Orthogonal construction of dual dynamic covalent linkages toward an “AND” logic-gate acid-/salt-responsive block copolymer

Article abstract of DOI:10.1016/j.polymer.2018.11.014

A novel block copolymer is fabricated by orthogonally coupling two end-functionalized polymers, glucose-capped methoxypoly(ethylene glycol) (mPEG_2k-diol) and phenylboronic acid-capped poly(ε-caprolactone) (PBA-PCL), with dual boronate ester and B–N/B–O dynamic covalent interactions. Due to their synergetic effect, this block copolymer can self-assemble into micelles in aqueous solution with high stability. More importantly, micellar disassembly can be only triggered in the presence of both signal inputs, acid and salt, displaying an “AND” logic-gate responsive behavior.

Full text of DOI:10.1016/j.polymer.2018.11.014

Accepted Manuscript
Orthogonal construction of dual dynamic covalent linkages toward an “AND” logic-
gate acid-/salt-responsive block copolymer
Wu-Cheng Nie, Fei Song, Qian Xiao, Jiao-Jiao Liu, Xiao-Hui Wang, Jiang-Ling Zhou,
Si-Chong Chen, Xiu-Li Wang, Yu-Zhong Wang
PII:
S0032-3861(18)31035-8
DOI:
https://doi.org/10.1016/j.polymer.2018.11.014
JPOL 21037
Reference:
To appear in: Polymer
Received Date: 6 August 2018
Revised Date: 30 October 2018
Accepted Date: 6 November 2018
Please cite this article as: Nie W-C, Song F, Xiao Q, Liu J-J, Wang X-H, Zhou J-L, Chen S-C, Wang X-L,
Wang Y-Z, Orthogonal construction of dual dynamic covalent linkages toward an “AND” logic-gate acid-/
salt-responsive block copolymer, Polymer (2018), doi: https://doi.org/10.1016/j.polymer.2018.11.014.
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ACCEPTED MANUSCRIPT
Orthogonal Construction of Dual Dynamic Covalent Linkages
toward an “AND” Logic-Gate Acid-/Salt-Responsive Block
Copolymer
Wu-Cheng Nie, Fei Song*, Qian Xiao, Jiao-Jiao Liu, Xiao-Hui Wang, Jiang-Ling
Zhou, Si-Chong Chen, Xiu-Li Wang*, Yu-Zhong Wang
Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key
Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly
Polymeric Materials (Sichuan), Sichuan University, Chengdu, 610064, China.
*
Corresponding Authors. Tel. & Fax: +86-28-85410755.
E-mail: songfei520@gmail.com and xiuliwang1@163.com
Abstract
A novel block copolymer is fabricated by orthogonally coupling two
end-functionalized polymers, glucose-capped methoxypoly(ethylene glycol)
(
mPEG -diol) and phenylboronic acid-capped poly(ε-caprolactone) (PBA-PCL),
2k
with dual boronate ester and B-N/B-O dynamic covalent interactions. Due to their
synergetic effect, this block copolymer can self-assemble into micelles in aqueous
solution with high stability. More importantly, micellar disassembly can be only
triggered in the presence of both signal inputs, acid and salt, displaying an “AND”
logic-gate responsive behavior.
Keywords: block copolymer; dynamic boronate ester; B-N/B-O interaction.

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1
. Introduction
Supra-amphiphiles, referring to the amphiphiles constructed by noncovalent
interactions or dynamic covalent bonds, are of particular interest because of their
relatively convenient and efficient synthesis, compared with the conventional covalent
counterparts [1-5]. Through precise molecular design, well-defined amphiphilic
supramolecular copolymers as well as their nano-aggregates can be readily fabricated,
which have gained intensive attention in many areas, including chemical analysis,
catalysis, biotechnology, drug transportation and surface modification [6-9]. Taking
advantage of the reversible nature, multifarious supramolecular copolymers with
different architectures responsive to environmental stimuli have been built in
succession. Typically, Zhang’s group developed enzyme-responsive polymeric
systems with electrostatic interactions between adenosine-5′-triphosphate and linear
hydrophilic chains [10, 11]; Zhang and coworkers reported a photo-responsive
amphiphile containing a pyrene-terminated surfactant and a malachite green molecule
which are connected with hydrophobic interactions [12]; Li et al. demonstrated, very
recently,
mechano-responsive
diblock
copolymers
by
employing
anthracene-maleimide mechanophore as the linkage [13,14]; more commonly,
pH-responsive supramolecular copolymers were constructed using benzoic imine [15],
oxime [16], and hydrazone bonds [17,18] as dynamic covalent interactions. However,
the actualization of single responsiveness is relatively simple for the supramolecular
copolymers, while integrating dual-/multi-stimuli responsiveness in a system is of
more difficulty because of the requirement of complicated and consummate molecular

