Chemically Triggered Synthesis, Remodeling, and Degradation of Soft Materials

Article abstract of DOI:10.1021/jacs.9b12122

Polymer topology dictates dynamic and mechanical properties of materials. For most polymers, topology is a static characteristic. In this article, we present a strategy to chemically trigger dynamic topology changes in polymers in response to a specific chemical stimulus. Starting with a dimerized PEG and hydrophobic linear materials, a lightly cross-linked polymer, and a cross-linked hydrogel, transformations into an amphiphilic linear polymer, lightly cross-linked and linear random copolymers, a cross-linked polymer, and three different hydrogel matrices were achieved via two controllable cross-linking reactions: reversible conjugate additions and thiol-disulfide exchange. Significantly, all the polymers, before or after topological changes, can be triggered to degrade into thiol- or amine-terminated small molecules. The controllable transformations of polymeric morphologies and their degradation herald a new generation of smart materials.

Full text of DOI:10.1021/jacs.9b12122

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Article
Chemically-Triggered Synthesis, Remodeling,
and Degradation of Soft Materials
Xiaolong Sun, Malgorzata Chwatko, Doo-Hee Lee, James
Bachman, James F. Reuther, Nathaniel A. Lynd, and Eric V. Anslyn
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12122 • Publication Date (Web): 03 Feb 2020
Downloaded from pubs.acs.org on February 3, 2020
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Chemically-Triggered Synthesis, Remodeling, and Degradation of
Soft Materials
Xiaolong Sun,† Malgorzata Chwatko,‡ Doo-Hee Lee,‡ James L. Bachman,‡ James F. Reuther,§ Na-
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‡
‡
thaniel A. Lynd,*, and Eric V. Anslyn*,
†The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and
Technology, Xi’an Jiaotong University, Xi’an, 710049, P.R. China.
‡Department of Chemistry/McKetta Department of Chemical Engineering, University of Texas at Austin, Austin,
Texas, 78712, United States of America.
§Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, 01854, United States of Amer-
ica.
KEYWORDS: Soft materials; click reaction; polymer synthesis; architecture remodeling; chemical degradation;
ABSTRACT: Polymer topology dictates dynamic and mechanical properties of materials. For most polymers, topology is a
static characteristic. In this article, we present a strategy to chemically trigger dynamic topology changes in polymers in
response to specific chemical stimulus. Starting with a dimerized PEG and hydrophobic linear materials, a lightly cross-
linked polymer, and a cross-linked hydrogel, transformations into an amphiphilic linear polymer, lightly cross-linked and
linear random copolymers, a cross-linked polymer, and three different hydrogel matrixes, were achieved via two controlla-
ble cross-linking reactions: reversible conjugate additions and thiol-disulfide exchange. Significantly, all the polymers, be-
fore or after morphological changes, can be triggered to degrade into thiol- or amine-terminated small molecules. The
controllable transformations of polymeric morphologies and their degradation heralds a new generation of smart materials.
INTRODUCTION
versibility and orthogonality of dynamic covalent chem-
istry is useful because these reactions enable independ-
ent control over differing functional groups in a single
chemical system.9-12 As alluded to above, if the orthogo-
nal interactions can be manipulated under physiologi-
cal conditions (i.e. in neutral aqueous media), they
could lead to new therapeutic materials platforms. Fur-
ther, if the molecular architecture can be morphed un-
der mild conditions, and with the ability to disassemble
the soft materials into small molecules on demand, con-
trollable material properties and triggered biodegrada-
tion are within reach. Each of these features is described
herein.
Stimuli-responsive polymers that are capable of adapt-
ing their structure, constitution, and reactivity on re-
ceiving an external stimulus have been emerging in var-
ious fields of study, such as biology, medicine, and man-
ufacturing.1-4 Currently, the control of their properties
has been limited to single chemical functional group in-
terchanges, and very limited structural/morphological
changes. Similarly, degradable polymers are an im-
portant goal within polymer science, and have been
used in a wide range of applications spanning medicine
and drug delivery, to microelectronics and environmen-
tal protection.5 In 2018, Lamb et al. reported that a bil-
lion plastic items are entangled on coral reefs across the
Asia-Pacific.6 Thus, it is increasingly critical that plastics
and other polymers be triggered to degrade at will.7
In the pursuit of such materials, Sumerlin reported that
macromolecular metamorphosis via reversible Diels-Al-
der transformations of polymer architectures can be
achieved in organic media at high temperature over the
period of few days.13-14 This exploration of topological in-
terchange was an important step-forward because a sin-
gle system could be converted to several macromolecu-
lar architectures depending upon reaction conditions.
