Article pubs.acs.org/Macromolecules
Precise Synthesis of Clickable Poly(n-hexyl isocyanate) Toshifumi Satoh,†,* Ryosuke Ihara,‡ Daisuke Kawato,§ Naoki Nishikawa,§ Daichi Suemasa,§ Yohei Kondo,§ Keita Fuchise,‡ Ryosuke Sakai,∥ and Toyoji Kakuchi† †
Division of Biotechnology and Macromolecular Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan Division of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan § Biological Chemistry and Engineering Course, Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, Japan ∥ Department of Material Chemistry, Asahikawa National College of Technology, Asahikawa 071-8142, Japan ‡
S Supporting Information *
ABSTRACT: Well-defined clickable poly(n-hexyl isocyanate) (PHIC) with an azido or alkyne end group as rod-like building block has been prepared by the living coordination polymerization using organotitanium catalysts. The obtained clickable PHIC was subjected to the Cu-catalyzed azide/alkyne cycloaddition for the synthesis of well-defined macromolecular architectures based on PHIC as a rod polymer, e.g., PHIC-bpoly(N-isopropyl acrylamide) as rod−coil block copolymer and 4-arm star PHIC as a rod star polymer. Additionally, PHIC-b-poly(L-lactide) and PHIC-b-poly(ε-caprolactone) as novel rod−coil block copolymers were respectively synthesized by the ring-opening polymerizations of L-lactide and εcaprolactone initiated from hydroxyl end-functionalized PHIC. All products were characterized by 1H NMR spectroscopy, MALDI−TOF mass spectrometry, and size exclusion chromatography equipped with multiangle laser light scattering and viscosity detectors.
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star (co)polymer8 including PHIC as a rod segment have been synthesized and characterized. For example, Lee et al. have reported the precise synthesis and the self-organization behavior of the rod−coil and rod−coil−rod block copolymers consisting of PHIC and poly(2-vinylpyridine).2a,6g In addition, Hadjichristidis et al. have reported well-defined complex macromolecular architectures based on PHICs such as polybutadiene-g-PHIC, polystyrene(PS)-b-(polyisoprene-gPHIC), and PS(PHIC)2 three miktoarm star copolymer.7a Although the macromolecular architectures including PHIC as a rod segment have a range of potential to exhibit unique properties, these synthetic methods still have limitations of the range of monomers; therefore, expansion of the synthetic methods for the macromolecular architectures based on PHIC is needed. In order to expand the synthetic methods for welldefined PHIC architectures, we suggest a postpolymerization strategy using the combination of organotitanium-catalyzed living coordination polymerization and Cu-catalyzed azide/ alkyne cycloaddition (CuAAC). In the present work, novel clickable PHICs (PHIC−N3, PHIC−CC, and PHIC−CC*) with an azido or alkyne end group have been precisely prepared by living coordination
INTRODUCTION Polyisocyanates are known to be typical rigid-rod or semiflexible polymers with a dynamic helical conformation in solution as well as in the solid state because of the combination of the steric repulsions between side groups and the doublebond character of consecutive amide bonds. Due to their unique structural properties, polyisocyanates have been widely studied in various fields such as helicity induction,1 liquid crystals,1c,s,2 and optical switches.1c,p,q,3 For examples, the optically active poly(alkyl isocyanate)s with a predominantly one-handed helical sense have been synthesized by the copolymerization of an achiral isocyanate and a small amount of an optically active isocyanate.1w In addition, the control over the preferred helical sense of a poly(n-hexyl isocyanate) (PHIC) by using a single light-driven molecular motor, covalently attached at the polymer end, has been recently accomplished in solution via a combination of photochemical and thermal isomerizations.1c Among the polyisocyanates, PHIC is one of the most studied and characterized polymer. Generally, the well-defined PHIC with a narrow polydispersity index can be synthesized via organotitanium-catalyzed living coordination polymerization4 and living anionic polymerization with sodium naphthalenide as an initiator using additives such as 15-crown-5 and sodium tetraphenylborate.5 Using these polymerizations, various types of the well-defined block copolymer,6 graft copolymer,1b,7 and © 2012 American Chemical Society
