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MIDA Boronate Stabilized Polymers as a Versatile Platform for
Organoboron and Functionalized Polymers
Congze He and Xiangcheng Pan*
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ABSTRACT: Boron-containing polymers have been demonstrated to be useful polymeric materials. However, the synthesis of
boronic acid- or ester-containing polymers is highly challenging
due to their instability and difficult characterization. This paper
introduces N-methyliminodiacetic acid (MIDA) as a unique
protecting group for the boronic acid to stabilize boron-containing
polymers. The synthesized MIDA boronate stabilized polymers
show remarkable stability to the air, moisture, and chromatography. Therefore, they could be used as a versatile platform for
synthesizing other boronic acid- or ester-containing polymers.
Postpolymerization modification also incorporates different functionalities by using this platform by Suzuki−Miyaura coupling. Both
stability and versatility make MIDA boronate stabilized polymers have great potential applications in material and biomedical fields
associated with borane chemistry.
■
GPC and showed the remarkable potential of self-assembly.20
However, harsh conditions using strong acids were still
required to synthesize or deprotect the hindered boronic
esters.21
The N-methyliminodiacetic acid (MIDA) group was widely
utilized as an alternative protecting group for boronic acid,
overcoming the potential degradation while it remained intact
and selectivity in cross-coupling reactions.22,23 MIDA boronate
was universally compatible with silica gel and stable in many
reaction conditions, and it was able to liberate the boronic acid
in mild conditions such as a weak basic environment.21 Herein,
it was elegantly utilized in Suzuki−Miyaura coupling,24,25
natural product syntheses,26 molecular syntheses by an
automated process,27 and catalyst-transfer polycondensation.28
Inspired by these pioneer and elegant works, we proposed
that the MIDA group could stabilize the boronic acidcontaining polymer, and the polymer with MIDA boronates
could be a general platform for various organoboron polymers.
Herein, styrene, acrylate, and methacrylate with a MIDA
boronate group were designed and synthesized as suitable
monomers in radical polymerizations. Reversible addition−
fragmentation chain transfer (RAFT) polymerization, a widely
used controlled radical polymerization method, was adopted to
INTRODUCTION
Organoboron polymers attracted particular attention since
they provided unique property in catalysis, sensing, luminescent materials, and biomedical application.1−3 Among them,
polymers with pendant boronic acids/esters accounted for the
majority4 since boronic acids/esters could serve as the
responsive sites of sensitive material5−9 or dynamic crosslinking points of self-assembled polymers10 and self-healing
materials.11
The direct polymerization of the boronic acid type of
monomers was a widely used synthetic approach to obtain the
boronic acid-containing polymers (Scheme 1A).12,13 However,
boronic acids were unstable and incompatible with most
synthetic reagents and silica gel, increasing the synthetic
difficulties.14 An alternative strategy was the incorporation of
boronic acids as small molecules into the polymer with reactive
functional groups (Scheme 1B). For example, aminophenylboronic acid was commonly used to react with carboxylic
groups to form the boronic acid-functionalized polymers or
hydrogels.15,16 The polymers with bare boronic acid pendant
groups were impossible to be characterized by gel permeation
chromatography (GPC); moreover, they were extremely
hygroscopic and easily involved in the boroxine formation.17,18
To overcome these obstacles, polymerization of trimethylsilyl-functionalized styrene was conducted by atom transfer
radical polymerization (ATRP), followed by a highly selective
boron-silica exchange with BBr3 (Scheme 1C). The reactive
dibromoboryl (BBr2) pendant group of the resulting polymer
could be transferred to pinacol boronate ester.19,20 Owing to
the pinacol group as the protecting group for the boronic acid
in the polymer, the synthesized copolymer could be defined by
© 2020 American Chemical Society
Received: March 22, 2020
Revised: April 26, 2020
Published: May 6, 2020
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Scheme 1. Approaches to Synthesize the Boronic Acid-Containing Polymers via (A) Direct Polymerization, (B) Incorporation
of Boronic Acids as Small Molecules, (C) Transformations of Functional and Reactive Groups, and (This Work) the MIDA
Boronate Approach
synthesize the MIDA boronate homopolymers and random
copolymers with predetermined molecular weights and narrow
molecular weight distributions. Polymers with MIDA boronate
as a pendant group were confirmed to be moisture- and airstable, and they could be transformed into polymers with other
kinds of organoborons, including boronic acids as well as
trifluoro- and pinacol boronates. Furthermore, postpolymerization modification incorporated different aromatic groups
through the Suzuki−Miyaura coupling reaction of MIDA
boronates.
