O-to-S Substitution Enables Dovetailing Conflicting Cyclizability, Polymerizability, and Recyclability: Dithiolactone vs. Dilactone

Article abstract of DOI:10.1002/anie.202109767

Developing chemically recyclable polymers represents a greener alternative to landfill and incineration and offers a closed-loop strategy toward a circular materials economy. However, the synthesis of chemically recyclable polymers is still plagued with certain fundamental limitations, including trade-offs between the monomer's cyclizability and polymerizability, as well as between polymer's depolymerizability and properties. Here we describe the subtle O-to-S substitution, dithiolactone monomers derived from abundant feedstock α-amino acids can demonstrate appealing chemical properties different from those of dilactone, including accelerated ring closure, augmented kinetics polymerizability, high depolymerizability and selectivity, and thus constitute a unique class of polythioester materials exhibiting controlled molecular weight (up to 100.5 kDa), atactic yet high crystallinity, structurally diversity, and chemical recyclability. These polythioesters well addresses the formidable challenges of developing chemically recyclable polymers by having an unusual set of desired properties, including easy-to-make monomer from ubiquitous feedstock, and high polymerizability, crystallinity and precise tunability of physicochemical performance, as well as high depolymerizability and selectivity. Computational studies explain why O-to-S modification of polymer backbone enables dovetailing desirable, but conflicting, performance into one polymer structure.

Full text of DOI:10.1002/anie.202109767

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: O-to-S Substitution Enables Dovetailing Conflicting Cyclizability,
Polymerizability, and Recyclability: Dithiolactone vs Dilactone
Authors: Yanchao Wang, Maosheng Li, Jinlong Chen, Youhua Tao,
and Xianhong Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202109767
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RESEARCH ARTICLE
O-to-S Substitution Enables Dovetailing Conflicting Cyclizability,
Polymerizability, and Recyclability: Dithiolactone vs Dilactone
[
a] [b]
[a]
[a]
, [a] [b]
[a]
Yanchao Wang,
Maosheng Li, Jinlong Chen, Youhua Tao,*
Xianhong Wang
[
a]
Y. Wang, M. Li, J. Chen, Y. Tao, X. Wang.
Key Laboratory of Polymer Ecomaterials,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences ,Changchun 130022 , P. R. China
E-mail: youhua.tao@ciac.ac.cn
[
b]
Y. Wang, Y. Tao.
University of Science and Technology of China, Hefei 230026, People’s Republic of China.
Supporting information for this article is given via a link at the end of the document.
ABSTRACT: Developing chemically recyclable polymers represents
commonly referred as readily available but “non-polymerizable”
monomer, produced polyester (PγBL) with complete chemical
recyclability (Figure 1). However, the ROP of γBL needs
energy-intensive, industrially undesirable low-temperature
conditions (-30 °C to -60 °C), and the delivered PγBL shows
a greener alternative to landfill and incineration and offers a closed-
loop strategy toward a circular materials economy. However, the
synthesis of chemically recyclable polymers is still plagued with
certain fundamental limitations, including trade-offs between the
monomer’s cyclizability and polymerizability, as well as between
polymer’s depolymerizability and properties. Here we describe the
subtle O-to-S substitution, dithiolactone monomers derived from
abundant feedstock α-amino acids can demonstrate appealing
chemical properties different from those of dilactone, including
accelerated ring closure, augmented kinetics polymerizability, high
depolymerizability and selectivity, and thus constitute a unique class
of polythioester materials exhibiting controlled molecular weight (up
to 100.5 kDa), atactic yet high crystallinity, structurally diversity, and
chemical recyclability. These polythioesters well addresses the
formidable challenges of developing chemically recyclable polymers
by having an unusual set of desired properties, including easy-to-
make monomer from ubiquitous feedstock, and high polymerizability,
crystallinity and precise tunability of physicochemical performance,
as well as high depolymerizability and selectivity. Computational
studies explain why O-to-S modification of polymer backbone
enables dovetailing desirable, but conflicting, performance into one
polymer structure.
[
4]
m
limited thermostability and crystallinity, with a low T of ~60 °C .