Received: March 19, 2012 Revised: April 16, 2012 Published: April 24, 2012 3677
dx.doi.org/10.1021/ma300555v | Macromolecules 2012, 45, 3677−3686
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Deionized water was used for thermoresponsivity measurement. All other reagents not mentioned were of synthetic grade and used without further purification. Instruments. The 1H NMR spectra were recorded using JEOL JNM-A400II instruments. Polymerization was carried out in an MBRAUN stainless steel glovebox equipped with a gas purification system (molecular sieves and copper catalyst) in a dry argon atmosphere (H2O, O2 99.5%; Kanto Chemical Co., Inc., (Kanto)) was distilled over sodium benzophenone ketyl under an argon atmosphere. Dichloromethane (CH2Cl2; Kanto), n-hexyl isocyanate (HIC; Tokyo Kasei Kogyo Co., Inc., (TCI)), εcaprolactone (ε-CL; TCI), propargyl alcohol (TCI), and N,N,N′,N″,N″-pentamethyldiethylene triamine (PMDETA; >98.0%, TCI) were distilled over calcium hydride (CaH2) under reduced pressure. L-Lactide (L-LA; TCI) was recrystallized from dry toluene twice. N,N-Dimethylcyclohexylamine (NCyMe2; Kanto) was dried over molecular sieve 3A and distilled under an argon atmosphere. (R)3-Butyn-2-ol (Kanto) was distilled before use ([α]20D, +47° (neat)). Trichloro(cyclopentadienyl)titanium(IV) (CpTiCl3; Kanto), acetic anhydride (Kanto), boron trifluoride diethyl etherate (BF3·OEt2; TCI), copper chloride(I) (CuCl; Sigma Aldrich Chemical Co., 99.995%), pentaerythritol (TCI), dry tetrahydrofuran (THF; Kanto), methanol (MeOH; Kanto), 4-dimethylaminopyridine (DMAP; Wako Chemical Co., 99+ %), benzoic acid (Kanto), and diphenylphosphate (DPP; TCI) were used as received. 6-Azido-1-hexanol,9 1-cyclohexyl3-(3,5-bistrifluoromethylphenyl) thiourea (CBT),10 azido end-functionalized poly(N-isopropylacrylamide) (PNIPAM−N3),11 and 5hexynoyl chloride12 were synthesized using previously reported techniques. N-Isopropylacrylamide (NIPAM; Kohjin Co.) was recrystallized twice from hexane/toluene (10/1, v/v) prior to use. 3678
dx.doi.org/10.1021/ma300555v | Macromolecules 2012, 45, 3677−3686
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3.36−4.06 (br, NCH2), 3.29 (t, −CH2N3), 2.28 (s, −C(O)CH3), 1.62 (br s, −NCH2CH2(CH2)3CH3), 1.29 (br s, −NCH2CH2(CH2)3CH3), and 0.88 (br s, −CH2CH3). FT-IR: 2958 (m), 2929 (m), 2098 (w), 1699 (s), 1462 (w), 1349 (m), 1246 (m), 1227 (m), 1287 (b), and 753 cm−1 (b). Synthesis of Propynyl End-Functionalized Poly(n-hexyl isocyanate) (PHIC−CC). The organotitanium catalyst 2 was synthesized according to the method of Novak et al.,4 as shown in Scheme S1 (see Supporting Information). To a 50-mL Schlenk flask, CpTiCl3 (518 mg, 2.36 mmol) was added, followed by addition of dry CH2Cl2 (1.5 mL) and stirring of the mixture to achieve homogeneity. After propargyl alcohol (133 mg, 2.36 mmol) was introduced, the reaction was carried out at room temperature for 3 h, and the solvent was then removed under vacuum overnight. HIC (3.00 g, 23.6 mmol) was added, and the flask was sealed off. The reaction was carried out for 24 h at 0 °C to afford a PHIC living chain as a soft solid material. Termination was achieved by the addition of a large excess of acetic anhydride (56.0 mL, 594 mmol) and BF3·OEt2 (2.20 mL, 17.8 mmol) and overnight reaction.13 The polymer was precipitated in cold MeOH and then precipitated twice using THF/MeOH. Finally, the polymer was filtered and dried under vacuum to afford a white powder: yield, 2.58 g, 85.9%; Mn,NMR, 3910 g·mol−1; Mw,SEC‑MALS (Mw/Mn,) in THF, 3880 g·mol−1 (1.06); dn·dc−1, 0.084; Mn,SEC (Mw/Mn) in THF, 4870 g·mol−1 (1.06). 