■
with MIDA boronate (M2 and M3), and methacrylate
monomer with MIDA boronate (M4).
Scheme 2 summarizes the synthetic approach for the
designed monomers and polymers with MIDA boronate.
Two MIDA boronate building blocks and monomer M1 were
prepared in excellent yields (>95%) by the dehydration
reaction between MIDA and the corresponding boronic
acids.29,30 Both building blocks with hydroxyl group reacted
with acryloyl chloride, providing the acrylate-type monomers
M2 and M3, while the phenol that reacted with methacryloyl
chloride gave the methacrylate-type monomer M4. All
synthesized monomers were stable and purified by silica gel
chromatography to obtain 67−87% isolated yields. The
structures of synthesized compounds were confirmed by 1H
NMR spectroscopy, showing two doublets at 4.34 and 4.14
ppm for the methylene (−CH2−) group and a singlet at 2.54
ppm for the methyl group in MIDA boronate. A broad peak at
12.96 ppm was also detected by 11B NMR spectroscopy,
further confirming the MIDA boronate group.
RAFT Homopolymerization of MIDA Boronate Monomers. Table 1 summarizes the RAFT polymerization of the
styrene and (meth)acrylate types of monomers with MIDA
boronate. The polymerization of M1 was conducted using 2-
RESULTS AND DISCUSSION
MIDA Boronate Monomers. Because the boronic acidcontaining monomers were known for their notoriously
difficult synthesis and purification, most reported boroncontaining monomers were mainly synthesized and used
without purification,13 such as 4-vinylphenylboronic acid and
3-acrylamidophenylboronic acid. MIDA boronate was remarkably convenient to prepare, purify, and store, representing an
efficient platform for multistep synthesis. Different MIDA
boronate monomers were synthesized in this work, such as 4vinylphenyl MIDA boronic ester (M1), acrylate monomers
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Scheme 2. Synthetic Route of MIDA Boronate Monomers and Polymers and Structures of Related Building Blocks, Chain
Transfer Agents, and Monomers Studied in This Work
Table 1. Synthesis of Homopolymers by RAFT Polymerizationa
entry
monomer
CTA
initiator
[M]0/[CTA]0/[I]0
time (h)
convb (%)
Mn,GPCc (K)
Mw/Mnc
1
2
3
4
5
6
7
8
9
10
11
12
13
M1
M1
M1
M1
M2
M2
M2
M3
M3
M3
M4
M4
M4
CTA-1
CTA-1
CTA-1
CTA-1
CTA-1
CTA-1
CTA-1
CTA-1
CTA-1
CTA-1
CTA-2
CTA-2
CTA-2
AIBN
AIBN
AIBN
AIBN
Et3B
Et3B
Et3B
Et3B
Et3B
Et3B
Et3B
Et3B
Et3B
20/1/0.2
50/1/0.2
100/1/0.3
200/1/0.4
50/1/1
100/1/3
200/1/6
50/1/2
100/1/2
200/1/3
50/1/2
100/1/3
200/1/5
10
18
20
32
2
2
2
2
2
2
2
2
6
95
94
60
93
95
84
97
94
99
97
94
87
98
2
8.2
14.5
31.1
18.3
23.2
38.4
15.2
26.3
38
17.9
30.4
57.8
1.21
1.14
1.26
1.27
1.18
1.35
1.46
1.49
1.47
1.51
1.45
1.55
1.35
a
Reaction conditions: [M]0 = 1 M in DMSO. AIBN conditions: thermal initiation at 70 °C, N2 atmosphere; Et3B conditions: at room temperature
and ambient atmosphere, Et3B: 1 mol/L in THF. bDetermined by 1H NMR spectroscopy. cDetermined by GPC in DMF, based on linear PS as a
calibration standard.
