Dynamic-covalent macromolecular stars with boronic ester linkages

Article abstract of DOI:10.1021/ja207005z

Macromolecular stars containing reversible boronic ester linkages were prepared by an arm-first approach by reacting well-defined boronic acid-containing block copolymers with multifunctional 1,2/1,3-diols. Homopolymers of 3-acrylamidophenylboronic acid (

Full text of DOI:10.1021/ja207005z

ARTICLE
pubs.acs.org/JACS
Dynamic-Covalent Macromolecular Stars with Boronic Ester Linkages
Abhijeet P. Bapat,† Debashish Roy,† Jacob G. Ray,§ Daniel A. Savin,§ and Brent S. Sumerlin*,†,‡
†Department of Chemistry, and ‡Center for Drug Discovery, Design, & Delivery, Southern Methodist University, 3215 Daniel Avenue,
Dallas, Texas 75275-0314, United States
§School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, Mississippi 39406,
United States
S Supporting Information
b
ABSTRACT: Macromolecular stars containing reversible boro-
nic ester linkages were prepared by an arm-first approach by
reacting well-defined boronic acid-containing block copoly-
mers with multifunctional 1,2/1,3-diols. Homopolymers of 3-
acrylamidophenylboronic acid (APBA) formed macroscopic
dynamic-covalent networks when cross-linked with multifunc-
tional diols. On the other hand, adding the diol cross-linkers to
block copolymers of poly(N,N-dimethylacrylamide (PDMA))-
b-poly(APBA) led to nanosized multiarm stars with boronic
ester cores and PDMA coronas. The assembly of the stars under
a variety of conditions was considered. The dynamic-covalent
nature of the boronic ester cross-links allowed the stars to
reconfigure their covalent structure in the presence of mono-
functional diols that competed for bonding with the boronic acid component. Therefore, the stars could be induced to dissociate via
competitive exchange reactions. The star formationꢀdissociation process was shown to be repeatable over multiple cycles.
’ INTRODUCTION
dissociation. Indeed, supramolecular motifs are promising as a
bottom-up approach for the design of nanomaterials. However,
stronger reversible links that respond to specific stimuli while
being otherwise stable might also be attractive for the design of
adaptive smart materials.
Macromolecular star architectures, with several polymer
chains connected to a central core, are interesting due to their
well-defined and compact structure, a high density of functional
groups, and unique properties in solution and in the melt. Star
polymers have been investigated for their potential utility in
biomedical and therapeutic applications1 and as nanoreactors,2
catalysts,3,4 photovoltaics,5 sensors,6 polymer electrolytes,7
compatiblizers for polymer blends,8 and higher order self-
assemblies.9 Controlled radical polymerization techniques (e.g.,
atom transfer radical polymerization,10 nitroxide mediated poly-
merization,3 and reversible additionꢀfragmentation chain transfer
polymerization11) have been extensively utilized for the synthesis
of well-defined macromolecular stars.12 Regardless of the po-
lymerization technique employed, the two general synthetic
routes to stars are the “arm-first” and “core-first” methods.
While the majority of macromolecular stars reported in the
literature were constructed via robust covalent bonds that link
the polymeric arms to the central core, star polymers with
degradable/cleavable arms13 or cores14 have also been investi-
gated. Adding a characteristic of dynamics (e.g., the potential for
controlled degradability) to star polymers is highly desirable in
many applications, including drug delivery and biotherapeutics.
In this respect, stars linked via supramolecular core interactions
have proven particularly promising. Hydrogen-bonding motifs,15
ionꢀdipole interactions,16 and metalꢀligand complexes17 have
all been exploited to prepare stars that are capable of controlled
The field of dynamic-covalent chemistry18 has been applied to
the design of reorganizable polymers19 that are composed of
reversible covalent linkages. While retaining the benefits of
reversibility, the higher strength of many dynamic-covalent bonds
as compared to supramolecular interactions20 potentially leads to
increased structural stability. Not surprisingly, the assembly of
dynamic-covalent architectures like macrocycles,21 dendrimers,22
and reorganizable polymers23 is a topic of growing interest. On
the basis of this concept, macromolecular stars constructed from
alkoxyamines,24 DielsꢀAlder25 adducts, and imines26 have been
investigated.
