ROMP-Derived Pyridylborate Block Copolymers: Self-Assembly, pH

Article pubs.acs.org/Macromolecules

ROMP-Derived Pyridylborate Block Copolymers: Self-Assembly, pHResponsive Properties, and Metal-Containing Nanostructures Gajanan M. Pawar, Roger A. Lalancette, Edward M. Bonder, John B. Sheridan,* and Frieder Jak̈ le* Department of Chemistry, Rutgers University Newark, 73 Warren Street, Newark, New Jersey 07102, United States

Downloaded by CENTRAL MICHIGAN UNIV on September 10, 2015 | http://pubs.acs.org Publication Date (Web): September 10, 2015 | doi: 10.1021/acs.macromol.5b01216

S Supporting Information *

ABSTRACT: The functionalization of organic polymers with polydentate ligands offers opportunities in areas ranging from supported catalysts to materials with desirable magnetic, redoxactive, stimuli-responsive, and self-healing properties. Herein, we present the synthesis and self-assembly of tris(2-pyridyl)borate (Tpyb)-functionalized homo and block copolymers, prepared via ring-opening metathesis polymerization (ROMP) of (bicyclo[2.2.1]hept-5-en-2-yl)-4-phenyl) (pyridin-1-ium-2y l ) d i ( p y r i d i n - 2 - y l ) b o r a t e ( M1 ) an d d im et h yl -7oxabicyclo[2.2.1]hept-5-ene-exo,exo-2,3-dicarboxylate (M2) using Grubbs third-generation catalyst. Controlled polymerization was confirmed by gel permeation chromatography (GPC; also referred to as size exclusion chromatography, SEC) and multinuclear NMR spectroscopy. The solution self-assembly in block-selective solvents (MeOH, THF) was investigated by dynamic light scattering (DLS), scanning electron microscopy (SEM), scanning tunneling electron microscopy (STEM), and transmission electron microscopy (TEM), and the microphase separation in a thin film was imaged by atomic force microscopy (AFM). Hydrolysis of the ester-substituted oxanorbornene block with NaOH led to a new copolymer with carboxylate functionalities that can be dispersed in water. The latter displays multiresponsive properties as each of the individual blocks can be reversibly switched between hydrophobic and hydrophilic states by simple adjustment of pH. Cross-linking of the block copolymer aggregates via metal ion complexation was accomplished, and the feasibility of metal ion exchange was demonstrated by energy-dispersive X-ray spectroscopy (EDX).

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trolled free radical polymerization,9 its self-assembly in block selective solvents, and complexation with metal ions. However, the synthesis required very high temperatures, and the scope of the second block was limited to nonpolar polystyrene. More versatile and controlled/living polymerization techniques are required to fully develop this class of Typb-functionalized block copolymers. Toward this end, we report here the synthesis of the first example of a Tpyb-functionalized norbornene monomer, (4-bicyclo[2.2.1]hept-5-en-2-yl)phenyl) (pyridin-1ium-2-yl)di(pyridin-2-yl)borate (M1). Controlled polymerization of M1 was achieved using Grubbs third-generation catalyst and the incorporation into an amphiphilic block copolymer accomplished by sequential polymerization of an oxanorbornene diester (M2) and M1. The block copolymer selfassembles in block-selective solvents, and the resulting aggregates are used to generate supramolecular metallopolymer networks. We also demonstrate that hydrolysis of the ester groups results in a novel multiresponsive block copolymer with pH-switchable self-assembly characteristics.

INTRODUCTION Polymers that are functionalized with chelating ligands1−9 enjoy increasing attention because of their utility as sensors,4 imaging agents,3 supports for metal ion remediation,10 and precursors to macromolecular catalysts.8 Treatment with suitable metal ions results in metallo-supramolecular materials11−16 that display promising stimulus-responsive2 and self-healing properties.17 Among the most commonly used chelate ligands are bipyridine,18,19 terpyridine,2,20−25 dipyrazolylpyridine,26 and diimidazolylpyridine derivatives.27,2 Tris(1-pyrazolyl)borate (scorpionate) ligands have drawn much interest in the field of catalysis, enzyme modeling, and material chemistry due to their high binding strength toward numerous metal ions.28−34 However, these powerful negatively charged chelate ligands have been sparingly studied in polymer chemistry due to their tendency to degrade via B−N cleavage and rearrangement reactions.35 Our recent discovery that replacement of the 1-pyrazolyl with 2-pyridyl groups generates more robust tris(2-pyridyl)borate (Tpyb) ligands provides an opportunity for development of a new class of metallopolymers.36,37 As potent ligands, they readily form stable sandwich-like complexes with different central metal ions, and a range of octahedral metal complexes (Tpyb)2M (M = Mg, Fe, Mn) based on Tpyb ligands have been reported.36,37 In our previous work, we discussed the preparation of a Tpybfunctionalized block copolymer by nitroxide-mediated con© XXXX American Chemical Society

Received: June 4, 2015 Revised: August 24, 2015

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DOI: 10.1021/acs.macromol.5b01216 Macromolecules XXXX, XXX, XXX−XXX

