Synthesis of Electron-Deficient Borinic Acid Polymers with

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Synthesis of Electron-Deficient Borinic Acid Polymers with Multiresponsive Properties and Their Application in the Fluorescence Detection of Alizarin Red S and Electron-Rich 8‑Hydroxyquinoline and Fluoride Ion: Substituent Effects Wen-Ming Wan,* Shun-Shun Li, Dong-Ming Liu, Xin-Hu Lv, and Xiao-Li Sun State Key Laboratory of Heavy Oil Processing, Centre for Bioengineering and Biotechnology, and College of Science, China University of Petroleum (East China), 66 Changjiang West Road, Qingdao Economic Development Zone, Qingdao, Shandong 266580, People’s Republic of China S Supporting Information *

ABSTRACT: Electron-deficient borinic acid monomers and corresponding polymers were synthesized with different substituents via a one-pot reaction of Grignard reagents with trimethoxyborane and reversible addition−fragmentation chain transfer polymerization, respectively. Further investigations of substituent effects of borinic acid polymers (PBAs) were carried out, indicating that the thermoresponsive properties of PBAs benefit from the increase of steric hindrance of PBA substituent, while the binding affinity of PBAs with Alizarin Red S, 8-hydroxyquinoline (HQ), and fluoride ion decreases with the increase of steric hindrance of substituent. Attributed to the strong dative N → B bond and the strongly luminescent boron quinolate, the application of phenyl PBA for fluorescence detection of HQ is realized with high sensitivity at the ppm level. These results therefore confirm that borinic acidcontaining polymer is a new type of stimuli-responsive polymer in the field of thermoresponsiveness over a wide temperature range and chemical sensor for diol and electron-rich compounds.

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INTRODUCTION Stimuli-responsive polymers, undergoing significant reversible or irreversible property changes upon relatively small external stimuluses, have gained considerable research efforts in the past decades.1−3 Stimuli-responsive polymers are therefore widely utilized as chemical or physical sensors, of which boroncontaining polymers have proven especially attractive because of their outstanding optical and sensory properties upon stimuli such as anions and sugars.4−12 For example, Jäkle and coworkers demonstrated pioneering works on the utilization of triarylborane polymers as stimuli-responsive materials in the detection of anions such as fluoride ion and cyanide ion.13−17 Sumerlin and co-workers demonstrated pioneering works on the utilization of boronic acid polymers as stimuli-responsive materials in the detection of sugars, while Kim and van Hest developed pioneering works on boroxole-functionalized polymers in the detection of sugars at physiological pH.18−25 Compared with exclusive studies on triarylborane polymers and boronic acid polymers, borinic acid polymers (PBA) have been mostly ignored, in spite of the high stability, catalytic Lewis acidity, and excellent substrate binding characteristics of borinic acid (BA) small molecules.26−29 Since the development of first PBA by Jäkle’s group in 2014, we and Jäkle have cooperatively demonstrated PBA as a new type of stimuli-responsive polymer in the fields of upper critical solution temperature (UCST) type thermoresponsive (tunable over a wide temperature range in DMSO, DMF, and THF with different amounts of water), © XXXX American Chemical Society

sensory (fluoride ion and acetylthiocholine chloride detection), supramolecular materials (with poly-Lewis base), and enzymatic biofuel cell application.30−34 Different from the general strategy to achieve multiresponsive polymers by incorporating different stimulus moieties into one polymer chain, such as temperature, pH, light irradiation, etc.,35−42 PBA homopolymer with one functional moiety exhibits multiresponsiveness. It is therefore highly desirable to investigate the substituent effects of PBA on its responsiveness. Electron-deficient organoboranes have gained ever-increasing research interests in the past decades due to their promising potentials in the fields of luminescent materials, nonlinear optical materials, electronic devices, and sensory materials.4−12 Attributed to vacant p-orbitals of tricoordinate boranes, they are known as Lewis acids and can be therefore utilized for fluoride and cyanide anions detection, as well as neutral Lewis bases, a process that typically accompanies color and luminescence changes.4 Generally, their Lewis acidity is relative to the electron deficiency of boron centers, while opposite to their stability in air and moisture. BA compounds and derivatives, consisting electron-deficient boron center and electron-rich hydroxide group, therefore benefit from moderate Lewis acidity and excellent stability in air and moisture, which Received: May 15, 2017 Revised: August 22, 2017