To
address
this
energy-intensive
trade-off,
challenge
fused-ring
and
γBL
depolymerizability/performance
derivatives were developed to improve polymerizability and
polyester crystallinity but still maintain the full chemical
[
6]
recyclability (Figure 1). Similar advances are achieved using
[
5, 7]
[8]
bridged-ring
or geminal-dimethyl-substituted thiolactone
monomers. These strategies are highly effective in specific
cases, but they each rely on specifically designed monomers,
thereby limiting their generality. Additionally, to produce
crystalline
recyclable
polymers
requires
either
stereocomplexation of enantiomers of chiral polymers in a
stoichiometric ratio or the burdensome
stereoselective polymerization of the racemic monomer.
and exquisite
[
6a, 9]
As
a
consequence, more general strategies for designing
chemically recyclable polymers that defy aforementioned
tradeoffs between the monomer’s cyclizability and
polymerizability and the polymer’s depolymerizability and
properties are highly required.
[
10]
Dilactones,
namely, cyclic dimer of α-hydroxy acids
Introduction
including glycolide, lactide, et al., are an exceptionally significant
class of monomers in the generation of aliphatic polyesters,
The development of a circular economy for plastics has
become critically significant.
[11]
[
1]
Challenges regarding the
which have the greatest potential to replace petroleum-based
recycling of commercial plastics have unlocked new
opportunities to develop chemically recyclable polymers with
closed-loop life cycles that can be completely depolymerized to
their monomers in an energy efficient manner. Previous
studies by Endo and Albertsson et al., suggested the possibility
that some polycarbonates could exhibit chemical recyclability via
ring-closing depolymerization, but this approach can be
[12]
polymers. Despite extensive studies on ROP of dilactones to
[13]
date,
dilactones synthesis on a large-scale is not easy, still
suffers from the low yields, low purity as well as harsh reaction
conditions (high temperature, high vacuum pressure) because of
[
2]
[14]
relatively high ring strain of dilactone (Figure 1). For instance,
Dove et al., demonstrated the improved synthesis of dilactone
[15]
derived from
L
-malic acid.
However, the yield of final
[3]
complicated by moderate depolymerization selectivity.
cyclization step in dilute conditions under reflux was only 30%.
[
4]
[5]
Recently, the groups of Chen and Lu have demonstrated
ingenious strategies for completely recyclable polymers using
ring-opening polymerization (ROP). However, the synthesis of
chemically recyclable polymers is still plagued with certain
fundamental limitations, including trade-offs between monomer’s
cyclizability (ring closure) and polymerizability, as well as
between polymer’s depolymerizability and performance.
Monomers with non/or low ring strain favor ring closure and can
be easily prepared, but they typically require harsh
polymerization methods. For example, Chen et al., elegantly
disclosed that ROP of unstrained γ-butyrolactone (γBL),
Very recently, our group proposed that S-carboxyanhydride
(
SCA) monomers derived from amino acids can be polymerized
in an ultrafast and selective manner to afford precision
[
16]
polythioesters with versatile functionality.
In particular, we
calculated the thermodynamic parameters of LacSCA (SCA from
lactic acid) and thiolactide, and came across that thiolactide, the
isostere of lactide, exhibited surprisingly near-equilibrium
thermodynamics.
Inspired by the presented challenges, we formulated the
following four working hypotheses. First, despite the simple
1
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RESEARCH ARTICLE
NaHS. Next, the precursors α-mercapto acids were effectively
converted to corresponding dithiolactone monomers bearing
different side chains via cyclization reaction with typical yields
∼
75-91% (Figure 2A). This is in contrast to classical dilactone
preparation where the cyclization yield is only ∼20-40% resulting
[
15, 19]
from relatively high ring strain of dilactone.
The monomers
were characterized with H, C NMR, and mass spectrometry
Figure S1-S12). Taken together, the synthesis of dithiolactone
1
13
(
monomers is simple and highly efficient and can be readily
scaled up to 100 g scale per batch in the lab.
Figure 1. Strategies for the synthesis of recyclable polymers.
O-to-S substitution, dithiolactone could demonstrate appealing
chemical performances different from those of dilactone,
including accelerated ring closure (by decreasing the ring strain),
and augmented kinetics polymerizability (by weak thioester
[
17]
bond ). These subtle and profound changes, could not only
render dithiolactone monomers easy-to-make, but also allow the
polymerization to proceed at mild conditions and yield high
molecular weight (MW) polymers (Figure 1). Worth noting here is
that during the preparation of this article, thiolactide was
attempted to polymerize to polythioesters but only low molecular
Figure 2. (A) Synthesis route and polymerization of dithiolactone. (B)
[
18]
weight (M
n
less than 5.0 kDa) oligomers were obtained.