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distillation, and the product remained as a viscous oil, which was used for the next reaction without further purification. Yield 2.18 g (80%). 1 H NMR (500.2 MHz, CDCl3): δ = 7.46 (d, J = 8 Hz, exo-Ph H), 7.39 (d, J = 8 Hz, 2H, endo-Ph H), 7.27 (overlapped, exo-Ph H), 7.14 (d, 2H, J = 8 Hz, endo-Ph H), 6.25 (dd, J = 3 Hz/5 Hz, 1H endo-NBE H-2 and exo-NBE H-2), 6.16 (dd, J = 3 Hz/5 Hz, exo-NBE H-3), 5.82 (dd, J = 3 Hz/5 Hz, 1H, endo-NBE H-3), 3.37 (m, 1H, NBE H-5), 3.09 (s, 1H, NBE H-4), 2.95 (s, 1H, NBE H-1), 2.19 (m, 1H, NBE H-6), 1.33−1.29 (m, NBE H-7 & H6), 0.24 (SiMe3). The endo:exo isomer ratio was determined by 1H NMR integration of the olefinic resonances in the 3-position at 5.82 and 6.16 ppm to be ca. 76:24. Synthesis of (4-Bicyclo[2.2.1]hept-5-en-2-yl)phenyl) (Tris(1-pyrid2-yl)borate (M1). Under N2 protection, to a solution of I (10.0 g, 41.2 mmol) in toluene (150 mL) was slowly added a solution of boron tribromide (10.4 g, 41.5 mmol) in toluene (100 mL), and the mixture was kept stirring for 5 h. The reaction mixture was slowly transferred to a suspension of pyridyl Grignard (33.6 g, 63.6 mmol (PyMgCl)2(THF)3.5) in toluene (400 mL). The resultant red-brown suspension was kept stirring overnight. The reaction mixture was directly passed through a small plug of alumina gel using first toluene and then dichloromethane as the eluents. The solvents were removed by rotary evaporation; the residue was taken up in CH2Cl2 and washed with aqueous NaHCO3. Subsequently, the product was taken in the aqueous phase by treatment with aqueous HCl (pH = 2) and washed with CH2Cl2. The mixture was neutralized by addition of aqueous NaHCO3 and then extracted three times with CH2Cl2. The organic layers were combined, dried over Na2SO4, and the solvent removed using a rotary evaporator. The residue was redissolved in CH2Cl2 and subjected to chromatography on alumina gel. The eluent was gradually varied from CH2Cl2 to pure THF (thin layer chromatography (TLC) was performed using CH2Cl2/THF = 80/20 on alumina). After evaporation of the solvents the product was obtained as a white solid. Yield: 3.25 g (19%). The product was further purified by recrystallization from CH2Cl2/hexanes at low temperature, and single crystals of M1 were obtained by recrystallization from acetone by slow solvent evaporation. 1H NMR (500.2 MHz, CDCl3): δ = 15 (br s, 1H, pyridyl N−H), 8.47 (d, J = 5 Hz, 3H, pyridyl H6), 7.59 (vt, J = 7.6 Hz, 3H, pyridyl H4), 7.37 (d, J = 8 Hz, 3H, pyridyl H3), 7.08 (vt, J = 6 Hz, 3H, pyridyl H5), 6.90 (m, 4H, Ph), 6.17 (dd, J = 3 Hz/6 Hz, 1H, NBE H-2), 5.80 (dd, J = 3 Hz/6 Hz, 1H, NBE H-3), 3.28 (m, 1H, NBE H5), 3.02 (s, 1H, NBE H-4), 2.88 (s, 1H, NBE H-1), 2.11 (m, 1H, NBE H-6), 1.43−1.40 (m, 2H, NBE H-7,7′), 1.26 (m, 1H, NBE H-6′). 13C NMR (125.718 MHz, CDCl3): δ = 184.3 (m, pyridyl-C2), 152.8 (m, Ph), 143.4 (pyridyl-C6), 140.6 (Ph), 136.7 (NBE), 135.9 (pyridylC4), 133.9 (NBE or Ph), 133.2 (NBE or Ph), 131.6 (pyridyl-C3), 127.0 (Ph), 119.5 (pyridyl-C5), 50.2 (NBE), 48.6 (NBE), 43.2 (NBE, 2 signals), 33.0 (NBE). 11B NMR (160.4 MHz, CDCl3) δ = −10.8. MALDI-MS (pos. mode, anthracene): m/z = 416.2353 (calcd for MH+, 12C281H2611B14N3 = 416.2220). The endo:exo isomer ratio was determined by integration of the olefinic resonance at 6.17 (endo- and exo-isomer) versus that at 5.80 ppm (only endo-isomer) in the 1H NMR spectrum to be >97:3, indicating that the exo-isomer had been removed almost quantitatively during the purification process. A small amount of another component is present that has a very similar structure consisting of a phenyltris(2-pyridylborate) fragment but contains a nonpolymerizable alkyl group instead of the norbornene moiety. This compound could not be removed from M1 by column chromatography and repeated recrystallization. The species was isolated after polymerization by removal of poly(M1) by precipitation and subsequent column chromatography (Figures S11 and S12). The identity of the alkyl substituent could not be determined unambiguously. Synthesis of Poly(M1). M1 (51.3 mg, 0.124 mmol) was dissolved in chloroform (1 mL) and added to a solution of RuCl2(PyBr)2(IMesH2) (CHPh) (1.4 mg, 1.6 μmol) in chloroform (1 mL) at room temperature. The polymerization was terminated after stirring for 14 h by the addition of 1 mL of ethyl vinyl ether. The polymer was then precipitated by dropwise addition to hexanes, collected by filtration and dried under vacuum. Yield: 32 mg (62%). Poly(M1): Mn(GPC) = 28 200 g/mol (Đ = 1.33); Mn (predicted based on monomer/initiator

