500 g/mol
Mw = 13 600 g/mol
Mn = 11 900 g/mol
Mw = 31 300 g/mol
Mn = 22 400 g/mol
Mw = 18 700 g/mol
Mn = 14 500 g/mol
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H
S
N
S
S
S
S
S
n
N
N
H
N
H
n
H
N
N
n
H
H
S
S
5
6
7
P
P
P
yield = 85%
Mw = 14 900 g/mol
Mn = 12 400 g/mol
yield = 69%
Mw = 18 200 g/mol
Mn = 13 800 g/mol
yield = 93%
Mw = 28 600 g/mol
Mn = 18 700 g/mol
O
O
O
O
S
S
S
S
S
S
S
N
H
N
H
N
N
H
n
N
H
N
n
H
n
H
8
9
10
P
P
P
yield = 99%
Mw = 49 400 g/mol
Mn = 26 100 g/mol
yield = 85%
Mw = 16 900 g/mol
Mn = 14 200 g/mol
yield = 82%
Mw = 16 500 g/mol
Mn = 13 800 g/mol
S
S
S
S
O
N
H
N
H
H
N
N
n
H
n
11
12
P
P
yield = 92%
Mw = 22 600 g/mol
Mn = 14 400 g/mol
yield = 95%
Mw = 86 200 g/mol
Mn = 41 200 g/mol
Figure 2. Chemical structures, yields, and molecular weights of P1-P12.
Table S5, and Figure S1). From the acid aspect, the para-phenylenediacetic acid 2a is
a better monomer compared with its meta-analogue 2b when polymerizing with
aliphatic diamine and sulfur, while the ortho-phenylenediacetic acid does not
polymerize at all, due to its severe steric hindrance.29,30 However, when aromatic
diamine is adopted, meta-phenylenediacetic acid 2b can deliver better polymerization
result than 2a, probably because of the improved polymer solubility of the nonlinear
shaped product. From the amine aspect, the aliphatic diamines generally work well in
these MCPs. On the other hand, the aromatic diamines with relatively weak alkalinity
are less reactive, which has not been reported in any sulfur-based MCPs before. In
comparison with the rapid ring-opening of S8 by aliphatic amine at room temperature
to form a dark brown solution, the aromatic amine does not show any reaction with
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sulfur even upon heating to 100 C (Figure S2). Nevertheless, in this MCP, with the
assistance of K2CO3, the aromatic amine could react with sulfur to form a dark brown
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solution, suggesting the formation of zwitterionic ammonium polysulfide chain. The
aromatic diamines could hence react efficiently with diacid and sulfur in the presence
of base to produce polythioamides. Inorganic or organic bases such as KOH, Na2CO3,
KF, t-BuOK, Cs2CO3, and DABCO (1,4-diazabicyclo[2.2.2]octane) could all facilitate
the polymerization of aromatic diamines (Table S6). All the aromatic diamines 3d-h
can provide good polymerization results, and the electron-rich aromatic diamines 3e
and 3f can generally produce polymers with higher Mws and yields compared with the
electron-poor aromatic diamines 3g and 3h. The MCP with aliphatic or aromatic
secondary diamines could not afford polymer product, probably due to their steric
hindrance. Moreover, a facilely available fluorene-containing dicarboxylic acid 2c was
prepared from a two-step synthesis to introduce additional functionality into the product
and test the functional group tolerance of the MCPs (Scheme S1), whose MCPs went
smoothly with both aliphatic and aromatic diamines. Polythioamides are obtained from
these MCPs with high yields up to 99% and Mws up to 86 200 g/mol, demonstrating the
general applicability of the MCPs and the rich chemical structures of the products
(Figure 2).
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Furthermore, with the commercially available inexpensive monomers and catalyst-
free mild condition of the MCPs, scalable polymerizations of 1, 2a, and 3a/3e were
conducted with 200-fold scale while keeping other reaction conditions unchanged.
While the yields of the products remain almost the same compared with that from the
0.5 mmol-scale MCPs, the Mws of the polymer improved (Table S7), suggesting that
the MCP is scalable which might potentially realize large scale production of
polythioamides conveniently and economically.
