Sulfur Conversion to Multifunctional Poly(O-thiocarbamate)s through Multicomponent Polymerizations of Sulfur, Diols, and Diisocyanides

Article abstract of DOI:10.1021/jacs.1c00243

Sulfur, which is generated from the waste byproducts in the oil and gas refinery industry, is an abundant, cheap, stable, and readily available source in the world. However, the utilization of excessive amounts of sulfur is mostly limited, and developing

Full text of DOI:10.1021/jacs.1c00243

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Article
Sulfur Conversion to Multifunctional Poly(O‑thiocarbamate)s
through Multicomponent Polymerizations of Sulfur, Diols, and
Diisocyanides
Jie Zhang, Qiguang Zang, Fulin Yang, Haoke Zhang, Jing Zhi Sun,* and Ben Zhong Tang*
Cite This: J. Am. Chem. Soc. 2021, 143, 3944−3950
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ABSTRACT: Sulfur, which is generated from the waste byproducts in
the oil and gas refinery industry, is an abundant, cheap, stable, and readily
available source in the world. However, the utilization of excessive
amounts of sulfur is mostly limited, and developing novel methods for
sulfur conversion is still a global concern. Here, we report a facile one-step
conversion from elemental sulfur to functional poly(O-thiocarbamate)s
through a multicomponent polymerization of sulfur, diols, and
diisocyanides, which possesses a series of advantages such as mild
condition (55 °C), short reaction time (1 h), 100% atom economy, and
transition-metal free in the catalyst system. Seven poly(O-thiocarbamate)s
are constructed with high yields (up to 95%), large molecular weight (up
to 53100 of Mw), good solubility in organic solvents, and completely new
polymer structures. The poly(O-thiocarbamate)s possess a high refractive
index above 1.7 from 600 to 1700 nm by adjusting the sulfur content. By incorporating tetraphenylethene (TPE) moieties into the
polymer structure, the poly(O-thiocarbamate)s can also be designed as fluorescent sensors to detect harmful metal cation of Hg2+ in
a turn-on mode with high sensitivity (LOD = 32 nM) and excellent selectivity (over interference cations of Pb2+, Au3+, Ag+).
Different from the previous reports, the exact coordination structure is first identified by single-crystal X-ray diffraction, which is
revealed in a tetracoordination fashion (two sulfur and two chloride) using a model coordination compound.
Among these methods, MCPs have stood out on sulfur
conversion. For example, polythioamides constructed by sulfur,
amines, and aldehydes were synthesized through a typical
MCP route.22,23 In the past five years, Tang et al. have
reported the MCPs of sulfur, amines and diynes,24 the MCPs
of sulfur, diamines, and isocyanides25 and the MCPs of sulfur,
aliphatic diamines, and acids26 in succession. MCPs are
becoming a powerful tool for sulfur conversion to functional
polymers. However, amines are invariably relied on among all
of the above-mentioned MCPs. As a result, the generalization
of sulfur conversion to polymers will be extremely limited.
Compared with amides, alcohols are cheaper, more eco-
friendly, storage-stable, and achievable. Similar to amines,
alcohols also possess nucleophilicity,29−31 which is proved by
its reaction with sulfur and isocyanides.32 So, alcohols are
expected reasonably to break the limitation on monomers and
to be considered as ideal raw materials.
INTRODUCTION
■
The utilization of sulfur can be dated back to ancient times, as
documented in Ancient Greek, Roman, and Chinese history.
Although studied for centuries, sulfur-related studies remain
hot spots in the areas of chemistry, biology, and geology.1−12
With the yearly increase of the oil and gas refinery industry, it
is reported that more than 80 million metric tons of sulfur are
globally produced.13,14 The excessive amounts of sulfur may
find their way out in polymer chemistry.15 As a simple, cheap,
and readily available starting material, sulfur is well suited as a
monomer in polymerization. Meanwhile, compared with
traditionally smelly or unstable sulfur-containing reagents
such as thiols, episulfide, isothiocyanates, and carbon
disulfide,16 elemental sulfur is an abundant source for sulfur-
containing polymers due to its odorlessness and stability.
