Diphenyl Phosphate as an Efficient Acidic Organocatalyst for

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Diphenyl Phosphate as an Efficient Acidic Organocatalyst for Controlled/Living Ring-Opening Polymerization of Trimethylene Carbonates Leading to Block, End-Functionalized, and Macrocyclic Polycarbonates Kosuke Makiguchi,† Yoshitaka Ogasawara,† Seiya Kikuchi,† Toshifumi Satoh,‡ and Toyoji Kakuchi‡,* †

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, 060-8628, Japan Division of Biotechnology and Macromolecular Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan

‡

S Supporting Information *

ABSTRACT: The ring-opening polymerization (ROP) of cyclic carbonates with diphenyl phosphate (DPP) as the organocatalyst and 3-phenyl-1-propanol (PPA) as the initiator has been studied using trimethylene carbonate (TMC), 5,5dimethyl-1,3-dioxan-2-one, 5,5-dibromomethyl-1,3-dioxan-2-one, 5-benzyloxy-1,3-dioxan-2-one, 5-methyl-5-allyloxycarbonyl-1,3dioxan-2-one, and 5-methyl-5-propargyloxycarbonyl-1,3-dioxan-2-one. All the polymerizations proceeded without backbiting, decarboxylation, and transesterification reactions to afford polycarbonates having narrow polydispersity indices. In addition, 6azido-1-hexanol, propargyl alcohol, and N-(2-hydroxyethyl)maleimide were used as functional initiators for the DPP-catalyzed ROP to produce the end-functionalized poly(trimethylene carbonate)s. For further modification of the azido end-functionlized polycarbonate, the macrocyclic poly(trimethylene carbonate) was synthesized by the intramolecular click cyclization of the αazido, ω-ethynyl poly(trimethylene carbonate). The DPP-catalyzed ROP was applicable for the block copolymerization of TMC and δ-valerolactone or ε-caprolactone to afford poly(trimethylene carbonate)-block-poly(δ-valerolactone) and poly(trimethylene carbonate)-block-poly(ε-caprolactone), and for that of TMC and L-lactide using DPP coupled with 4-dimethylaminopyridine without quenching to produce poly(trimethylene carbonate)-block-poly(L-lactide).

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INTRODUCTION

activation of the hydroxyl groups in the initiators or polymer chain ends through hydrogen bonding. Furthermore, the thiourea/amine catalyzed-ROP proceeded through dual activation of monomer and hydroxyl groups of the polymer/ initiator.15,16 Meanwhile, hydrogen chloride,17,18 carboxylic acids,19−21 organic sulfonic acids,22 and organic phosphoric acids23−25 were catalyzed as a Brønsted acid, which activated the cyclic esters in the controlled/living ROP using an alcohol initiator to afford well-defined polyesters. In addition, very strong Brønsted acids, such as trifluoromethane sulfonic acid (TfOH), 2 6 , 2 7 pent afluo rophenylbis(tri flyl)met hane (C6F5CHTf2),28 and triflimide (HNTf2),29,30 were efficiently controlled the ROP of LA, ε-CL, and δ-VL, and their acidity

Organocatalytic polymerizations have been developed as one of the useful methods for producing metal-free polymers, since Hedrick and Waymouth reported the ring-opening polymerization (ROP) of a lactide (LA) using 4-dimethylaminopyridine (DMAP) as the organocatalyst.1 There are many efforts to elucidate the appropriate combinations of organocatalysts and monomers for producing precisely controlled polymerization systems, in which the ROPs of cyclic esters, such as LA, εcaprolactone (ε-CL), and δ-valerolactone (δ-VL), have been sufficiently studied using various types of organocatalysts.2−4 For example, DMAP,1 guanidine,5−7 N-heterocyclic carbene,8−11 and phosphine12 were used as nucleophilic organocatalysts, which were effective for the controlled/living ROP of cyclic esters through activation of the monomers. In addition, phosphazene13,14 and amidine5 performed as strong basic organocatalysts for the controlled/living ROP of cyclic esters by © 2013 American Chemical Society

Received: January 8, 2013 Revised: February 13, 2013 Published: February 25, 2013 1772