The tail gas of the up-scaled polymerization was collected to the aqueous solutions
of AgNO3 and Ca(OH)2, respectively. While no significant change was observed in the
solution of AgNO3, the solution of Ca(OH)2 became turbid, indicating that CO2 was
released from the polymerization and there was no H2S gas generated. With all the
experimental evidences from the polymerization, we’ve proposed a mechanism for the
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reaction of sulfur, benzyl acid, and amine as shown in Scheme S2. First, cyclic S8
underwent a ring opening reaction with aliphatic amine or base to provide a zwitterionic
ammonium polysulfide chain A.6,40 The proton of benzyl acid is removed under basic
condition to form B, which was then reacted with A to release CO2 and generate
intermediate C. In the presence of aliphatic amine or base, C was reacted with A again
to form intermediate D,1,41 followed by its transformation to thiocarbonyl group-
containing intermediate E. Amine was then attack on the thiocarbonyl group to afford
thioamide product F.
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The excess amount of acid in the MCP of S8, diacid and aliphatic diamine is
beneficial because the crucial step is the reaction between the deprotonated acid and the
zwitterionic ammonium polysulfide chain A to produce CO2, and in the reaction
solution, diacid and aliphatic diamine could form insoluble salt rapidly which may
inhibit further reaction. On the other hand, for aromatic diamines with relatively poor
alkalinity, they would not form insoluble salts with diacid and remain dissolved in the
solution, hence no excess amount of acid is needed. However, while the alkalinity of
aliphatic diamine is enough to assist the formation of zwitterionic ammonium
polysulfide chain, as well as the reaction steps from intermediate C to D, and then to E,
the aromatic diamines with weak alkalinity could not undergo these reactions without
the assistance of additional base such as K2CO3.
Structural Characterization of the Polythioamides. The polymer structures have
been characterized through the comparison with their small molecular model
compounds 4 and 5, which were synthesized according to the literature.39 Single crystal
structure of 5 was obtained to prove its structure and a strong intermolecular S…H-N
hydrogen bond with a distance of 2.643 Å was observed (Figure S3 and Table S8). The
IR spectrum of P1 is quite similar with that of the model compound 4: the absorption
bands of the stretching vibration of the C=O group from 2a disappeared, the stretching
vibration peak of the −NH2 group from 3a at 3341 cm-1 had converted to the stretching
vibration peaks of −NH− at 3271 cm−1, indicating the consumption of monomers and
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the generation of thioamide groups (Figure S4). Similarly, in the 1H NMR spectrum of
P1, the proton resonance of −COOH in 2a at  12.30 and that of −NH2 in 3a at  1.44
disappeared, and the −NH− proton resonance of thioamide group had appeared at 
10.35, which was in good accordance with the same resonance peak in 4 at  10.23
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(Figure 3A-D). In the C NMR spectra, the carbon resonance of −COOH from 2a at
 172.78 has converted to the characteristic C=S carbon peak in P1 at  196.11 (Figure
3E-H). Similar comparison of the aromatic polythioamide P8 and its model compound
5 could be found in Figure S5 and S6. For P2-12, the absorption bands of −NH−
stretching vibration emerged at 3190-3464 cm-1 in the IR spectra (Figure S7-S9), the
1
characteristic −NH− protons resonance at  10.33-12.18 in the H NMR spectra, and
13
the characteristic C=S carbons resonance at  196.11-197.82 in the C NMR spectra
(Figure S10-S14) all emerged, proving their polythioamide structures. Furthermore, X-
ray photoelectron spectroscopy (XPS) was used to elucidate the existence form of sulfur
in these polymers, and a high resolution scan of the S 2p region clearly identifies the S
binding energy of P1 (161.61 and 162.88 eV), P5 (161.65 and 162.65 eV) and P8
(161.73 and 162.98 eV), which are similar with that of 5 (161.65 and 162.80 eV), but
different from that of elemental sulfur (164.00 and 165.16 eV), suggesting that the
elemental sulfur has been completely converted to thioamide groups in these polymers
(Figure S15).
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C1
OH
O
O
*
*
HO
A
C1
Ha
Ha
*
E
F
*
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NH2
H2N
Hb
Hb
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Chemical Shift (ppm)
Chemical Shift (ppm)
Figure 3. 1H NMR spectra of (A) 2a, (B) 3a, (C) 4, and (D) P1. 13C NMR spectra of
(E) 2a, (F) 3a, (G) 4, and (H) P1 in DMSO-d6. The solvent peaks were marked with
asterisks.