Sulfur-containing polymers have drawn increasing attention for
their various optoelectronic applications and unique metal-
coordination capacity.17−19 However, the methods for
preparing sulfur-containing polymers using elemental sulfur
are extremely limited.20 Only a handful of research has been
reported including vulcanization, inverse vulcanization,15,21
multicomponent polymerizations (MCPs) of elemental
sulfur,22−26 and copolymerization of episulfide and elemental
sulfur.27,28
Received: January 8, 2021
Published: March 4, 2021
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Herein, MCPs of sulfur, alcohols, and isocyanides are
explored. The incorporation of elemental sulfur by MCPs
permitted the placement of sulfur atoms in polymer main
chains easily and efficiently, producing a series of poly(O-
thiocarbamate)s with high molecular weight (Mw) thiocarba-
mate during a short period in high yields. It is noteworthy that
only base was used in the polymerization as catalyst but
without any transition-metal catalysts. Furthermore, to the best
of our knowledge, poly(O-thiocarbamate)s were unprecedent-
edly prepared; thus, they could become a new family of
functional polymers. In addition, thanks to the involvement of
sulfur, poly(O-thiocarbamate)s possess high refractive index,
showing potential as advanced optical materials. More
importantly, elemental sulfur efficiently conversed to thione,
and taking advantage of the unique coordination between
thione and mercury cation (Hg2+), poly(O-thiocarbamate)s
were designed to be fluorescent chemosensors for Hg2+ in a
turn-on mode with high sensitivity and good selectivity.
Different from the previous reports,25,33−46 the coordination
structure was first revealed using single-crystal X-ray diffraction
for model coordination compound, which clarifies the
detection mechanism.
solvents (N,N′-dimethyl formamide, dimethyl sulfoxide, and
N-methylpyrrolidone). Surprisingly, the best result was
obtained in N,N′-dimethyl formamide (DMF), and polymer
with a Mw of 24600 was furnished in 90% yield (entry 13,
Table S1).
To explore monomer scope and endow products with
functions, a series of diols 1a−f and aromatic/aliphatic
diisocyanides 2a,b were selected as monomers (Scheme 1).
Scheme 1. Polymerization of Sulfur, Diols, and
Diisocyanides
The MCPs of all seven combinations of sulfur, six diols, and
two diisocyanides were carried out in DMF at 55 °C for 1 h
under nitrogen atmosphere. All of the polymerizations
proceeded smoothly and rapidly to produce poly(O-
thiocarbamate)s. The yields of the polymers were in the
range of 81−95%, and the Mw was high, up to 53100 (Table 1,
RESULTS AND DISCUSSION
■
MCPs of Sulfur, Diols and Diisocyanides. First, the
reaction mechanism was investigated using a model reaction of
sulfur, diol 3, and isocyanide 4 (Scheme S1). Sulfur and NaH
were initially mixed in solvent, generating sodium polysulfide.
Five minutes later, isocyanide was added and then attacked by
sodium polysulfide. Isothiocyanate 5 was separated as the sole
key intermediate (Figure S1-A). It is noteworthy that the
feeding sequence is of crucial importance for the formation of
5. If sulfur/NaH/isocyanide were fed together, there were
impurities as shown in Figure S1-B. Afterward, ethanol was
added, producing the end product M1 with the promotion of
NaH. After understanding the reaction mechanism of the three
components, we set about developing the MCPs of sulfur,
diols, and diisocyanides.
Table 1. Polymerization of Sulfur, Diols, and
a
Diisocyanides
yield
(%)
b
b
c
polymer
monomers
Mw
Mn
Đ
Mn′
P1
P2
P3
P4
P5
P6
P7
S + 1a + 2a
S + 1b + 2a
S + 1c + 2a
S + 1d + 2a
S + 1e + 2a
S + 1f + 2a
S + 1a + 2b
90
91
90
87
85
95
81
24600
27600
26400
23900
20800
53100
13400
14200
16700
16800
13300
13200
26500
8600
1.73
1.65
1.57
1.80
1.57
2.00
1.56
13134
6401
7224
7663
9179
17394
7509
Polymerization conditions are key parameters, which have
direct effects on the derived polymers. To systematically
optimize various conditions, economic sublimed sulfur, cheap
alcohol 1a, and easily prepared diisocyanide 2a were chosen as
an example. The polymerization was first carried out by a
stoichiometric ratio of monomers at 40 °C in THF, producing
a low Mw polymer in 53% yield (entry 1, Table S1). The excess
amount of sulfur and NaH could significantly improve the yield
and Mw of the product. When the ratio of [S]/[NaH]/[1a]
reached 4:4:1, the Mw of polymer was as high as 10000 (entry
3, Table S1). Increasing or decreasing monomer concentration
could not promote the polymerization, so 0.2 M was chosen as
the optimal concentration. The temperature had a big
influence on the MCPs. At room temperature, both the yield
and the Mw of the product were undesired. After the
temperature was raised to 55 °C, the yield was dramatically
increased to above 90%, probably due to the enhanced
solubility of sodium polysulfide in THF. A better result could
not be obtained if the temperate continued to rise (entries 6−
8, Table S1). Thus, the polymerization was carried out at 55
°C in subsequent explorations. Time-optimizing experiments
indicated that 1 h was enough for the completion of the MCP
(entry 10, Table S1). The short period of polymerization was
strong moyivator for the popularization of the MCP. Apart
from THF, the solvent was changed into three other polar
a
Carried out in 1 mL of DMF at 55 °C under nitrogen atmosphere
([1] = 0.2 mmol/L, [S] = 0.8 mmol/L, [1]/[2] = 1:1, [S]/[NaH] =
b
1:1). Mw and Đ (Mw/Mn) of polymers were estimated by GPC in
c
DMF on the basic of a polystyrene calibration. Mn′ of polymers were
1
calculated by the method of integral area in H NMR (Figure S5).