Boron-containing polymers serve as precursors for nanostruc-
tured materials with potential applicability in sensing, electronic
devices, catalysis, and biomedical applications.27 Boronic acid
functional polymers have also been used as solid-supported
protecting groups and separation agents in organic synthesis.28
We are exploring the applicability of boronic acid-functionalized
polymers for the synthesis of dynamic-covalent macromolecular
assemblies. The reversible reaction of boronic acids with 1,2- and
Received: July 26, 2011
Published: November 21, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Poly(N,N-dimethylacrylamide)
(PDMA) (3) and Subsequent Chain Extension with 3-
Acrylamidophenyl Boronic Acid (APBA) (4) To Give PDMA-
b-poly(APBA) (5)
Figure 1. Size exclusion chromatography traces of the poly(N,N-
dimethylacrylamide)(PDMA) macro-chain transferagent and thepinacol
ester of PDMA-b-poly(3-acrylamidophenylboronic acid) (PDMA74-
b-PAPBA18) block copolymer.
Table 1. Results for the Synthesis of Poly(N,N-
dimethylacrylamide) (PDMA) and PDMA-b-poly(3-acryla-
midophenylboronic acid) (PDMA-b-PAPBA)
1,3-diols to form esters in aqueous and organic media is well-
known.29 The reversible nature of boronic esters has been
exploited in the design of “self-repairing” poly- or oligo-
dioxaborolane polymers,30 blue-emissive materials,31 covalent
organic frameworks,32 sensors,33 and RNA mimics.34 Dehydra-
tion of boronic acids35 has also been used to prepare dynamic-
covalent macromolecular assemblies via six-membered boroxine
formation.36,37 We previously reported the successful RAFT
polymerization of vinyl monomers containing protected and
free boronic acid functionality and their pH- and glucose-
responsive behavior.38,39 Subsequently, we reported the synth-
esis of a boronic acid functional RAFT chain transfer agent
(CTA), which led to well-defined ω-boronic acid functionalized
polymers and their subsequent dehydration to yield dynamic-
covalent three-arm stars via boroxine formation.37
b
polymer
Mn,theoa (g/mol)
Mn (g/mol)
Mw/Mn
PDMA74
8700
11 900
11 000
15 100
11 100
15 700
7700b
11 200c
11 300b
15 200c
12 500b
15 900c
1.11
1.35
1.10
1.34
1.16
1.28
PDMA74-b-PAPBA18
PDMA110
PDMA110-b-PAPBA20
PDMA122
PDMA122-b-PAPBA18
a Calculated from stoichiometric ratios and monomer conversion.
b Determined by SEC. c Calculated from 1H NMR spectroscopy.
(Scheme 1). The resulting PDMA macroCTA (3) was further
chain extended with APBA (4) to yield the desired PDMA-b-
PAPBA block copolymer (5). A mixture of DMF/water (95:5 v/v)
was used as the polymerization solvent during extension with
APBA, with the small amount of water being necessary to avoid
gelation due to boroxine formation. NMR spectroscopy indi-
cated good agreement between the experimental and theoretical
Mn. Prior to analysis by SEC, the boronic acid units of the block
copolymer were protected by esterification with pinacol to
prevent column adsorption. SEC traces of the protected block
copolymer demonstrated a clean shift toward lower elution
volume as compared to the PDMA macroCTA, indicating chain
extension was efficient (Figure 1). The final block copolymer had
an average composition of PDMA74-b-PAPBA18. Similarly, two
additional PDMA-b-PAPBA block copolymers with different
block compositions were also synthesized (Table 1).
Formation of Dynamic-Covalent Macromolecular Stars.