Screening of organobase catalysts.
Second, the depolymerizability and selectivity in chemical
recycling of the obtained polythioesters should be high because
the cyclizability of the six-membered dithiolactone is
thermodynamically favored. Third, introduction of sulfur atoms
into polymer chain could lead to atactic yet crystalline materials,
thereby enhancing thermal and mechanical properties. Fourth,
structurally diversity dithiolactone monomers could be easily
prepared from ubiquitous feedstock α-amino acids or α-hydroxy
acids which possess rich functionality, allowing for precise
regulating of performance and post-polymerization modification.
Then, we used rac-thiolactide (M1), the isostere of rac-lactide,
as a model monomer. ROP of M1 was probed by investigating
the thermodynamics of its polymerization with a organobase
catalyst, 4-(N, N-dimethylamino) pyridine (DMAP), which has
been demonstrated to be effective for the polymerization of
[
20]
lactide,
indicating standard-state thermodynamic parameters
o
-1
o
-1 -1
of ΔH
S37). The ceiling temperature (T
97 °C for an initial M1 concentration ([M]
respectively. These results highlighted the high polymerizability
P
= -4.92 kJ mol and ΔS
) was calculated to be -67°, or
) of 1.0, or 5.0 M,
P
= -23.86 J mol K (Figure
c
1
0
Here we disclose that dilactone substitution, the subtle O-to-S
change, can decrease the ring strain while preserving
polymerizability under mild conditions, and thus constitute a
unique class of polythioesters exhibiting controlled molecular
weight (up to 100.5 kDa), atactic yet high crystallinity,
structurally diversity, and chemical recyclability. These
polythioesters obtained via ROP of dithiolactone, defy above-
mentioned tradeoffs by having an unusual set of desired
performances, including easy-to-make monomer, high and
controlled polymerizability, crystallinity, as well as high
depolymerizability and selectivity.
[4]
of M1 than γBL, as indicated by a substantially elevated T
-67 °C (1.0 M) vs. -136 °C (1.0 M) for the polymerization of M1
and γBL, respectively.
c c
: T
=
Our investigations started with examination of the M1
polymerization at 25 °C in dichloromethane (DCM) at monomer
concentration [M] = 5.9 M. To our delight, the employment of
benzyl mercaptan as the initiator and DMAP as the catalyst led
to rapid and controlled polymerizations of thiolactide at different
monomer to initiator ratios ([M]
0
/[I]
0
) ranging from 30:1 to 600:1
/[I] ratio of 50:1,
(
Table 1, entries 3-8). For example, at the [M]
0
0
the ROP achieved 58% conversion in 10 min, affording a
unimodal polythiolactide with a low molecular weight distribution
Results and Discussion
Đ (M
w n 0 0
/M ) = 1.28 (entry 4). Particularly, when [M] /[I] was 600/1,
The α-mercapto acid used to prepare substituted dithiolactone
monomers were commercial products or synthesized from
naturally occurring α-amino acid. For example, 2-
mercaptoisovaleric acid was prepared by diazotization of DL-
M
n
as high as 73.3 kDa was obtained (entry 8). Overall, we have
demonstrated that organobase DMAP-mediated ROP of
dithiolactone provides a facile and reliable approach toward
preparation of high MW polythioester under mild conditions.
2
valine with NaNO in HBr solution, followed by substitution with
2
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RESEARCH ARTICLE
a
Table 1. Ring-opening Polymerization Results of M1
c
d
Entry
Catalyst
[M] /[I] /[Cat]
Time
M
(kDa)
Mn,SEC
(kDa)
Đ (M /M )
Conv.
0
0
0
n,calcd.
w
n
b
c
(
min)
60
60
5
1
2
3
4
5
6
7
8
9
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
50/0/1
50/1/0
-
-
-
0
-
-
-
0
30/1/0.2
50/1/0.2
75/1/0.2
100/1/0.2
250/1/1
600/1/2
250/1/1
250/1/1
250/1/1
3.2
4.2
7.3
10.5
14.1
32.5
73.3
31.8
30.2
29.2
b
1.30
1.28
1.35
1.33
1.39
1.42
1.38
1.53
1.55
58%
58%
57%
57%
52%
50%
52%
54%
54%
10
30
60
90
180
90
30
15
5.2
7.6
10.1
23.0
52.9
23.0
23.9
23.9
Et N
3
1
0
1
TBD
1
t-BuP2
a
0 0
All ROPs were initiated using benzyl mercaptan in dry dichloromethane in a glovebox at 25 °C. Calculated molecular weight based on [M] /[I] ratio and
c
o
d
1
conversion. Measured by SEC (THF, 35 C), PS calibration. Monomer conversion measured by H NMR.