Figure S2). These features demonstrate the general monomer
applicability and high efficiency. All polymers possess excellent
solubility in common solvents and good thermal stability
(Figure S3).
Characterization of the Poly(O-thiocarbamate)s. To
facilitate the structural characterization, model compound M1
was prepared under the same reaction conditions (Scheme S2)
and its structural accuracy was proved by the single-crystal
structure (Table S2). Then the 1H/13C NMR spectra of
monomers 2a and 1a, model compound M1, and P1 in
DMSO-d6 were compared as shown in Figure 1. The
1
resonance of −OH at δ 5.13 ppm disappeared in the H
NMR spectra of M1 and P1, while the resonance of CN at δ
167.95 entirely disappeared in the 13C NMR spectra of M1 and
P1. The comparison indicated that monomers 1a and 2a have
been consumed and polymerized. More importantly, a new
broad peak “a” at δ 10.43 assigned to −NH− emerged in the
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1
Figure 1. H NMR spectra of monomers 2a (A), 1a (B), model compound M1 (C), and polymer P1(D) in DMSO-d6; 13C NMR spectra of
monomers 2a (E), 1a (F), model compound M1 (G) and polymer P1(H) in DMSO-d6.
1H NMR spectrum of P1, which corresponded to the set of
sulfur conversion through the present MCP, P5 possessed a
1
high refractive index of 1.725 to 1.701 from 600 to 1700 nm
(Figure 2), in line with expectations. Furthermore, P3 presents
peaks “a′” at δ 10.50−10.36 in the H NMR spectrum of M1.
Meanwhile, a set of peaks at δ 189.77−188.39 assigned to the
thiocarbonyl “c” of P1 was well-matched with “c′” of M1 at δ
189.28−188.04 in 13C NMR (Figure 1G). As there was a
conformational equilibrium between cis- and trans-orientations
in the O-thiocarbamate group, a pronounced conformational
split of the signal was observed in the NMR spectra of M1 and
P1, which had previously been found in analogous structures
such as urea, thiourea, amide, and thioamide.47−49 All of these
results manifested that the O-thiocarbamate linkage had been
formed indeed. In addition, the structural characterization by
FTIR also showed the vanishing of CN/−OH and the
formation of CS/−NH− (Figure S4), reaching the same
conclusion.
For other poly(O-thiocarbamate)s P2−P7, the characteristic
Figure 2. Light refraction spectra of thin solid films of poly(O-
thiocarbamate)s.
1
−NH− peaks at δ 9.55−7.05 all emerged in their H NMR
spectra (Figure S5), and the characteristic CS peaks at δ
179.6−183.6 all emerged in their 13C NMR spectra (Figure
S6), besides the vibration of CS at 1505−1490 cm−1, and
the vibration −NH− at 3354−3218 cm−1 all emerged in their
FTIR spectra (Figure S7−12), confirming the expected
poly(O-thiocarbamate) structure.
higher n of 1.755 to 1.730 at the same wavelength window
when more sulfur atoms were introduced accompanying
diphenyl sulfide in 2c. The poly(O-thiocarbamate)s prepared
from this sulfur-based MCP are hence promising candidates
for advanced optical materials.