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DOI: 10.1021/acs.macromol.7b01002 Macromolecules XXXX, XXX, XXX−XXX

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125.76, 115.15, 22.08, 21.21 ppm. 11B NMR (128 MHz, CDCl3): δ = 48.76 ppm. Synthesis of the PBAs with Different Substituents by Reversible Addition−Fragmentation Chain Transfer (RAFT) Polymerization.50−53 The phenyl BA (1.25 g, 6 mmol), CTA (10.95 mg, 30 μmol), and AIBN (0.5 mg, 50 μL of freshly prepared 10 mg/ mL AIBN stock solution in THF, 3 μmol) at the feeding molar ratio of 200/1/0.1 and 1 mL of THF were put into a Schlenk tube. Then the solution was degassed by three freeze−evacuate−thaw cycles. After the Schlenk tube was sealed, it was immersed in an oil bath at 80 °C and kept stirring for 12 h. Polymerization was stopped by putting the Schlenk tube into an icy water bath. After the product was purified by precipitation into excessive petroleum ether, filtered, and dried under vacuum, 0.82 g of powder-like phenyl PBA was obtained with a yield of 65.7%. 1H NMR (400 MHz, [D6]-DMSO): 7.95−6.03 (broad, −C6H5, −C6H4−, 9H), 5.71−5.14 (broad, B−OH, 1H), 1.32−1.03 (broad, −CHCH2, 2H) 0.93−0.56 (broad, −CHCH2, 1H). 13C NMR (100 MHz, [D6]-DMSO): δ = 134.74 (broad), 127.89 (broad), 22.76 ppm. 11B NMR (128 MHz, [D6]-DMSO): δ = 72.63−23.80 (broad) ppm. Mesityl PBA was synthesized by changing the BA with the substituent of phenyl group into the BA with the substituent of mesityl group (1.50 g, 6 mmol) with the same procedures. 1.01 g of mesityl PBA was obtained as a solid with a yield of 66.4%. 1H NMR (400 MHz, CDCl3): 7.71−7.14 (broad, −C6H4−, 2H), 7.00−6.51 (broad, −C6H2−, 2H, −C6H4−, 2H), 5.94−5.62 (broad, B−OH, 1H), 2.41− 2.19 (broad, phCH3, 3H), 2.19−1.77 (broad, phCH3, 6H), 1.34−1.07 (broad, −CHCH2, 2H), 0.94−0.76 (broad, −CHCH2, 1H). 13C NMR (100 MHz, CDCl3): δ = 139.05, 137.79, 135.29, 127.07 (broad), 22.10, 21.22 ppm. 11B NMR (128 MHz, CDCl3): δ = 65.58−30.79 (broad) ppm. Characterizations. Nuclear Magnetic Resonance (NMR). The 1H NMR and 13C NMR measurements were performed on a Bruker AscendTM 400 spectrometer in CDCl3 or [D6]-DMSO using tetramethylsilane as an internal reference. The 11B NMR spectra were recorded in boron-free quartz NMR tubes with BF3·Et2O (δ = 0) in C6D6 as an external reference. UV−Vis Absorption Spectra. The spectra were acquired in THF with sample concentration of 100 μM (based on borinic acid repeating unit) on a Shimadzu UV-2450 UV−vis spectrophotometer. Fluorescence Measurements. The measurements were recorded in THF with sample concentration of 10 μM (based on borinic acid repeating unit) on a Hitachi F2500 fluorescence spectrofluorometer. Gel Permeation Chromatography (GPC). The molecular weight and molecular weight distribution were determined on a Viscotek TDA 302 triple detector array equipped with one TSK-Gel GMHHRN column at 30 °C, and THF was used as eluent at a flow rate of 1.0 mL/min. Monodispersed polystyrene standards were used in the calibration of molecular weight and molecular weight distribution. Thermoresponsive Measurements. The measurements of the polymer solution (1 mg/mL) in DMSO, DMF, and THF with different amounts of water (vol %) were performed using an oil bath with a digital thermocontroller, where the temperature with an obvious turbidity change in the heating process was used to determine the UCST.