MATERIALS AND METHODS

Materials. Grubbs third-generation catalyst, RuCl2(PyBr)2(IMesH2) (CHPh) (IMesH2 = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene),38 dimethyl-7oxabicyclo[2.2.1]hept-5-ene-exo,exo-2,3-dicarboxylate (M2),39 and 2pyridylmagnesium chloride35 were synthesized as previously reported. The isomeric purity of M2 was confirmed by 1H NMR analysis. Ether solvents were distilled from Na/benzophenone prior to use. Hydrocarbon and chlorinated solvents were purified using a solvent purification system (Innovative Technologies; alumina/copper columns for hydrocarbon solvents). Chlorinated solvents were distilled from CaH2 and degassed via several freeze−pump−thaw cycles. All other solvents and chemicals were purchased from commercial sources and used without further purification. Reactions and manipulations of boron halide species were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inertatmosphere glovebox (MBraun). Fluorinated grease was used for reactions involving boron tribromide. All other procedures were carried out under ambient conditions. Methods. NMR spectra were recorded either on a Bruker Avance III HD NMR spectrometer (Bruker BioSpin, Billerica, MA) operating at a proton frequency of 500.2 MHz and equipped with a 5 mm broadband gradient SmartProbe (Bruker, Billerica, MA), a Varian INOVA 500, or a Varian INOVA 600 spectrometer equipped with a boron-free probe. All NMR spectra were referenced internally to the solvent peaks. The abbreviations Ph (phenylene), NBE (norbornene), s (singlet), d (doublet), t (triplet), vt (virtual triplet), m (multiplet), and br (broad) are used in the signal assignments. High-resolution matrix-assisted laser desorption ionization−mass spectrometry (MALDI-MS) data were obtained on an Apex Ultra 7.0 Hybrid FTMS and MALDI-TOF (time-of-flight) MS data on a Bruker Ultraflextreme. Single crystal X-ray diffraction intensities for M1 were collected on a Smart Apex2 CCD diffractometer at 100 K using Cu Kα (1.541 78 Å) radiation. Details of the X-ray diffraction experiment and selected bond lengths [Å] and angles [deg] are provided in the Supporting Information. Crystallographic data for the structure of M1 have been deposited with the Cambridge Crystallographic Data Center as supplementary publications CCDC 1420073. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ ccdc.cam.ac.uk). GPC analyses with refractive index (RI) detection were performed in DMF (20 mM NH4[PF6], 60 °C, 0.50 mL/min) and using a Waters Empower system equipped with a 717plus autosampler, a 1525 binary HPLC pump, a 2487 dual λ absorbance detector, and a 2414 refractive index detector. Two styragel columns (Polymer Laboratories; one 5 μm Mixed-C and one 10 μm Mixed-B), which were kept in a column heater at 60 °C, were used for separation. The columns were calibrated with narrow poly(methyl methacrylate) (PMMA) standards (Polymer Laboratories, Varian Inc.). DLS measurements were performed at 25.0 ± 0.1 °C with a Malvern Zetasizer Nano-ZS instrument, equipped with a 4 mW, 633 nm He−Ne laser, and an Avalanche photodiode detector at an angle of 173°. SEM and STEM were performed on a Hitachi S-4800 field emission scanning electron microscope (FE-SEM, Hitachi Co. Ltd. S-4800). AFM was performed on a Nanoscope IIIa multimode SPM (Digital Instruments) operated in the “scanasyst” peak force tapping mode and TEM on a FEI Tecnai 12 electron microscope operated at 80 kV. UV−vis absorption data were acquired on a Varian Cary 5000 UV−vis/NIR spectrophotometer. Cyclic voltammetry (CV) experiments were carried out on a CV-50W analyzer from BASi. The three-electrode system consisted of an Au disk as working electrode, a Pt wire as counter electrode, and an Ag wire as a pseudoreference electrode. Synthesis of (4-Bicyclo[2.2.1]hept-5-en-2-yl)phenyl)trimethylsilane (I). Freshly cracked cyclopentadiene (10.0 g, 151 mmol) and trimethyl(4-vinylphenyl)silane (2.00 g, 11.3 mmol) were added to a 25 mL glass vessel equipped with a magnetic stir bar. The reaction mixture was stirred and heated in a microwave reactor at 170 °C for 4 h. Unreacted starting materials were removed by fractional B