High Refractive Index. High refractive index (n) materials
play an irreplaceable role in the achievement of large-scale
photonic integration due to their compact structure.50,51
However, the vast majority of organic polymers exhibit
relatively low n-values of 1.5−1.6.52,53 In contrast, high
refractive index polymers (HRIPs, where n ≥ 1.7) have
drawn increasing attention for their various optoelectronic
applications such as organic light-emitting diodes, microlens
components in both charge-coupled devices, high-performance
complementary image sensors,54,55 all-polymer optoelectronic
devices,56 and polymer-based optical waveguides.50 It is well-
known that introduction of sulfur atoms is one of most
effective strategies to produce HRIPs. Taking advantage of
Mercury Detection Application of Poly(O-
thiocarbamate)s and Its Coordination Mode. Tetraphe-
nylethylene (TPE) is a typical luminogen exhibiting an
aggregation-induced emission (AIE) feature.57 When TPE
was introduced into polymers as an intentional design, P4 and
P5 showed the expected AIE characteristics. As shown in
Figures 3 and S13, the polymers did not fluoresce in the
mixtures of tetrahydrofuran (THF)/water when the water
fraction (fw, by volume) was low. With the increase of fw,
aggregates of polymers formed due to the decrescent solubility.
A sharp enhancement of fluorescence under UV light was
observed for the system when fw was higher than a certain
value. Taking P4 as an example, there was an obvious
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Figure 3. Plots of relative PL intensity versus water fractions (by
volume) in THF/water mixtures of (A) P4 and (B) P5.
Concentration of polymers: 10 μM. Excitation wavelength: 360 nm.
aggregation/fluorescent turning point at 60% water content.
This turning point could be used for turn-on detection based
on AIE mechanism.
Poly(O-thiocarbamate)s are expected to show metal
complexation because the thione groups have strong binding
affinity to heavy metal ions.57 If P4 dispersing in THF/H2O
(fw = 60%) did coordinate some metal ions, aggregation would
occur to polymer segments because the metal cations played
the role of cross-linking points by coordinating with adjacent
thione group. Meanwhile, the coordination decreased the
water solubility of the poly(O-thiocarbamate)s for the reduced
free thione groups. As a result, the fluorescence of P4 would be
turned on as it was excited under UV light.
Figure 4. (A) PL spectra of P4 in THF/water mixtures (v/v: 4/6)
with the presence of different metal ions (10 μM). (B) Relative
intensity (I/I0 − 1) at 474 nm of P4 in THF/H2O mixture (v/v: 4/6,
10 μM) in the presence of different metal ions (10 μM) and the
corresponding fluorescence photos taken under UV irradiation. I0 =
PL intensity in the absence of metal ions. Excitation wavelength: 360
nm. (C, D) PL spectra and relative intensity of P4 when KBr (100
μM) was added in each test mixture, other all conditions remain same
with A and B respectively.
To prove the above deduction, P4 was selected as an
example to demonstrate the interaction between poly(O-
thiocarbamate)s and different metal ions. The preliminary
results are shown in Figures 4 and S14−S16. As revealed by
the experimental data, the solution of P4 responded to both
Hg2+ and Ag+ with positive signals (Figure 4A,B). However,
when a screening agent of KBr was used, Hg2+ could be
detected solely, and Ag+ was successfully shielded; meanwhile,
other metal ions remained nonfluorescent, regardless of
addition of the screening agent or not (Figure 4C,D). Using
this method, the detection of Hg2+ in the presence of various
metal ions was successful with AIE-active polymer P4 (Figure
S14). Then the response speed was tested, and the
fluorescence turn-on reached equilibrium after 10 min (Figure
S15). The fluorescence intensity at 474 nm showed a great
linear correlation with the concentrations of Hg2+ from 0 to 7.5
μM. The limit of detection (LOD) was calculated to be 32 nM
according to the equation of LOD = 3SB/m (Figure S16).
Mercury is a global contaminant because it can travel a long
distance through the atmosphere, resulting in wide distribution
of Hg in water, air, and soil.58,59 Since knowledge of the
toxicity of mercury was intensified following the deaths of
hundreds of people caused by Minamata disease in Japan in
the 1950s, the hazards of mercury have been highly addressed
by scientists around the world.60 To date, environmental
mercury contamination is still a great threat to our world, and
numerous studies have been made to detect mercury ions.33−46
As indicated by the experimental results, TPE-containing
poly(O-thiocarbamate)s prepared from this MCP route can be
used to detect mercury ion in a turn-on mode with high
selectivity and sensitivity, which are promising for fluorescent
chemosensors for Hg2+.
Hg2+ detection. However, the exact coordination between
probes and heavy-metal ions has historically been vague. Some
hypotheses were previously reported without explicit struc-
tures. To identify the coordination mode between P4 and
mercury ion, tabular crystals of model compound M1 were
dissolved and mixed with HgCl2 in ethanol/water solution
(Scheme S4), and acicular crystals were obtained eventually.