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trimethylene carbonates, respectively, and (2) the synthesis of a macrocyclic PTMC by the click cyclization of the α-azido,ωethynyl PTMC. In addition, we synthesized diblock copolymers consisting of PTMC and polyester segments using δ-VL, ε-CL, and L-lactide (LLA) without quenching.

strongly affected the polymerization characteristics of these monomers; TfOH effectively controlled the living ROP of racLA, but not ε-CL,22 and C6F5CHTf2 was effective for δ-VL and ε-CL, but not rac-LA,28 while the acidity of HNTf2, which is situated between TfOH and C6F5CHTf2, was suitable for the controlled/living ROP of LA, ε-CL, and δ-VL.30 To expand the scope of applicable monomers for the organocatalytic ROP, we focused on cyclic carbonates and their polymers of aliphatic polycarbonates from the viewpoint of biodegradable polymeric materials. The ROP of cyclic carbonates, such as trimethylene carbonate (TMC), has been studied using nucleophilic and basic organocatalysts, such as a tertiary amine,31−33 guanidine,32−34 amidine,34−36 phosphazene,32,33 N-heterocyclic carbene,34 and thiourea/amine,34 in which these organocatalysts activated the monomers and/or the initiating/propagating groups. The ROP of cyclic carbonates smoothly proceeded at room temperature using amidine, while the other polymerizations required a relatively high polymerization temperature (50−150 °C), leading to carbonate polymers with relatively wide polydispersity indices. In addition, Bourissou et al. reported the ROP of TMC using acidic organocatalysts, such as TfOH and methane sulfonic acid (MSA);37 TfOH triggered a undesirable decarboxylation of the carbonate group due to the extremely strong acidity of TfOH, resulting in broadening of the molecular weight distributions of the obtained polymers, while MSA was effective for affording poly(trimethylene carbonate)s (PTMCs) with controlled molecular weights (Mn, ∼8640) and narrow polydispersity indices (99.5%; water content, < 0.001%) was purchased from Kanto Chemical Co., Inc., and distilled over sodium benzophenone ketyl under an argon atmosphere. Dichloromethane (CH2Cl2; >99.5%, water content, 98%, TCI), propargyl alcohol (1b; >98%, TCI), δ-valerolactone (δ-VL; 99%, Kanto Chemical Co., Inc.), and εcaprolactone (ε-CL; 99%, Tokyo Kasei Kogyo Co., Ltd. (TCI)) were distilled over CaH2 under an argon atmosphere. L-Lactide (LLA; >98%, TCI), trimethylene carbonate (TMC; >98%, TCI), and 4dimethylaminopyridine (DMAP; >99%, Wako) were purified by recrystallization from dry toluene prior to use. Diphenyl phosphate (DPP; >99%, TCI), benzoic acid (>99%, TCI) and a weak base anion exchange resin, Amberlyst A21 (Organo Co., Ltd.), were used as received. 