However, the use of elevated temperatures, organic sol-
vents, and the lack of polymer degradation following
transformation, limits the applications that can be envi-
sioned.
The physical properties of polymers are strongly de-
pendent on several features: their molecular weight dis-
tribution, tacticity, topology, chemical functionality,
and density of cross-linking.8 It is rare that these fea-
tures can be triggered to change by altering the bonding
patterns within the backbone structure, as well as
changing cross-linking group functionality, and thereby
manipulate material properties. To realize transfor-
mations of macromolecular topology, exploiting the re-
“Click chemistry” is a term used to describe reactions
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that are high yielding, wide in scope, and create byprod-
an analogous manner to 121, 27 (Scheme 2, Fig. S1-S3). This
ucts that can be easily removed.15-17 The Cu(I)-catalyzed
Azide/Alkyne Cycloaddition (CuAAC) 'click' reaction
has been used extensively in the field of polymer sci-
ence.18-20 Recently, our group reported an alternative
click strategy, and an associated chemically-triggered
“declick”, which involves the coupling of amines and
thiols via conjugate acceptor 1 that functions in both
aqueous and organic media.21-22 (Scheme 1). The two thi-
oethers in 1 can be released using various reagents,23 and
amine/thiol conjugates such as 2 could be declicked
with dithioltreitol (DTT). Noticeably, decoupling of
conjugate acceptor 1 releases two equivalents of thiol
which can facilitate thiol-disulfide exchange reac-
tions.23-25 Thus, we envisioned the use of structures such
as 1 to couple and decouple amines/thiols in polymers,26
as well as facilitate disulfide dynamic exchange, as two
reversible covalent bonding interactions for independ-
ent control of polymer morphology. With these reac-
tions in mind, we pursued linear polymers and cross-
linked networks creation, along with chemically-trig-
gered metamorphosis and degradation. The dramatic
topological manipulations reported herein, and associ-
ated changes in physical properties, induced by chemi-
cally-triggered transformations, represent a promising
approach to generate tunable ‘smart’ materials.
subunit was the central, modifiable linker that was used
in coupling or cross-linking polymer entities that allows
for amine/thiol scrambling or disulfide exchange to
chemically trigger morphological changes of the poly-
mer/gel and subsequently “declick” degradation.
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Scheme 2. Synthesis of the conjugate acceptor 3
To generate characteristic spectroscopic signals that
would allow us to monitor the transformations of the
polymers and gels, we analyzed the spectral differences
of the chromophores involved in the clicking and
declicking reactions of 1 with small molecules via UV-
Vis spectrophotometry. The vinylogous thio-, oxo-, and
amino-derivatives shown in Scheme 1 all have distinct
absorbance spectra. For example, the reaction of 3 in
phosphate-buffered saline (PBS) with DTT, β-mercap-
toethanol (BME), ethanedithiol (EDT), cysteine (Cys)
and cysteamine (CYA), gave distinguishable UV-Vis ab-
sorbance spectra. The λmax for 3 was at 350 nm, while
reagent-induced declicking led to blue-shifted spectra
and different λmax values depending on the structure of
the cyclized products: DTT (250 nm, a tricyclic product,
4),28 BME (290 nm, 5), EDT (330 nm, 6), Cys or CYA (280
nm, 7) (Fig. S4). The relative speed of decoupling was
DTT > BME > EDT ~ CYA > Cys. Thus, the chemical
group interchanges involved in the polymer/network
topological changes could be monitored by simple UV-
Vis absorbance spectroscopy, and the rate of decoupling
could be controlled, if desired. As described below, the
λmax values allowed us to follow the extent of chemical
functional group interchange.
