Thiol–Anhydride Dynamic Reversible Networks

Article abstract of DOI:10.1002/anie.202001388

The reaction of thiols and anhydrides to form ring opened thioester/acids is shown to be highly reversible and it is accordingly employed in the fabrication of covalent adaptable networks (CANs) that possess tunable dynamic covalent chemistry. Maleic, succinic, and phthalic anhydride derivatives were used as bifunctional reactants in systems with varied stoichiometries, catalyst, and loadings. Dynamic characteristics such as temperature-dependent stress relaxation, direct reprocessing and recycling abilities of a range of thiol–anhydride elastomers, glasses, composites and photopolymers are discussed. Depending on the catalyst strength, 100 % of externally imposed stresses were relaxed in the order of minutes to 2 hours at mild temperatures (80–120 °C). Pristine properties of the original materials were recovered following up to five cycles of a hot-press reprocessing technique (1 h/100 °C).

Full text of DOI:10.1002/anie.202001388

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Accepted Article
Title: Thiol-Anhydride Dynamic Reversible Networks
Authors: Christopher N. Bowman, Maciej Podgórski, Sudheendran
Mavila, Sijia Huang, Nathan Spurgin, and Jasmine Sinha
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001388
Angew. Chem. 10.1002/ange.202001388
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Thiol-Anhydride Dynamic Reversible Networks
[
a,b]
[a]
[a]
[a]
[a]
Maciej Podgórski,
Sudheendran Mavila, Sijia Huang, Nathan Spurgin, Jasmine Sinha, and
[
a]
Christopher N. Bowman*
[
[
a]
b]
Prof. C.N. Bowman, Dr. M. Podgórski, Dr. S. Mavila, Dr. J. Sinha, S.Huang, N. Spurgin
Department of Chemical and Biological Engineering
University of Colorado
UCB 596, Boulder 80309 CO, USA
E-mail: Christopher.bowman@colorado.edu
Dr. M. Podgórski
Department of Polymer Chemistry
Maria Curie-Sklodowska University
pl. Marii Curie-Sklodowskiej 5, 20-031 Lublin, Poland
Supporting information for this article is given via a link at the end of the document.
Abstract: The reaction of thiols and anhydrides to form ring opened
thioester/acids is shown to be highly reversible and it is accordingly
employed in the fabrication of covalent adaptable networks (CANs)
that possess tuneable dynamic covalent chemistry. Maleic, succinic
and phthalic anhydride derivatives were used as bifunctional
reactants in systems with varied stoichiometries, catalyst, and
loadings. Dynamic characteristics such as temperature-dependent
stress relaxation, direct reprocessing and recycling abilities of a
range of thiol-anhydride elastomers, glasses, composites and
photopolymers are discussed. Depending on the catalyst strength,
intractable mixture of products. Accordingly, model studies were
performed whereby phthalic anhydride (1a) was reacted with
°
thiol 2a in acetonitrile at 90 C for 2 hours in the absence of a
base or catalyst. Aliquots of various volumes were extracted,
concentrated, and dissolved in DMSO-d
6
to give three
homogeneous samples at different concentrations (25, 50, and
1
100 mM); H-NMR of these samples revealed different
conversions to the product (25, 38, and 54%, respectively). This
simple experiment demonstrated that the ring opening reaction
was spontaneous, highly reversible, and likely proceeding
through a reversible ring opening/closing mechanism. This
effective reversibility was attributed to the planar structure of the
phthalic anhydride, which after ring opening enforces close
vicinity between the aromatic thioester anhydride and the
1
00% of externally imposed stresses were relaxed in the order of
minutes to hours at mild temperatures (80-120°C). Pristine
2
properties of the original materials were recovered following up to
five cycles of a hot-press reprocessing technique (1h/100°C).
neighbouring carboxylic group.
A
similar ester-anhydride
reversion was recently shown in polyester networks designed
Covalent Adaptable Networks (CANs) are
a
subset of
from multi-functional alcohols and phthalic anhydride
thermosetting polymer networks that exhibit covalent bond
reshuffling capabilities while retaining typical thermoset end-use
[11]
derivatives
.