6-Azido-1-hexanol (1a),38 N-(2-hydroxyethyl)maleimide (1c),39 5,5-dimethyl-1,3-dioxan-2-one (3a),40 5,5-dibromomethyl-1,3dioxan-2-one (3b),41 5-benzyloxy-1,3-dioxan-2-one (3c),42 5-methyl5-allyloxycarbonyl-1,3-dioxan-2-one (3d),35 and 5-methyl-5-propargyloxycarbonyl-1,3-dioxan-2-one (3e)43,44 were synthesized using previously reported techniques. All other reagents were of synthetic grade and used without further purification. Instruments. The number-average molecular weight (Mn,NMR) was determined from the 1H NMR spectra recorded by a JEOL JNMA400II instrument. The 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 < 1 ppm). The moisture and oxygen contents in the glovebox were monitored by an MB-MO-SE 1 and an MB-OXSE 1, respectively. The size exclusion chromatography (SEC) was performed at 40 °C in CHCl3 (0.8 mL·min−1) using a Jasco GPC-900 system equipped with a set of two Shodex KF-805L columns (linear, 8 mm × 300 mm). The SEC in DMF containing lithium chloride (0.01 mol·L−1) was performed at 40 °C using a Jasco high performance liquid chromatography (HPLC) system (PU-980 Intelligent HPLC pump, CO-965 column oven, RI-930 Intelligent RI detector, UV-2075 Plus Intelligent UV/vis detector, and Shodex DEGAS KT-16) equipped with a Shodex Asahipak GF-310 HQ column (linear, 7.6 mm ×300 mm) and a Shodex Asahipak GF-7 M HQ column (linear, 7.6 mm ×300 mm) at the flow rate of 0.4 mL·min−1. The polydispersity (Mw/Mn) of the polymers was calculated on the basis of a polystyrene calibration. The preparative SEC was performed in CHCl3 (3.5 mL·min−1) at 23 °C using a JAI LC-9201 equipped with a JAI JAIGEL-3H column (20 mm × 600 mm; exclusion limit, 7 × 104) and a JAI RI-50s refractive index detector. The viscosity of the polymer solution was determined by SEC in THF (1.0 mL·min−1) at 40 °C using an Agilent 1100 series instrument equipped with two Shodex KF-804 L columns (linear, 8 mm ×300 mm; exclusion limit, 4 × 105) and a Viscostar viscosity detector (Wyatt Technology). The IR spectra were recorded using a Perkin-Elmer Paragon 1000. The matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI−TOF MS) of the obtained polymers was performed using an Applied Biosystems Voyager-DE STR-H equipped with a 337-nm nitrogen laser (3 ns pulse width). One hundred shots were accumulated for the spectra at a 25 kV acceleration voltage in the reflector mode and calibrated using polystyrene (average Mn 3,600, Waters Associates) as the internal standard. Samples for the MALDI− TOF MS were prepared by mixing PTMC (10 mg·mL−1, 10 μL), a matrix (2,5-dihydroxybenzoic acid, 30 mg·mL−1, 20 μL) and cationizing agent (sodium iodide, 4.0 mg·mL−1, 10 μL) in THF. The MALDI target was spotted with 1.0 μL of solution and allowed to air-dry. Polymerization of Trimethylene Carbonate (TMC). A typical procedure for the polymerization is as follows: TMC (51.0 mg, 0.500