Scheme 1. A general schematic of the coupling-decou-
pling reactions exploited herein. The addition of an amine
to 1 or 3 leads to formation of 2 through thiol release. Next,
thiols can be scrambled using structures such as 1 or 2 with
varying R-groups. The addition of various reagents release thi-
ols and amines in aqueous conditions and can create species
4, and the generalized structures 5, 6, and 7, each with distinc-
tive UV-Vis λmax values. (4: 250 nm, 5: 290 nm, 6: 330 – 350 nm,
and 7: 280 – 300 nm)
RESULTS AND DISCUSSION
New conjugate acceptor (CA) and clicking-declicking
reactions
The core unit and basis for topological versatility for the
polymers and gels was the conjugate acceptor 3, which
was synthesized from Meldrum’s acid, carbon disulfide
and 6-iodo-1-hexyne in DMSO at room temperature in
Scheme 3. The reactions between 3 and amine derivatives
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Scheme 4. Schematic representation of the polymer synthesis and topological changes achieved via click-declick
chemistry and disulfide-thiol scrambling. Dimerized PEG (12) and hydrophobic linear polymer (13), lightly cross-linked
polymer (14), and hydrogel (15) were synthesized via CuAAC. New amphiphilic linear polymer (16), lightly cross-linked (17),
random copolymer (18), cross-linked polymer (19) and hydrogels (20, 21, 22) were created via amine scrambling using the
conjugate acceptor core of 3, and/or thiol-disulfide reactions. Note: Given the potential hazards of poly(azide) compounds, the
reactions were carried out in the fume hood, treated carefully, stored and used in safe places.29
3, Fig. S5). Further, upon addition of a declicking trigger
(DTT), the LC-MS peak corresponding to 10 and 11 van-
ished and a peak indicative of the tricyclic product 4 was
observed (Fig. S6), confirming that 3 retained similar
chemical properties to 1 in the context of click and
declick reactions.
Next, LC-MS was employed to monitor the reactions
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between 3 and small molecular amines, such as benzyl
amine (8) and 1,3,5-triaminomethylbenzene (9), to con-
firm the expected mono-thiol displacements. After five
minutes, LC-MS peaks corresponding to complete prod-
uct formation (10 and 11, CA = conjugate acceptor de-
rived from 3) were identified for both amines (Scheme
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Polymer design, analysis and transformation. An
overview
plications such as plastic remediation, drug delivery, tis-
sue engineering, or microelectronics.2, 5
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To start, we created four polymer types by using stand-
ard CuAAC reactions: dimerized PEG 12, linear hydro-
phobic 13, hyper-branched 14, and a cross-linked hydro-
gel 15 (Schemes 4 and 5). There were several reasons to
choose these four architectures as our starting point.
Linear polymers such as polyethylene, nylons, and pol-
yesters possess high densities, high tensile strengths,
and high melting points.30 Lightly cross-linked poly-
mers have been widely applied in many applications,
such as light emitting materials, biomaterials, and com-
posites.31 Hydrogels are commonly used scaffolds in tis-
sue engineering, drug delivery, as well as sensing.32-33
Important for our triggered morphology changes, these
four starting polymeric architectures contain repeating
core conjugate-acceptors analogous to 1 and 3. These ac-
ceptors could undergo reactions with amine-terminated
polymers or branched monomers via the click reaction
that releases a thiol. In turn, the released thiols can be
used in oxidation/reduction reactions through disulfide
exchange. Thus, starting with structures 12-15, an am-
phiphilic linear 16, a lightly cross-linked homopolymer
17, a random copolymer 18, a cross-linked polymer 19,
and hydrogels 20, 21, 22 were generated by amine
scrambling and thiol disulfide formation (Scheme 4,
and Table S1). Furthermore, all the polymeric architec-
tures — before or after morphological changes — were
degraded into small molecules or homopolymers in-
duced by declick triggers, hence presaging potential ap-
Dimerized PEG transformation to aggregated struc-
tures
The dimerized PEG 12 was formed by CuAAC of
poly(ethylene glycol) methyl ether azide (23, Mn ca.