The reaction kinetics were carefully investigated for the M1
polymerization at targeted degrees of polymerization (DP) of
50. A linear semilogarithmic plot of monomer concentration to
polymerization time was observed (Figure 1A), and the
molecular weights of formed polymers linearly increased with
the conversion of monomer (Figure 1B), suggesting a wellcontrolled polymerization process. Figure 1C shows that the
GPC curves of obtained polymers shifted to the higher
molecular weight range with narrower distributions, finally
(dodecylthiocarbonothioylthio)-2-methylpropionic acid
(CTA-1) as RAFT chain transfer agents (CTA) and 2,2′azobis(2-methylpropionitrile) (AIBN) as the thermal radical
initiators in dimethyl sulfoxide (DMSO) with pretreatment of
removing oxygen. After 10 h of polymerization under
conditions ([M1]0:[CTA-1]0:[AIBN]0 = 20:1:0.2) at 70 °C,
the conversion of M1 reached 95%, providing the polymers
P1-CTA with Mn = 2000 and Mw/Mn = 1.21 (entry 1, Table
1).
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Figure 1. (A) Pseudo-first-order kinetic plot. (B) number-average molecular weights and polydispersity as a function of conversion. (C) GPC
traces of P1-CTA. (D) 1H NMR spectra of P1-CTA in DMSO-d6.
providing the polymers P1-CTA with Mn = 8200 and Mw/Mn
= 1.14 (entry 2, Table 1). The polymerizations under similar
conditions but with a higher ratio of [monomer]0/[CTA]0
(DP = 100 and 200) provided polymers with narrow molecular
distributions and higher molecular weights (entries 3 and 4,
Table 1).
The precipitation of the reaction mixture in tetrahydrofuran
(THF) or methanol (MeOH) provided the purified polymers
P1-CTA as light-yellow powders, which were only dissolved in
dimethylformamide (DMF) or DMSO. 1H NMR spectroscopy
was conducted to confirm the structure of the obtained
polymers, showing two broad peaks at 6.43 and 7.09 ppm for
aromatic protons (a, b) and two peaks at 4.33 and 4.03 ppm
for the methylene group (c) of MIDA boronate (Figure 1D).
We previously reported that autoxidation of triethylborane
(Et3B) was able to initiate and mediate RAFT polymerization
at ambient atmosphere without any pretreatment of degassing.31,32 This method was applied to polymerize M2 and M3
with various DPs (50, 100, and 200) in the presence of CTA-1,
reaching conversion over 90% in 2 h and providing welldefined polymers (entries 5−10, Table 1). Because CTA-1 was
not a suitable RAFT chain-transfer agent for methacrylate,
RAFT polymerization of M4 was conducted with S-dodecyl-S′(α-methyl-α-cyanobutyric acid) trithiocarbonate (CTA-2) to
yield polymers with predetermined molecular weights (entries
11−13, Table 1). These results suggested that RAFT
polymerization could provide the homopolymers from MIDA
boronate monomers, and the poly(meth)acrylate backbone
with MIDA boronic esters was first synthesized and reported.
Random Copolymers with MIDA Boronates. To solve
the solubility of the homopolymers and to explore the
copolymerization of MIDA boronate monomers with other
traditional monomers, the random copolymerizations were
conducted, and the results are summarized in Table 2. MIDA
boronate monomer M1 and styrene (St) were copolymerized
with CTA-1 and AIBN as the thermal radical initiator under
conditions ([St]:[M1]:[CTA-1]:[AIBN] = 40:10:1:0.4),
reaching 53% conversion for styrene and 63% conversion for
M1 to provide the random copolymer P1-r-PS-CTA with Mn =
2200 and Mw/Mn = 1.18 (Table 2, entry 1). The polymers P1r-PS-CTA with higher molecular weights were also obtained
with higher degrees of polymerizations (DP = 100, 200, 500,
entries 2−4, Table 2).