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design. To date, a series of supramolecular copolymers that can simultaneously sense
two signals (including physical, chemical and biological stimuli) have been created,
including: dual thermo/photo [19], thermo/pH [20], thermo/redox [21], photo/CO [22],
2
pH/enzyme [23-25], adamantaneamine/Na S O [26], and reactive oxygen
2
2
4
species/glutathione [27] responsive systems. Nevertheless, the dual responses are
irrelevant with each other, meaning that the stimuli-responsive properties are of single
channel (“OR” logic-gate mode). Because of the complexity of application
environments (animal/human bodies for instance) for drug delivery, making the
responses follow an “AND” logic-mode, i.e. two signal inputs are simultaneously
prerequisite for outputs, is urgently required but remains a great challenge. Till now,
additionally, such molecular AND gates have been almost used in fluorescent probes
[
28]. In contrast, there have been very limited works reporting AND logic
gate-mediated block copolymers [29-31].
Boronate ester bond, an interesting pH-responsive dynamic covalent bond formed
between boronic acid and diols, has been used in carbohydrate sensors [32-34],
nano-reactors [35], and drug delivery systems [36-39]. Due to the weak affinities
between boronic acid and diols, however, using boronate ester bond to fabricate block
copolymers has been limited reported [40-43]. Boronate esters are Lewis acidic
compounds, which can form B-N interaction with N-donor ligands [44]. As a result,
the improved stability of boronate esters as well as the higher affinity between
phenylboronic acid and polyols can be realized by forming the dative B-N bond [45,
4
6]. Herein, we propose, for the first time, to utilize the boronate ester and B-N/B-O

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interaction as the linkages for developing a novel amphiphilic block copolymer
(Scheme 1), which can self-assemble into micelles in neutral aqueous solution. Such
+
micelles are disintegrated rapidly upon simultaneous exposure to two signals (H and
salt that are generally involved in organisms) but remain stable in the case of the
single-signal input, presenting an “AND” logic-gate pH/salt-responsive property.
Scheme 1. Orthogonal connection of mPEG -diol and PBA-PCL to form block
2
k
copolymer and schematic illustrations of its self-assembly and disassembly behaviors.
2
. Experimental
.1 Materials
2
Methoxypoly(ethylene glycol) amine（mPEG-NH , M = 2000）was purchased from
2
n
JenKem Technology Co., Ltd. D-(+)-gluconic acid δ-lactone and tin 2-ethylhexanoate
Sn(Oct) ) were purchased from Sigma-Aldrich Co., Ltd. ε-Caprolactone (ε-CL,
(
2

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purity > 99%) was provided from Acros Co., Ltd. (Beijing, China) and distilled under
reduced pressure prior to use. Pinacol was purchased from Alfa Co., Ltd.
4
-(Hydroxymethyl)phenylboronic acid was purchased from Adamas Reagent Co., Ltd.
Other chemicals were purchased from Kelong Chemical Industries Reagent Co. Ltd
(
(
(
Chengdu, China) and used as received unless specified. Toluene and tetrahydrofuran
THF) were dried by refluxing over sodium and distilled before use. Triethylamine
TEA) was dried by refluxing over CaH and distilled prior to use.
2
2
.2 Characterization
1
11
H, and B NMR spectra were recorded on a Bruker Advance 400 and 600
spectrometer (Bruker, Germany) using tetramethylsilane (TMS) as an internal
reference. FTIR spectra were obtained on a Nicolet6700 Fourier Transform Infrared
spectrophotometer (Thermo Scientific Co., USA). X-ray photoelectron spectroscopy
(XPS) measurement was performed using a VG Scientific Escalab MK II
spectrometer equipped with an Mg Kα excitation source (1253.6 eV). Average
molecular weights of samples were measured by a Waters HPLC system equipped
with a model 1515 pump, a 2414 refractive index (RI) detector, and two columns
(Styragel HT2 and Styragel HT4). THF was used as the eluent at a flowing rate of 1.0
-1
-1
mL min , and sample concentration was 2.5 mg mL . The average molecular weights
(
M and M ) were calculated by using monodisperse polystyrenes as standards.
n
w
Hydrodynamic diameter of micelles was measured at 25 ºC by a Zetasizer Nano ZS90
Malvern Instruments, Ltd., U.K.) with a He-Ne laser (633 nm) and 90°collecting
(