Scheme 1. Synthesis of Poly(trimethylene carbonate) (PTMC) Using 3-Phenyl-1-propanol (PPA) as the Initiator and Diphenyl Phosphate (DPP) as a Catalyst

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−CH2CCH−), 4.05 (m, 2H, >NCH2CH2CH2CH2CH2CH2−), 4.12−4.35 (m, 4H × n, (−OCH2CH2CH2O−)n), 4.43 (t, 2H, J = 6.4, >NCH2−), 7.31 (s, 1H, triazole ring). Block Copolymerization of Trimethylene Carbonate (TMC) and δ-Valerolactone (δ-VL) or ε-Caprolactone (ε-CL). A typical procedure for the polymerization is as follows: TMC (51.0 mg, 500 μmol) was added to a stock solution of DPP (10.0 μL, 10.0 μmol) in toluene at room temperature in a glovebox. A toluene stock solution of PPA (10.0 μL, 10.0 μmol) was then added to the solution to initiate the polymerization under an argon atmosphere. After the first polymerization was stirred for 36 h, the block copolymerization was then started with 50 equiv. of δ-VL (45.3 μL, 500 μmol). Before the polymerization was quenched by the addition of Amberlyst A21 after 4 h, a portion of the polymerization mixtures was added using a small amount of triethylamine to determine the monomer conversion, which was directly determined from the 1H NMR measurement. The crude polymer was isolated by reprecipitation in cold methanol to give poly(trimethylene carbonate)-block-poly(δ-valerolactone) (PTMC-bPVL). Yield: 34.5%. Mn,NMR = 9760 g·mol−1; Mw/Mn = 1.09. 1H NMR (CDCl3): δ (ppm) 1.59−1.78 (m, 4H × m, (−COCH2CH2CH2CH2O−)m), 1.98−2.15 (m, 2H, ArCH2CH2-; m, 2H × n, (−OCH2CH2−)n), 2.34 (t, 2H × m, J = 7.2 Hz, (−COCH2−)m), 2.71 (t, 2H, J = 7.6, ArCH2−), 3.65 (m, 2H, −CH2OH), 4.03−4.17 (m, 2H,ArCH2CH2CH2−; m, 4H × n, (−OCH 2 CH 2 −) n ), 4.24 (t, 2H × (m − 1), J = 6.0 Hz, (−CH2O−)m‑1), 7.17−7.34 (m, 5H, aromatic). A similar condition for the diblock ROP of TMC and δ-VL was used for the block copolymerization of TMC and ε-CL to give poly(trimethylene carbonate)-block-poly(ε-caprolactone) (PTMC-bPCL). Yield: 52.1%. Mn,NMR = 10500 g·mol−1; Mw/Mn = 1.12. 1H NMR (CDCl3): δ (ppm) 1.34−1.45 (m, 2H × m, (−COCH 2 CH 2CH 2 CH2 CH2 O−)m ), 1.56−1.76 (m, 4H × m, ( −COC H 2 CH 2 C H 2 C H 2 CH 2 O− ) m ), 1.98−2.13 (m, 2H, ArCH2CH2−; m, 2H × n, (−OCH2CH2−)n), 2.31 (t, 2H × m, J = 7.6 Hz, (−COCH2−)m), 2.71 (t, 2H, J = 8.0, ArCH2−), 3.65 (m, 2H, −CH2OH), 4.02−4.15 (m, 2H,ArCH2CH2CH2-; m, 4H × n, (−OCH2CH2−)n), 4.24 (t, 2H × m, J = 6.4 Hz, (−CH2O−)m), 7.17−7.34 (m, 5H, aromatic). Block Copolymerization of Trimethylene Carbonate (TMC) and L-Lactide (LLA). TMC (51.0 mg, 500 μmol) was added to a stock solution of DPP (10.0 μL, 10.0 μmol) in toluene at room temperature in a glovebox. A toluene stock solution of PPA (10.0 μL, 10.0 μmol) was then added to the solution to initiate the polymerization under an argon atmosphere. After the first polymerization stirred for 16 h, the block copolymerization was then started with 50 equiv. of LLA (72.1 mg, 500 μmol), 2 equiv of DPP (5.00 mg, 20.0 μmol), 6 equiv. of DMAP (7.33 mg, 60.0 μmol), and CH2Cl2 (111 μL). Before the polymerization was quenched by the addition of Amberlyst A21 and benzoic acid after 74 h, a portion of the polymerization mixture was added using a small amount of triethylamine and benzoic acid to determine the monomer conversion, which was directly determined from the 1H NMR measurement. The crude polymer was isolated by reprecipitation in cold methanol to give poly(trimethylene carbonate)-block-poly(L-lactide) (PTMC-b-PLLA). Yield: 48.1%. Mn,NMR = 11000 g·mol−1; Mw/Mn = 1.10. 1H NMR (CDCl3): δ (ppm) 1.57 (q, J = 7.3, 3H × m, (−CH3)m), 1.95−2.12 (m, 2H, ArCH2CH2−; m, 2H × n, (−OCH2CH2−)n), 2.76 (t, 2H, J = 6.5, ArCH2−), 4.15 (m, 2H, ArCH2CH2CH2−), 4.24 (t, 4H × n, (−OCH2CH2−)n), 4.46 (m, −CH(CH3)OH), 5.11−5.21 (q, 1H × m − 1, J = 7.3, (−CH(CH3)O−)m‑1), 7.13−7.29 (m, 5H, aromatic).