1H NMR (CDCl3, 400 MHz), δ (ppm): 4.78 (br s, −OCH2CCH), 3.10−4.20 (br, −NCH2), 2.54 (s, −CCH), 2.28 (s, −C(O)CH3), 1.62 (br s, −NCH2CH2(CH2)3CH3), 1.29 (br s, −NCH2CH2(CH2)3CH3), and 0.88 (br s, −CH2CH3). Synthesis of (R)-Butynyl End-Functionalized Poly(n-hexyl isocyanate) (PHIC−CC*). The organotitanium catalyst 3 was synthesized according to the method of Novak et al.,4 as shown in Scheme S1 (see Supporting Information). To a 50-mL Schlenk flask, CpTiCl3 (78.4 mg, 0.357 mmol) was added, followed by addition of dry CH2Cl2 (2 mL), and stirring to achieve homogeneity. After a CH2Cl2 stock solution of (R)-3-butyn-2-ol (117 μL, 0.363 mmol, 3.10 mol·L−1 in CH2Cl2) was introduced, the reaction was carried out at room temperature for 3 h, and the solvent was then removed under vacuum overnight. HIC (800 mg, 6.29 mmol) was added and the flask was sealed off. The reaction was carried out for 24 h at 0 °C to afford a PHIC living chain as a soft solid material. Termination was achieved by the addition of a large excess of acetic anhydride (5.1 mL, 540 mmol) and BF3·OEt2 (0.66 mL, 5.35 mmol) and overnight reaction.13 The polymer was precipitated in cold MeOH and then precipitated twice using THF/MeOH. Finally, the polymer was filtered and dried under vacuum to afford a white powder: yield, 776 mg, 89.3%; Mn,NMR, 6270 g·mol−1; Mw,SEC‑MALS (Mw/Mn,) in THF, 6270 g·mol−1 (1.05); dn·dc−1, 0.084; Mn,SEC (Mw/Mn) in THF, 6810 g·mol−1 (1.13). 1H NMR (CDCl3, 400 MHz), δ (ppm): 5.44 (q, −C(O)OCH(CH3) −), 3.30−4.10 (br, −NCH2−), 2.52 (s, −CCH), 2.28 (s, 3H, −C(O)CH3), 1.62 (br s, −NCH2CH2(CH2)3CH3), 1.29 (br s, −NCH2CH2(CH2)3CH3), and 0.88 (br s, 150H, CH2CH3). Synthesis of Hydroxyl End-Functionalized PHIC (PHIC−OH). To a 50-mL Schlenk flask, PHIC−N3 (800 mg, 0.143 mmol, Mn,NMR = 5,100 g mol−1, 1.0 equiv) and CuCl (43.4 mg, 0.429 mmol, 3.0 equiv) were added and dried under vacuum at room temperature overnight. The solution of PMDETA (179 μL, 0.857 mmol, 6.0 equiv), propargyl alcohol (29.5 μL, 0.500 mmol, 3.5 equiv), and THF (6.70 mL) was bubbling in an argon atmosphere for 15 min and was then introduced using a cannula. The reaction was allowed to proceed at room temperature for 48 h and was terminated by bubbling in air. The reaction mixture was purified by silica gel flash chromatography (THF) to remove the copper catalyst. The polymer was isolated by reprecipitation from THF into cold methanol. Finally, the polymer was filtered and dried under vacuum to afford a light yellow powder: yield, 745 mg, 91.5%; Mn,NMR, 5200 g·mol−1; Mw,SEC‑MALS (Mw/Mn,) in THF, 5200 g·mol−1 (1.06); dn·dc−1, 0.098; Mn,SEC (Mw/Mn) in THF, 6900 g·mol−1 (1.11). 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.52 (s, triazole ring), 4.81 (d, CH2OH), 4.37 (t, −CH2-triazole ring), 4.19 (br s, −C(O)OCH2−), 3.66 (br s, −NCH2−), 2.28 (s, −C(O)CH3), 1.62 (br s, −NCH2CH2(CH2)3CH3), 1.29 (br s, −NCH2CH2(CH2)3CH3), and 0.88 (br s, −CH2CH3). FT-IR: 2957
(m), 2930 (m), 1699 (s), 1459 (w), 1348 (m), 1245 (m), 1180 (m), and 758 cm−1 (b). Synthesis of PHIC-b-PLLA. A typical procedure for the polymerization is as follows:10 In the glovebox, PHIC−OH (100 mg, 19.0 μmol, Mn,NMR = 5,250, 1.0 equiv) was added to L-lactide (149 mg, 1.04 mmol, 55 equiv), NCyMe2 (23.0 μL, 156 μmol, 8.25 equiv), and CBT (38.4 mg, 104 μmol, 5.5 equiv) in CH2Cl2 at 27 °C. After 72 h, benzoic acid (3.46 mg, 28.0 μmol, 1.5 equiv) was added to quench the polymerization, and the polymer was precipitated in 100 mL of MeOH. The polymer was isolated by reprecipitation from THF in cold MeOH/n-hexane (v/v = 9/1): yield, 185 mg, 74.1%; Mn,NMR, 13 300 g·mol−1; Mw/Mn (SEC) in THF, 1.08. 1H NMR (CDCl3, 400 MHz), δ (ppm): 7.52 (s, triazole ring), 5.28 (s, triazole ring−CH2O), 5.19 (q, −C(O)CH(CH3)O−), 4.37 (t, triazole ring−CH2), 4.19 (s, −C(O)OCH2−), 3.66 (br. s, −NCH2−), 2.28 (s, −C(O)CH3), 1.62 (m, −(CH2)3CH3, −OCH2(CH2)4−), 1.29 (s, −NCH2CH2−), and 0.88 (s, −CH2CH3). Synthesis of PHIC-b-PCL. A typical procedure for the polymerization is as follows:14 In the glovebox, ε-CL (0.15 mL, 1.38 mmol) was added to PHIC−OH (112.3 mg, 2.77 × 10−2 mmol, Mn,NMR = 4060 g·mol−1) in dry toluene at 27 °C. A toluene stock solution of DPP (27.7 μL, 2.77 × 10−2 mmol) was then added to the solution to initiate the polymerization under an argon atmosphere. After 8 h, the polymerization was quenched by the addition of Amberlyst A21. The polymer was isolated by reprecipitation from CH2Cl2 in cold MeOH: yield, 170 mg (63.0%); Mn,NMR, 9220 g·mol−1; Mw/Mn (SEC) in THF, 1.07. 