1,000 g mol-1) and 3 in a tert-butanol/water mixture
(Scheme 5). Proton NMR spectroscopy (Fig. S7) dis-
played the formation of triazole moieties and LC-MS
spectrum (Fig. S8, S9) showed the product with
mass/charge ratio (m/z) = 2565.40 for [12+H]+, and
1327.24 for [12+2H]2+. Next, gel permeation chromatog-
raphy (GPC) displayed a shift to a lower elution time for
12 compared to 23 (Fig. 1E), indicating the successful
click of 23 with 3.
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To transform 12 to an amphiphilic polymer, amine-ter-
minated polystyrene 24 (Mn ca. 5,000 g mol-1) was uti-
lized to scramble one thiol of 12 to generate 16 (Scheme
4, and Fig. S10). To monitor the transformation, UV-Vis
absorbance was used, which revealed a blue-shifted ra-
tiometric signal corresponding to product formation.
Specifically, the shift in absorbance of the bis-vinylo-
gous thiol ester, as found in structure 1 (as in 6) (λmax
=
330 - 350 nm), relative to an amine/thiol version of the
conjugate acceptor, as found in structure 2 (as in 7) (λmax
= 310 nm), indicated the formation of polymeric product
16 (Fig. 1A and S11). In GPC analysis, 16 displayed a lower
retention time and higher molecular weight (Mn = 5,800
g mol-1) with Ð = 1.3 (Fig. 1E). Subsequently, 16 was in-
Scheme 5. Synthesis of the initial set of subunits. Reagents and conditions: a. CuSO4.5H2O, sodium ascorbate, tert-butanol:H2O
0
0
= 1:1 (vol), 20 C; b. CuI, tris(benzyltriazolylmethyl)-amine (TBTA), sodium ascorbate, THF:DMF:H2O = 2:2:1 (vol), 20 C; c. CuI,
TBTA, sodium ascorbate, THF:DMF:H2O = 1:1:1 (vol), 20 0C; d. CuSO4.5H2O, sodium ascorbate, tert-butanol:H2O = 1:1 (vol), 20 0C.
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vestigated using DLS to evaluate the size and am- amine-induced scrambling and complete thiol release
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phiphilic properties (Fig. 1F, S12 and S13). The correla-
tion in size determined by DLS in aqueous solution (ca.
90 nm) suggests that 16 existed in assemblies.34-36 Thus,
a hydrophilic freely aqueous soluble compound was
triggered to generate aggregation in-situ by an ex-
change of a single block unit.
(Fig. S15). This topological change releases dangling thi-
ols in the polymer, and we took advantage of this to fur-
ther cross-link the material. Thus, oxidation of the dan-
gling thiols to disulfides was performed by addition of
H2O2, generating lightly cross-linked polymer 17 (Mn =
21.0 kg mol-1, Ð = 1.18, Fig. 1H, Table S2 and Fig. S16). In
Figure 1H, the prominent shoulders are likely due to
smaller molecular weight polymers/oligomers with dif-
ferent extents of cross-linking. The approximate molec-
ular weight of these shoulders is 200 g/mol and below.
To further determine the role of H2O2, we checked if
sulfoxides were formed by oxidation of the thioethers of
the Meldrum’s acid derivatives. LC-MS was used to
monitor the reaction between 1 and H2O2. This was
conducted using the same conditions used in creating
polymer 17, and there was no chemical reaction (Fig.