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-1
optics. The sample concentration was fixed as 0.5 mg mL for the DLS measurement.
Fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence
spectrometer (Varian Co., USA). Transmission electron microscopy (TEM) was
carried out on a Tecnai G2 F20 S-TWIN Transmission Electron Microscope (FEI Co.,
USA) operated at an acceleration voltage of 200 kV. Samples were dropped on
carbon-coated copper grids and stained with 2% (w/v) phosphotungstic acid.
2
.3 Synthesis of mPEG -diol
2k
mPEG -diol was synthesized by a modified route according to a previous report
2k
(Scheme S1A) [47]. Methoxypoly(ethylene glycol) amine (0.5 g, 0.25 mmol, Mn =
2
000) and 0.18 g D-(+)-gluconic acid δ-lactone (1 mmol) were dissolved in 30 mL of
MeOH, followed by the addition of 70 µL of triethylamine. The reaction was allowed
to proceed for 72 h at room temperature. After the evaporation of MeOH, the
remaining crude product was diluted with deionized water and dialyzed against
deionized water (1,000 MWCO) for 24 h, and then lyophilized (yield: 0.41 g, 60%).
2
.4 Synthesis of phenylboronic acid-terminated poly(ε-caprolactone) (PBA-PCL)
2
-(4-Hydroxymethylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (pin-HMPBA)
was prepared according to a previous literatures [48]. At first, pin-HMPBA-PCL was
synthesized via the ring-opening polymerization (ROP) of ε-CL using pin-HMPBA as
the initiator and Sn(Oct) as the catalyst (Scheme S1B). In typical, pin-HMPBA
2
(0.468 g, 2 mmol) was added into a Schlenk tube equipped with a magnetic stir bar,

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which was then dried on a vacuum line at 50 ºC for 1 h. The flask was then purged
with dry argon, followed by the addition of dry toluene and a certain amount of
freshly distilled ε-CL. After heating, 0.2 M Sn(Oct) was rapidly injected through a
2
septum. The polymerization was carried out for 24 h at 110 ºC. The reactor was then
cooled to room temperature, and the reaction mixture was poured into excessive
diethyl ether to give the product, which was dissolved in 20 mL chloroform and then
precipitated in excess cold diethyl ether. The product, pin-HMPBA-PCL, was dried
overnight under vacuum at 30 ºC (yield: 85%).
To a THF/water solution (20 mL/5 mL) containing pin-HMPBA-PCL (0.15 mmol),
NaIO (192 mg, 0.9 mmol) was added to remove the pinacol group completely. The
4
mixture was stirred at room temperature for 1 h. Then, 1 M aqueous HCl (198 µL)
was added and stirred at room temperature for additional 24 h. Then the mixture was
diluted by water (10 mL) and extracted with CHCl . The organic layer was collected,
3
dried over anhydrous Na SO and filtrated. The filtrate was concentrated under
2
4
reduced pressure and precipitated in diethyl ether. The product was finally dried in
vacuum at 30 ºC to a constant weight (yield: 90%).
2
.5 Synthesis of D-gluconolactone butyl amide (DGL-BA)
DGL-BA was synthesized by a modified route according to previous reports (Scheme
S1C) [49, 50]. 1.78 g D-(+)-gluconic acid δ-lactone (10 mmol) and n-butylamine
(0.92 g, 12.5mmol) were dissolved in 50 mL of MeOH. The reaction was allowed to