MIDA boronate monomer M2 or M3 was copolymerized
with methyl acrylate (MA) under Et3B/oxygen initiated RAFT
polymerization conditions. The copolymerizations also
reached nearly 80% monomer conversion in 2 h and provided
well-controlled random copolymers P2-r-PMA-CTA or P3-rPMA-CTA with different molecular weights and narrow
distributions (entries 5−10, Table 2). Methyl methacrylate
(MMA) was adopted to copolymerize with M4 because of
their similar structures. Similar to the homopolymerization of
M4, the copolymerization with MMA was regulated by CTA-2
to yield P4-r-PMMA-CTA (Mn = 8200, Mw/Mn = 1.30, entry
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Table 2. Synthesis of Random Copolymers by RAFT Polymerizationa
t (h)
convb (%)
Mnc (K)
Mw/Mnc
40/10/1/0.4
20
2.2
1.18
AIBN
80/20/1/0.3
45
4.8
1.17
M1, St
AIBN
160/40/1/0.3
50
12.6
1.14
4
M1, St
AIBN
400/100/1/0.3
50
21
1.20
5
M2, MA
Et3B
40/10/1/1
2
6
M2, MA
Et3B
80/20/1/2
2
7
M2, MA
Et3B
160/40/1/3
2
8
M3, MA
Et3B
40/10/1/2
2
9
M3, MA
Et3B
80/20/1/2
2
10
M3, MA
Et3B
160/40/1/4
2
11d
M4, MMA
Et3B
40/10/1/3
2
M1: 53
St: 63
M1: 51
St: 45
M1: 40
St: 41
M1: 25
St: 29
M2: 81
MA: 70
M2: 85
MA: 73
M2: 83
MA: 79
M3: 90
MA: 93
M3: 70
MA: 84
M3: 76
MA: 82
M4: 71
MMA: 60
entry
monomer
initiator
1
M1, St
AIBN
2
M1, St
3
[CM]0/[MD]0/[CTA-1]0/[I]0
6.2
1.16
14
1.16
20
1.20
7.4
1.22
11
1.23
20.1
1.26
8.2
1.30
a
Reaction conditions: [MD]0 = 0.2 M in DMSO; CM: comonomers including St, MA, and MMA; MD: MIDA boronate monomers; M1−M4;
AIBN conditions: thermal initiation at 70 °C, N2 atmosphere; Et3B conditions: at room temperature and ambient atmosphere; Et3B: 1 mol/L in
THF. St: styrene; MA: methyl acrylate; MMA: methyl methacrylate. bDetermined by 1H NMR. cDetermined by GPC in DMF, based on linear PS
as a calibration standard. dCTA-2 was used instead of CTA-1.
Figure 2. Pictures of moisture absorption experiments. (A) Original appearance of the MIDA boronate-containing polymer P1 and (B) 30 days
later. (C) Original appearance of poly(4-vinylboronic acid) and (D) 2 days later.
Stability of the MIDA Boronate Polymers. We
speculated that the MIDA boronate-containing polymers
would have more stability compared to the boronic acidcontaining polymers because of the MIDA boronates as the
11, Table 2). These successful copolymerization results
suggested that all four MIDA boronate monomers could be
easily copolymerized with other regular monomers and
incorporated into any polymeric structures.
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Figure 3. 1H NMR spectrum of (A) original MIDA boronate-containing copolymer P1-r-PS (in DMSO-d6), (B) transformation to ethylene glycol
boronic ester (in CDCl3), (C) transformation to pinacol boronate (in CDCl3), and (D) hydrolysis to boronic acids (in DMSO-d6). (E) 19F NMR
spectrum of trifluoroborate-containing copolymer (in DMSO-d6).
Table 3. PPM of MIDA Boronate-Containing Polymers by Suzuki−Miyaura Couplinga
polymer precursor
entry
c
1
2
3
4
5
6
7
8
b
ArBr
composition
Mn
methyl 4-bromobenzoate
methyl 4-bromobenzoate
bromostyrene
bromobenzyl alcohol
bromofluorene
bromostyrene
bromobenzene
bromofluorene
P1-r-PS-CTA
P1-r-PS
P1-r-PS
P1-r-PS
P1-r-PS
P3-r-PMA
P3-r-PMA
P3-r-PMA
5400
5400
6600
6100
6100
8800
8200
8200
product
b
PDI
Mnb
PDIb
1.18
1.18
1.26
1.26
1.26
1.49
1.28
1.28
11100
5600
6900
7200
6900
10300
8700
9200
1.21
1.38
1.25
1.22
1.22
1.19
1.34
1.30
Reaction conditions: [ArBr]:[B(MIDA)]:[Pd(OAc)2]:[RuPhos] = 2:1:0.06:0.12, 60 °C, t = 6−7 h, >95% conversion of MIDA. bDetermined by
GPC. cConducted without the removal of end group.
a
diaminonaphthalenyl (DAN) boronate was stable in the basic
environment. Therefore, it is worth to realize the transformation and hydrolysis of MIDA boronate in the polymers,
making MIDA boronate-containing polymers as a general
platform for different organoboron-containing polymers.