The basicity of the catalysts had a significant effect on the
polymerization rate but mediocre Đ﹥1.5 (entry 10-11). This
result indicates the prevalence of extensive
polymerization. Under equal conditions, the polymerization
DMSO
catalyzed by triethylamine (TEA, pK
a
= 9.0) with basicity
transthioesterification under stronger base that compete with
chain propagation. A similar trend in organocatalytic thiolactone
close to DMAP, afforded very similar polymerization results
entry 9). While, the employment of stronger base like 1,5,7-
[5]
(
ROP was also observed by Lu et al.
CH3CN
triazabicyclo[4.4.0]dec-5-ene (TBD, pK
phosphazene base (t-BuP , pK
2 a
a
=
26.0) and
CH3CN
= 33.5) gave boosted
Figure 3. ROP of M1. (A) M
M] /[I] /[Cat] ratio of 100/1/0.2. Inset: Overlay of SEC profiles at different M1 conversions. (B) MALDI-TOF-MS of iodoacetamide-capped polythiolactide prepared
by [Cat] /[I] ratio of 1/1.
n 0 0 0 0 n
and Đ vs. [M] /[I] ratio. Inset: Overlay of SEC profiles at different [M] /[I] ratios. (B) M and Đ vs. Conversions of M1 at
[
0
0
0
0
0
3
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RESEARCH ARTICLE
a
Table 2. Controlled Polymerization of Dithiolactones Mediated by DMAP/BnSH
c
d
Entry
Monomer
[M] /[I] /[Cat]
Time
M
Mn,SEC
(kDa)
Đ (M /M )
Conv.
0
0
0
n,calcd.
w
n
b
c
(min)
(kDa)
15.2
21.0
21.5
102.2
17.2
28.6
13.1
b
1
2
3
3
4
5
6
M2
M3
100/1/0.5
100/1/0.5
100/1/0.5
500/1/0.5
100/1/0.5
100/1/0.5
50/50/1/0.5
20
19.2
24.0
23.2
100.5
16.3
34.5
17.8
1.32
1.38
1.26
1.36
1.41
1.39
1.42
74%
80%
50
M4
90
92%
M4
1200
40
88%
M5
75%
M6
50
89%
M1+M2
20
58%/77%
a
c
0 0
All polymerizations were performed in dry dichloromethane in a glovebox at 25 °C. Calculated molecular weight based on [M] /[I] ratio and conversion.
o
d
1
Measured by SEC (THF, 35 C), PS calibration. Monomer conversion was measured by H NMR.
Encouraged by the above promising results, we studied the
polymerization of M1 in more detail. With 0.2 equiv DMAP
ring-closing depolymerization (Figure 4B). The almost
instantaneous depolymerization (< 2 min) of the resultant
polythiolactides suggested an easier route than most other
relative to benzyl mercaptan, the
polythiolactide increased linearly with [M]
the unimodal distributions (Figure 3A). Meanwhile, the evolution
of M showed a linear relationship with monomer conversion
while Đ kept relatively low (<1.4) (Figure 3B). The PhCH S−
M
n
values of obtained
[
4, 6a, 8]
0
/[I] ratios, along with
0
recyclable polymers.
Counter-intuitively, depolymerization
proceeded in high selectivity, achieving a mixture of rac/meso-
thiolactides in a ultra-high ratio of ca. 98:2 irrespective of the
initial transthioesterification reactions (Figure S28), which could
be further seperated by recrystallization to afford the pure rac-
thiolactide in 95% isolated yield. The chemical recyclability of
polythiolactide was also studied by bulk depolymerization
method. We applied bulk depolymerization at 95 °C using a
catalytic amount of DMAP via a vacuum distillation setup.