S17). Also, an additional experiment was carried out
with an excess of H2O2, and again no reaction was
found. All the above data shows that polymer 17 was
constructed by two covalent attachments, one through
Hydrophobic polymer transformations to lightly
cross-linked and amphiphilic co-polymers
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The linear hydrophobic polymer 13 was obtained
through step-growth CuAAC polymerization between 3
and 1,4-di(azidomethyl)benzene 25, which afforded a
yellow solid (Mn = 5,300 g mol-1, Ð = 1.35) after precipi-
tation and purification. (Scheme 5, Fig. S14). Simple ad-
dition of 1,3,5-triaminomethylbenzene 9 to 13 resulted
in a distinctive change in the polymer backbone as well
as cross-linking. UV-Vis time-kinetics displayed a blue-
shift due to amine scrambling (i.e. analogous of 1 to 2),
giving a clean isosbestic point at λ = 325 nm (Fig. 1B). 1H
NMR spectroscopy titrations of 13 with 9 demonstrated
(C)
(D)
(B)
(A)
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Retention time (min)
Size (d.nm)
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G'' for 19
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G' for 20
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Ang. Frequency (rad/s)
Angular Frequency (rad/s)
Figure 1. UV-Vis absorbance time kinetics, GPC, and rheometry for the polymer remodeling. (A) 12 (0.25 mg/mL) to 16
using amine terminated polystyrene 24 (1.0 mg/mL); (B) 13 (0.4 mg/mL) to 17 using 1,3,5-triaminomethylbenzene 9 (20 μM); (C)
13 (0.2 mg/mL) to 18 using PEG diamine 26 (0.2 mg/mL); (D) 14 (0.14 mg/mL) to 19 using 9 (41 μM) before adding hydrogen
peroxide. The time kinetics were run in chloroform every 20 mins for (A); every 10 mins for (B) and (D); every 30 mins for (C);
(E) GPC for 12, 16, 23 and 24 in chloroform; (F) DLS for 16 in water; (G) GPC for 13 and 18 in chloroform; (H) GPC for 13 and 17 in
DMF (containing 0.01 M LiBr); (I) GPC for lightly cross-linked polymer 14 in chloroform; (J) Storage modulus G′ and loss mod-
ulus G″ for cross-linked polymer 19; (K) Storage modulus G′ and loss modulus G″ for swelled hydrogel 15, 20, 21 and 22 in
HEPES buffer.
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Scheme 6. Schematic depiction of polymer degradation induced in the presence of DTT or tris(2-carboxyethyl)phos-
phine (TCEP)/ethanolamine in neutral HEPES buffer. The degradation process was triggered either by the cyclization be-
tween a bis-vinylogous derivatives of 3 with DTT, or disulfide cleavage by reduction with TCEP, or ethanolamine-induced decou-
pling of structures such as 1, respectively.
the thiol/amine conjugate such as 2, and the other
through disulfide bonds. Thus, we triggered both back-
bone and cross-linking interchanges from a hydropho-
bic polymer to a lightly cross-linked network.
and increase at 310 nm (Fig. 1C) while GPC displayed a
lower retention time and higher molecular weight (Mn
= 38.9 kg mol–1, Ð = 1.39, Table S2) for 18, indicating the
formation of longer random copolymers (Fig. 1G). Addi-
1
tionally, analysis of the H NMR spectrum (Fig. S18) re-
In the next transformation, poly(ethylene glycol) dia-
mine 26 (Mn = 6,000 g mol-1) was used to functionalize
13, again via thiol displacement. Following the reaction
between 13 and 26 in 1:1 ratio, product 18 was precipi-
tated from cold methanol. UV-Vis spectroscopy again
demonstrated a decrease in the absorbance of 350 nm
vealed the presence of peaks corresponding to PEG
backbone protons. These data demonstrate successful
chemically-triggered remodeling of the linear polymers
through amine/thiol scrambling.
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Figure 2. GPC, 1H NMR spectroscopy, and LC-MS for the polymer degradation. (A) GPC of 12 and 16 before and after degra-
dation; (B) Mass spectra for tricyclic product 4 observed in LC-MS; (C) Masses for 33 and 35 observed in LC-MS, resulted from
decomposition of 13, 17 and 18; (D) 1H NMR for the thiol-terminated PEG product 32 released from 12; (E) 1H NMR for the four-
arm PEG thiol 29 degraded from hydrogel 15; (F) Mass spectra for 34 released from 14 and 19 in LC-MS.
Lightly cross-linked polymer transformations to
cross-linked polymer
in a water/tert-butanol mixture (Scheme 5). The volume
of the hydrogel was expanded after swelling in water
due to the hydrophilic network, (see supplementary
material) resulting in a frequency-independent storage
modulus of 2.2 kPa (Fig. 1K).
Lightly cross-linked polymer 14 was synthesized
through the copper-catalyzed click reaction between 3
and 1,3,5-tris(azidomethyl)benzene 27 (Scheme 5, Fig.