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Macromolecules ratio of 80:1) = 33 000 g/mol. Polymerization studies with different initiator/catalyst ratios were performed at room temperature for 3 h using otherwise similar conditions. 1H NMR (499.973 MHz, CDCl3): δ = 8.5 (br m, pyridyl H6), 7.8−7.3 (br m, pyridyl H3 and H4), 7.2 (br m, pyridyl H5), 6.9 (br, Ph), 5.5 and 5.1 (br m, −CHCH−), 3.4− 1.0 (br m, NBE). 11B NMR (160.4 MHz, CDCl3): δ = −11.7. Low-Temperature Polymerization. M1 (10.2 mg, 0.025 mmol) was dissolved in CDCl 3 (0.5 mL) and added to a solution of RuCl2(PyBr)2(IMesH2) (CHPh) (0.27 mg, 0.31 μmol) in CDCl3 (1 mL) at −27 °C. Quantitative conversion of monomer was confirmed by 1H NMR after 10 min. The polymer was precipitated by dropwise addition to hexanes, collected by filtration, and dried under vacuum. For Poly(M1): Mn(GPC) = 29 400 g/mol (Đ = 1.29); Mn (predicted based on monomer/initiator ratio of 80:1) = 33 000 g/mol. Synthesis of Poly(M2-b-M1). M2 (200.5 mg, 0.945 mmol) was dissolved in chloroform (5 mL) and added to a solution of RuCl2(PyBr)2(IMesH2) (CHPh) (7.0 mg, 7.9 μmol) in chloroform (2 mL). The reaction mixture was stirred for 1 h, and a small amount of the mixture was removed for GPC (Mn = 46 100 g/mol, Đ = 1.15) and NMR measurements. A solution of M1 (100.0 mg, 0.241 mmol) in chloroform (3 mL) was added, and the reaction mixture was stirred for an additional 4 h. The polymerization was terminated with ethyl vinyl ether (3−4 drops) and stirred for another 15 min. The polymer was precipitated by dropwise addition to hexanes, collected by filtration, and dried under vacuum. Yield: 201 mg (67%). Poly(M2-bM1): Mn(GPC) = 60 300 g/mol (Đ = 1.16), Mn (predicted based on monomer/initiator ratio of 120:30:1) = 38 000 g/mol. 1H NMR (500.2 MHz, CDCl3): δ = 8.5−8.2 (br m, pyridyl H6), 7.7−7.3 (br m, pyridyl H3 and H4), 7.2−6.7 (br m, pyridyl H5 and Ph), 5.91 and 5.61 (br s, −CHCH− of poly(M2)), 5.08 and 4.73 (br s, −CHO− of poly(M2)), 3.71 (br s, −CH3 of poly(M2)), 3.10 (br s, −CHCO of poly(M2)), 3.0−1.0 (br m, NBE), olefinic signals for poly(M1) not observed. 11B NMR (160.5 MHz, CDCl3): δ = −10.8. AFM Measurement. Poly(M2-b-M1) was dissolved in DMF and drop-cast on a freshly cleaned mica surface. The sample was dried overnight under vacuum prior to AFM measurement. Self-Assembly of Poly(M2-b-M1) in Selective Solvents. A solution of poly(M2-b-M1) (15 mg) in 5 mL of DMF was added into a dialysis tube (Fisherbrand regenerated cellulose membrane with 3500 Da molecular weight cutoff). The dialysis tube was placed in stirring MeOH (PStTpyb shell) or THF (PS shell) for 3 h. The solvent was replaced three times at 3 h intervals with 500 mL of fresh solvent. Aliquots of the aggregate solution were placed in a 1 cm quartz cuvette and analyzed by DLS. Copper mesh grids with a carbon coating were dipped into the respective aggregate solution (diluted 2-fold) and allowed to air-dry prior to STEM and SEM analysis. Hydrolysis of Poly(M2-b-M1). Synthesis of Poly(M3-b-M1). To a solution of poly(M2-b-M1) (15 mg) in 5 mL of DMF was added a solution of NaOH (25 mg) in a mixture of THF and methanol. The resulting polymer solution was stirred for 3 h and then added into a dialysis tube (Fisherbrand regenerated cellulose membrane with 3500 Da molecular weight cutoff). The dialysis tube was placed in stirring water for 3 h. The deionized water was replaced three times at 3−4 h intervals with 500 mL of fresh deionized water. The hydrolysis of the block copolymer was monitored by IR spectroscopy (Figure S26). Aliquots of the aggregate solution (diluted 2-fold) were placed in a quartz cuvette and analyzed by DLS. Copper mesh grids with a carbon coating were dipped into the respective aggregate solution (diluted 2fold) and allowed to air-dry prior to STEM and SEM analysis. pH-Responsive Properties of Poly(M3-b-M1). The self-assembly of poly(M3-b-M1) was studied at acidic and basic pH. Assembly at acidic pH (pH = 1) was accomplished by addition of 20 μL of 6 N HCl solution to 0.40 mL of polymer aggregate solution (∼1.0 mg/mL). Then, 6 N KOH solution was added to the polymer solution until it reached pH = 12. Analysis by DLS, SEM, STEM, and TEM was performed before and after addition of HCl. Complexation of Poly(M2-b-M1) with Fe(II). Formation of Poly(M2-b-M1)-Fe. A solution of FeCl2 (15 mM) and NEt3 (20 μL) in methanol (1 mL) was added to the preformed aggregates of