Solubility and Thermal Stability. poly(p-phenylene terephthamide) (PPTA) fiber,
known as a popular high-performance material called Kevlar, enjoys a series of unique
advantages such as low density, high strength, good toughness, high temperature
resistance, which has found wide application in military, aeronautics, astronautics,
automobiles, and sports.42 However, due to the rigid linear structure of PPTA, its strong
covalent bonds along the fiber direction, and N-H···O hydrogen bonds in the transverse
direction, the material can hardly be dissolved in any organic solvent, and the solution-
involved characterization and application are hence inapplicable. When O atoms in
PPTA were replaced by S atoms, the intermolecular hydrogen bonds were weakened,43
which could bring improved solubility and processability to the corresponding material.
Indeed, despite the presence of intermolecular and intramolecular hydrogen bonds, the
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polythioamides still show excellent solubility in common polar solvents such as DMSO
and DMF, etc, including P5 which possesses a similar structure as PPTA, with only
oxygen atoms been replaced by sulfur atoms. PPTA was synthesized according to the
literature,44 which is totally insoluble in DMF, while P5 can be well-dissolved in DMF
to form an orange solution (Figure S16), enabling characterization such as GPC and 1H
NMR. In addition, self-supporting smooth thin film of P8 can be obtained by simple
solvent evaporation approach, demonstrating good processability of these
polythioamides.
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The polythioamides also exhibit good thermal stability, probably owing to their
intermolecular or intramolecular hydrogen bonding between thioamide moieties. Their
decomposition temperatures at 5% weight loss (Td) under nitrogen were ranging from
298 to 391 °C. Generally, the polythioamides with more aromatic rings show higher
thermal resistance, and the polymer prepared from aromatic diamines generally possess
higher char yields compared with that synthesized from aliphatic diamines. The glass
transition temperature (Tg) of polythioamides could also be modulated from 59 to 248
oC, by varying the monomer structures (Figure S17).
Gold Recovery Application of Polythioamides. Besides its value as currency metal,
gold is widely used because of its good ductility, thermal conductivity and chemical
stability.45 The current annual demand for gold is 4 000 tons, of which 1 500 tons comes
from the recycling of spent industrial products,45 considering that the gold content in
the circuit boards of cellular phones is 300−350 g/t, which is much higher than that in
gold ore with the range of 0.3−17 g/t.46-49 Gold refineries from the discarded electronic
wastes are crucial, however, mercury and sodium cyanide, or alternative leachants
including thiosulfate, bromide, iodide, and sodium hypochlorite are frequently used to
recover gold, which lead to environment pollution and limited efficiency.45,50 Green
approaches are hence in great demand, and polythioamides with their strong metal
coordination property may be ideal candidates to retrieve gold ions.51
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P8
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P1 405
P5 597
P8 468
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Weight (mg)
Time (min)
Figure 4. (A) Extraction efficiency of Au3+ with different amounts of solid powder of
polythioamides P1, P5 and P8. (B) Time-dependent extraction of Au3+ with 10 mg solid
powders of P5 and P8. Inset: the gold extraction process with polythioamide. [Au3+]0 =
100 mg/L. VAu3+ = 10 mL.
Polythioamides P1, P5 and P8 with varied polymer structures were selected as
examples to investigate their metal complexation, and different amount of solid powder
of these polymers were added into the aqueous solutions of Au3+, respectively. The
solid was removed by centrifugation after stirring at room temperature for 1 h and the
remaining metal ion concentrations were measured by Atom Absorption Spectroscopy
(AAS) to calculate the extraction efficiency. The extraction capacities of the polymers
are 405 (P1), 597 (P5) and 468 mg·Au3+/g (P8), respectively (Figure 4A, Table S9),
suggesting that the polythioamides prepared from aromatic diamines show better
performance compared with the polythioamide prepared from aliphatic diamines. P5
and P8 were then selected as examples to investigate the speed and sensitivity of the
gold extraction process, and both polymers can achieve a high extraction efficiency >98%
in less than 1 min and reach equilibrium in 10 min with >99% extraction efficiency
(Figure 4B, Table S10). The extraction processes with DMF solution of P8 are also
quite sensitive, which can even work with low [Au3+]0 as 10 ppb, reaching 90.94%
efficiency (Table S11). The solid powder and DMF solution of P8 enjoy comparably
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high extraction efficiency (Table S9, entry 5 and Table S11, entry 1). The practical
condition for gold extraction usually involves extreme pH environment, and the gold
extraction experiments were hence compared under acidic and neutral condition. Good
stability of the polymer and high reproducibility of the extraction efficiency can be
obtained with varying pH values. Even with pH value of 0.10, the extraction efficiency
of P8 could still reach 99.96%, demonstrating the robustness of the polythioamide
materials (Figure S18).