S19). GPC with multiangle light scattering detection
gave the molecular weight for 14 (Mn = 14.9 kg mol-1, Ð
Next, DTT was used to degrade the hydrogel by cleaving
the linkage at conjugate acceptor core 3, leading to the
quantitative generation of four-arm PEG thiol 29 (Fig.
2E) in pH 7.3 HEPES (4-(2-hydroxyethyl)-1-piperazine-
ethanesulfonic acid) buffer. After removal of 4 and re-
sidual DTT by dialysis, the resulting system containing
numerous free thiols was cross-linked through disulfide
bonds by addition of H2O2, generating a new network-
ing matrix: hydrogel 20 (Table S1, Fig. S22). Gel 20
demonstrated a softened elastic modulus of only 260 Pa
corresponding to lower overall cross-link density (Fig.
1K). Continuing with the theme, we employed four-arm
poly(ethylene glycol) amine 30 (Mn ca. 2.0 kg mol-1) to
construct hydrogel 21, using multiple amines separated
by long PEG chains that expanded the matrix of the hy-
drogel through scrambling thiols on the conjugate ac-
ceptor. Visibly, hydrogel 15 became a viscous liquid after
mixing with 30 for six hours (1:1 ratio to the conjugate
acceptor), liberating the thiols along with a noticeable
odor. Subsequent addition of H2O2 linked the free thiols
as disulfides in a second crosslinking, leading to hydro-
gel 21 (Fig. S23). Rheometric testing gave an even softer
storage modulus (75 Pa) for the swelled hydrogel, which
supported our proposed role of 30 to expand the hydro-
gel matrix. Next, four-arm PEG thiol 31 (Mn ca. 5.0 kg
mol-1, 1:1 ratio to the conjugate acceptor) was utilized to
morph the matrix of hydrogel 15, of which the thiols
1
= 1.16, Fig. 1I, Table S2). Using H NMR analysis ala the
method of Moore,37 an approximate degree of branching
of 65% was obtained. Similar to the reaction of 12 or 13,
UV-Vis kinetics of the reaction of 14 with the triamine 9
1
demonstrated a blue shift, and new H NMR spectros-
copy resonances appeared in a time-dependent manner
(Fig. 1D and S20). The analyses confirmed the remolding
of the architecture and the release of thiol moieties.38
The product was directly oxidized with H2O2 resulting
in a highly cross-linked polymer 19 in a DMF/THF mix-
ture which exists as a gel (Fig. S21). Rheometric tests
found the storage modulus (Gʹ) to be an average of 5.5
kPa and the loss modulus (Gʹʹ) to be 1.5 kPa on average,
slightly increasing with the change of angular frequency
in the range of 0.1 – 100 rad/s (Fig. 1J). In summary,
lightly cross-linked polymeric backbones were mor-
phed, resulting in gels in situ by simple addition of a
cross-linking unit.
Hydrogel transformations to mono- and double net-
working hydrogels
In addition to polymers, the strategies discussed here
were applied to matrix hydrogels. Initially, hydrogel 15
was synthesized via click reaction between 3 and four-
arm poly(ethylene glycol) azide 28 (Mn ~ 10.0 kg mol-1)
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would exchange between a bis-vinylogous thiol esters in more crosslinking chemical bonds, we could control
1
2
3
4
5
6
7
8
15 and thiol-terminated derivative 31. Subsequently, re-
leased free thiols enabled the second cross-linking
through formation of disulfides, generating hydrogel 22
(Fig. S24). With this different cross-linking, the storage
modulus became to 230 Pa. Thus, numerous hydrogels
of differing physical properties resulting from backbone
alternations were readily created via simple mixing of
reagents.
and tune the degradation through different declicking
reactions.
First, 12 was treated with DTT in HEPES buffer (pH 7.3)
for three hours and the solution was then placed in di-
alysis tubing (1.0ꢀkDa molecular weight cut-off) and sus-
pended in deionized water for 40 hours (changing the
solvent every ten hours) to remove small molecules (Fig.