With the X-ray single-crystal structural analyses, the coordina-
tion structure was veritably confirmed (Figure 5A, Table S3).
Different from previous reports, two S atoms and two Cl atoms
simultaneously participated in the coordination, while N atoms
did not, affording an unforeseen tetracoordination structure.
More subtly, well-defined features were revealed by the
crystallographic data. The complex geometry was unsym-
metrical; for example, the distances between Hg and two S
atoms were 2.538 and 2.457 Å, respectively, and there were
intra- and intermolecular interactions through Cl···H and Cl···
S (Figure 5B). Hereto, the coordination mode between thione
groups of P4 and mercury ion was clearly characterized (Figure
5C). This is an unprecedented insight of the interactions
between Hg2+- and sulfur-containing functionalities. It may
lead to important advances in dealing with environmental
mercury contamination.
CONCLUSIONS
■
In this work, economic one-step conversion from elemental
sulfur to functional poly(O-thiocarbamate)s through a
transition-metal free multicomponent polymerization of sulfur,
diols, and diisocyanides is realized. The detailed conditions for
the polymerization reaction were systematically explored. The
experimental results indicated that this MCP possesses unique
It is of great significance to address the coordination mode
between thione groups and Hg2+ because of the importance of
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Figure 5. (A) X-ray single crystal structure of M2. (B) Crystal packing of M2. (C) Schematic diagram of detection on Hg2+ with P4.
advantages of mild conditions, short reaction time, high
efficiency, high atom economy, and convenience, producing
soluble poly(O-thiocarbamate)s with high yields, large Mw, and
great diversity of well-defined polymer structures. Therefore, a
novel MCP has been established. The present MCP route
breaks the dependency on amide monomers, taking the
generalization of sulfur conversion one step further. Mean-
while, the poly(O-thiocarbamate)s containing higher sulfur
content show a high refractive index above 1.7 from 600 to
1700 nm and are ideal candidates as advanced optical
materials. The poly(O-thiocarbamate)s can also be designed
as turn-on type fluorescent chemo-sensors by introducing TPE
moieties into the polymer skeleton. On the basis of the AIE-
activity of TPE moieties, the selective, sensitive, and “turn-on”
detection of mercury ion are successfully realized. More
importantly, the coordination structures between poly(O-
thiocarbamate)s and mercury ion are proven to be a
tetracoordination mode using a single crystal of model
coordination compound, which is first revealed here.
AUTHOR INFORMATION
■
Corresponding Authors
Jing Zhi Sun − MOE Key Laboratory of Macromolecules
Synthesis of Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou
310027, China; orcid.org/0000-0001-5478-5841;
Email: sunjz@zju.edu.cn
Ben Zhong Tang − MOE Key Laboratory of Macromolecules
Synthesis of Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou
310027, 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, Kowloon, Hong Kong,
China; orcid.org/0000-0002-0293-964X;
Email: tangbenz@ust.hk
Authors
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c00243.
■
Jie Zhang − MOE Key Laboratory of Macromolecules
Synthesis of Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou
310027, China; orcid.org/0000-0003-1641-1042
Qiguang Zang − MOE Key Laboratory of Macromolecules
Synthesis of Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou
310027, China
Fulin Yang − MOE Key Laboratory of Macromolecules
Synthesis of Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou
310027, China
sı
Materials, instruments, synthetic and experimental
procedures, and characterization data; procedures for
detection on mercury ion; optimization on the polymer-
ization; the proposed reaction mechanism; thermogravi-
metric analysis of P1−P7; FT-IR spectra and 1H and 13
C
NMR spectra of 1c−1e, 2b, M1, M2, and P1−P7
(Figures S17−S28) (PDF)
Accession Codes
Haoke Zhang − MOE Key Laboratory of Macromolecules
Synthesis of Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou
310027, China; orcid.org/0000-0001-7309-2506
CCDC 2023847 and 2034126 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Complete contact information is available at:
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Author Contributions
Monomer and polymer synthesis, structure characterization,
property investigation and manuscript preparation were
performed by J.Z. Figure improvement, graphical abstract
design, and fluorescent detection were performed by Q.Z. F.Y.
helped with polymer synthesis and data analysis. Theoretical
calculation and crystallographic data treatments were per-
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review, and editing was performed by J.Z.S. and B.Z.T.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Foundation of China (22021715 and 51573158) and Zhejiang
Province Department of Science & Technology (major
scientific and technological project: 2020C03030).
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