ensure their comprehensive applications as Lewis acid catalysts, in Ca2+ regulation, cross-coupling reactions, transferring both aryl groups and organic light-emitting devices (OLEDs).43 Attributed to the so-called sensory signal amplification effects of luminescent polymers,17 when BA moieties are incorporated into polymer chain, the high binding affinity with diol compound28 and the electron deficiency of PBA derived from BA are therefore highly anticipated to enable its fluorescence detection of diol compound, Alizarin Red S (ARS), and electron-rich molecules, such as 8-hydroxyquinoline (HQ) and fluoride ion. Herein, we describe the synthesis of electron-deficient PBAs with different substituents (Scheme S1), the investigation of their multiresponsive properties, and their applications as chemical sensors in the detection of diol compound, ARS, and electron-rich compounds, HQ and fluoride ion. The electrondeficient boron center and strong dative N → B bond of luminescent boron quinolate ensure the high sensitivity in the fluorescence detection of HQ.

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EXPERIMENTAL SECTION

Materials. Bromobenzene, bromomesitylene, 2,4,6-triisopropylbromobenzene, 4-bromostyrene, magnesium, tetrabutylammonium fluoride, ARS, HQ, and trimethylborate were purchased from Aladdin or Sinopharm Chemical Reagent Co. Ltd. and used without further purification. The THF was distilled from Na/benzophenone prior to use. The azobis(isobutyronitrile) (AIBN, Aladdin-reagent, 98%) was used as initiator and recrystallized in methanol. The chain transfer agent (CTA) was synthesized according to literature procedures.44−48 The PBA (R: triisopropylphenyl (Tip); R′: phenyl) (Mn,GPC = 28 900 g/mol, Đ = 1.24) was prepared as reported previously.33 Other regular chemical reagents were used as received. Synthesis of the BAs with Different Substituents.49 To one flame-dried 2-neck round-bottom flask containing 0.87 g (36 mmol) of freshly peeled Mg scraps under nitrogen protection was added 10 mL of dry THF. Then 4-bromostyrene (3.92 mL, 30 mmol) dissolved in 20 mL of THF was added to the flask at room temperature through a syringe. Meanwhile, bromobenzene (3.16 mL, 30 mmol) dissolved in 30 mL of THF was injected into another flask with same amount of magnesium under a nitrogen atmosphere. Then the reactions were carried out on a magnetic stirrer at room temperature for 2 h. After that, the 4-bromostyrene and bromobenzene Grignard reagent were added dropwise to the flask containing trimethyl borate (3.34 mL, 30 mmol) and THF (20 mL) successively through a syringe at 0 °C. Then the solution reacted continually for 4 h under magnetic stirring, followed through filtration and extraction with dichloromethane/ water. After being washed with saturated ammonium chloride aqueous solution and the deionized water for three times, the organic solution was dried with anhydrous sodium sulfate and concentrated under reduced pressure. Then the crude product was subjected to column chromatography on silica gel with petroleum ether/ethyl acetate mixture (10/1 v/v) as the eluent to afford 3.92 g of the BA with the substituent of phenyl group with a yield of 62.7%. 1H NMR (400 MHz, CDCl3, δ): 7.86−7.72 (m, −C6H5, −C6H4−, 5H), 7.61−7.37 (m, −C6H5, −C6H4−, 4H), 6.85−6.71 (dd, −CHCH2, 1H), 5.94−5.76 (t, −CHCH2, B−OH, 2H), 5.38−5.27 (d, −CHCH2, 1H). 13C NMR (100 MHz, CDCl3): δ = 140.07, 136.65, 134.71, 131.12, 127.96, 125.61, 115.18 ppm. 11B NMR (128 MHz, CDCl3): δ = 45.67 ppm. BA with mesityl substituent was synthesized by changing the bromobenzene into bromomesitylene (4.59 mL, 30 mmol) with the same procedures. 5.06 g of the BA with mesityl substituent was synthesized with a yield of 67.4%. 1H NMR (400 MHz, CDCl3, δ): 7.75−7.70 (d, −C6H4−, 2H), 7.45−7.40 (d, −C6H4−, 2H), 6.90−6.84 (s, −C6H2−, 2H), 6.81−6.70 (dd, −CHCH2, 1H), 5.88−5.78 (d, −CHCH2, B−OH, 2H), 5.36−5.28 (d, −CHCH2, 1H), 2.36−2.30 (s, −phCH3, 3H), 2.24−2.18 (t, −phCH3, 6H). 13C NMR (100 MHz, CDCl3): δ = 140.89, 139.10, 137.98, 136.86, 135.58, 129.06, 127.25,