Boronic acids react with 1,2- and 1,3-diols to form boronic esters
in organic media. We hypothesized that addition of a compound
with more than one diol functionality (i.e., a multifunctional
diol) should lead to cross-linking via boronic ester formation be-
tween the pendant boronic acid units of PAPBA homopolymers
(Scheme 2).
Herein, we demonstrate that well-defined boronic acid-
containing block copolymers are highly versatile building blocks
for the construction of macromolecular stars via an arm-first
approach. The addition of multifunctional 1,2/1,3-diols to block
copolymers that contain one segment with pendent boronic acid
moieties leads to stars that are composed of covalent, yet readily
reversed, boronic ester linkages. The assembly of the stars under
a variety of conditions was considered. The dynamic-covalent
nature of the boronic ester cross-links allowed the stars to
reconfigure their covalent structure in the presence of mono-
functional diols that competed for bonding with the boronic acid
component. Therefore, the stars could be induced to dissociate
via competitive exchange of the multidiol cross-linkers with
monodiols.
The stars formed by this approach had a narrow size distribu-
tion and many similarities to block copolymer micelles. How-
ever, as compared to micelles that are essentially supramolecular
assemblies, these boronic ester stars behaved as covalent struc-
tures of considerable molecular weight, while maintaining their
dynamic and reversible nature.
’ RESULTS AND DISCUSSION
On the other hand, when cross-linking the boronic acid
moieties in a PDMA-b-PAPBA block copolymer by addition
of multifunctional diols, the passive PDMA segments were
Synthesis of PDMA74-b-PAPBA18 Block Copolymer via
RAFT. DMA (1) was polymerized at 60 °C in DMF using
DMP (2) as the chain transfer agent and AIBN as the initiator
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Scheme 2. Formation and Degradation of Dynamic-
Covalent Macromolecular Networks and Stars via Boronic
Ester Formation and Transesterification
Figure 3. Plots of correlation function decay rate Γ versus q2 and
transmission electron microscopy images of PDMA74-b-PAPBA18 (A,B),
PDMA110-b-PAPBA20 (C,D), and PDMA122-b-PAPBA18 (E,F) cross-
linked with tris-diol.
fraction of free boronic acid groups can be quite challenging,39
DLS and TEM were used to investigate star formation. For
example, with a 0.5 w/v % solution of PDMA74-b-PAPBA18
cross-linked with tris-diol, an increase in Dh from 5 to 9 nm for
the unimers to 24 nm was observed after star formation (Figure
S3A). Linearity in the correlation function decay rate (Γ) versus
q2 plot for solutions of stars from three PDMA-b-PAPBA block
copolymers (PDMA74-b-PAPBA18, PDMA110-b-PAPBA20, and
PDMA122-b-PAPBA18) (Figure 3) confirmed Brownian diffusion
of spherical particles with low polydispersities. Spherical aggre-
gates with average sizes of approximately 20ꢀ30 nm were
observed by TEM (Figure 3 and Figures S3ꢀ5B). Solutions of
the stars were colloidally stable, as evidenced by a lack of
macroscopic cross-linking or settling over time. In contrast, a
control experiment of a bis-diol (diglycerol) being added to a
solution of PAPBA homopolymer resulted in gelation and
macroscopic precipitation, as expected (Figure S6 in the Sup-
porting Information).
Boronic acid-containing polymers may also undergo dehydra-
tion to cross-link via boroxine formation at elevated temperatures
in anhydrous media. Therefore, to determine if cross-linking of
the block copolymers could be attributed to boroxine formation
instead of esterification with the multifunctional diols, PDMA74-
b-PAPBA18 in methanol at two different concentrations (10 and
50 mg/mL) was heated to 50 °C in the absence of diol. No
change in turbidity or size (as determined by DLS) was observed
over periods of up to 21 h in both cases, suggesting a lack of
Figure 2. Multifunctional and monofunctional diols used for formation
and dissociation of boronic ester macromolecular stars.
expected to prevent macroscopic precipitation, limiting network
formation to the nanoscale (Scheme 2). The resulting core-cross-
linked structures would therefore be stars formed by an arm-first
approach.