MIDA boronate polymers were reacted with pinacol or
ethylene glycol via in situ deprotecting MIDA boronate with
trimethylamine in the presence of water, providing pinacol or
ethylene glycol boronic ester, respectively. This transformation
was followed and determined by 1H NMR spectroscopy
(Figure 3). The resonances of Ha and Hb of MIDA boronate
(Figure 3A) completed disappeared, and they were replaced
with a singlet (Hc) at 3.49 ppm, indicating the methylene on
glycol (Figure 3B), or a broad peak (Hd) at 1.36 ppm,
indicating the methyl group on pinacol (Figure 3C). The
pinacol or ethylene glycol boronic ester-containing polymer
effective protecting groups for boronic acids. To study the
stability of the MIDA boronate-containing polymers, we
conducted the comparative experiment of moisture absorption
for P1 and poly(4-vinylboronic acid). Both polymers were left
under an ambient atmosphere and temperature in the fume
hood. After 30 days, MIDA boronate-containing polymers P1
were still white powders with negligible difference in weights
(Figure 2A,B), while 1.21 g of poly(4-vinylboronic acid) as a
yellowish powder gained 310 mg weight and changed to a
yellowish gel in only 2 days (Figure 2C,D). The stage change
of poly(4-vinylboronic acid) might be due to the formation of
boroxine or covalent cross-linking in the presence of water.18
Transformations of MIDA Boronate Polymers. Various
boronate esters as protecting groups for boronic acid had
different stability depending on the conditions.21 For example,
MIDA boronate could tolerate the acidic environment while
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The transformation to trifluoroborate was performed with
KHF2 in a mixture solvent of water and THF. The target
product was confirmed by 19F NMR spectroscopy as a broad
peak at −138 ppm, referring to fluorine on trifluoroborate
(Figure 3E), and the obtained polymer was also dissolved in
chloroform. The hydrolysis of MIDA boronate on the pendant
group was conducted with NaOH in water/THF, similar to the
conditions for the hydrolysis of small molecules.33 The
resulting polymers could be obtained by precipitating in
methanol and also were confirmed by 1H NMR spectroscopy
(Figure 3D), showing that all the characteristic peaks of the
MIDA boronate disappeared. For the hydrolysis of P2-r-PMA,
the MIDA boronates were unstable in strong basic conditions;
therefore, the saturated NaHCO3 aqueous solution provided a
weaker basic environment to obtain the corresponding boronic
acid-containing copolymers.
Postpolymerization Modification by Suzuki−Miyaura
Coupling. Suzuki−Miyaura coupling was an effective method
for postpolymerization modification (PPM).34 Particular
functional groups could be site specifically generated as the
polymer pendant group, and then precision macromolecules
with targeted properties could be obtained. Mori reported the
successful synthesis of the thioether polymers with halogen in
pendant group and functionalized by Suzuki−Miyaura
coupling and Buchwald−Hartwig coupling to provide the
block copolymerizations with two distinct optoelectronic
functionalities.35 Similarly, Magenau synthesized the polymer
with 4-bromophenyl as the pendent group by RAFT
polymerization and followed by Suzuki−Miyaura PPM to
incorporate different functional groups with excellent efficiency
(functionalization >90%).36 In all the above cases, the small
molecules used in PPM were boronic acids that reacted with
aryl halides as moieties in polymers. It will be more valuable to
utilize aryl halides as small molecules in PPM since there will
be more choices for aryl halides compared to the boronic acidcontaining compounds.