1H NMR (CDCl3, 400 MHz), δ (ppm): 7.60 (s, triazole ring), 5.25 (s, CH2OH), 4.35 (s, triazole ring−CH2−), 4.10 (t, −OCH2−), 3.95−3.45 (br, −NCH2-), 2.31 (t, −C(O)CH2− and −C( O)CH3)), 1.80−1.50 (br, −NCH2CH2(CH2)3CH3 and −C(O)CH2CH2CH2CH2CH2O−), 1.50−1.30 (m, −C(O)CH2CH2CH2CH2CH2O−), 1.29 (br s, −NCH2CH2(CH2)3CH3), and 0.88 (br s, −CH2CH3). Synthesis of Pentaerythrityl Tetra-5-hexynoate (4). Under a nitrogen atmosphere, 5-hexynoyl chloride (1.00 g, 7.66 mmol) was added to a solution of pentaerythritol (220 mg, 1.62 mmol) and DMAP (786 g, 6.43 mmol) in dry CH2Cl2 (18 mL). The reaction mixture was stirred at ambient temperature for 7 h. The reaction mixture was then filtered and the solvent evaporated. The resulting crude residue was then purified via chromatography on a silica gel column using a hexane/ethyl acetate solution (3/1, v/v, Rf = 0.26) as eluent. The solvent was removed by rotary evaporation, and the final product was dried under vacuum overnight to yield 4: yield, 0.410 mg, 50.0%. 1H NMR (DMSO-d6, 400 MHz), δ (ppm): 1.69 (tt, J = 7.3 Hz, J = 14.5 Hz, (−CH2CH2CH2−, 8H), 2.19 (dt, J = 7.1 Hz, J = 2.6 Hz, (−CH2CCH, 8H), 2.41 (t, J = 7.5 Hz, −COCH2−, 8H), 2.80 (t, J = 2.7 Hz, −CH−, 4H), and 4.09 (s, −CH2O−, 8H). 13C NMR (DMSOd 6 , 100 MHz): δ (ppm) 17.01 (−CH 2 CCH), 23.35 (−CH2CH2CH2−), 32.32 (−COCH2−), 41.73 (−C(CH2)4−), 62.09 (−CH2O−), 71.71 (−CH−), 83.53 (-CCH), and 171.94 (−CO). Anal. Calcd for C29H36O8 (512.59): C, 67.95; H, 7.08. Found: C, 67.69; H, 7.21. Synthesis of PHIC-Star. Compound 4 (5.91 mg, 1.15 × 10−5 mol, 0.12 equiv), PHIC−N3 (Mn,NMR = 4000 g·mol−1, 400 mg, 1.00 × 10−4 mol, 1 equiv), and CuCl (26.3 mg, 2.66 × 10−4 mol, 2.7 equiv) were stirred in dry THF (3.0 mL) under an argon atmosphere. Into the reaction mixture was added PMDETA (92.2 mg, 5.32 × 10−4 mol, 5.4 equiv) with stirring at room temperature for 89 h. The reaction mixture was exposed to air, diluted with THF, and passed through a silica gel column to remove the copper catalyst. The solvent was removed by rotary evaporation, and the residue was purified by the preparative SEC in CHCl3 to afford PHIC-star as a light yellow powder: yield, 138 mg 72.7%. Mw,SEC‑MALS (Mw/Mn) in THF = 14 250 g·mol−1 (1.06); dn·dc−1, 0.090. 1H NMR (CDCl3, 400 MHz), δ (ppm): 7.34 (s, triazole ring), 4.33 (t, −CH2−triazole ring), 4.00−4.20 (m, −C(O)OCH2− and C(CH2)4O−), 3.66 (br s, −NCH2−), 2.75 (t, −OC(O)CH2CH2CH2−triazole ring), 2.41 (t, −OC(O)CH2−), 2.28 (s, −C(O)CH3), 1.90−2.02 (m, −OC(O)CH2CH2CH2−triazole ring and −CH2CH2OC(O)−), 1.62 (br s, 3679
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Table 1. Bulk Polymerization of n-Hexyl Isocyanate (HIC) Catalyzed by 1−3a catalyst 1
2
3
[HIC]/[catalyst] 10 20 30 50 10 30 50 10 17
Mn,NMRb 2790 3110 4000 7430 3910 6450 12800 4200 6270
Mw,SEC‑MALS (Mw/Mn)c 2810 3070 3790 7440 3880 6240 12800 4260 6270
(1.04) (1.05) (1.03) (1.07) (1.06) (1.10) (1.07) (1.02) (1.05)
[η]c mL·g−1 5.3 6.5 7.4 14.8 6.0 16.9 21.8 6.0 9.5
Kc mL·g−1 1.49 5.15 2.25 3.29 1.75 6.74 4.86 2.33 1.61
× × × × × × × × ×
−1
10 10−2 10−3 10−4 10−2 10−4 10−5 10−3 10−3
αc 0.45 0.60 0.98 1.21 0.71 1.16 1.38 0.91 1.00
Polymerization conditions: temp., 0 °C; time, 24 h; atmosphere, Ar. bDetermined by 1H NMR spectrum in CDCl3. cThe weight-average molecular weight Mw,SEC‑MALS, polydispersity Mw/Mn, intrinsic viscosity [η], and Mark−Houwink-Sakurada constants α and K ([η] = K·Mw,SEC‑MALSα) were determined in THF by SEC equipped with MALS and viscosity detectors.
a
−NCH2CH2(CH2)3CH3), 1.29 (br s, −NCH2CH2(CH2)3CH3), and 0.88 (br s, −CH2CH3). Synthesis of PHIC-b-PNIPAM. PHIC−CC (Mn,NMR = 6450 g·mol−1, 230 mg, 3.56 × 10−5 mol, 1.7 equiv), PNIPAM−N3 (Mn,NMR = 13700 g·mol−1, 280 mg, 2.04 × 10−5 mol, 1 equiv),11 and CuCl (6.20 mg, 6.24 × 10−5 mol, 3 equiv) were stirred in dry THF (3.0 mL) under an argon atmosphere. Into the reaction mixture was added PMDETA (21.6 mg, 1.25 × 10−4 mol, 6 equiv) and with stirring at room temperature for 66 h. The reaction mixture was exposed to air, diluted with THF, and passed through a silica gel column to remove the copper catalyst. The solvent was removed by rotary evaporation and the purification by preparative SEC in CHCl3 afforded PHIC-bPNIPAM as a white powder: yield, 255 mg (62.1%). Mw,SEC‑MALS (Mw/ Mn) in THF = 17 540 g·mol−1 (1.08); dn·dc−1, 0.088. 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.83 (s, triazole ring), 5.31 (br s, −OCH2-triazole ring), 4.45 (br s, −NHCH2CH2−triazole ring), 3.00− 4.20 (m, −NCH(CH3)2 and −NCH2−), 2.00−2.40 (m, including 2.28 (s, −C(O)CH3)), 1.62 (br s, −NCH2CH2(CH2)3CH3), 1.29 (br s, −NCH2CH2(CH2)3CH3), 1.14 (br s, −NCH(CH3)2), and 0.88 (br s, −CH2CH3).
Figure 1. 1H NMR spectrum of PHIC−N3 (Mn,NMR = 4000 g·mol−1, Mw,SEC‑MALS = 3790 g·mol−1) in CDCl3.