poly(M2-b-M1) in methanol (0.5 mL). A red precipitate formed, which was collected by centrifugation and analyzed by SEM and EDX. Complexation of M1 with Cu(II). Synthesis of Cu(M1)2. To a solution of M1 (32.5 mg, 0.078 mmol) and triethylamine (25 mg, 0.25 mmol) in methanol (5 mL) was added a solution of Cu(ClO4)2·6H2O (14.5 mg, 0.039 mmol) in MeOH (2 mL). Upon stirring for 2 h, a light green precipitate formed, which was collected by centrifugation, washed with fresh MeOH, and dried under high vacuum. Yield: 28 mg (80%). 1H NMR (500.2 MHz, C6D6): δ = 28 (very br, pyridyl H), 16 (very br, pyridyl H), 7.67 (br, 8H, Ph), 6.21 and 6.13 (NBE, 4H, −CHCH−), 3.97 (br, 2H, NBE H-5), 3.16 (br, 2H, NBE H-4), 2.85 (br, 2H, NBE H-1), 2.15 (br, 2H, NBE H-6), 1.71 and 1.61 (br, NBE H-7), 1.36 (br, NBE H-6), two pyridyl-H protons were not observed due to excessive paramagnetic peak broadening. 11B NMR (192.4 MHz, C6D6): δ = −26.8. MALDI-TOF MS (pos. mode, anthracene): m/z = 892.5738 (calcd for MH+, 12C561H5011B214N6 = 892.3651). Complexation of M1 with Fe(II). Synthesis of Fe(M1)2. To a solution of M1 (20.3 mg, 0.049 mmol) and triethylamine (25 mg, 0.25 mmol) in THF (5 mL) was added a solution of FeCl2 (8.2 mg, 0.065 mmol) in THF (2 mL). The resultant red solution was kept stirring for 5 h. The reaction mixture was passed through a small plug of alumina gel using THF as the eluent. After evaporation of the solvents the product was obtained as a red solid. The crude product was washed three times with MeOH and dried under high vacuum. Yield: 18.2 mg (84%). 1H NMR (500.2 MHz, THF-d8): δ = 7.93 (d, J = 8 Hz, 4H, Ph), 7.54 (d, J = 8 Hz, 6H, pyridyl H), 7.27 (m, 10H, pyridyl H and Ph), 7.13 (br, 6H, pyridyl H), 6.46 (t, 6H, J = 6 Hz, pyridyl H), 6.30 (m, 2H, NBE H-2, −CHCH−), 5.98 (m, 2H, NBE H-3, −CH CH−), 3.56 (overlapped, 2H, NBE H-5), 3.21 (s, 2H, NBE H-4), 2.99 (s, 2H, NBE H-1), 2.30 (m, 2H, NBE H-6), 1.65−1.45 (overlapped, 6H, NBE H-6,7). 13C NMR (125.718 MHz, THF-d8): δ = 184.8 (m, pyridyl), 156.2 (pyridyl), 149.7 (m, Ph), 140.6 (Ph), 136.5 (NBE), 136.2 (Ph), 133.7 (pyridyl), 133.1 (NBE), 127.1 (Ph), 122.4 (pyridyl), 117.4 (pyridyl), 50.1 (NBE), 48.9 (NBE), 43.5 (NBE), 43.4 (NBE), 32.7 (NBE). 11B NMR (160.5 MHz, THF-d8): δ = −7.5. MALDI-TOF MS (anthracene): m/z = 884.5195 (calcd for M+, 12C561H5011B214N6 = 884.3646). Exchange of Cu(II) with Fe(II) in Cu(M1)2. To a solution of complex Cu(M1)2 (18 mg, 0.020 mmol) in THF (5 mL) was added a suspension of FeCl2 (5.0 mg, 0.039 mmol) in THF (2 mL). A color change from murky-green to orange-red occurred upon stirring for 4 h. The solvent was evaporated under high vacuum, and the residue was redissolved in toluene, passed through a small pad of alumina gel, and once again dried under vacuum. Yield: 10 mg (56%). The NMR and MS data were identical to those of complex Fe(M1)2 prepared by reaction of M1 with FeCl2. Complexation of Poly(M1) with Cu(II) and Exchange of Cu(II) with Fe(II). Poly(M1)-Cu was prepared by following the procedure for Cu(M1)2 using poly(M1) (20.0 mg), Cu(ClO4)2·6H2O (17.5 mg, 0.047 mmol), and triethylamine (30 μL, 0.21 mmol). Yield: 15 mg (71%). Poly(M1)-Cu (15.0 mg) was then suspended in a solvent mixture of THF/MeOH (5 mL), and a solution of FeCl2 (10 mg, 0.079 mmol) in methanol (2 mL) was added. Upon stirring for 2 days, the color of the suspension changed from light green to grayish yellow. The product was thoroughly washed with MeOH, THF, and CH2Cl2 and studied by EDX spectroscopy.

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RESULTS AND DISCUSSION The synthesis of the borate monomer M1 is outlined in Scheme 1. First, the silyl-functionalized norbornene I was prepared by Diels−Alder reaction of cyclopentadiene and trimethyl(4-vinylphenyl)silane. Unreacted starting materials were removed by fractional distillation, leaving behind compound I as a viscous oil that consisted of a mixture of 76% endo and 24% exo-product according to integration of the olefinic resonances in the 1H NMR spectrum. The endo:exo ratio is typical of Diels−Alder reactions between styrene and cyclopentadiene.40 Compound I was then subjected to a C


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Macromolecules


Scheme 1. Synthesis and Homopolymerization of M1 (Terminated with Vinyl Ethyl Ether)