S25). The thiol-terminated product 32 was confirmed by
1H NMR spectroscopy (Fig. 2D) and GPC, where a longer
retention time (Fig. 2A) indicated success of the decou-
pling reactions. Next, 16 was declicked using DTT for 36
hours in HEPES buffer (50% acetonitrile as co-solvent,
Fig. S26). After dialysis, GPC data showed that the result
had a lower molecular weight (Fig. 2A). Next, polymers
13, 14, 17, 18 and 19 were all processed in HEPES buffer
in the presence of DTT (overnight for 13 and 14; two days
for 17 and 18; five days for 19). As expected, the polymers
containing amine/thiol conjugate acceptors such as
structure 2, were more difficult to decompose than the
polymers containing thiol ester conjugate acceptors
such as with 1, due to the amine being less labile as a
leaving group. Yet, all species did entirely degrade with
DTT. A macroscopic surface-accessible degradation oc-
curred, which can be seen by the naked-eye and ana-
lyzed by LC-MS. LC-MS data showed the tricyclic prod-
uct 4 (Fig. 2B) with a mass/charge ratio (m/z) = 305.0
for all polymers declicked by DTT, and bis-thiol termi-
nated derivative 33 released from 13, 17 and 18 (m/z =
417.2, Fig. 2C, Fig. S27–S29); three-arm thiol derivative
34 released from 14 and 19 (m/z = 586.2, 608.2 Fig. 2F,
9
Significantly, all of the topological remodeling of the
polymer and hydrogel backbones and cross-linkers were
performed in neutral aqueous condition at ambient
temperature by simply adding new monomers, cross-
linkers, polymers or an oxidant. The resulting spatio-
temporal regulation of material properties is promising
in the applications of drug delivery system, cells encap-
sulation, and cell migration.
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Macromolecular degradation into small molecules
Remodeling and transforming polymeric architectures
via chemically-triggered synthesis has been described
above. Another critical property of a polymer is its abil-
ity to degrade, which facilitates controlled removal of
plastic pollutants once the material has served its pur-
pose. Further, such triggerable material degradation
should hold wide applicability in biomedicine and drug-
delivery. As we have stressed throughout, all of the bis-
vinylogous thiol esters analogous to 1, bis-vinylogous
thiol amine structures containing linkages as in 2, and
disulfide bonds, could be decoupled by various chemi-
cal reagents. Further, with the co-existence of two or
Figure 3. Monitoring hydrogel degradation by rheometer. (A) Time-based kinetics for degradation of hydrogel 15 in the
presence of DTT; (B) TCEP-induced disulfide reduction for hydrogel 20 degradation; (C) Two cross-linked networks: disulfide
bond and amine/thiol conjugate acceptor linked in hydrogel 21 were degraded by TCEP and DTT, respectively; (D) Two cross-
linked networks: a bis-vinylogous thiol-ester such as 1 and disulfide bond linking hydrogel 22 were degraded by ethanolamine and
TCEP, respectively. Storage modulus and loss modulus for hydrogels 20, 21 and 22 were scanned before and after degradation.
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Fig. S30–S31), respectively. We also observed the azide- cross-linked polymers and hydrogels; further even al-
1
2
3
4
5
6
7
8
containing side products 35 and 36, 37 (Fig. 2C, Fig. S27–
S31) from declick reaction mentioned above, due to in-
complete CuAAC click reaction.
lowing interconversion of random co-polymers with
other lightly cross-linked polymers. In addition, in a fac-
ile fashion 4 different hydrogels could be created via
manipulating the components allowed to react with
structures containing the reactivity of 1. The choice of
amines and thiols used herein are specific to this study,
but clearly the possibilities are vast given the numbers
of amine and thiol units that can be imagined. Lastly,
and potentially just as important, all the soft materials
are degradable with various chemical triggers, albeit
specifically DTT was used in this study. Owing to the
mild reaction conditions and ease of use in a wide vari-
ety of applications, this method is expected to have nu-
merous material applications.