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proceed for 24 h at room temperature. After the evaporation of MeOH, the crude
product was washed with isopropanol (yield: 1.95 g, 72%).
2
.6 Synthesis of DGL-BA-PBA
Typically, an equal molar amount of DGL-BA (2 mmol) and PBA (2 mmol) were
added in 50 mL of toluene at room temperature. The mixture was heated under reflux
using a Dean Stark apparatus for 6 h. After that, the reaction was cooled to room
temperature and the solvent was evaporated to give the product (yield: 0.66 g, 88%).
2
.7 Preparation of mPEG -diol/ PBA-PCL block copolymer
2k
Typically, an equal molar amount of mPEG -diol (0.05 mmol) and PBA-PCL (0.05
2
k
mmol) were dissolved in 20 mL anhydrous THF at room temperature. 4 Å molecular
sieves (1g) were added as a water-absorbing agent, and the reaction was performed
for 24 h at room temperature. The reactant was filtered and the mPEG -diol/
2
k
PBA-PCL block copolymer was obtained by the precipitation in diethyl ether. The
product was filtered and dried in vacuum at 30 ºC (yield: 75%).
2
.8 Preparation of blank and Nile red-loaded micelles
A general dialysis method was used to prepare mPEG -diol/ PBA-PCL micelles.
2
k
Briefly, 5 mg of block copolymer was dissolved in 1.0 mL DMSO. The resultant
solution was added into 3 mL deionized water under vigorous stirring. After 30 min,
the solution was transferred into a dialysis bag (MW=3500 Da) and dialyzed against

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deionized water for 24 h. The deionized water was replaced every 4 h within the first
1
2 h period and every 6 h within the following 12 h. To prepare the Nile red-loaded
micelles, the block copolymer (5 mg) and Nile red (0.2 mg) were dissolved in 1.0 mL
DMSO, and the resultant solution was added into 3 mL deionized water at a rate of 1
-1
mL h under vigorous stirring. After 1 h, the solution was transferred into a dialysis
bag (MW=3500 Da) and dialyzed against deionized water for 48 h.
2
.9 Determination of critical micelle concentration (CMC)
CMC of block copolymer was determined by a fluorescence spectroscopy using
-5
-2
pyrene as a probe. The sample concentration was varied from 2.0×10 to 5.0×10 mg
-1
-7
-1
mL , and the final pyrene concentration was kept at 6×10 mol L . The measurement
was carried out at room temperature under an emission wavelength of 372 nm. The
CMC value was calculated from the inflection point of polymer concentration where
the relative excitation fluorescence intensity ratio (I /I ) was changed.
3
37 333
2
.10 Nile red release experiment
Prior to the determination on Nile red release behavior, the encapsulation of the dye
was monitored using a fluorescence spectrophotometer. The Nile red-loaded micelles
were placed into a dialysis bag (MW=3500 Da), followed by being immersed into 50
mL solution with different pH values. At predetermined time intervals, a 2 mL dye
encapsulated solution inside the dialysis bag was taken out and the amount of released

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Nile red at different pH values was determined by fluorescence measurement under
the excitation wavelength of 550 nm.
3
. Results and Discussion
As a hydrophilic block, mPEG -diol is synthesized by reacting mPEG -NH with
2
k
2k
2
D-(+)-gluconic acid δ-lactone in the presence of triethylamine (Scheme S1A). On the
other hand, the hydrophobic block, PBA-PCL, is synthesized in three steps (Scheme
S1B):
(i)
synthesis
of
2
-(4-hydroxymethylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (pin-HMPBA)
with a dehydration condensation reaction, (ii) ROP of ε-caprolactone (ε-CL) on
pin-HMPBA to yield pin-HMPBA-PCL with Sn(Oct) as the catalyst, and (iii)
2
deprotection of pin-HMPBA-PCL to expose terminal boronic acids groups. The
chemical structures of the intermediates and targeting products are confirmed by
NMR and GPC results (Figures S1-S4 and Table S1). To prove the two
homopolymers can form a block copolymer, mPEG -diol is mixed with an equivalent
2
k
amount of PBA-PCL in tetrahydrofuran (THF). Compared to the homopolymer
counterparts, only a single signal is detected by for gel permeation chromatography
(GPC) analysis for the mixture (Figure 1A). These results indicate the boronate ester
bond is constructed successfully between mPEG -diol and PBA-PCL, and the
2
k
molecular weight of the resulting block copolymer is 7.7 kDa (Table S1), suggesting
the formation of a copolymer with a longer chain. In general, boronate ester bond is
not stable in polar solvents [45], however, the molecular weight of the block