Scheme 2. Synthesis of Block Copolymer Poly(M2-b-M1) (Terminated with Vinyl Ethyl Ether)

was polymerized with Grubbs third-generation catalyst (0.83 mol %) in chloroform at room temperature. A small amount of the reaction mixture was withdrawn for GPC and 1H NMR measurements. Complete conversion of M2 was verified by the disappearance of the olefinic resonances in the 1H NMR spectra, and the molecular weight of the homopolymer was determined by GPC to be Mn = 46 100 (Đ = 1.15). Monomer M1 was then added and the reaction mixture was stirred for 3 h. Precipitation into hexanes gave the desired block copolymer in 67% isolated yield. 1H NMR analysis of poly(M2-b-M1) showed broad resonances for both the poly(M2) and poly(M1) block (Figure S18). A resonance at −10.8 ppm in the 11B NMR spectrum corroborates the presence of the tetracoordinated boron-functionalized block (Figure S19). The GPC traces of the homopolymer (Mn = 46 100, Đ = 1.15) and block copolymer (Mn = 60 300, Đ = 1.16) are compared in Figure 1;

boron/silicon exchange reaction41,42 with BBr3 in toluene, and the borylated product (II) was directly added to a solution of 2pyridylmagnesium chloride in toluene. Workup by extraction into aqueous HCl, followed by neutralization, re-extraction into CH2Cl2, and subsequent chromatography on alumina gel, gave M1 in ca. 20% yield over two steps. The monomer was further purified by recrystallization from a mixture of CH2Cl2/hexanes at low temperature and then fully characterized by highresolution MALDI-MS and multinuclear NMR spectroscopy (Figures S2−S7). An 11B NMR resonance at a chemical shift of −10.8 ppm confirmed the presence of tetracoordinate boron. Correlation spectroscopy (H,H-COSY) and nuclear Overhauser effect spectroscopy (H,H-NOESY) allowed for complete assignment of the 1H NMR signals, and integration of the olefinic resonances indicated that M1 consists of >97% endoisomer (Figures S2−S4).43 The structure was further confirmed by a single crystal X-ray analysis (Figure S8). The homopolymerization of M1 was carried out with 1.25 mol % Grubbs third-generation catalyst in chloroform at room temperature for 14 h. The polymerization was terminated by the addition of 1 mL of ethyl vinyl ether. The product, poly(M1), was isolated as a white powder in 62% yield. The molecular weight was determined by GPC in DMF containing 20 mM NH4PF6 at 60 °C. The number-average molecular weight and dispersity index of poly(M1) relative to narrow PMMA standards were 28 200 g/mol and Đ = 1.33, respectively, suggesting that M1 could be polymerized in a controlled manner via ROMP. The “living” character of the polymerization was further confirmed by varying the initial monomer/catalyst ratio (catalyst/monomer = 1/9.4, 1/11.5, 1/ 14.4, 1/19.1, and 1/28.7). As seen in Figure S9, the molecular weight of the polymer increased with an increase in the initial catalyst/monomer ratio, which demonstrates that well-defined Tpyb-functionalized polymers with targeted molecular weight can be prepared via ROMP. Further investigations showed that complete monomer conversion can be achieved within less than 10 min based on the disappearance of the vinyl resonances (Figure S10), even when the polymerization is carried out at −27 °C. However, the dispersity of the product was only slightly lower (Đ = 1.29).44 Encouraged by these results, we pursued the synthesis of a diblock copolymer from M1 and dimethyl-7-oxabicyclo[2.2.1]hept-5-ene-exoexo-2,3-dicarboxylate (M2). First, monomer M2

Figure 1. GPC RI traces of poly(M2) (blue) and poly(M2-b-M1) (black).

they clearly confirm chain extension with formation of the block copolymer. Based on the GPC results, the degree of polymerization for the constituent blocks is DPn(M2) = 217 and DPn(M1) = 34. The molecular weights based on GPC are generally higher than those expected based on the ratio of monomer to catalyst ([M2]/[M1]/[cat.] = 120/30/1), which is likely due in part to structural differences between the D