mmol) was added to a stock solution of DPP (10.0 μL, 10.0 μmol) in toluene at room temperature in the glovebox. A toluene stock solution of PPA (10.0 μL, 10.0 μmol) was then added to the monomer solution to initiate the polymerization under an argon atmosphere. Before the polymerization was quenched by the addition of Amberlyst A21 after 36 h, a portion of the polymerization mixtures was added to a small amount of triethylamine to determine the monomer conversion, which was directly determined from the 1H NMR measurement. The crude polymer was isolated by reprecipitation in cold methanol to give poly(trimethylene carbonate) (PTMC). Yield: 85.7%, Mn,NMR = 5220 g·mol−1; Mw/Mn = 1.09. 1H NMR (CDCl3): δ (ppm) 1.92 (q, 2H, J = 6.4, −CH2CH2OH), 1.97−2.11 (m, 2H, ArCH2CH2−; 2H × (n − 1), (−OCH2CH2−)n‑1), 2.71 (t, 2H, J = 7.6, ArCH2−), 3.74 (t, 2H, J = 6.0, −CH2OH), 4.13−4.32 (m, 2H,ArCH2CH2CH2-; m, 4H × n − 1, (-OCH2CH2CH2O−)n‑1; m, 2H, −CH2CH2CH2OH), 7.16−7.37 (m, 5H, aromatic). Synthesis of End-Functionalized Poly(trimethylene carbonate) (2). A typical procedure for the polymerization is as follows: TMC (204 mg, 2.00 mmol) was added to a stock solution of DPP (40.0 μL, 40.0 μmol) in toluene at room temperature in the glovebox. A toluene stock solution of 1a (40.0 μL, 40.0 μmol) was then added to the solution of TMC and DPP to initiate the polymerization under an argon atmosphere. Before the polymerization was quenched by the addition of Amberlyst A21 after 36 h, a portion of the polymerization mixture was added to a small amount of triethylamine to determine the monomer conversion, which was directly determined from the 1H NMR measurement. The crude polymer was isolated by reprecipitation in cold methanol to give the end-functionalized poly(trimethylene carbonate) with the azido group (2a). Yield: 68.0%. Mn,NMR = 5070 g·mol−1; Mw/Mn = 1.08. 1H NMR (CDCl3): δ (ppm) 1.41 (m, 4H, N3CH2CH2CH2CH2−), 1.65 (m, 4H, N3CH2CH2CH2CH2CH2−), 1.92 (q, 2H, J = 6.4, −CH2CH2OH), 1.97−2.16 (m, 2H × (n − 1), (−OCH2CH2−)n‑1), 3.28 (t, 2H, J = 7.6, N3CH2−), 3.74 (t, 2H, J = 5.6, −CH2OH), 4.05 (m, 2H, N3CH2CH2CH2CH2CH2CH2−), 4.12− 4.39 (m, 4H × (n − 1), (−OCH2CH2CH2O−)n‑1; m, 2H, −CH2CH2CH2OH). Synthesis of α-Azido, ω-Ethynyl Poly(trimethylene carbonate) (N 3 −PTMC−CCH). To a mixture of 1-ethyl-3-(3dimethylaminopropyl)carbodiimide hydrochloride (EDC; 23.0 mg, 120 μmol) and DMAP (14.7 mg, 120 μmol) were added a degassed solution of 2a (Mn,NMR, 5070, Mw/Mn 1.08; 120 mg, 23.7 μmol) in CH2Cl2 (3.5 mL) and then 5-hexynoic acid (8.70 μL, 80.0 μmol) under an argon atmosphere. After the reaction mixture was stirred for 40 h at 20 °C, the reaction solution was evaporated and purified by reprecipitation from CH2Cl2 in cold methanol to give N3−PTMC− CCH. Yield: 81.7%. Mn,NMR = 5320 g·mol−1; Mw/Mn = 1.09. 1H NMR (CDCl3): δ (ppm) 1.41 (m, 4H, N3CH2CH2CH2CH2−), 1.64 (m, 4H, N3CH2CH2CH2CH2CH2−), 1.85 (quin, 2H, J = 7.2, −CH2CH2CC), 1.98 (dt, 2H, −CCH), 2.00−2.13 (m, 2H × n, (−OCH2CH2−)n), 2.27 (m, 2H, −CH2CC), 2.46 (t, 2H, J = 7.2, −CH2CH2CH2CC) 3.28 (t, 2H, J = 7.6, N3CH2−), 4.05 (m, 2H, N 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 −), 4.12−4.39 (m, 4H × n, (−OCH2CH2CH2O−)n). Synthesis of Macrocyclic Poly(trimethylene carbonate) (cyclic-PTMC). A solution of N3−PTMC−CCH (82.4 mg, 15.5 μmol) in degassed CH2Cl2 (8.0 mL) was added to a solution of copper(I) bromide (CuBr; 120 mg, 0.837 mmol) and pentamethyldiethylenetriamine (PMDETA; 175 mg, 1.01 mmol) in degassed CH2Cl2 (200 mL) using a syringe pump at the rate of 8.0 μL·min−1 at 20 °C under flowing argon. After completing the addition, the mixture was stirred for another 3 h. The mixture was then washed with aqueous NaHCO3 and brine, and the organic layer was dried over MgSO4. The solvent was removed under reduced pressure. The crude product was purified using preparative SEC (eluent, CHCl3) to remove the remaining residue to give cyclic-PTMC. Yield: 18.2%. Mn,NMR = 5340 g·mol−1; Mw/Mn = 1.11. 1H NMR (CDCl3): δ (ppm) 1.39 (m, 4H, >NCH2CH2CH2CH2−), 1.65 (m, 4H, >NCH 2 CH 2 CH 2 CH 2 CH 2 −), 1.89 (quin, 2H, J = 6.8, −CH2CH2CCH−), 1.97−2.16 (m, 2H × n, (−OCH2CH2−)n), 2.39 (t, 2H, J = 7.2, −CH2CH2CH2CCH−) 2.76 (t, 2H, J = 7.2,