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RESULTS AND DISCUSSION Synthesis of Clickable Poly(n-hexyl isocyanate)s with an Azido or Alkyne End Group. In designing suitable macromolecular substrates for polymer functionalization using CuAAC click chemistry, the living coordination polymerization of n-hexyl isocyanate (HIC) using organotitanium(IV) catalysts was examined. For the preparation of α-functionalized poly(nhexyl isocyanate)s (PHIC), the CpTiCl3 derivatives (1−3) of three clickable alcohols, 6-azido-1-hexanol, propargyl alcohol, and (R)-3-butyn-2-ol, were used as catalysts, as shown in Scheme 1. These compounds were active catalysts for the polymerization of HIC and have been used to synthesize clickable poly(n-hexyl isocyanate) (PHIC), that is, azido, propynyl, and (R)-butynyl end-functionalized PHIC (PHIC− N3, PHIC−CC, and PHIC−CC*, respectively). When HIC was added to catalysts 1−3, a yellowish orange solution was formed and a waxy white solid was isolated typically in 85−98% yields after the termination reaction at the ω-end using acetic anhydride.13 The polymerization results are summarized in Table 1. The Mn,NMR and Mw,SEC‑MALS values of the obtained PHICs increased with the increasing initial ratio of [HIC]/[catalyst], and the polydispersity indices were very narrow with Mw/Mn values ranging from 1.02 to 1.10 (Figure S1, see Supporting Information). The chemical structures of the obtained polymers were assigned to PHIC by the 1H NMR measurements (Figures 1−3, respectively) and the characteristic peaks due to the initiating ligand were observed as the peaks of the polymer
chain end. For the PHIC−N3 obtained with catalyst 1, the peaks due to the methylene protons next to the azido group at the α-chain end appeared at 3.29 ppm (m in Figure 1), and the peak due to the acetyl protons at the ω-chain end was observed at 2.28 ppm (g in Figure 1), while the peaks of the ethynyl protons on the α-chain end of the obtained PHIC−CC and PHIC−CC* were observed at 2.54 (i in Figure 2) and 2.52 ppm (i in Figure 3), respectively. These results indicated that
Figure 2. 1H NMR spectrum of PHIC−CC (Mn,NMR = 3910 g·mol−1, Mw,SEC‑MALS = 3880 g·mol−1) in CDCl3. 3680
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Figure 3. 1H NMR spectrum of PHIC−CC* (Mn,NMR = 4200 g·mol−1, Mw,SEC‑MALS = 4260 g·mol−1) in CDCl3.
the obtained polymers should possess an azido or ethynyl group at the α-chain end and an acetyl group at the ω-chain end. Additionally, a MALDI−TOF MS measurement provided direct evidence that the obtained PHICs possess a clickable group at the α-chain end and an acetyl group at the ω-chain end. Figures 4, S2, and S3 (Supporting Information) show the MALDI−TOF MS spectra of PHIC−N3, PHIC−CC, and PHIC−CC*, respectively. On the basis of the MALDI−TOF MS analysis, one series of peaks was observed, having a regular interval of ca. 127.10 for the molar mass corresponding to the HIC unit. In addition, the m/z values of the peaks perfectly agreed with the theoretical molecular weight of PHIC possessing a clickable group and an acetyl group at the chain ends. These results based on the SEC, 1H NMR, and MALDI− TOF MS measurements strongly suggested that the coordination polymerization of HIC proceeded in a living manner without any side reactions and that the well-defined clickable PHICs were obtained. The intrinsic viscosity ([η]) and the Mark−Houwink− Sakurada exponent α of PHIC−N3, PHIC−CC, and PHIC− CC*, which were estimated by SEC equipped with MALS and viscosity detectors, increased with the increasing molecular weight, and the PHICs with Mn,NMR > 4000 g·mol−1 showed α > 0.91, which is typical for semiflexible or rigid-rod polymers. The chiral characteristics of PHIC−CC* with a (R)butynyl end group were studied by circular dichroism (CD). As reported before,1a,b the chiral chain end in PHICs had a stereochemical influence upon the helical conformation of the polymer backbone due to the covalent chiral domino effect. The chiral chain end in PHIC−CC* also exerted a helical induction ability and the PHIC−CC* showed a negative Cotton effect at ca. 255 nm, as shown in Figure S4 (Supporting Information, which is a characteristic n-π* transition of the polyisocyanate backbone and had a conformation with an Mhelical sense.1a,b The PHIC−CC* with Mn,NMR of 4200 and 6270 g·mol−1 showed similar CD intensities. The data are consistent with the previous report for the chirally initiated PHIC.1a,b Therefore, the PHIC−CC* is a suitable chiral rod segment for the synthesis of chiral macromolecular architectures. Synthesis of PHIC−OH via Click Reaction Using PHIC− N3. In order to confirm the value of the obtained clickable PHICs, some CuAAC reactions using clickable PHICs were
Figure 4. MALDI−TOF MS spectrum at reflector mode of PHIC−N3 (Mn,NMR = 4000 g·mol−1).