Polythiolactide depolymerized cleanly into the corresponding
thiolactide monomer with >90% isolated yield (Figure S29).
n
2
group at the α-end of the polymer was clearly seen at δ = 4.11
1
ppm in H NMR (Figure S13). Furthermore, the outstanding
terminal thiol reactivity and controllability of the ROP were
demonstrated by the efficient end-capping employing
iodoacetamide,
PhCH S−/−CH
affording
polythiolactide
bearing
(
2
2
CONH ) end groups, as evidenced by MALDI-
2
TOF MS (Figure 3C). These results again demonstrate that
organocatalytic ROP of M1 exhibited a controlled feature.
However, there still existed inevitable transthioesterification side
reactions. MALDI-TOF MS of the polythiolactide revealed that
the ions corresponding to odd-numbered linear polymer with m/z
separated by 88 mass units, which were generated by
competitive transthioesterifications of the polythiolactide chains.
The observed remarkable selectivity in depolymerization is
presumably due to the following two reasons: (1) organobase
DBU can catalyze quick, essentially quantitative epimerization of
meso-thiolactide to rac-thiolactide, and (2) the rac-thiolactide is
more stable than meso-analog thermodynamically. This meso-
to-rac-thiolactide transformation is reminiscent of that proposed
for the epimerization of meso-LA to rac-LA mediated by the B/N
As a control experiment, the ROP of M1 using DMAP or
benzyl mercaptan alone, under comparable conditions, didn’t
occur (Table 1, entries 1-2). These results combined with NMR
[
22]
Lewis pair, elegantly demonstrated by Chen et al., where [B]
Lewis acid serves to improve the α-H acidity of substrate meso-
LA and hence accelerates the depronation. Moreover,
precipitation of rac-LA from solution is necessary to further
enhance the meso-to-rac-LA conversion. By contrast, the
employment of simple organobase like DBU, without the
assisting of Lewis acid, enables quantitative meso-to-rac-
thiolactide conversion, because of the increased α-H acidity of
thiolactide.
and MALDI-TOF MS analysis reveal
a
initiator/chain-end
[
21]
activation mechanism,
where DMAP activates the benzyl
mercaptan initiator or chain end.
A remarkable feature of the obtained polythiolactide is its
chemical recyclability for a cradle-to-cradle life cycle. It is widely
known that the industrial generation of
L L
-lactide ( -LA), is
furnished through metal-catalyzed depolymerization of a low
[
22]
MW poly(
depolymerization selectivity, and provides a mixture of LA
stereoisomers that are desired -LA but also a considerable
L
-lactide). However, this approach is subject to low
To address the above hypothesis, density functional theory
(DFT) calculations were carried out to evaluate the
thermodynamic stability of the (S,S)- and (R,S)-thiolactides
(Figure 5). Specifically, (S,S)-isomer is about 2.3 kcal/mol more
L
amount of meso-LA as a byproduct, thus requiring substantial
separation and purification. Intuitively, we thought that the
selectivity in depolymerizing of the resulting polymer
polythiolactide should also be low since the polymerization
process existed inevitable transthioesterification reactions, thus
affording the mixture of rac-thiolactide and meso-thiolactide
[
23]
stable than the (R,S)-isomer,
confirming that the rac-
thiolactides represent the major constituents. Moreover, the
caculated equilibrium ratio between (R,R)- and (S,S)-isomers to
the (R,S)-isomer is 98-2% at B3LYP, consistent with the
experimental value. Furthermore, DFT calculation indicated that
(
Figure 4A). In this context, high selectivity in chemical recycling
of the polythiolactide is of great interest. Polythiolactide was
diluted to 100 mg/mL in CDCl in the presence of DBU (1 mol %
in the boat-like conformation of (S,S)-isomer, the two CH
3
groups are localized in the equatorial positions with a H-H
distance of 2.22Å. On the contrary, in the boat-like conformation
3
relative to monomer units). A quantitative recovery of the
thiolactide monomer was achieved within 2 min at 25 C via
of (R,S)-isomer, one CH
position whereas the other occupies an axial position in which
3
group is arranged in the equatorial
o
4
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RESEARCH ARTICLE
1
Figure 4. (A) Scheme of polythiolactide depolymerization. (B) Overlay of the H NMR spectra in CDCl
3
of polythiolactide (bottom), recycled thiolactide after
depolymerization (middle), and the started thiolactide as a reference (top).
thermodynamically favored. Interestingly, the energy difference
between rac- and meso-lactide was substantially lower, i.e., 1.0
kcal/mol (Figure 5), indicating the low depolymerization
selectivity of polylactide. Collectively, owing to large energy
discrepancy between rac- and meso-thiolactide, along with the
increased α-H acidity, depolymerization of polythiolactide is
accompanied by the simultaneous quantitative meso-to-rac-
thiolactide conversion, thus achieves high depolymerization
selectivity.