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RESULTS AND DISCUSSION Synthesis of PBA with Different Substituents. PBA has been proven especially attractive as a new type of promising stimuli-responsive polymer in the areas of thermoresponsive, fluoride ion-responsive, sensory, supramolecular, and enzymatic biofuel cell materials. With a general structure of RR′BOH, the properties of PBA are therefore expected to be readily finetuned by varying the substituents R and R′. In comparison with the previous synthesis of BA monomer in three steps,30 the one-pot synthesis of BA monomers via sequential reactions of desired aromatic Grignard reagents with trimethoxyborane in a selective manner, as shown in Scheme 1, was adapted for the B

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Scheme 1. Illustration of the Synthetic Route of Borinic Acid Monomers (BAs) and Corresponding Borinic Acid Polymers (PBAs) with Different Substituents (Phenyl, Mesityl, and Tip)

synthesis of PBA with different substituents in this work.49 The electron-deficient BAs monomers (R: phenyl, mesityl, and Tip substituents; R′: phenyl) were successfully synthesized with high yields (>53%). Their chemical structures were confirmed by 1H, 11B, and 13C NMR results (Figure 1). In the 1H NMR

Figure 2. GPC curves of (A) phenyl PBA and (B) mesityl PBA.

spectra of PBAs are compared to those of the BA monomers in Figure 1. The successful preparation of PBAs with both phenyl and mesityl substituents is confirmed by the disappearance of vinyl protons at 6.77, 5.85, and 5.32 ppm, the appearance of broad main chain signals at 1−2 ppm regions, and broad signals for phenyl and methyl protons on the side chains. The broader 11 B NMR signals of PBAs are likely due to neighboring group effects in the polymer chain (Figure S2). Further verification of the preparation of PBAs is indicated by unimodal symmetric GPC curves with narrow PDI, i.e., Mn,GPC = 27 100 and Đ = 1.17 for phenyl PBA and Mn,GPC = 26 100 and Đ = 1.16 for mesityl PBA, as shown in Figure 2. To verify the controlled/ living characteristics of RAFT polymerization of borinic acid monomers, the RAFT polymerization kinetics investigations were carried out by taking mesityl borinic acid monomer as an example (Figure S3). It is clear that this RAFT polymerization exhibits first-order polymerization kinetics (Figure S3A). The different experimental molecular weight from the theoretical molecular weight should be due to the hydrodynamic radius difference between PBA and polystyrene standards (Figure S3B). PBAs obtained at different polymerization time exhibit unimodal symmetric GPC curves with narrow PDIs (Figure S3C). These results therefore confirm that PBAs with controlled molecular weight could be achieved via RAFT polymerization. By changing the R substituents, properties of PBAs can be readily tuned. Taking solubility as an example, phenyl PBA is not soluble in chloroform, while mesityl PBA and Tip PBA are, what hints the importance of substituents on the properties of PBA. Photophysical Properties. Attributed to the vacant porbital of boron center, the π-conjugation length of aromatic substituents is extended, resulting in electron-deficient moieties with photophysical properties. The photophysical properties of BA and PBA were investigated (Figure 3). It is clear that phenyl BA exhibits an maximum absorption at 257 nm in THF

Figure 1. 1H NMR spectra of BA monomers (R: phenyl substituent (C); R: mesityl substituent (D)) and corresponding PBAs (R: phenyl substituent (A); R: mesityl substituent (B)).