[
1]
properties and solvent insolubility . Unlike conventional
[
1b, 2]
thermosets, CANs
application of an external stimulus, such as temperature,
can be reshaped or repurposed by
[
3]
[
4]
[5]
light, or changes in pH. , which activates the internal dynamic
covalent chemistry (DCC). Consequently, there exist
considerable number of DCCs available for implementation into
a
[
6]
CANs , although many of these reactions, with some notable
[
7]
[8]
[9]
exclusions (urethanes , thiourethanes , siloxanes , etc),
require multistep monomer synthesis and complex methods for
implementation. In the present communication, a previously
unreported dynamic chemistry based on the reversible ring
opening reaction of anhydrides with thiols is studied. It is shown
here that different commercial anhydrides efficiently act as
tuneable dynamic crosslinks at different temperatures, opening
the possibility for broad application of this chemistry in material
science and polymer chemistry (Scheme 1).
Scheme 1. A. The unexpected concentration dependent reversion of the thiol-
1
anhydride reaction in polar media as evidenced by H-NMR; B. proposed
Previous reports regarding the room temperature dynamic
mechanisms of the reversible addition of thiols to anhydrides via thiol-thioester
exchange (1) or reversible anhydride ring closure (2).
[
10]
exchange of thiols and thioesters within network polymers
were largely based on monomers derived from the ring opening
of succinic anhydride with 3-mercaptopropionic acid to form
functional thioesters. While further exploring the scope of this
ring opening, it was observed that the reaction of phthalic
anhydride (1a) with 3-mercaptopropionic acid (2a) gave highly
variable yields of the product and attempts to further
functionalize the diacid (3a) through esterification resulted in an
To determine the reversibility of the thiol-anhydride adducts
derived from aliphatic anhydrides, methyl succinic anhydride
(
MSA) was reacted in another model reaction with methyl 3-
mercaptopropionate (MMP) (Figure S1). The combination of
equimolar concentrations of MMP and MSA, this time with
1
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inclusion of
5 mol % DMAP catalyst, resulted in two
regioisomers whose thermal reversibility was investigated by
NMR (Figure S2). Though devoid of the structural planarity of
the aromatic anhydrides, the aliphatic thioester anhydrides were
also found to be highly reversible, resulting in close to 40 %
substrate regeneration at 120°C. Both model experiments
clearly demonstrated the general reversibility and fidelity of the
thiol-anhydride adducts in structurally diverse anhydrides.
Following discovery of this highly reversible and spontaneous
reaction, application was sought in material science, specifically
with interest in utilizing inexpensive multifunctional anhydrides
as dynamic crosslinks. Accordingly, maleic anhydride (MAn) was
reacted with
a tetrathiol PETMP (pentaerythritol tetrakis(3-
mercaptopropionate)) at various stoichiometries, in the presence
of N-dimethyl aminopyridine-DMAP (5 mol%) and DCM
(
monomers/solvent - 50/50 vol%). After solvent evaporation and
drying, the concomitant thiol-Michael and thiol-anhydride
reactions resulted in the formation of thioether/thioester-based
networks with side-chain carboxylic groups. (Figure S3).
Because the thiol-Michael reaction of the MAn generated a
substituted (thioeter) analog of the succinic anhydride, this
provided justification for the expected dynamic behaviour of
such
two-component
thiol-anhydride
systems.
The
thermomechanical properties of MAn/PETMP networks of varied
stoichiometries are illustrated by DMA results in Figure 1A and
S4. As expected, increasing the thiol concentration leads to less
crosslinked networks with progressively lower glass transition
R
temperatures and rubbery storage moduli (E ). As opposed to
[
10]
our previous reports on thioester containing CANs , here, thiol-
anhydride polymerizations form thioesters in situ without any
need for involved monomer synthesis. Further exploration of
these materials is seen in Figures 1B-C, which illustrate
exemplary DMA stress relaxation curves, showing these
networks to behave dynamically in response to increased
temperature. It is evident that rapid stress relaxation occurs at
relatively low temperatures, and even in the absence of free
thiols (Figure 1B). By taking into consideration the thermal
reversibility of the model thioester anhydride compounds, the
reversible addition is proposed as the main dynamic exchange
mechanism. However, in this instance such dynamic behaviour
may also be related to thiol-thioester exchange, as illustrated in
Scheme 1, as an auxiliary exchange mechanism, especially
when abundant thiols are present. Indeed, increasing the thiol
excess increases the stress relaxation rate, although such
behaviour is concomitant with the reductions in crosslinking
densities, and therefore prevents any efficient decoupling of
Figure 1. A. Storage modulus and tangent delta results of MAn/PETMP
dynamic networks of varied thiol-anhydride stoichiometries (the # after the
monomer acronym indicates % monomer excess). B. DMA stress relaxation
profiles for MAn/PETMP CANs at 100°C. C. Temperature dependent stress
relaxation for MAn/PETMP-50 formulations with 5 mol% DMAP, and D. 5
mol% TEMPO.