A notable feature of our approach is to provide manifold
opportunities to implement polymerization of a large number of
dithiolactones under very mild conditions. To expand the
monomer scope, we explored the ROP of rac-thioethylglycolide
(
(
M2), rac-thioisobutylglycolide (M3), rac-thioisopropylglycolide
M4), rac-thioallylglycolide (M5) and rac-
thiohydroxymethylglycolide (M6), respectively (Figure 2A). In all
cases, rapid polymerizations were realized, affording a series of
polythioesters (PM2-PM6, Figure S15-S24), with M4 exhibiting
Figure 5. B3LYP/6-31G(d)\B3LYP/6-311++G(2d,2p) optimized conformation
of (S,S)-thiolactide, (R,S)-thiolactide, (S,S)-lactide and (R,S)-lactide.
the highest monomer conversion (92%) of the series; the M
n
the H-H distance is shorter, i.e., 2.07Å, indicating a stronger
flagpole interaction. Such conformation difference shows that
the selective formation of rac-thiolactide over meso-analog is
values of all the polythioesters were close to the expected MWs.
(Table 2). All the polythioesters were depolymerized into the
corresponding dithiolactone monomers at 50 C, catalyzed by
o
5
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RESEARCH ARTICLE
DBU (1 mol % relative to monomer units) (Figure S30-S34).
Random co-polythioester of M1 and M2 (Table 2, entry 6) could
also be depolymerized using DBU, providing the mixture of
symmetrical and asymmetrical dithiolactone monomers (Figure
S35-S36). More importantly, DMAP-mediated ROP of M5
resulted in polythioester with pendant vinyl groups, enabling
further functionalization via various transformations. In brief, the
DMAP-catalytic ROP of dithiolactones is indeed a very powerful
new approach for the preparation of a wide range of functional,
recyclable polythioesters from ubiquitous feedstock.
The thermal properties of the resultant polythioesters were
examined by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The TGA suggested that these
recyclable polythioesters exhibit high thermal stability, with Td,5%
o
∼
182-254 C (Figure S39-S44). Figure 6 shows the T
g
of various
o
polythioesters. T
thioallylglycolide, and it raised to 17.0 C for the polythioesters
from thioisobutylglycolide, and it was further increased to 41.9
g
was -19.2 C for the polythioesters from
g
Figure 6. T values of the recyclable polythioesters.
o
high MW PM4 (M
measured for low MW (M
n
= 100.5 kDa) was 1.7-fold higher than that
= 18.5 kDa) counterpart. Especially,
o
C for the polythioesters from thiohydroxymethylglycolide (Figure
n
S45-S49). The
g
T s of all these recyclable polythioesters
the elongation at break of high MW PM4 was approximately 37
times higher than its low MW analog (Figure 7D).
spanned an extended temperature range of ∼61 °C, suggesting
the important influence of side chain groups on the mobility of
polythioester segments. Of note, although DMAP-catalyzed
ROP of M4 led to atactic polythioester PM4 (Figure S19-S20), it
unexpectedly is crystalline with a T
a heat of fusion (ΔH
second heating scan at 10°C/min. Moreover, powder x-ray
diffraction (XRD) measurement of PM4 exhibited sharp
diffraction peaks at 2θ of 12.6° and 19.3° (Figure 7B), further
demonstrating that the atactic polythioester with pendant
isopropyl groups is a semicrystalline polymer. These results
show that polythioester with pendant isopropyl groups exhibits
the unusual ability to crystallize, even is not stereoregular. The
O-to-S modification not only renders six-membered
dithiolactone monomer easy-to-make and facilitates highly
selective
polymerizability. As
catalyzed by DMAP needed heating at 35 °C for 4 days to reach
depolymerization,
but
also
increases
the
m
1
of 115.6 °C (Figure 7A) and
) of 15.9 J g determined via DSC from a
−
a
control experiment, ROP of lactide
f
[
14a]
9
3% monomer conversion for a [M]
In comparison, DMAP-mediated polymerization of thiolactide at
5 °C was found to reach equilibrium within 1 h for a related
M] /[I] /[Cat] ratio of 100/1/0.2. To address the underlying
0 0 0
/[I] /[Cat] ratio of 10/1/1.