spectrum of phenyl BA, its chemical structure was verified by the phenyl protons at 7.41 and 7.80 ppm, the vinyl protons at 6.77, 5.85, and 5.32 ppm, and the B−OH proton at 5.86 ppm. In the 1H NMR spectrum of mesityl BA, its chemical structure was verified by the phenyl protons at 7.70, 7.42, and 6.86 ppm, the vinyl protons at 6.77, 5.85, and 5.32 ppm, the B−OH proton at 5.86 ppm, and the methyl protons at 2.18 and 2.31 ppm. The single broad peaks at around 46 and 49 ppm in the 11 B NMR spectra of phenyl BA and mesityl BA proved the presence of boron and its electron deficiency as tricoordinate boron. To eliminate the molecular weight distribution effect on the properties of PBAs, PBA polymers with moderate molecular weight and controlled molecular weight were prepared by RAFT polymerization of BA monomers using AIBN as the initiator and 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DMP) as the CTA at a feeding molar ratio of [BA]/[DMP]/[AIBN] = 200/1/0.1 in THF at 80 °C (Scheme 1, Figure 1, Figure 2, and Figure S3). The 1H NMR C

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content from ∼0−2.5%, in DMF from 0 to 100 °C with ∼3− 17% H2O content, and in THF from 0 to 60 °C with ∼4−19% H2O content. With decrease of hydrophobicity, 1 mg/mL PBA (R: Tip; R′: phenyl) solution in DMSO exhibited tunable UCST ∼25 to ∼100 °C with ∼1−11% H2O content, in DMF from 30 to 100 °C with ∼11−23% H2O content, and in THF from 10 to 60 °C with ∼27−35% H2O content. With further decrease of hydrophobicity, 1 mg/mL PBA (R: mesityl; R′: phenyl) solution in DMSO exhibited tunable UCST ∼25 to 94 °C with ∼4−17% H2O content, in DMF from ∼20 to 92 °C with ∼15−42% H2O content, and in THF from ∼25 to 60 °C with ∼42−60% H2O content. For PBA (R: phenyl; R′: phenyl), no thermoresponsive behavior was observed. Obviously, substituents of PBA have high influence on its thermoresponsive properties. With the trend of decreasing hydrophobicity of R substituents from Tip to mesityl and phenyl and R′ substituents from biphenyl to phenyl, respectively, more and more amounts of H2O content are needed to achieve critical solution temperature. Fluorescence Detection Properties. Compared with the widespread investigations on utilization of boronic acid polymers for diol molecules detection, borinic acid derivatives have been mostly ignored in spite of high affinity with diol molecules.28 We therefore investigated the bonding affinity of PBA with ARS for the first time, one of widely used diol dyes. On the other hand, attributed to the electron deficiency of boron center, the investigations on the fluorescence detection of electron-rich HQ and fluoride ion were carried out as well. Attributed to the extension of π-conjugation length of aromatic substituents by the vacant p-orbital of boron center, PBAs with different substituents in solution are bluely luminescent under irradiation with UV lamp at 365 nm (Figure 5G). When diol dye ARS is added to the THF solution of PBA with phenyl substituent, the luminescence of the solution changes from blue to pink under irradiation with UV lamp at 365 nm, indicating the successful binding between borinic acid moiety and diol moiety with significant fluorescence changes (Figure 5G and Scheme 2). This strong affinity between the borinic acid moiety and diol moiety is consistent with the results reported by Taylor.28 To confirm the binding processes between phenyl PBA and ARS, UV titration and fluorescence titration were carried out, and the results are shown in Figure 5. Because the UV spectrum of PBA overlaps with that of ARS and ARS has independent absorption at 430 nm, the titrations were therefore carried out by adding PBA to ARS solution. As is shown in Figure 5A, ARS shows maximum absorption at 430 nm. With the addition of phenyl PBA, the absorbance decreases gradually at 430 nm and increases gradually at 502 nm. Because of the luminescent binding product, the luminescent intensity increases correspondingly, as shown in Figure 5B. These data therefore confirm the successful binding between ARS with borinic acid moieties on phenyl PBA side chains. Further verification of the binding affinity of electrondeficient borinic acid moiety with electron-rich HQ and fluoride ion was carried out. As shown in Figure 5G, the luminescence of phenyl PBA solution changes from blue to green under irradiation with UV lamp at 365 nm, indicating the successful binding between electron-deficient borinic acid moiety and electron-rich HQ accompanied by significant fluorescence changes. Further addition of fluoride ion causes significant luminescence changes from green to nonluminescent, indicating that the binding affinity between phenylborinic acid moiety and HQ is challenged by fluoride ion and fluoride

