Thermoresponsive dynamic covalent single-chain polymer nanoparticles reversibly transform into a hydrogel

Article abstract of DOI:10.1002/anie.201207953

To gel and back: Polymer nanoparticles can be reversibly transformed into a chemically cross-linked hydrogel. This transformation is triggered by the application of heat, which causes the polymer chains to aggregate, and the dynamic nature of the covalent cross-linking serves to reorganize the polymer chains into a hydrogel network. Copyright

Full text of DOI:10.1002/anie.201207953

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Angewandte
Communications
DOI: 10.1002/anie.201207953
Constitutional Dynamic Materials
Thermoresponsive Dynamic Covalent Single-Chain Polymer
Nanoparticles Reversibly Transform into a Hydrogel**
Daniel E. Whitaker, Clare S. Mahon, and David A. Fulton*
Emerging applications in materials, biomedical, and nano
sciences demand new “intelligent” polymeric materials, which
possess the capacity to adapt or undergo a macroscopic
response to changes in their environments.[1] In this regard,
there exist already a number[2] of conventional stimuli-
responsive polymers that can respond to input stimuli, such
as changes in temperature or pH, by undergoing a reversible
phase transition. Although well-studied and exploited,[3] these
polymers still present a rather limited palette of structure,
property, and responsiveness, and chemists have been actively
researching alternative means to endow polymers with the
virtues of adaptive and responsive features.
a union of physical and dynamic covalent interactions has led
to complex molecular architectures when combined with
small molecules,[8] but has seen little exploitation on macro-
molecular scales. Herein, we demonstrate how the capacity of
DCBs to undergo component exchange can, in combination
with conventional thermo-responsive polymers, be harnessed
to prepare water-soluble dynamic covalent polymeric nano-
particles that can undergo a remarkable reversible trans-
formation into a macroscopic hydrogel network.[9]
We have produced single-chain polymer nanoparticles
(SCPNs) (NP1–NP4, Scheme 1) based upon previously
described[10] water-soluble OEGMA300 copolymers (P1–P4,
An approach of increasing importance towards intelligent
polymeric materials is the utilization of so-called dynamic
covalent bonds (DCBs),[4] which, when judiciously placed
either into or between polymer chains, present key features
that can be exploited to endow polymeric species with
responsive and adaptive properties.[5] A key feature of
DCBs is their capacity to undergo component exchange
processes involving the exchange of one reaction partner for
another (for example, the reaction of an imine with an amine
to produce a different imine and amine), which can present
polymeric species with opportunities to adapt their constitu-
tions by reshuffling components, and thus change their
properties.[6] As these exchange process usually require the
help of a suitable catalyst to aid kinetics, the option exists to
halt these processes and kinetically fix the products by
quenching the catalyst, an option not available with non-
covalent bonds. A further advantage of employing DCBs
within responsive and adaptive polymeric materials is that the
strength of the covalent bond ensures the resultant materials
possess chemical robustness, an important issue when con-
sidering possible applications.
The development of increasingly sophisticated intelligent
polymeric species will require the combination of different
stimuli-responsive elements. We became intrigued by the
possibility of developing such materials through the combi-
nation of conventional polymers that adapt to stimuli with
a physical response,[7] and DCBs able to continuously adjust
their constitution in the presence of a suitable catalyst. Such
Scheme 1. Conjugation of polymer chains P1–P4 with 1 to form intra-
molecularly cross-linked SCPNs NP1–NP4, and their subsequent
reversible transformation into an intermolecularly cross-linked hydro-
gel.
Table 1) that feature reactive aromatic aldehyde functions,
which serve as handles for the formation of acyl hydrazone
cross-links. These copolymers are prepared by the copoly-
merization of the monomers OEGMA300 and p-(2-methacry-
loxyethoxy)benzaldehyde (MAEBA; see the Supporting
Information), and are of similar molecular weights (Mw 37–
61 kDa), but differ in their densities of displayed aldehyde
functions, as measured by the molar weight percentage of the
aldehyde-containing monomer within the copolymer chains.
[*] D. E. Whitaker, C. S. Mahon, Dr. D. A. Fulton
Chemical Nanoscience Laboratory, School of Chemistry, Newcastle
University, Newcastle upon Tyne, NE1 7RU (UK)
E-mail: d.a.fulton@ncl.ac.uk
Homepage: http://www.dafresearchgroup.com
[**] We thank the Engineering and Physical Sciences Research Council
for generous support (EP/G066507/1).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207953.
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Table 1: Characterization of polymers P1–P4 and nanoparticles NP1–NP4 and NP1’–NP4’.