Four different multifunctional 1,2- or 1,3-diols, tris-diol,
TCOH, diglycerol, and D-sorbitol, were chosen for star formation
(Figure 2). Slightly turbid, bluish solutions were observed
upon mixing the diols and block copolymer solutions at 50 °C
(Figure S2). After the solutions were cooled to room tempera-
ture, no change in size was observed over extended times,
suggesting the stars were kinetically frozen under the conditions
employed. Because accurate SEC analysis of polymers with a
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Figure 5. Solution size distributions of PDMA74-b-PAPBA18 in metha-
nol in the presence of tris-diol as a cross-linker at a fixed ratio of boronic
acid groups [ꢀB(OH)2]:diol = 1:0.3 equiv after 16 h. The distribution
of PDMA74-b-PAPBA18 in the absence of cross-linker is shown for
comparison.
Figure 4. Hydrodynamic size distributions as a function of the ratio of
boronic acid [ꢀB(OH)2] to diol with 1,10,100-(1,3,5-triazinane-1,3,
5-triyl)tris(3-(2,3-dihydroxypropylthio)propan-1-one) (tris-diol) as a
cross-linker at a concentration of 0.85ꢀ1 w/v % PDMA74-b-PAPBA18.
boroxine formation even at a considerably higher PDMA74-b-
PAPBA18 concentration (50 mg/mL) than that used for forma-
tion of the stars by addition of multifunctional diol cross-linker
(Figure S7). These results suggest a mechanism of star formation
by PDMA74-b-PAPBA18 cross-linking via esterification with the
multifunctional diols. Therefore, the resulting nanostructures
should consist of PDMA shells passivating a cross-linked PAPBA-
diol core (Scheme 2). Further insight into the assembly and star
formation process was obtained by considering the effects
of (i) diol:boronic acid stoichiometry and (ii) overall block co-
polymer concentration.
A minimum ratio of each diol to boronic acid was required for
complete transition from polymeric unimers to stars (Figures 4,
and S10A and S11ꢀS13A and C in the Supporting Information).
It was observed that the concentration of cross-linking diol
needed to induce star formation was dependent on the specific
diol being considered. At a constant initial concentration of
PDMA74-b-PAPBA18 of 1 w/v %, the approximate concentration
of diol needed to induce efficient star formation, as determined
by DLS, increased in the order tris-diol ≈ 0.3 equiv > TCOH ≈
0.4 equiv > D-sorbitol ≈ 0.7 equiv > diglycerol ≈ 2.5 equiv.
Because the tris-diol was most efficient at inducing star forma-
tion, this cross-linker was investigated in greater detail.
Similar to the concept of critical micelle concentration for
amphiphilic block copolymers in solution, efficient star forma-
tion under the conditions considered also required a minimum
concentration of the boronic acid block copolymer. At a tem-
perature of 50 °C and a constant ratio of boronic acid to tris-diol,
the concentration of PDMA74-b-PAPBA18 was increased until
aggregates were observed by DLS. All DLS measurements were
taken at 16 h, well after no further changes were observed during
model experiments of star formation (Figure S8). At a constant
ratio of boronic acid to tris-diol = 1:0.3 equiv, a slight increase in
size was observed by DLS for PDMA74-b-PAPBA18 at a con-
centration of 0.1 mg/mL (Figures 5 and S9). However, when the
Figure 6. Aromatic region of the 1H NMR spectra of (a) PDMA74-b-
PAPBA18, (b) tris-diol, and (c) stars formed by addition of the tris-diol
to PDMA74-b-PAPBA18. Attenuation of the APBA repeat unit phenyl
signals suggests the star cores are desolvated. Spectra A and B were
recorded in methanol-d4 with [PDMA74-b-PAPBA18] = 50 mg/mL and
[tris-diol] = 22 mg/mL respectively, while (C) was recorded for a 1:1 v/
v mixture of the PDMA74-b-PAPBA18 and tris-diol solutions. Complete
spectra are included in Figure S10 in the Supporting Information.
concentration of block copolymer was raised to 0.5 mg/mL, stars
of 18 nm were observed. Increasing the concentration of block
copolymer to 1 and 2 mg/mL did not result in a further increase
in size. These results suggest that with the given block copolymer,
diol, and solvent combination, steric stabilization provided by the
PDMA corona leads to a maximum average star size of approxi-
mately 18ꢀ25 nm.