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To investigate the selectivity of the polythioamides among different metal ions,
solid powder of P8 was directly added into the respective aqueous solutions of 16
different metal ions including Na+, K+, Mg2+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+,
Zn2+, Cd2+, Hg2+, Pb2+, Ce3+, and Au3+ with the metal ion concentration of 100 mg/L.
The solid was removed by centrifugation after 1 h and the remaining concentrations of
metal ions in the supernatant were tested to calculate the extraction efficiency. Of all
the 16 metal ions, strong coordination between P8 and Au3+/Hg2+ were observed with
the extraction efficiencies of Au3+ (>99.99%) and Hg2+ (86.01%) (Figure S19, Table
S12). Encouragingly, from the mixed solution of the 16 different metal ions, P8 could
extract Au3+ selectively with the extraction efficiency >99.99%, while the efficiency is
only 7.01% for Hg2+, and below 1.71% for the other metal ions (Figure 5A, Table S12).
For the colored metal ion solutions, only the color of Au3+ solution faded, while the
solutions of Cr3+, Fe3+, Co2+, Ni2+, and Cu2+ showed no change after the treatment with
P8 (Figure 5A, inset). The polythioamides can hence extract Au3+ rapidly (1 min),
efficiently (>99.99%), sensitivity (10 ppb), and selectively with high capacity (~0.60
g·Au3+/g) under practical acidic conditions, which is promising for gold recycling from
the electronic wastes.
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after
P8
Au3+
Cr3+
Fe3+
Co2+
Ni2+
Cu2+
Na+ K+ Mg2+ Ca2+ Cr3+ Mn2+ Fe3+ Co2+ Ni2+ Cu2+ Zn2+ Cd2+ Hg2+ Pb2+ Ce3+ Au3+
Metal ions
C
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filter
dry
1000 oC
air, 4 h
magnify
× 100
91.64 wt% Au
naked eye
95.62
8.32
0.22%
microscope
0.00%
Ni2+
Cu2+
Metal ions
Au3+
Figure 5. (A) Selective extraction of Au3+ by P8 in an aqueous solution with 16 mixed
metal ions. Inset: Six colored metal ion solutions before and after the treatment with
P8. [Mn+]0 = 100 mg/L, VMn+ = 10 mL, mP8 = 10 mg. (B) The initial concentrations of
Ni2+, Cu2+ and Au3+ and the selective extraction of Au3+ by P8 from the leaching
solution of a discarded CPU. (C) The gold recovery process with P8.
This gold extraction approach was hence applied to extract Au3+ directly from a
discarded computer processing unit, whose metal scrap chippings were soaked in an
aqueous solution of N-bromosuccinimide and pyridine,52 to obtain a blue solution with
1478.04 mg/L of Cu2+, 95.62 mg/L of Ni2+, and 8.32 mg/L of Au3+. After the solution
was treated with the solid powder of P8, only 2.88% of Au3+ was left in the solution,
while the concentrations of Ni2+ and Cu2+ were almost unchanged, demonstrating that
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the polythioamide can selectively extract Au3+ with 97.12% efficiency at practical
condition in the presence of high concentration of interference metal ions (Figure 5B,
Table S13). Most importantly, elemental gold can be recovered by facile pyrolysis of
the resulting P8-Au3+ complex. After the complex was heated at 1000 oC in air for 4 h,
a yellow solid with metallic golden luster was obtained with the gold content of 91.64
wt% measured by AAS (Figure 5C).
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To reveal the working mechanism of the gold extraction process, XPS of
polythioamide P1 before and after its coordination with Au3+ was studied. From the
XPS spectrum of P1 to that of P1-Au3+, besides that the characteristic binding energy
for Au 4f (84.45, 88.14 eV) was emerged in the latter, the spectra of S 2p had changed
both the spectrum profile and the binding energy from 161.61, 162.88 eV to 161.42,
162.52 eV, while that of N 1s did not show obvious difference (Figure 6A-C), indicating
that S atoms from P1 participated in the coordination with Au3+, while N atoms did not.