demonstrated. First, we carried out the click reaction of PHIC− N3 with propargyl alcohol as a simple alkyne derivative to prepare hydroxyl end-functionalized PHIC (PHIC−OH), which can be used as a macroinitiator for the ring-opening polymerization of lactide and lactone leading to the welldefined rod−coil block copolymers, as shown in Scheme 2. Table 2 summarizes the results of the CuAAC reactions. The reaction was carried out using CuCl and PMDETA in THF at room temperature ([propargyl alcohol]/[PHIC−N3]/[CuCl]/ [PMDETA] = 3.5/1.0/3.0/6.0). The reaction was allowed to proceed for 48 h, and the polymer was obtained as a light yellow powder in ca. 90% isolated yield. Figures S5 (Supporting Information) and 5 show the SEC trace and the 1H NMR spectrum of the resultant polymers, respectively. The SEC curve of the product displayed a similar monodisperse sharp peak with a narrow polydispersity compared to that of PHIC− N3 as a starting polymer, and there is no peak in the low molecular weight region, strongly supporting the fact that the decomposition of PHIC did not proceed. For the 1H NMR measurement (Figure 5), the signals of the methylene protons (m in Figure 1) next to the azido group completely disappeared 3681
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Scheme 2. Synthesis of PHIC−OH, PHIC-b-PLLA, and PHIC-b-PCL
polymer at m/z = 3331.08 in Figure 6 corresponds to the 24mer of PHIC−OH (3331.5028, calcd. for [M + K]+). These results based on the SEC, 1H NMR, IR, and MALDI−TOF MS analyses showed that the click reaction successfully proceeded and that the well-defined PHIC−OH was obtained. In order to confirm the activity of PHIC−OH as a macroinitiator, the living ring-opening polymerizations of Llactide (L-LA) and ε-caprolactone (ε-CL) were demonstrated using CBT/NCyMe210 and DPP14 catalysts, respectively. The polymerization results are summarized in Table 3. The polymerizations homogeneously proceeded and the objective block copolymers were quantitatively obtained. After polymerization, the SEC trace of PHIC−OH is clearly shifted toward high molecular weight without any tailing, as shown in Figure S7, Supporting Information. The 1H NMR spectrum of the obtained product consisted of the peaks of PHIC and poly(Llactide) (PLLA) or poly(ε-caprolactone) (PCL), as shown in Figure 7. In addition, the molecular weights of the obtained block copolymers were very close to the expected ones with narrow polydispersities (1.07−1.10), as listed in Table 3. Furthermore, the composition of the block copolymers was easily controlled by changing the ratio of monomer and PHIC−OH ([M]/[PHIC−OH]). On the basis of the results, the ring-opening polymerizations of L-LA and ε-CL using PHIC−OH macroinitiator successfully proceeded to produce the well-defined rod−coil block copolymers, PHIC-b-PLLA and PHIC-b-PCL, respectively; i.e., the PHIC−OH macroinitiator for the ring-opening polymerization is very useful for synthesizing novel rod−coil block copolymers. Synthesis of PHIC-Star via Click Reaction Using PHIC− N3. To obtain a well-defined 4-arm star PHIC (PHIC-star) via arm-first strategy based on click chemistry, the reactions of PHIC−N3 with the tetraalkyne-functionalized core material, i.e., pentaerythrityl tetra-5-hexynoate (4), were carried out in dry THF at room temperature for 89 h in the presence of CuCl and PMDETA, as shown in Scheme 3. To ensure complete consumption of the alkyne residues in 4, excess PHIC−N3 was used, i.e., the [alkyne]/[PHIC−N3]/[CuCl]/[PMDETA] molar ratio was 0.5/1.0/2.7/5.4. The obtained materials were purified by preparative SEC in CHCl3 to afford PHIC-star as a light yellow powder. Figure S8 (Supporting Information shows the SEC traces of the PHIC−N3 and the obtained materials.
Table 2. Synthesis of PHIC−OH via CuAAC Reaction of PHIC−N3 with Propargyl Alcohola PHIC−N3
PHIC−OH
Mn,NMRb
Mw,SEC‑MALSc
Mw/Mnc
Mn,NMRb
Mw,SEC‑MALSc
Mw/Mnc
3600 5100 7500 10000 12500
3900 4800 8200 11600 12500
1.07 1.02 1.04 1.12 1.13
3700 5200 7600 10000 12500
4000 5200 8600 11000 12600
1.03 1.06 1.06 1.04 1.10
a
Synthesis conditions: solvent, dry THF; catalyst, CuCl, PMDETA; [propargyl alcohol]/[PHIC−N3]/[CuCl]/[PMDETA] = 3.5/1.0/3.0/ 6.0 ; reaction time, 48 h; room temperature. bDetermined by 1H NMR spectrum in CDCl3. cDetermined by SEC−MALS in THF.
Figure 5. 1H NMR spectrum of PHIC−OH (Mn,NMR = 5200 g·mol−1, Mw,SEC‑MALS = 5200 g·mol−1) in CDCl3.
in the spectrum, and the signal of the methine proton (p′ in Figure 5) in the triazole ring was observed at 7.52 ppm. Additionally, the signals due to the methylene protons (q′ and m′ in Figure 5) next to the triazole ring appeared at 4.81 and 4.37 ppm, respectively. Furthermore, the disappearance of the peak attributable to the azido group (2098 cm−1 in Figure S6, Supporting Information) was confirmed in the IR spectrum of the product. As further evidence, the MALDI−TOF MS spectrum of PHIC−OH showed only one series of peaks, which have a regular interval of ca. 127.10 for the molar mass corresponding to the HIC unit, and the peak of the resultant 3682
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Table 3. Synthesis of PHIC-b-PLLA and PHIC-b-PCL using PHIC−OH Macroinitiator M a
L-LA a L-LA a L-LA b
ε-CL ε-CLb ε-CLb
[M]/[PHIC−OH]
convn/%
Mn,th.c
Mn,NMRd
Mw/Mne
20 35 55 50 100 150
>99 >99 >99 90.2 94.9 83.5
8200 10300 13100 9210 14900 18400
8200 11300 13300 9220 14400 18600
1.10 1.10 1.08 1.07 1.07 1.07
a Copolymerization conditions: solvent, dry-CH2Cl2; initiator, PHIC− OH (Mn,NMR = 5250 g·mol−1, Mw/Mn = 1.10); [M] = 0.7 mol·L−1; catalyst, CBT, NCyMe2; [PHIC−OH]/[CBT]/[NCyMe2] = 1.0/5.5/ 8.3; time 72 h; temp, 27 °C; atmosphere, Ar. bCopolymerization conditions: solvent, dry-toluene; initiator, PHIC−OH (Mn,NMR = 4060 g·mol−1, Mw/Mn = 1.09); [M] = 1.0 mol·L−1; catalyst, DPP; [PHIC− OH]/[DPP] = 1/1; time 8 h; temp, 27 °C; atmosphere, Ar. cMn,th = ([M]/[PHIC−OH]) × convn × (MW of monomer) + Mn, initiator. d Determined by 1H NMR spectrum in CDCl3. eDetermined by SEC in THF using PSt standards.