Polymer
MWACM [%][a]
Mn [Da][b,j]
Mw [Da][c,j]
PDI[d,j]
LCST [8C][e,k]
DLCST [8C][f,k]
TGEL [8C][g]
tSOL [days][h]
tSCPN [days][i]
P1
P2
P3
P4
20.5
16.5
9.0
33800
47100
31600
29700
45300
60900
40800
37100
1.34
1.30
1.26
1.21
32.0
37.5
49.0
52.0
12.5
10.0
3.0
48.5
52.5
57.0
66.0
20.0
5.0
2.0
0.5
>60
30
7
4.5
2.5
3
[a] Molar weight % of aldehyde-containing monomer. [b] Number-average molecular weight. [c] Weight-average molecular weight. [d] Polydispersity
index. [e] Lower critical solution temperature. [f] Change in LCSTupon formation of SCPNs NP1–NP4. [g] Gelation temperature. [h] Time taken for re-
dissolution of gel. [i] Time taken for complete re-equilibration to SCPNs NP1’–NP4’. [j] As determined by GPC in DMF containing LiBr (1 glÀ1
;
0.6 mLminÀ1) calibrated against near monodisperse poly(methyl methacrylate) standards. [k] Determined by turbidimetric analysis in AcOH/NH4OAc
buffer (pH 4.5). LCST=temperature at 50% transmittance.
This series of copolymers allows us to explore the effects of
cross-linking density upon the reversible cross-linking pro-
cess. All copolymers possess sufficiently low polydispersities
(PDI < 1.4) for their use in this study. These linear copolymers
display thermoresponsive behavior, and reversibly precipitate
from aqueous solution when heated above their lower critical
solution temperatures (LCSTs; Table 1), a quality which
provides a thermoresponsive feature to the resultant SCPNs.
Acylhydrazone bonds are well-known[11] to undergo exchange
processes at acidic pH (with optimum kinetics at pH 4.5), but
are kinetically fixed at pH 7. This feature endows the
nanoparticles with a binary response to pH that can be
usefully combined with the thermoresponsive nature of the
copolymers.
SCPNs NP1–NP4 were prepared (Scheme 1) by the
addition of low-molecular-weight dihydrazide 1 to a solution
of copolymers P1–P4 (0.1–1 wt%) in AcOH/AcONH4 buffer
Figure 2. Partial 1H NMR spectra (400 MHz, D2O, pH 4.5, 208C) of
a) copolymer P2, b) SCPN NP2 and c) SCPN NP2’.
(0.1m, pH 4.5), thus causing intramolecular cross-linking
through the formation of dynamic covalent acylhydrazone
bonds. The cross-linking process was followed by gel perme-
ation chromatography (GPC), which revealed an increased
retention time relative to linear copolymers (Figure 1; see
also the Supporting Information, Figures S8–S11), an obser-
vation that is consistent with other work[12] in the field of
SCPNs and indicates the collapse of the polymer chain along
with concomitant reduction in its hydrodynamic volume.
Further evidence for the formation of SCPNs was seen by
its corresponding nanoparticles (Figures 2 and S4–S7). For
example, when P2 was reacted with dihydrazide 1, a broad-
ening of signals associated with the aromatic protons (d = 7.1,
7.8 ppm) within the polymer scaffold was observed, along
with the disappearance of the signal corresponding to the
aromatic aldehyde (d = 9.8 ppm) and the appearance of
a signal (d = 8.1 ppm, labeled “x” in Figure 2) corresponding
to hydrazone formation. Estimation of the LCSTs of NP1–
NP4 was performed by cloud point determination at pH 4.5.
In all cases, the LCSTs of the SCPNs were observed (Table 1)
to be higher than that of their linear precursors, presumably
on account of the relative hydrophilicity of cross-linker 1.
Constitutional reorganization of the nanoparticles into
a hydrogel network was triggered simply by raising the
temperature of the nanoparticle solutions above the esti-
mated LCSTs at pH 4.5. As expected, a white precipitate was
seen to form in all cases. Upon raising the solution temper-
ature, no change was seen until the gelation temperature[13]
was reached, whereupon after 5 min the precipitate was seen
to form an opaque solid material. Vial inversion tests
indicated that this material possessed hydrogel-like character-
istics, which suggests the reorganization of the polymer chains
within the nanoparticles from intra- to intermolecularly cross-
linked. When samples of the hydrogel (75 mg) derived from
NP1 were transferred to a selection of organic solvents (1 mL
each of MeOH, CH2Cl2, DMSO, and DMF) dissolution was
not observed after more than 40 days, compared with full
1
comparing the H NMR spectra of the linear copolymer and
Figure 1. Differential refractive index gel permeation chromatography
(GPC) traces in DMF (0.6 mLminÀ1, containing LiBr 1 gLÀ1).
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dissolution of P1 in < 10 min in these same solvents. This
simple chemical test further suggests the existence of
chemical cross-links within the gel material, and implies that
the gel does not exist as a physical aggregation of SCPNs.