1
Additional structural information was obtained by H NMR
spectroscopy of the aggregates. The spectrum of PDMA74-b-
PAPBA18 in methanol-d4 showed all of the peaks expected for the
PDMA and PAPBA blocks. Upon addition of a diol cross-linker,
the signals of PAPBA were significantly attenuated, which was
most readily observed by the near disappearance of the phenyl
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(see Scheme 2) are shown in Figure 7 (similar data with pinacol
as the monodiol are shown in Figure S15). Upon introduction of
the AMPOH monodiol and heating at 50 °C for 2 h, stars of
approximately 17 nm were dissociated to unimolecular chains of
approximately 4 nm (Figure 7). This reduction in size is
consistent with dissociation of the stars upon competitive
cross-link cleavage by the monodiol. Importantly, after dissocia-
tion to unimers, addition of more tris-diol to the same solution
led to regeneration of stars with sizes of ∼17 nm. The dissocia-
tion/regeneration process was observed to be reversible over at
least six cycles for the stars formed with tris-diol as the cross-
linker.
The large excess of tris-diol:ꢀB(OH)2 during the sixth cycle
did not appear to prevent cross-linking and star formation by
saturation of the polymer backbone with diol units. To further
confirm this, methanol solutions of PDMA74-b-PAPBA18 and tris
diol were directly mixed at a ꢀB(OH)2:diol ratio of 1:24
(corresponding to the final star formation step in Figure 7; see
the Supporting Information and Figure S14A for details) and
heated for 19 h at 50 °C. DLS measurements confirmed the
formation of stars with an average hydrodynamic diameter of
19 nm. Addition of the monodiol AMPOH (5.7 equiv) to the
resulting solution and heating at 50 °C for 18 h (analogous to the
final star dissociation step in Figure 7; see the Supporting
Information and Figure S14A for details) led to dissociation of
the stars into unimers of 4 nm. The dissociation of stars to
unimers also led to the reappearance of the phenyl proton signals
from the PAPBA block in the 1H NMR spectrum (Figure S14B).
While stars formed with tris diol as the cross-linker demonstrated
repeated dissociation/regeneration, limited success was ob-
served in the case of stars formed with other cross-linking diols.
For example, AMPOH readily dissociated the stars formed with
TCOH, diglycerol, or D-sorbitol. However, further addition of a
large excess of these diol cross-linkers did not lead to star
regeneration under the experimental conditions considered.
These results suggest that careful selection of the cross-linking
and monodiols is necessary to ensure sustained reversibility over
many cycles. These results have led to more in-depth investiga-
tions regarding the kinetic and thermodynamic factors governing
boronic ester formation and transesterification in organic media
and will be the subject of a future report.
Figure 7. Solution size measured by dynamic light scattering during
investigations of the reversibility of dynamic-covalent star formation and
dissociation with PDMA74-b-PAPBA18 (original concentration = 20
mg/mL) in methanol. Solid lines indicate the addition of 1,10,100-(1,3,
5-triazinane-1,3,5-triyl)tris(3-(2,3-dihydroxypropylthio)propan-1-one)
(tris-diol) cross-linker, and dashed lines indicate the addition of
2-amino-2-methyl-1,3-propanediol (AMPOH) monodiol cross-link
cleaving agent. Specific information for each step of the study is provided
in the Supporting Information.
signals at 6.8ꢀ8.1 ppm (Figure 6 and Figures S10ꢀ13B in the
Supporting Information). The reduction in intensity of the
PAPBA signals suggested the cores of the stars were significantly
desolvated, more closely resembling core-cross-linked micelles
than low-density nanogels. This is consistent with the observa-
tion of macroscopic precipitation instead of swollen gel forma-
tion when PAPBA homopolymer was reacted with a multi-
functional diol in methanol, as was previously discussed
(Figure S6).