2
[
0
0
0
reason for enhanced polymerizability, we studied the ROP of
thiolactide and lactide by DFT calculations. To simplify the
calculation, we used methyl mercaptan and methanol as
nucleophiles, respectively. The free energies of key
intermediates (INTs) and transition states (TSs) in chain growth
obtained polythioester PM4 can be manufactured into
dumbbell-shaped specimen by injection molding at 150 °C
Figure 7C). In the tensile test, the measured tensile strength for
a
(
Figure 7. (A) DSC thermogram of PM4. (B) Powder XRD profile of PM4. (C) The photograph of PM4 as a dumbbell-shaped specimen obtained by injection
molding at 150 °C (D) Stress-strain curves of low MW PM4 (S , red curve,δb =12.8 MPa, ε =3.8%, M =18.5 kDa) and high MW PM4 (S , blue curve, δb =21.3
A
b
n
B
MPa, ε
b
=140.1%, M
n
=100.5 kDa). Break point indicated by “X”. Inset data between 0 and 5.0% strain.
6
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Angewandte Chemie International Edition
10.1002/anie.202109767
RESEARCH ARTICLE
Figure 8. Quantum chemical simulations of DMAP-catalyzed ring-opening of lactide and thiolactide.
were shown in Figure 8. The change in free energy of the
thiolactide polymerization was calculated to be 0.8 kcal/mol
the unusual ability to crystallize, even is not stereoregular.
Additionally, the large number and diversity of dithiolactone
monomers that are easily prepared from α-amino acids which
inherently possess rich pendent functionality, allow for precise
regulating of properties. The easy-to-make monomer derived
from ubiquitous feedstock, atactic yet high crystallinity, and
unique depolymerization chemistry paves the way to the
development of chemically recyclable synthetic plastics.
(
INT1 to INT3). Such near equilibrium thermodynamic further
support that polythiolactide is a desired polymer structure with
chemical recyclability for a cradle-to-cradle life cycle. The energy
barrier for propagation was18.5 kcal/mol for thiolactide (INT1 to
TS1) and 24.9 kcal/mol for lactide (2INT1 to 2TS1), respectively.
The ∼6.4 kcal/mol discrepancy in energy barrier, implies that
chain growth of thiolactide is kinetically more favourable than
that of lactide, which guaranteed that the thiolactide
polymerization would undergo in a mild fashion as we observed.
The intrinsically dynamic and weak thioester bond is the origin of
the enhanced kinetics polymerizability. Meanwhile, the
calculated energy barrier for ring-opening involving nucleophilic
attack of methanol onto the carbonyl group of thiolactide was 2.5
kcal/mol lower than that of lactide (22.4 vs 24.9 kcal/mol) (Figure
S50). Taken together, DFT calculations disclosed ring closure
Acknowledgement
This work is supported by the National Natural Science
Foundation of China (Grants 91856113, and 51873211), and
Jilin Science and Technology Bureau (Grant No.
20200301023RQ).
(
cyclizability)
of
dithiolactone
over
dilactone
is
Conflict of interest
thermodynamically favored, while ring opening of thiolactide
over lactide is kinetically facile. Thus, the subtle O-to-S
substitution enables dovetailing desirable, but conflicting,
properties into one monomer structure.
The authors declare no competing financial interests.
Keywords:
Dithiolactone;
Ring-opening
polymerization;
Chemically recyclable; Polythioesters; Crystallinity
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RESEARCH ARTICLE
RESEARCH ARTICLE
[
a] [b]
[a]
Yanchao Wang,
Maosheng Li,
, [a] [b]
[
a]
Jinlong Chen,
Xianhong Wang
Youhua Tao,*
a]
[
Page No. – Page No.
O-to-S Substitution
Enables
Dovetailing Conflicting Cyclizability,
Polymerizability, and Recyclability
Here we describe the subtle O-to-S
substitution, dithiolactone monomers
derived from abundant feedstock can
demonstrate
appealing
chemical
properties different from those of
dilactone, including accelerated ring
closure,
polymerizability,
augmented
kinetics
high
depolymerizability and selectivity,
and thus constitute a unique class of
polythioester materials exhibiting
controlled molecular weight, atactic
yet high crystallinity, structurally
diversity, and chemical recyclability.
9
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