Figure 7. 1H NMR spectra of PHIC-b-PLLA (upper, Mn,NMR = 13 300 g·mol−1) and PHIC-b-PCL (lower, Mn,NMR = 14 400 g·mol−1) in CDCL3.
Figure 6. MALDI−TOF MS spectrum at reflector mode of the obtained PHIC−OH (Mn,NMR = 3700 g·mol−1, Mw,SEC‑MALS = 4000 g·mol−1).
Scheme 3. Synthesis of PHIC-Star via Click Reaction of PHIC−N3 with Core Material 4
The SEC trace of the PHIC−N3 as the starting material is clearly shifted toward the high molecular weight region without any tailing, indicating that the click reaction successfully proceeded. Figure 8 shows the 1H NMR spectra of 4, PHIC−N3, and the obtained material. In the spectrum of the obtained material (Figure 8c), the signals of the methylene protons (m in Figure 8b) next to the azido group of PHIC−N3 completely disappeared and the signals of the methylene protons shifted to 4.33 ppm (m′ in Figure 8c). Additionally, the signals of the terminal ethynyl protons (x in Figure 8a) of 4 disappeared and the signal of the methine proton (x′ in Figure 8c) in the triazole ring was observed at 7.34 ppm. Furthermore, the signals of the methylene protons (w in Figure 8a) of 4 shifted to downfield at 2.75 (w′ in Figure 8c). These results 3683
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the synthesis and characterization of star PHICs with many arms. Synthesis of PHIC-b-PNIPAM via Click Reaction of PHIC−CC. To obtain novel rod−coil block copolymer PHIC-b-PNIPAM, the click reaction of PHIC−CC (Mn,NMR = 6450 g·mol−1, Mw/Mn =1.10) with azido end-functionalized PNIPAM (PNIPAM−N3, Mn,NMR = 5800 g·mol−1, Mw/Mn =1.12)11 was carried out in THF at room temperature using the CuCl/PMDETA complex as the catalyst system ([PHIC− CC]/[PNIPAM−N3]/[CuCl]/[PMDETA] = 1.7/1.0/3.0/ 6.0), as shown in Scheme 4. After the reaction, the mixture was Scheme 4. Synthesis of PHIC-b-PNIPAM via Click Reaction of PHIC−CC with PNIPAM−N3
Figure 8. 1H NMR spectra of (a) 4, (b) PHIC−N3 (Mw,SEC‑MALS = 3790 g·mol−1), and (c) PHIC-star (Mw,SEC‑MALS = 14 250 g·mol−1) in CDCl3.
purified by column chromatography and preparative SEC to give the product as a white solid (Mw,SEC‑MALS = 10 130 g·mol−1, Mw/Mn =1.04). In the 1H NMR spectrum of the resultant block copolymer (Figure 9b), the signal of the methine proton (i in
showed that the click reaction smoothly proceeded and that the PHIC-star was obtained. Three PHIC-stars with a different arm length were synthesized and the results are summarized in Table 4. The Mw,SEC‑MALS values of the obtained PHIC-stars were in the range 11 620−27 590 g·mol−1, which were about four times those of the corresponding PHIC−N3 as an arm. The [η], α, and hydrodynamic diameter (Dh) values increased with the increasing molecular weight of PHIC-star, e.g., [η] of 11.8− 32.5 mL·g−1, α of 0.55 − 0.84, and Dh of 3.7−6.9 nm for the Mw,SEC‑MALS of 11 620−27 590 g·mol−1. Most importantly, the [η] values of the PHIC-stars were smaller than that of linear PHICs with a similar molecular weight, e.g., 21.8 mL·g−1 for the PHIC−CC with the Mw,SEC‑MALS of 12 800 g·mol−1 (Table 2), but 11.8 mL·g−1 for the PHIC-star with Mw,SEC‑MALS of 11620 g·mol−1, as shown in Figure S9, Supporting Information. The [η] values of the PHIC-stars were also smaller than that of the 3-arm star PHIC.8d In addition, the α values of the PHICstar were smaller than that of linear PHICs with a similar molecular weight, though the PHIC-star has rigid rod arms. The results indicated that PHIC-star has less physical entanglement and a more compact structure based on the branched structure. As described in this section, the arm-first strategy based on click chemistry is useful for preparing a star PHIC consisting of arms having same molecular weight. We are currently studying
Figure 9. 1H NMR spectra of (a) PNIPAM−N3 (Mn,NMR = 2100 g·mol−1) and (b) PHIC-b-PNIPAM (Mw,SEC‑MALS = 9210 g·mol−1) in CDCl3.