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gives smaller particles with a Dh of ca. 28 nm, which contain the shorter pyridylborate block at the core. The blocky nature of poly(M2-b-M1) was also studied by 1H NMR spectroscopy in CD3OD and THF-d8 (Figures S20 and S21). As expected, only the Tpyb-functionalized block of poly(M2-b-M1) was detected in CD3OD. However, in the case of THF-d8, 1H NMR signals of both blocks were observed, which suggests that the micelle core is partially solvated, possibly due to the shorter block length or the formation of Hbonds between the THF solvent and the acidic N−H proton. The morphology of the polymer aggregates was further studied by SEM, STEM, and AFM (Figure 4). All these methods reveal the formation of regular spherical nanoparticles and confirm the smaller particle size in THF in comparison to MeOH. To convert poly(M2-b-M1) into an amphiphilic block copolymer that is amenable to self-assembly in water, the ester block was hydrolyzed with NaOH. The hydrolysis process with generation of poly(M3-b-M1) was monitored by infrared spectroscopy, which confirmed the disappearance of the CO stretch at 1725 cm−1 for the ester groups (Figure S26). After dialysis with deionized water, DLS analysis of poly(M3-b-M1) revealed a number-average Dh of 28 nm (Figure 3b), which is consistent with the data obtained for poly(M2-b-M1) in THF prior to hydrolysis and indicates that the Tpyb-functionalized block forms the core region of the aggregates. SEM, TEM, and STEM measurements further confirmed the aqueous selfassembly of the block copolymer into well-defined spherical micelles (Figure S28). Interestingly, the amphiphilic block copolymer poly(M3-bM1) shows pH-responsive properties that result in reversible core−shell interconversion. As illustrated in Figure 5, at strongly acidic pH, the tris(2-pyridyl)borate groups are protonated, rendering the poly(M1) block hydrophilic, whereas the protonated poly(M3) block becomes hydrophobic. Thus, at low pH the block copolymer is expected to form reverse micelles with poly(M3) at the core and poly(M1) in the corona. DLS, TEM, SEM, and STEM techniques were used to monitor this pH-dependent self-assembly of the polymer. As shown in Figure 5, the size of the aggregates increases dramatically from 28 to 330 nm upon acidification. We also note that evidence of further agglomeration was found under acidic conditions (Figures S29 and S30). This process is reversible, as addition of base regenerates poly(M1) as the hydrophobic and poly(M3) as the hydrophilic block. Multiresponsive polymers of this type that can undergo “flip-flop” micellization upon changes in the solution pH are of significant current interest with respect to the development of new smart materials and drug delivery vehicles.46−48 Tris(2-pyridyl)borate (Tpyb) ligands9,36,37 are also promising building blocks in metallo-supramolecular chemistry because of their excellent chemical stability and high binding affinity toward various metal ions. To verify the metal ion complexation behavior, we first prepared the molecular complexes Cu(M1)2 and Fe(M1)2 by treating M1 with Cu(ClO4)2·6H2O and FeCl2, respectively, in the presence of Et3N. Both complexes were fully characterized by multinuclear NMR, MALDI-TOF mass spectrometry, and UV−vis spectroscopy (Figures S31−S37). The molecular weight data from MALDI-TOF MS measurements indicated the attachment of two ligands, which is consistent with our prior observation that the pyridylborate ligands readily form neutral octahedral 2:1 complexes in the presence of M(II) ions.36,37 Sharp signals in the 1H and 13C NMR spectra clearly demonstrated that


norbornene-type polymers and the PMMA standards. We also studied a reverse block copolymerization protocol in which monomer M1 was polymerized first with Grubbs thirdgeneration catalyst for 2.5 h at room temperature in chloroform, and then monomer M2 was added to the “living” poly(M1). Block copolymer formation was confirmed by GPC analysis (Figure S22), but we observed a small amount of residual unreacted homopolymer, presumably due to side reactions in the later stage of the polymerization of M1 that result in loss of the reactive chain end.45 Because of the bimodal distribution, we did not use this block copolymer for further studies. AFM was used to investigate the morphology of poly(M2-bM1) as a thin film cast from DMF solution (2.0 mg/mL). As shown in Figure 2, microphase separation of the blocks is

Figure 2. AFM micrograph (peak force tapping mode) of poly(M2-bM1).

clearly observed. The height image of the sample showed the width and height of the larger features to be in the order of 25− 35 nm and 1.25−2.25 nm, respectively. Based on the different solubility characteristics of the individual blocks, poly(M2-b-M1) is also expected to undergo self-assembly in selective solvents. Methanol is a good solvent for poly(M1) but a poor solvent for the hydrophobic block poly(M2). Conversely, tetrahydrofuran is a good solvent for poly(M2) but a poor solvent for poly(M1). To study the selfassembly, poly(M2-b-M1) was dissolved in DMF as a common solvent and then dialyzed with the respective selective solvent, THF or methanol. According to DLS measurements (Figure 3a), dialysis with MeOH results in particles with a numberaverage hydrodynamic diameter (Dh) of 45 nm. These aggregates are expected to feature the oxanorbornene diester block in the core, surrounded by a solubilizing Tpybfunctionalized shell. Conversely, dialysis with tetrahydrofuran

Figure 3. Aggregate size distribution of (a) poly(M2-b-M1) in THF and CH3OH and (b) poly(M3-b-M1) at varying pH. E


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Figure 4. Illustration of the self-assembly of poly(M2-b-M1) in block-selective solvents. From left to right: photographs of dialyzed solutions, SEM, STEM, and AFM images.

Figure 5. Illustration of the self-assembly of poly(M3-b-M1) in acidic and basic aqueous medium. From left to right: photographs of acidic and basic solutions and SEM and TEM images.

were treated with a solution of FeCl2 in methanol, followed by addition of NEt3 as a base. A red precipitate formed, which proved to be insoluble in all common organic solvents, indicating successful complexation with Fe(II) and crosslinking of the polymer aggregates. This is consistent with our prior studies with molecular pyridylborate ligands, which showed that complexation to Fe(II) always results in the formation of octahedral 2:1 complexes, even when the metal ion is in large excess.36,37 SEM analysis of the precipitate (Figure 6) provided evidence for formation of an extended