We hypothesize that this constitutional reorganization of
SCPNs into a hydrogel arises on account of the combination
of supramolecular and molecular dynamics. When heated
above their LSCTs the nanoparticles become hydrophobic
and aggregate, a process occurring on the supramolecular
level. The highly localized nanoparticle concentration then
facilitates intermolecular reorganization of the acylhydrazone
crosslinks at the molecular level, with intramolecular cross-
links cleaved to create a significant number of isoenergetic
interchain cross-links, thus leading to the formation of
a macroscopic hydrogel.
the molecular dynamics of the SCPNs and thus prevented the
gelation process from occurring. To further highlight that
dynamic covalent cross-links are required to facilitate gel
formation, the acyl hydrazone bonds within NP1 were
reduced by reaction with NaBH3CN to form non-reversible
covalent amine cross-links. When these nanoparticles were
raised above their LCST, the resulting precipitate quickly
redissolved when the temperature was lowered back below
the LCST, a result which indicates that nanoparticles absent in
dynamic covalent cross-links cannot form hydrogels.
In summary, we have reported that SCPNs are able to
reversibly undergo a transition into a chemically cross-linked
hydrogel upon raising the temperature of their aqueous
solutions above their LCSTs at mildly acidic pH, a process
which is facilitated both by the thermoresponsive nature of
the polymer chains and the capacity of dynamic covalent
acylhydrazone bonds to undergo component exchange pro-
cesses. This synergy of conventional stimuli-responsive poly-
mer with DCBs results in a polymeric material possessing
unique adaptive features. Such triggered gel formation
requires the simultaneous application of both low pH and
temperature, and the polymeric species described here are
thus also a rare example of a material which requires the
simultaneous application of two orthogonal stimuli to trigger
response.[15] The ability to obtain a response by using
a combination of stimuli applied simultaneously could lead
to greater specificity regarding where and when events are
triggered, a feature which would be highly advantageous in
fields such as drug delivery, and we speculate that such
materials will become increasing topics of future interest.
The reverse transformation of the hydrogel network back
into SCPNs (NP1’–NP4’)[14] was triggered by simply cooling
the sample to room temperature, with the hydrogel material
slowly redissolving and returning to the sol state after 0.5–
20 days (Table 1). The increased time taken for the reverse
transformation indicates that the de-cross-linking of polymer
chains is a considerably slower process than the forward
gelation process. We hypothesize that this slowdown is on
account of the multivalency of the inter-polymer bonding,
whereby the high cross-linking densities within the hydrogel
inhibit separation and solvation of individual polymer chains,
as all intermolecular cross-links must be broken before this
separation is possible. This hypothesis is supported by the
observation that, as the level of cross-linking decreases from
NP1–NP4, so too does the time taken for the resultant
hydrogels to re-dissolve upon cooling (Table 1). To inves-
tigate the reversibility of this gelation process in more detail,
Received: October 2, 2012
Revised: November 13, 2012
Published online: December 6, 2012
1
GPC traces, H NMR spectra, and dynamic light scattering
(DLS) data were gathered from the aqueous SCPN solutions
formed after redissolution of hydrogel. The GPC chromato-
grams showed highly polydisperse nanoparticles (Figure S12)
to be present after initial re-dissolution, which slowly re-
equilibrated to afford GPC traces (Figures 2 and S8–S11) very
similar to that of the starting SCPNs NP1–NP4. DLS analysis
of NP1’–NP4’ showed (Figures S14–S16) a single monomodal
particle distribution identical to that of the starting SCPNs
NP1–NP4. Similarly, the 1H NMR spectra of NP1’–NP4’ were
found to be similar to those of the initially-formed SCPN
NP1–NP4 (Figures S4–S7). Taken together, all of these
experiments indicate that upon cooling the hydrogel reor-
ganizes back into a solution of SCPNs.
By kinetically fixing the dynamic acylhydrazone
exchange, it was possible to trap the polymer chains in
either their SCPN or hydrogel forms. For example, heating
a solution of NP3 to 658C at pH 7, at which point the
acylhydrazone bonds are kinetically fixed, did not result in
hydrogel formation and the SCPNs merely formed a white
precipitate that readily redissolved upon cooling to room
temperature. Furthermore, when a solution of NP3 at pH 4.5
was heated at 538C to form the hydrogel and then the pH of
the supernatant adjusted to 7, the gel did not redissolve for
several months when cooled back to room temperature.
Readjustment of the pH to 4.5 resulted in the gel dissolving
after one day. Maintaining the pH at 7 has, in effect, frozen
Keywords: constitutional dynamic chemistry · hydrogels ·
nanoparticles · polymers · sol-to-gel transition
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