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copolymer can be determined by GPC measurement with THF as the solvent.
Therefore, it is speculated that, other than the boronate ester, more interactions also
exist between the two blocks.
1
Figure 1. (A) GPC profiles and (B) H NMR spectra of mPEG -diol, PBA-PCL and
2
k
mPEG -diol/PBA-PCL block copolymer in CDCl .
2k
3
First, we notice that although the proton peaks of the aromatic protons of
phenylboronic acid are of no difference before and after the deprotection of the borate
diester bond (Figure S4), an obvious up-field shift (about 0.5 ppm) is detected after
1
introducing an equivalent of mPEG -diol (Figure 1B). The H NMR spectra of the
2
k
homopolymer blocks and the block copolymers are provided in Figure S5. From the
integral areas of PBA-PCL and PEG, the coupling efficiency of the two blocks is
calculated to be 91.9%. To further understand the secondary interaction, small
molecular models, phenylboronic acid and D-gluconolactone butyl amide (DGL-BA)
are synthesized (Scheme S1C) and used instead of the homopolymers because of the
low resolution of NMR measurement for their terminal groups. From the NMR
spectra in Figure S6-S9, we can confirm the successful syntheses of the small

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molecular models. As shown in Figure S10, some characteristic absorption peaks are
−
1
−1
−1
determined at 1359 , 1308 and 1026 cm for DGL-BA-PBA, corresponding to
B-O, C-O-B and B-C bonds, respectively [51, 52]. Meanwhile, compared with the
DGL-BA, the absorption band belonging to hydroxyl group of DGL-BA-PBA is
weakened, which indicates the condensation reaction between phenylboronic acid and
DGL-BA. Additionally, the characteristic peaks of the amide group of DGL-BA,
-1
which are observed at 1651 (C=O stretching) and 1529 cm (-NH-CO-stretching),
-1
blue-shift to 1658 and 1547 cm for DGL-BA-PBA, implying the deformed
π-conjugated structure of the amide group as well as the existence of an additional
intramolecular interaction of the amide with some atom. As a further investigation,
XPS spectroscopy is conducted on DGL-BA-PBA. As shown in Figure S11, the
primary B 1s and N 1s peaks appear at 192.1 eV and 398.6 eV in XPS spectra of
phenylboronic acid and DGL-BA. While for DGL-BA-PBA, we can see three B 1s
peaks, which represent three different sorts of boron atoms including free
phenylboronic acid (192.1 eV), boronate ester bond (189.9 eV), and B-N/B-O
interaction (186.9 eV); the N 1s peak shifts up to 400.3 eV. These results suggest the
existence of intramolecular interaction between the amide and the boron atom [53, 54].
On the one hand, the boronate ester can be stabilized by dative B-N bond [45, 46] or
an intramolecular B-O interaction [55, 56]. On the other hand, Kataoka et al.
suggested that an amide group attached to Neu5Ac at the C-5 position may stabilize
the B(OH) complexation with the glycerine moiety via an intramolecular B-N/B-O
2
interaction [57]. Herein, the intramolecular B-N/B-O interaction is expected to