Figure 2) in PHIC−CC completely disappeared, and the signal of the methine proton (i′) in the triazole ring was
Table 4. Synthesis of PHIC-Star via CuAAC Reaction of PHIC−N3 with Compound 4 PHIC−N3 as arm Mw,SEC‑MALS (Mw/Mn) 3070 (1.05) 3790 (1.03) 7440 (1.07) a
a
−1
a
[η] (mL·g ) 6.5 7.4 14.8
PHIC-star a
−1
K (mL·g ) −2
5.15 × 10 2.25 × 10−3 3.29 × 10−4
α
a
0.60 0.98 1.21
Mw,SEC‑MALS (Mw/Mn) 11620 (1.03) 14250 (1.06) 27590 (1.06)
a
[η] (mL·g−1) a
11.8 14.7 32.5
Ka (mL·g−1) −2
6.55 × 10 2.17 × 10−2 5.97 × 10−3
αa
Dhb (nm)
0.55 0.69 0.84
3.7 4.6 6.9
Determined by SEC-MALS-viscometer in THF. bDetermined by DLS in THF (3.0 g·L−1 at 25 °C). 3684
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Table 5. Synthesis of PHIC-b-PNIPAM via the Click Reaction of PHIC−CC with PNIPAM−N3 PHIC-CC
a
PNIPAM−N3
PHIC-b-PNIPAM
Mn,NMRa (Mw/Mn)b
[η]b mL·g−1
Kb mL·g−1
αb
Mn,NMRa (Mw/Mn)b
Mw,SEC‑MALS (Mw/Mn)b
[η]b mL·g−1
Kb mL·g−1
αb
6450 (1.10)
16.9
6.74 × 10−4
1.16
2100 (1.18) 5800 (1.12) 13700 (1.22)
9210 (1.03) 10130 (1.04) 17540 (1.08)
16.5 18.8 19.9
7.36 × 10−4 2.27 × 10−3 1.33 × 10−1
1.10 0.98 0.51
Determined by 1H NMR spectrum in CDCl3. bDetermined by SEC−MALS−viscometer in THF.
Additionally, the ring-opening polymerization of L-LA and εCL with PHIC−OH macroinitiator led to novel rod−coil block copolymers, e.g., PHIC-b-PLLA and PHIC-b-PCL. The combination of the organotitanium-catalyzed coordination polymerization and the CuAAC reaction generate a new possibility for the design and synthesis of macromolecular architectures based on PHIC. We are currently studying the synthesis of chiral macromolecular architectures using PHIC− CC* and the self-aggregation and self-organization behavior of the obtained rod−coil block copolymers and rod star polymers.
alternatively observed at 7.83 ppm. In addition, the methylene protons (y in Figure 9a) next to the azido group of PNIPAM− N3 shifted at 4.45 ppm (y′). Furthermore, the peak attributable to the azido group of PNIPAM−N3 disappeared in the IR spectrum of the resultant block copolymer. Thus, the product was assigned as the targeted rod−coil block copolymer PHICb-PNIPAM. The PHIC-b-PNIPAMs with different compositions were synthesized by the click reaction of a PHIC−CC with PNIPAM−N3 having a different molecular weight. The results of the click reactions are listed in Table 5. All the click reactions successfully proceeded and PHIC-b-PNIPAMs with Mw,SEC‑MALS of 9210 −17540 g·mol−1 were obtained. On the basis of the SEC-MALS measurement, the α value of the PHICb-PNIPAM decreased with the increasing molecular weight, i.e., the increasing composition of PNIPAM, though the [η] increased with the increasing molecular weight. The results indicated that the increasing composition of PNIPAM as a random coil polymer lead to the lower stiffness. Table 6 lists the solubility of the obtained PHIC-b-PNIPAM. All the block copolymers were soluble in THF, which is a good
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Synthetic scheme of organotitanium catalysts 1−3, SEC traces of PHIC−CC, PHIC−N3, PHIC−OH, PHIC-b-PLLA, PHIC-b-PCL, and PHIC-star, MALDI−TOF MS spectra of PHIC−CC and PHIC−CC*, CD and UV−vis spectra of PHIC−CC*, IR spectra of PHIC−N 3 , and double logarithmic plots of [η] vs Mw,SEC‑MALS for PHIC−N3, PHIC−CC, and PHIC-star (Scheme S1 and Figures S1− S9). This material is available free of charge via the Internet at http://pubs.acs.org.
Table 6. Solubility of PHIC-b-PNIPAMa PHIC-b-PNIPAM (Mw,SEC‑MALS)
THF
H2O
n-hexane
PHIC50-b-PNIPAM19 (9210) PHIC50-b-PNIPAM57 (10130) PHIC50-b-PNIPAM136 (17540)
○ ○ ○
× × ○
× × ×
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone and Fax: +81-11-706-6603.
−1
Conditions: room temperature; concentration, 0.1 g·L ; ○, soluble; × , insoluble. a
ASSOCIATED CONTENT
S Supporting Information *
Notes
The authors declare no competing financial interest.
■
solvent for PHIC and PNIPAM, while they were insoluble in nhexane, which is a good solvent for PHIC but a poor solvent for PNIPAM. Only the PHIC50-b-PNIPAM136 with a high composition of PNIPAM is soluble in water at 20 °C and the Dh value measured by DLS is ca. 68 nm (0.1 g·L−1 at 20 °C), indicating that a micelle with a PHIC core and a PNIPAM shell should be formed. When the solution was heated, the solution became cloudy and the lower critical solution temperature (LCST) was ca. 40 °C. As described in this section, PHIC−CC is also a suitable rod segment for the click reaction and is useful for preparing well-defined rod−coil block copolymers.
ACKNOWLEDGMENTS This work was supported by the Global COE Program (Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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REFERENCES
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CONCLUSIONS In this study, the synthesis of well-defined clickable PHICs (PHIC−N3, PHIC−CC, and PHIC−CC*) with controlled molecular weights and narrow polydispersities was achieved in high yields by the organotitanium-catalyzed living coordination polymerization using catalysts 1-3. The obtained clickable PHICs were suitable rod segments for the synthesis of well-defined macromolecular architectures based on PHIC as a rod polymer, e.g., PHIC-b-PNIPAM as a rod−coil block copolymer and PHIC-star as a 4-arm rod star polymer. 3685
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