Fe(M1)2 is diamagnetic in solution, which is consistent with a low-spin d6 configuration. In contrast, the 1H NMR spectrum of the d9 complex Cu(M1)2 showed very broad and strongly shifted resonances, especially for the pyridyl groups, due to the paramagnetism of Cu(II). Similarly, the 11B NMR spectrum of Fe(M1)2 displayed a sharp peak at −7.5 ppm, which is in the same range as that of M1 (−10.8 ppm), whereas the signal for Cu(M1)2 experienced a strong paramagnetic shift and appeared at −26.8 ppm. The UV−vis absorption spectrum for the dark red complex Fe(M1)2 in CH2Cl2 revealed intense bands with maxima at ca. 480 and 425 (sh) nm, which can be assigned to metal-to-ligand charge transfer.36,37,49 A very broad band with a maximum at ca. 600 nm was observed for the purple complex Cu(M1)2. This band is due to the 2T2g ← 2Eg transition of the d9 complex ion and appears at slightly higher energy than for (HB(pz)3)2Cu (620 nm),50 which is consistent with a very strong ligand field of M1. The redox properties were studied by cyclic voltammetry in CH2Cl2 using [Bu4N]PF6 as the electrolyte (Figure S38). The complex Fe(M1)2 exhibited a redox potential of −350 mV vs Fc/Fc+, which is attributed to the Fe(II)/Fe(III) redox couple and indicates the facile oxidation of the complex. For Cu(M1)2 a partially reversible redox wave with a potential of −650 mV vs Fc/Fc+ can be assigned to reduction of Cu(II) to Cu(I). To study the metal ion complexation properties of the polymer, aggregate solutions of poly(M2-b-M1) in methanol

Figure 6. SEM image of Fe(II) cross-linked block copolymer aggregates. F


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copolymer, poly(M2-b-M1), forms well-defined spherical particles in THF and MeOH as block-selective solvents. Hydrolysis of the ester functionalities in the poly(M2) block with NaOH resulted in a multiresponsive block copolymer, poly(M3-b-M1), that self-assembles in water and is responsive to changes in pH. Aggregates with the Tpyb block in the core or shell can be reversibly produced by variation of the pH. The facile and reversible protonation that is described is unique to this ligand class as protonation is facilitated by the negative charge of the borate ligand, enabling the application of polyfunctional pyridyl ligands to multiresponsive pH-dependent self-assembly. In addition, cross-linking of the poly(M2-bM1) aggregates by metal ion complexation was explored, and the possibility of metal ion exchange was demonstrated. Different from the more commonly employed class of terperpyridyl ligands, the complexation with M(II) ions results in neutral (counterion-free), metal-rich cross-linked polymer networks. On the basis of these findings, we envision potential future applications of these types of functional block copolymers in areas ranging from pH-controlled drug release to polymer-based metal ion remediation systems.

nanoparticle network as a results of Fe(II) cross-linking. The presence of Fe(II) was further confirmed by EDX measurements (Figure S23). Finally, we explored the propensity of the polymer-supported Tpyb ligands to undergo metal exchange reactions. In a proofof-principle study, we investigated the exchange of Cu(II) with Fe(II) ions in the polymer networks. The ligands in the Jahn− Teller d9 complex Cu(M1)2 are much more weakly bound than in the low-spin d6 complex Fe(M1)2; therefore, Cu(II) should be readily replaced by addition of Fe(II). Indeed, treatment of a solution of Cu(M1)2 in THF with a suspension of FeCl2 in THF led to a color change from murky green to orange-red, indicating the successful transfer of the Tpyb ligands from Cu(II) to Fe(II). The metal ion exchange was further confirmed by UV−vis spectroscopy (Figure S37), MALDITOF MS, and multinuclear NMR measurements. We then studied the ability of Cu(II) cross-linked poly(M1) to undergo similar metal exchange reactions (Figure 7). Treatment of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01216. 1 H NMR data as well as SEM and STEM images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.J.). *E-mail: [email protected] (J.B.S.). Notes

Figure 7. Exchange of Cu(II) for Fe(II) in metal-complexed poly(M1) polymer network. Photographs of polymer before and after treatment with FeCl2.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation under Grant CHE-956655. The SEM (MRI 1039828) and one of the NMR instruments (MRI 1229030) used in these studies were acquired with partial support from the National Science Foundation. We are grateful to Dr. Huixin He, Amir Khoshi, and Mehul Patel for help with the acquisition of some of the AFM data, to Markos Papadakis for the initial synthesis of I, and to Roman Brukh for assistance with the acquisition of SEM data.

poly(M1) with Cu(ClO4)2·6H2O in MeOH resulted in a green precipitate, attributed to the Cu(II) cross-linked polymer network (Figure S24), which was separated by centrifugation and thoroughly washed with fresh MeOH. Then, the metal exchange was attempted by adding a solution of FeCl2 in methanol to a suspension of the cross-linked polymer in a solvent mixture of THF/MeOH. The green solid slowly changed color to light beige upon stirring for 2 days. The product was thoroughly washed with MeOH, THF, and CH2Cl2 and then studied by EDX spectroscopy. The predominant occurrence of the iron peaks in the EDX spectrum, and the color change indicated successful exchange of metal ions (Figure S25).

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ABBREVIATIONS ROMP, ring-opening metathesis polymerization; Tpyb, tris(2pyridyl)borate; NBE, norbornene; TLC, thin layer chromatography.

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CONCLUSIONS In conclusion, using Grubbs third-generation catalyst, we accomplished the controlled ROMP of a pyridylboratefunctionalized norbornene monomer, M1, to give the corresponding tripodal chelate ligand-functionalized polymer. Using similar methods, an amphiphilic pyridylborate block copolymer was obtained via sequential ROMP of the oxanorbornene diester monomers M2 and M1. DLS, SEM, STEM, and AFM studies showed that the resulting block

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ROMP-Derived Pyridylborate Block Copolymers: Self-Assembly, pH

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