To further identify the coordination structure between polythioamide and Au3+,
model compound 5 was mixed with Au3+ in ethanol/water solution to afford a yellow
compound 5-Au3+ after removal of the solvent (Figure 6D). The XPS spectrum of S 2p
region had significantly changed from 5 to 5-Au3+ while that of N 1s region was not
affected, indicating that S atom is the coordination site with Au3+ (Figure S20).
Moreover, the −NH− proton resonance of 5 at  11.63 was shifted to  10.12 in the 1H
NMR spectrum of 5-Au3+, and the C=S carbon resonance of 5 at  196.80 was shifted
13
to  165.09 in the C NMR spectrum of 5-Au3+ (Figure S21). Most importantly, the
MALDI-TOF MS spectrum of 5-Au3+ was measured to be 683.1103 g/mol, which is in
great accordance with the molecular weight of the gold complex consisted of one Au3+
and two molecules of 5 (Figure S22). With these insights, it could be concluded that
two S atoms from polythioamide could form a strong binary coordination with one Au3+
to crosslink the polymer chains and hence extract Au3+ efficiently and selectively from
aqueous solutions of mixed metal ions (Figure 6E).
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Figure 6. (A) Au 4f, (B) S 2p, and (C) N 1s XPS of P1 and P1-Au3+. (D) The synthesis
of model compound 5-Au3+ and (E) the proposed mechanism for the extraction of Au3+
with polythioamides.
CONCLUSIONS
In conclusion, catalyst-free multicomponent polymerizations of elemental sulfur,
dicarboxylic acids, and diamines were reported for the economic and convenient
conversion from sulfur to functional polythioamides with great structural diversity,
large molecular weights, and excellent yields. Compared with the aromatic diynes and
diisocyanides monomers of the reported sulfur-based MCPs, the dicarboxylic acids in
these MCPs are superior candidates, on accounts of their commercially availability and
abundant natural sources, enabling scalable preparation of polythioamides.
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Furthermore, besides the commonly used aliphatic diamine monomers, aromatic
diamine monomers that have not been used in any sulfur-based MCPs, can undergo this
multicomponent polymerization smoothly with the assistant of base, affording unique
polythioamide structures with rigid backbones. The polythioamides enjoy good
solubility despite of the rigid structure and intermolecular hydrogen bonds, facilitating
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comprehensive characterization with GPC, H and 13C NMR spectra, XPS, and IR
spectra of their well-defined structures. Moreover, thanks to the strong binary
coordination of thioamide moieties with Au3+, these polythioamides can be utilized in
gold recovery for the sensitive, selective, efficient extraction of Au3+ from the practical
acidic leaching solution of discarded electronic wastes, which could further afford
elemental gold with large capacity after pyrolysis of the polythioamide-Au3+ complexes.
The future challenge of this MCP is to make it applicable to extensive aliphatic acids.
The multicomponent polymerizations of sulfur, acids, and amines have paved the way
of economic and scalable conversion from sulfur waste to functional polythioamide
materials, providing solutions to both problems of sulfur utilization and gold recovery,
demonstrating their great academic importance and potential industrial interests.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on
the ACS Publications website. Materials, instruments, synthetic and experimental
procedures, and characterization data; effect of monomer loading ratio, concentration,
time, temperature, monomer structures, base and scales on the polymerization; the
proposed reaction mechanism; GPC curves of P1-12; FT-IR spectra, 1H and 13C NMR
spectra of 2c, 4, 5, 5-Au3+, and P1-12; all the original MALDI-TOF MS spectra, 1H and
13C NMR spectra; thermogravimetric analysis of P1-12; XPS spectra of sulfur, 5, P1,
P5, P8, 5-Au3+, and polythioamide-Au3+ complexes; effect of different dosages of
polythioamides, time, [Au3+]0, and pH value of gold extraction; selectivity test with
different metal ions and gold extraction from discarded CPU by P8.
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AUTHOR INFORMATION
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Corresponding Authors
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* (R.H.) E-mail msrrhu@scut.edu.cn.
Notes
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The authors declare no competing financial interest
ACKNOWLEDGMENTS
This work was partially supported by the National Science Foundation of China
(21490573, 21822102, 21774034, 21490574, and 21788102), the Natural Science
Foundation of Guangdong Province (2016A030306045 and 2016A030312002), and the
Innovation and Technology Commission of Hong Kong (ITC-CNERC14SC01).
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OH
O
O
S
n
HO
S
DMSO
gold recovery
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N
S
S
H
H
N
S
S
S
S
H2N
NH2
S
S
commercially available monomers
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