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stabilize the phenylboronic acid complexation with the glucose moiety against
hydrolysis. Considering to obtain more structural information of hydroxyl and amide
1
13
bonds, H and C NMR measurements are performed using DMSO-d as the solvent
6
for DGL-BA-PBA. As shown in Figure 2, the existence of the B-N/B-O interaction is
1
confirmed from the H NMR spectra, where the amide NH peak shows a down-field
shift (deshielded by +0.13 ppm). The increased electron density is speculated for the
1
1
boron atom due to the formation of B-N/B-O bond, as a result, in the B NMR results,
1
1
the resonance of B of PBA at 28.57 ppm displays an up-field shift to 9.35 ppm
(Figure S12). These results confirm that the amide group of DGL-BA is of a great
importance for the increased stability of DGL-BA-PBA on the basis of the B-N/B-O
interaction [57]. Furthermore, we investigate in detail an involvement of the hydroxyl
1
13
groups on C-6, -7, -8, -9, and -10 in the complexation. From H and C NMR spectra
(
Figure 2), the peaks at C -OH and C -OH of DGL-BA-PBA are observed to
8
9
disappear, while the H-8 and H-9 peaks shows down-field shifts 0.27 and 0.29 ppm.
Additioanally, C-8 and C-9 peaks of DGL-BA-PBA shift by 8.41 and 6.48 ppm,
which is in good agreement with the deshielding effect of cyclic boronate esters [58,
5
9]. These results suggest that binding sites between DGL-BA and phenylboronic
acid locate at C-8, 9. As a result, dual bonds, in terms of boronate ester and B-N/B-O
interaction, are proposed to form between the hydrophilic and hydrophobic polymeric
blocks.

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1
13
Figure 2. H (A, B, and C) and C (D, E, and F) NMR spectra of DGL-BA,
DGL-BA-PBA, and phenylboronic acid in DMSO-d , respectively.
6
After confirming the dual interactions, we further investigate the self-assembly
behavior of such block copolymer. As determined by the pyrene fluorescence probe
method, the critical micelle concentration (CMC) of the block copolymer is
-4
-1
approximately calculated as 3.89×10 mg mL (Figure S13). As shown in Figure 3A,
the solution of mPEG -diol/PBA-PCL exhibits an obvious Tyndall effect, indicating
2
k
the existence of colloidal nanoparticles. From the dynamic laser scattering (DLS)
result, a unimodal size distribution with the mean diameter of roughly 72 nm is
observed for mPEG -diol/PBA-PCL micelles. In addition, as presented from the
2
k
transmission electron microscope (TEM) analysis, the micelles are generally of
spherical morphology with the size of 30-60 nm, which is reasonably smaller than the
DLS result because of the dehydration (Figure 3B) [60-62].

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Figure 3. (A) DLS result (the inset photo shows the Tyndall effect), and (B) the
morphology of mPEG -diol/PBA-PCL block copolymer micelle observed by TEM
2
k
(negatively stained with phosphotungstic acid (PTA)).
As well known, boronate ester bond is responsive to pH change in aqueous solution
63]. Therefore, the mPEG -diol/PBA-PCL micelles are expected to disassemble at
[
2
k
relatively low pH values. To in detail examine the pH-responsive performance
induced by the cleavage of the boronate ester, the micelles are incubated at different
pH values and monitored by DLS. Interestingly, the sizes of the micelles are nearly
unchanged when pH values are no lower than 5.0 (Figure 4A). Compared with the
previous work on the breakage of boronate ester [36], the stronger tolerance of the
micelles against acid can be attributed to the local formation of B-N/B-O interaction
[
64-66]. In spite of this, the dual bonds cannot stabilize the micelles when further
decreasing the pH value to 4.0, that is, the micelle size increases sharply from 72 nm
to about 600 nm after 3-h incubation. Correspondingly, swollen, dissociated, and
adhered aggregates are observed, compared to the homogeneous morphology captured

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under the neutral condition. This cloudy aqueous solution is freeze-dried, and the
obtained white solid is then subjected to GPC analysis. As can be seen in Figure S14,
a bimodal distribution is observed for the sample, implying that the bond cleavage is
triggered by acid. To investigate whether the micelles have a dual-responsive property,
a salt (NaCl) is introduced as the second signal in the incubation solution. Concretely,
the micelle size remains almost unchanged in the case of the salt concentration below
-1
0
.5 mg mL (Figure S15). However, a remarkable increase of the micelle size is
-1
observed with the increased salt concentration from 0.5 to 3.0 mg mL . Furthermore,
-1
despite in the presence of NaCl (0.5 mg mL ), the micelle size is unchanged at
neutral condition; while just decreasing the pH value to 6.5 can induce an obviously
increased size of the micelles (Figure 4B). The size change is even much greater than
the counterpart at the NaCl-free and pH 5.0 condition. Accordingly, serious
aggregation occurs for the micelles when the exposure to the dual-signal input,
suggesting the AND logic-gate pH/salt responsive performance of the
mPEG -diol/PBA-PCL micelles.
2k