The aggregation number of the stars was estimated by static
light scattering during flow injection polymer analysis,40 a chro-
matography technique capable of providing Mw when coupled
with light scattering instrumentation. For the specific case of
PDMA74-b-PAPBA18 cross-linked with tris-diol at a ratio of
ꢀB(OH)2:diol = 1:0.57 equiv, the weight average molecular
weight (Mw) was determined to be 2.145 ꢁ 106 g/mol. Dividing
this number by the molecular weight of the PDMA74-b-PAPBA18
arms yielded an estimated 191 arms/star.
Reversibility of Dynamic-Covalent Macromolecular Star
Formation. The readily accessible reversibility of boronic esters
imparted a dynamic nature to the resulting covalent macromo-
lecular stars. Thus, the stars could be dissociated into individual
polymer chains (unimers) after displacement of the multifunctional-
diol cross-linker during a competitive reaction with a monodiol
(Scheme 2). The ability of a given diol to displace another from
a boronic ester (i.e., transesterification) in organic media is
well established.41 In the case of the PDMA74-b-PAPBA18 stars,
displacement of the tris-diol cross-linker by addition of a
competitive monodiol could lead to cross-link cleavage and a
conversion of the stars to unimers with boronic ester pendant
groups.
Preliminary studies indicated stars could also be readily
formed in other alcohols that were common solvents to the
block copolymer and diol cross-linkers. In both ethanol and
isopropanol, star formation was observed upon addition of tris-
diol (Figure S16). These results indicate the versatility and scope
of this approach to dynamic covalent stars via boronic ester
chemistry.
’ CONCLUSIONS
The macromolecular stars described here rely on the self-
assembly of diols with boronic acid-containing polymers. While
reactions between species with more than one diol function-
ality lead to macroscopic network formation with boronic acid
homopolymers, block copolymers with one boronic acid seg-
ment lead to the formation of well-defined stars by the arm-first
approach.
To investigate dynamic-covalent reversibility, the PDMA74-b-
PAPBA18 stars formed with tris-diol as the cross-linker were
exposed to two monodiols. The DLS results for star dissociation
due to cross-link cleavage upon transesterification with AMPOH
A variety of applications can be envisioned for dynamic-
covalent stars on the basis of boronic esters. Similar to polymeric
micelles that have similar coreꢀshell morphologies, it may be
possible to encapsulate cargo (e.g., drugs, dyes, fragrances, catalysts)
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Journal of the American Chemical Society
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within the star cores. However, as opposed to supramolecular
polymer assemblies that may be susceptible to dissociation upon
dilution, the covalently cross-linked stars should be more robust
and capable of withstanding significant dilution. Nevertheless,
these covalent assemblies can still be induced to dissociate by
exposure to an external species that competes for binding with
functional groups along the polymer. Macromolecular constructs
based on boronic acids are particularly interesting in this respect,
given the abundance of potential polyol species encountered in
nature (e.g., sugars, polysaccharides, nucleic acids). Further,
while the work described here focuses on assemblies in solution,
applications in the bulk can also be envisioned, as the dynamic
nature of boronic ester-based polymers may have particular
utility in the area of rehealable materials.
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Corresponding Author
bsumerlin@smu.edu
’ ACKNOWLEDGMENT
This material is based upon work supported by the National
Science Foundation (CAREER DMR-0846792) and an Alfred P.
Sloan Research Fellowship (B.S.S.). Funding was provided by the
NSF DMR MRI-0421406 (USM) for the purchase of the JEOL
JEM-2100 electron microscope (D.A.S.).
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