The polymer with CTA as chain-end (P1-r-PS-CTA)
reacted with methyl 4-bromobenzoate under Pd-catalyzed
coupling conditions by using RuPhos as a ligand. However, the
molecular weight of the functionalized polymer was doubled
compared to the original molecular weight (Table 3, entry 1,
and Figure 4A), and this similar behavior was observed by
Mori and Magenau.35,36 We hypothesized that the trithioester
might reduce to thiol (−SH), and two polymer chains with
thiol group were coupled in the basic condition during the
cross-coupling. Moreover, Suzuki−Miyaura coupling was
hindered by the catalyst toxicity due to the sulfur atom in
CTA as a chain-end group or thiol group, which might reduce
the coupling efficiency.37 To eliminate the effects of CTA in
the cross-coupling reaction, the trithioester chain-end group
was removed for all investigated polymers by using an excess of
triethylborane under aerobic conditions (Scheme 3).38 The
color of the solution immediately disappeared, and the UV−vis
spectra further confirmed the successful and efficient chain-end
removal (Figure 4B).
Suzuki−Miyaura coupling was conducted for chain-end
removed polymers such as P1-r-PS and P3-r-PMA with various
aryl bromides, including methyl 4-bromobenzoate, bromostyrene, bromobenzyl alcohol, and bromofluorene (Scheme 3),
and the results are summarized in Table 3. The coupling
reaction between methyl 4-bromobenzoate and P1-r-PS
without any sulfur chain-end provided the corresponding
functionalized polymer (Mn = 5600 from 5400, Mw/Mn = 1.38,
Figure 4. (A) GPC traces of the PPM of copolymers with or without
chain-end group. (B) Example of UV−vis spectra of P2-r-PMA-CTA
(with end group) and P2-r-PMA (without end group). (C) 1H NMR
spectra of functionalized and original P3-r-PMA.
became soluble in chloroform, further indicating the successful
transformations.
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Scheme 3. Removal of RAFT Agent on Polymers and the Postpolymerization Modification by Suzuki Coupling
entry 2, Table 3), indicating no cross-linking between two
polymer chains this time. All other PPMs via Suzuki−Miyaura
were efficient and successful, providing the desired functionalized polymers with slightly increased molecular weight. The
1
H NMR resonances of MIDA boronate or boronic acid were
not observed after the coupling reaction, indicating almost full
conversion in Suzuki−Miyaura coupling with excess aryl
bromides. All modified polymers after PPM were purified by
precipitating in methanol three times, and then they were
characterized by 1H NMR spectroscopy. Figure 4C exhibits the
spectra of original P3-r-PMA and functionalized polymer after
PPM, showing that characteristic peaks of styrene (peaks b, c,
and d) replaced the peaks of MIDA boronate (peak i). Another
obvious evidence for the successful coupling was the change of
solubility, as the polymer precursor could be dissolved in
DMSO but not chloroform. In contrast, the functionalized
polymers could be dissolved in chloroform but not in DMSO
anymore.
■
AUTHOR INFORMATION
Corresponding Author
Xiangcheng Pan − State Key Laboratory of Molecular
Engineering of Polymers, Department of Macromolecular
Science, Fudan University, Shanghai 200438, China;
orcid.org/0000-0003-3344-4639; Email: panxc@
fudan.edu.cn
Author
Congze He − State Key Laboratory of Molecular Engineering of
Polymers, Department of Macromolecular Science, Fudan
University, Shanghai 200438, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.0c00665
Notes
■
The authors declare no competing financial interest.
■
■
CONCLUSION
The MIDA boronate-containing monomers were reported and
synthesized as a new group of organoboron monomers suitable
for (controlled) radical polymerization. RAFT polymerizations
were performed successfully to synthesize both homopolymers
and copolymers with MIDA boronate in a good control. MIDA
boronate-containing polymers were easily synthesized, characterized, and handled. Importantly, they showed remarkable
stability to air and moisture compared to other organoboroncontaining polymers.
MIDA boronates as polymer pendant groups efficiently
transformed to other organoboron functional groups such as
pinacol, ethylene glycol boronic ester, trifluoroborate, and
boronic acid, making MIDA boronate-containing polymer a
versatile platform for various organoboron polymers. Furthermore, other functionalizations could be incorporated using
the MIDA boronate polymer as a general platform by Suzuki−
Miyaura coupling as the postpolymerization modification.
The marriage of MIDA boronate chemistry and polymer
synthesis offers a new and practical strategy of synthesizing
organoboron-containing and functionalized polymers, which
could have broad applications in material and biomedical fields
associated with borane chemistry.
■
Experimental details (PDF)
ACKNOWLEDGMENTS
We thank the support from the National Natural Science
Foundation of China (21871056, 21704017, and 91956122).
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