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Figure 4. (A) Size changes at different pH aqueous solutions and PTA-stained TEM
image in pH 4.0 aqueous solutions for 3h. (B) Size changes at different pH solutions
-1
with 0.5 mg mL sodium chloride and PTA-stained TEM image in pH 5.0 aqueous
solutions for 3 h for the mPEG -diol/PBA-PCL block copolymer micelle. (C) A
2
k
possible association and disassociation mechanism for the mPEG -diol/PBA-PCL
2k
blcok copolymer.

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Based on the above results, a possible association and disassociation mechanism is
proposed for the mPEG -diol/PBA-PCL block copolymer (Figure 4C). On one hand,
2
k
as reported previously [67, 68], the formation of boronate ester bond generally takes
place in alkaline solutions. However, if dative B-N bonds are formed, the resultant
3
tetrahedral sp B atoms allow the further formation of boronate ester bonds at neutral
condition. On the other hand, the presence of dative B-N bonds could lower the pKa
of secondary amines [69], thereby increasing the acid resistance of the dynamic
structure. In our system, therefore, the two synergistic interactions between
mPEG -diol and PBA-PCL strengthen their association, generating the high stability.
2k
Therefore, the copolymer can form micelles without destruction even under pH value
as low as 5.0. Further decreasing the pH value can result in the protonation of the
amide groups as well as the following breakage of the boronate ester bonds.
-
Additionally, the presence of NaCl induces the binding of the Cl anions to the amide
groups [70], which is favorable to destroying the B-N/B-O and boronate ester bonds
at relatively weak acidic conditions.
To further investigate the potential biomedical applications of this block copolymer,
Nile red is used as a model drug to evaluate the controlled release behavior of the
above resulting micelles. The in vitro drug release of Nile red-loaded micelles is
studied. As shown in Figure 5A, in acidic environment, the micelles are in swollen
state, inducing the relatively fast release of Nile red, that is, 17.2% of Nile red at pH 7
while 75.3% at pH 4. Compared with that, the release becomes obviously faster if
NaCl is introduced in the releasing medium. As shown in Figure 5B, 35.1% of loaded

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Nile red is released at pH 7 in the presence of NaCl, and the release amount increases
remarkably to 72.4% and 84.1% when pH values decrease to 6.5 and 5.0, respectively.
Furthermore, when we control the introduction of acid- and NaCl-signals at different
stages, the release of Nile red can follow an “on-command” release mode (Figure S16)
under periodic stimuli. This synergistic effect by pH and NaCl induces the AND
logic-gate pH/salt responsive drug release function of the micelles, which is desirable
in drug delivery applications.
Figure 5. Drug release behavior of Nile red from mPEG -diol/PBA-PCL micelles at
2k
-1
different pH values in the (A) absence and (B) presence of 0.5 mg mL NaCl.
4
. Conclusions
In conclusion, to construct an “AND” logic stimuli-responsive block copolymer, we
employ boronate ester and B-N/B-O interaction as the orthogonal linkage to lock a
hydrophilic block and a hydrophobic block together. The block copolymer can
self-assemble into micelles in aqueous solution. In the case of the pH value above 5.0,
+
the micelles are stable against the single signal input, H or salt, while disassembly

ACCEPTED MANUSCRIPT
occurs quickly when two signals are simultaneously exerted. Because salt and pH
variations are generally involved in physiological environments, we believe that using
synergistic interactions will contribute to the development of more efficient drug
delivery systems and biomedical devices.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the financial support by the National Key Research and
Development Program of China (2016YFC1100702), National Science Foundation of
China (51773134), and the Sichuan Province Youth Science and Technology
Innovation Team (No. 2017TD0006).
Appendix A. Supplementary information
Supplementary data associated with this article can be found, in the online version
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Highlights
¢
Boronate ester and B-N/B-O bonds are constructed as dynamic linkages to
synthesize SAP
¢
¢
The SAP can self-assemble into micelles in aqueous solution with high stability
Micellar disassembly can be only triggered by AND logic-gate acid-/salt-stimuli