DTT was also successfully used to declick the hydrogels
into small molecules. The 3D network 15 was processed
in HEPES buffer in the presence of DTT, and after 20
minutes the yellow gel disassembled, with no disassem-
bly of the control sample without DTT (inset picture in
Fig. S32). LC-MS verified the generation of 4 (Fig. S32),
and following dialysis, 1H NMR spectroscopy confirmed
formation of 29 (Fig. 2E). Furthermore, kinetic studies
from the rheometer displayed a decreasing of the stor-
age modulus (G′) due to degradation of hydrogel 15 fol-
lowing addition of DTT, indicating gradual breakage of
the matrix (Fig. 3A, Fig. S33). Next, the viscoelasticity of
a soft hydrogel 20 (storage modulus G′ = 28 Pa) was
transformed rapidly into a solution state (G′ < G′′) due
to the cleavage of disulfide bonds by tris(2-carboxy-
ethyl)phosphine (TCEP) (Fig. 3B). Following dialysis,
the solution state was reversed into a gel under hydro-
gen peroxide due to the reformation of disulfide bonds
(Fig. S34).
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ASSOCIATED CONTENT
This Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Additional experimental procedures and characterization
data are provided in the supplementary materials. Materials
and Methods, Supplementary Text, Figs. S1 to S35, Tables S1 to
S2, Captions for Data S1 to S35, Synthetic preparations.
The crosslinked matrixes 21 and 22 could also be tuned
and controlled by different decoupling reagents. The
matrix of hydrogel 21 (G′ ~ 130 Pa), containing both the
amine/thiol conjugate and disulfide bonds, were tuned
separately by TCEP for the reduction of disulfides (G′
~ 70 Pa), and subsequently by DTT for the cleavage of
the thiol-amine conjugate analogous to 2 (G′ ~ 5 Pa, Fig.
3C and S35). While for hydrogel 22 (G′ ~ 1000 Pa), etha-
nolamine was employed to cut off the bis-vinylogous
thiol ester core analogous to 1 (G′ ~ 500 Pa), and then
TCEP was used to reduce the disulfide bonds (G′ ~ 5 Pa),
decomposing the cross-linked hydrogel entirely into
small molecules (Fig. 3D and S36). The tunable and de-
gradable properties of the hydrogels, particularly the
ability to tune the two dynamically cross-linked samples
under physiological conditions, are potentially applica-
ble in drug delivery systems and tissue bioengineering.
AUTHOR INFORMATION
Corresponding Author
*anslyn@austin.utexas.edu (E.V.A);
*lynd@che.utexas.edu (N.A.L)
ORCID
Xiaolong Sun: 0000-0003-4003-6924
Doo-Hee Lee: 0000-0002-5116-2064
James F. Reuther: 0000-0001-5611-0290
Nathaniel A. Lynd: 0000-0003-3010-5068
Eric V. Anslyn: 0000-0002-5137-8797
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors acknowledge the use of facilities and instrumen-
tation supported by the National Science Foundation through
the Center for Dynamics and Control of Materials: an NSF
MRSEC under Cooperative Agreement No. DMR-1720595, and
NIH grant number 1 S10 OD021508-01. We gratefully
acknowledge the support of the Welch Regents Chair (F-0046)
to EVA and the Welch Foundation (F-1904) to NAL. XLS
thanks the National Natural Science Foundation of China No.
21907080.
CONCLUSION
We have demonstrated in macromolecular constructs
the versatility of being able to click thiols, and amines
and thiols, via structures analogous to 1 to make mor-
phable soft materials. By simply controlling the se-
quence of addition of monomers, cross-linkers or poly-
mers, we could chemically trigger interconversion of
backbone architectures and crosslinks, control polymer
morphologies, and alter macroscopic properties, all at
ambient temperature in aqueous media. In specific, the
reversible conjugate additions inherent in the organic
chemistry of 1 and its analogs introduced herein, cou-
pled with thiol-disulfide scrambling, led to very simple
approaches that interconvert linear hydrophobic, hy-
drophilic, and amphiphilic polymers, as well as lightly
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Insert Table of Contents artwork here
1
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Amphiphilic polymer
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Hydrogel
Hydrogel
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Hydrophilic polymer
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Hydrogel
DTT
DTT
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Hydrogel
Hydrophobic polymer
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Hyperbranched
Random copolymer
Hyperbranched
Cross-linked polymer, 18
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