[
12]
either effect . It would be expected that the off-stoichiometry in
a thermodynamically driven process would favour the product
(
thioester) formation, thus imposing an obvious constraint on the
reversibility. Such an effect cannot be ruled out in the network
where the anhydride is in excess (no free thiols and reduced
crosslinking) which exhibits longer relaxation times than the
more crosslinked, but also more reversible, stoichiometric
system (Figure 1B). Conversely, free thiols allowing for
associative thiol-thioester exchange and reduced crosslinking
could effectively counter the reduced reversibility and result in
networks with a higher propensity for stress reduction.
Further, the rates of stress relaxation also vary for
compositionally analogous networks with catalyst types (Figures
1
a
C and 1D). The Arrhenius activation energies (E ) were
estimated at around 100-115 kJ/mol when DMAP or TEMPO
were used (Table S2). As seen from comparison of Figure 1C
and 1D, in a 50% thiol excess system 5 mol % of a nucleophile
(
DMAP) is a more efficient catalyst, in this implementation, than
[
13]
5
mol
%
of
a
base generator such as TEMPO
.
2
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Figure 2. A. Direct reprocessing of MAn/PETMP-50 by hot press molding at 100°C for 1 h. B. Stress-strain dependence of pristine and recycled MAn/PETMP-50
dynamic elastomers (b). C. Permanent shape reconfiguration of a MAn/PETMP-sillica particle filled composite. The composite was initially deformed around a
cylinder at 100°C for a prescribed amount of time (up to 3 h). After each time interval the constraints were removed and the resulting shape at temperature of the
experiment photographed. The shape retention ratio after 3h at 100 °C was 94 %. The material was then returned to its flat shape by deformation (flattening) and
heating for 3 h.
The mechanisms of catalytic activation are clearly different for
either type of catalyst. Here, the nucleophile (pKa = 9.4)
performs better than the base – a product of TEMPO reduction
characterized by an impressive 7.6 (±0.7) GPa Young’s Modulus
and 130 (±9) MPa peak stress from a three-point bending test
(Figure S7). As demonstrated in Figure 2C, this dynamic
composite was thermally reshaped through several intermediate
and permanent functional shapes eventually returning the
material to its original flat rectangular shape. Each of the shapes
in Figure 2C were found to be permanent and stable at elevated
temperatures in unconstrained configurations.
(
tetrametylpiperidine, pKa = 11.4). Even though less efficient
than DMAP, using TEMPO, a source of a thermally latent
[
13]
base,
facilitates casting of the resin as solvent is no longer
necessary. Here, TEMPO-based samples were cast in thin films
and cured thermally at 80-100 °C for 1 h. Interestingly, even
catalyst free samples exhibit dynamic characteristics as shown
in Figure S5, further supporting the reversible addition
mechanism as outlined above.
This new thiol-anhydride dynamic chemistry was further
implemented in the fabrication of dynamic thiol-ene
photopolymers. Three examples of dynamic thiol-ene networks
based on three anhydride derivatives (1b-d) and HDT or PETMP
were readily synthesized (Figure 3A-C). Each of the
photopolymer resins was formulated using solventless
Overall, having excess thiol (rather than anhydride) in
combination with a catalytic species clearly accelerates the
relaxation (exchange) rates which is suggestive of an
associative pathway (Scheme 1B, bottom). Although a more
detailed mechanistic study would be required to distinguish the
differences between these mechanisms, especially in systems of
defined crosslinking densities, it seems that both pathways are
active and play a role in the overall dynamic characteristics of
thiol-anhydride materials. Because of the enhanced dynamic
performance of systems with free pendant thiols and
2
conditions and cured by exposure to UV irradiation (10 mW/cm ,
365 nm) in the presence of DMAP (5 mol%) and DMPA
photoinitiator (0.2 wt%). Structurally distinctive anhydrides were
found to influence the thermodynamic equilibria and thus gave
rise to variations in the temperature-dependent thioester-
anhydride adduct formation and reversibility. A stoichiometric
g R
and more crosslinked ASA/PETMP system (T = 42 °C; E = ~8
nucleophilic catalysts,
a
50% excess thiol composition
MPa) was characterized by significantly accelerated rates of
stress relaxation when compared with the loosely crosslinked
(
MAn/PETMP-50, 5 mol% DMAP) was selected to further
demonstrate self-healing and reprocessability properties of
these materials (Figure 2 and S6). As presented in Figure 2A,