Figure 3. Photophysical properties of BA and PBA. (A) Absorption spectra of phenyl BA and phenyl PBA, (B) absorption spectra of mesityl BA and mesityl PBA, (C) emission spectra of phenyl BA (excited at 257 nm) and phenyl PBA (excited at 246 nm), and (D) emission spectra of mesityl BA (excited at 267 nm) and mesityl PBA (excited at 241 nm).

solution. In comparison, phenyl PBA shows maximum absorption at 246 nm because the π-system of the individual chromophores becomes smaller upon polymerization of the vinyl group. Meanwhile, phenyl BA exhibits an maximum emission at 314 nm, and the phenyl PBA exhibits an similar emission, but the peak is relatively broader, probably due to the neighboring effect caused by the very close boron chromophores in the polymer side chain. Similarly, mesityl BA shows a maximum absorption at 267 nm and an emission at 314 nm under the maximum absorption in THF, while the mesityl PBA shows maximum absorption and emission at 241 and 314 nm. Thermoresponsive Properties. Attributed to the reversible cross-linker effect of water with B−OH moieties, PBA′ was reported to exhibit upper critical solution temperature (UCST) type thermoresponsive property, which varies in a wide temperature range in solvents such as DMSO, DMF, and THF with different amount of water.30−34 To investigate whether PBAs with different substituents inherit this intriguing thermoresponsive property, PBA solutions (0.1 wt %) in water miscible solvents, including DMSO, DMF, and THF with different amounts of water, were studied (Figure 4, Tables S1 and S2). The PBA of different R substituents showed different temperature sensitivity. According to previous reports,30−34 1 mg/mL PBA (R: Tip; R′: biphenyl) solution in DMSO exhibited tunable UCST from 20 to 100 °C by varying the H2O

Figure 4. (A) Digital photos of thermoresponsive behavior of mesityl PBA in DMSO. (B) Plots illustrating mesityl PBA in DMSO, DMF, and THF ([PBA] = 1 mg/mL) in the presence of different amounts of water (v/v). D

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ion has greater affinity than HQ with phenylborinic acid moiety (Figure 5G and Scheme 2). The UV titration and fluorescence titration were applied to follow these changes, as shown in Figure 5C,D. With the addition of HQ, a new absorption band at 394 nm appears and increases, which is attributed to the formation of boron quinolate (Scheme 2). Meanwhile, the formed boron quinolate moiety is strongly luminescent. The luminescent intensity increases correspondingly with a maximum emission band at 501 nm. Further addition of fluoride ion causes the deformation of boron quinolate moiety, exhibiting the decrease of absorption band at 394 nm and emission band at 501 nm (Figure 5E,F and Scheme 2). When a substituent of PBA is replaced by a mesityl group, a substituent with greater steric hindrance than the phenyl group, the binding affinity of the mesitylborinic acid moiety of PBA changes significantly, as shown in Figure 6C and Scheme 2. The

Figure 6. (A) UV titration and (B) fluorescence titration (excited at 512 nm) of the binding processes between mesityl PBA and ARS (9.7 × 10−5 mol/L). (C) Digital illustration of the fluorescence detection of diol-ARS and electron-rich HQ by PBA with mestiyl substituent under irradiation with UV lamp at 365 nm.

Figure 5. (A) UV titration and (B) fluorescence titration (excited at 502 nm) of the binding processes between phenyl PBA and ARS (7.5 × 10−5 mol/L). (C) UV titration and (D) fluorescence titration (excited at 394 nm) of the binding processes of phenyl PBA with electron-rich HQ (1.1 × 10−4 mol/L). (E) UV titration and (F) fluorescence titration (excited at 394 nm) of the deformation of phenyl boron quinolate moiety by adding fluoride ion. (G) Digital illustration of the fluorescence detection of ARS and electron-rich HQ and fluoride ion in THF solution of PBA with phenyl substituent under irradiation with UV lamp at 365 nm.

luminescence of mesityl PBA changes from blue to pink when ARS is added, while there is no luminescence changes when HQ is added. From the UV titration and fluorescence titration results in Figure 6A,B, it is clear that the binding product of mesitylborinic acid moiety with ARS is formed with absorption band increase at 512 nm and the decrease at 430 nm, and with