direct reprocessing by hot press molding was conveniently
achieved at 100°C for 1 hour. The same material was assessed
for its tensile properties after multiple reprocessing cycles
g R
MAn/PETMP-50 composition (T = 39 °C; E = ~2 MPa) (Table
S2). Furthermore, the off-stoichiometric, thiol abundant systems
have apparently lower activation energies than their
stoichiometric analogs but the differences seem minimal (around
10 kJ/mol). Because of its planar structure, the phthalic
anhydride derived thiol-ene network exhibits the highest
reversibility at ambient-to-moderate temperatures. Small
(
Figure 2B). A slight decline in the stress-strain profiles was
observed initially (up to the third cycle), but it was attributed to a
possible off-equilibrium of the system with a number of “cleaved”
bonds still present. When re-equilibrated by time-extended slow
cooling (30 min), the properties of the pristine CAN were
recovered. Further, a dynamic composite was readily prepared
by thermal curing of a MAn/PETMP stoichiometric resin with 50
wt% thiolated silica microparticles (avg. d = 0.4 µm) and 2 mol%
molecule model studies indicate
a 55-60% thiol-anhydride
conversion at ambient conditions when such mixtures are left to
equilibrate for an extended time (Figure S8). The thiol-phthalic
anhydride mixture requires extended reaction times in the
presence of DMAP before UV exposure to achieve equilibrium
conversion; only then subsequent UV exposure in the presence
of DMPA results in thiol-ene crosslinking.
TEMPO.
This resulted in a composite glass that was
3
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Figure 3. Schematics of dynamic thiol-ene photopolymer fabrication based on A. MAn (1b), B. ASA (1c) and C. APA (1d). In D. the DMA stress relaxation curves
are shown for ASA/PETMP stoichiometric system at temperatures ranging from 70 to 110 °C. The inset in Fig. D. shows an Arrhenius fit of the relaxation times at
-
1
e
a
and the respective activation energy (E ).
Once degelled at elevated temperatures, the oligomers must be
re-equilibrated at 30 °C for up to 9 h to repolymerize back to a
crosslinked network. The depolymerization was monitored in the
DMA. For example, at a 0.01 N preload force, the drop-off in the
rubbery modulus occurs at around 80 °C for the phthalic
anhydride thiol-ene network whereas at the same preload force
the MAn/PETMP-50 network breaks down at 160 °C (Figure S9).
Because of low reversibility threshold the phthalic anhydride
based thioester systems are not practical for many high
performance applications, although they could have excellent
utility in soft materials or organogels where increased chain
mobility would facilitate required reaction yields and reversibility.
anhydride CANs show impressive ability to self-heal, reshape
and recycle. No discoloration nor any other property
deterioration is observed after repeatable reprocessing cycles.
Such dissociative CANs are characterized by rapid dynamic
response at elevated temperatures, and little to no dynamic
response at temperatures below or nearing glass transitions.
Keywords: covalent adaptable networks • dynamic composites •
photopolymers • stress relaxation • recycling
Acknowledgments: We would like to acknowledge the support of
National Science Foundation (NSF CHE 13012296). We thank Dr. B.
Worrell for his assistance with data interpretation and manuscript
preparation.
Lastly, the retro-Michael reaction, which was previously reported
[
14]
[15]
in the construction of dynamic thiol-Michael
and thiol-yne
networks, was also considered as possible contributing
a
mechanism in the MAn-based dynamic system reported here.
To compare the potential for the reversibility of thiol-Michael
derived thioethers and anhydride-based thioesters, a control
ester only system was prepared by anhydride ring-opening with
ethylene glycol and subsequent thiol-Michael crosslinking with
PETMP (Figure S10). Here, stress relaxation profiles were
generated at elevated temperatures and stress relaxation solely
attributed to the retro-Michael reaction is noted. However,
significantly higher temperatures (160°C) are required to achieve
comparable bond exchange rates and match those of the
anhydride thioester CANs.
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Entry for the Table of Contents
Thiol-anhydride dynamic reversible networks were fabricated and assessed for their dynamic characteristics. Three anhydride
derivatives (maleic, succinic and phthalic) were used to create dynamic thermosets of diverse propensities for thioester anhydride link
reversion. Thus an impressive degree of control over their dynamic responsiveness was achieved.
6
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