Scheme 2. Proposed Mechanism of Structural Response of Substituted PBAs to Diol Compound, ARS and Electron-Rich Compounds, HQ and Fluoride Ion, and the Corresponding Digital Illustration under Irradiation with UV Lamp at 365 nm

E

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simplified method involving one-pot reaction between trimethoxyborane and Grignard agents, and corresponding PBAs with different substituents have been prepared with wellcontrolled molecular weight and molecular weight distribution via RAFT polymerization. Investigations on the substituent effects of PBAs on the thermoresponsive behaviors, and the binding affinity of borinic acid moieties with both model diol compound, ARS, and electron-rich HQ and fluoride ion are carried out. With the gradual increase of steric hindrance of PBA from phenyl to mesityl and Tip substituents, its thermoresponsive behavior over a wide temperature range in different solvents is even more pronounced, while its binding affinity with ARS, HQ, and fluoride ion decreases gradually. Attributed to the strong dative N → B bond and strongly luminescent boron quinolate, the application of phenyl PBA for fluorescence detection of HQ is realized with high sensitivity at the ppm level. These results therefore confirm that borinic acidcontaining polymer is a new type of stimuli-responsive polymer in the field of thermoresponsiveness over a wide temperature range and chemical sensor for diol and electron-rich compounds. This work therefore opens a new avenue in the designing and applications of stimuli-responsive polymers.

luminescent intensity increase at 562 nm correspondingly, which is similar to that of phenyl PBA. While for HQ addition, no changes are observed. These results therefore confirm the successful binding of mesitylborinic acid with model diol compound, ARS, while no affinity with electron-rich HQ, possibly due to the fact that binding site of HQ is larger than that of ARS (Scheme 2). In comparison, phenyl PBA exhibits successful binding affinity with both model diol compound, ARS, and electron-rich HQ and fluoride ion, the steric hindrance of the substituent on PBA therefore plays an important role in the binding affinity of borinic acid moiety with diol and electron-rich compounds. Further increase of the steric hindrance of PBA is realized with Tip substituent. As is shown in Figure 7 and Scheme 2,

Figure 7. Digital illustration of the fluorescence detection of diol-ARS and electron-rich HQ by PBA with Tip substituent under irradiation with UV lamp at 365 nm.

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ASSOCIATED CONTENT

S Supporting Information *

there are no changes when ARS and HQ are added to Tip PBA solution, indicating no binding affinity of Tip borinic acid moiety with ARS and HQ. According to the above results, substituents have therefore significant effects on the binding affinity of borinic acid moiety with diol and electron-rich compounds. With the gradual increase of steric hindrance from phenyl to mesityl and Tip substituents, no binding affinity is observed for mesityl PBA with HQ and no affinity for Tip PBA with both ARS and HQ. Attributed to the widespread application of HQ in the field of cosmetics, the detection of HQ with high sensitivity is of great importance.54,55 According to the above data, it is worth mentioning that binding of phenyl PBA with HQ causes about 6060-fold of luminescence enhancement, while about 280-fold for ARS. Attributed to the strong dative N → B bond and the strongly luminescent boron quinolate, the application of phenyl PBA for fluorescence detection of HQ at ppm level was therefore carried out. As is seen in Figure 8, clear luminescence enhancement and linear luminescent intensity vs HQ content are observed at ppm level, which confirms the fluorescence detection of HQ with high sensitivity.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01002. Figures S1−S3, Tables S1 and S2, and Scheme S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.-M.W.). ORCID

Wen-Ming Wan: 0000-0002-3692-4454 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding support from NSFC 51503226, Shandong Provincial NSF ZR2015EQ018, China University of Petroleum (East China) starting funding, and “the Fundamental Research Funds for the Central Universities”.

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CONCLUSION In conclusion, electron-deficient borinic acid monomers with different substituents have been successfully prepared via a

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Figure 8. Detection of HQ through binding of phenyl PBA with HQ: (A) emission spectra (excited at 394 nm); (B) emission intensities (excited at 394 nm). F

DOI: 10.1021/acs.macromol.7b01002 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01002 Macromolecules XXXX, XXX, XXX−XXX