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RESULTS AND DISCUSSION Ring-Opening Polymerization of Trimethylene Carbonate using Diphenyl Phosphate. In order to study the catalytic performance of diphenyl phosphate (DPP) as the acidic organocatalyst for the ring-opening polymerizations (ROP) of cyclic carbonates, we carried out the polymerization of trimethylene carbonate (TMC) using 3-phenyl-1-propanol (PPA) as the initiator in toluene using the [TMC]0/[PPA]0/ 1774

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suggested that no decarboxylation had occurred because there was no signal at 3.47 ppm due to the ether linkages formed by the decarboxylation. Additionally, one series of peaks perfectly agreed with the molecular weight of the polymer from TMC possessing the residue of PPA and the hydroxyl chain end for the MALDI−TOF MS measurement of the polymer from the TMC; e.g., for the 39-mer, the measured value of 4138.45 corresponded to the calculated value of 4138.31, as shown in Figure 2, parts b and c, strongly suggesting that PPA only initiated the ROP of TMC. These results mean that the DPPcatalyzed ROP of TMC proceeded in a living manner without any side reactions, such as backbiting, decarboxylation, and transesterification reactions, and PPA obviously acted as the initiator. The controlled/living manner was used for producing the PTMCs with different molecular weights by varying the initial molar ratio of TMC and PPA ([TMC]0/[PPA]0) from 20 to 100 (Table 1, runs 1, 2, and 4). The molecular weights of the resultant PTMCs linearly increased with the increasing [TMC]0/[PPA]0, and the Mn,NMR values fairly agreed with the Mn,calcd ones predicted from the [TMC]0/[PPA]0 values and the monomer conversions; the Mn,NMRs of 2100, 3000, and 9640 g mol−1 corresponded to the Mn,calcds of 1990, 3190, and 9850 g mol−1, respectively. In addition, their SEC traces are narrow, as shown in Figure 3, and the Mw/Mn values are as low as 1.09 − 1.13 even for the relatively high molecular weights; for example, the Mw/Mn was 1.13 for the PTMC with the Mn,NMR of 9640 g mol−1 (Table 1, run 4). The controlled/living nature for the DPP-catalyzed ROP of TMC was additionally confirmed by the kinetic and postpolymerization experiments. For the kinetic plots, a distinct first-order relationship between the reaction time and monomer conversion was observed, meaning that the monomer consumption rate was constant during the polymerization. In addition, the Mn,NMR of PTMC linearly increased with the reaction time, the Mn,NMR values of the PTMC fairly agreed with the Mn,calcd ones, and the Mw/Mn of PTMC showed low values ranging from 1.07 to 1.15, as shown in Figure S1, Supporting Information. Furthermore, the chain extension experiment supported the controlled/living nature of the DPPcatalyzed ROP of TMC. For the first polymerization with [TMC]0/[PPA]0/[DPP]0 = 50/1/1 in toluene, the PTMC with Mn,NMR = 4820 g mol−1 and Mw/Mn = 1.09 was obtained along with the monomer conversion of 96.4%. After the second polymerization by the subsequent addition of 50 equivalents of TMC toward PPA (in CH2Cl2; [TMC]0 = 5.0 mol·L−1), the PTMC with Mn,NMR = 8920 g mol−1 and Mw/Mn = 1.09 was afforded, as shown in Figure S2, Supporting Information, indicating that the chain end group of PTMC truly maintained a living nature. Thus, the DPP-catalyzed ROP of TMC was revealed to possess a living nature, producing a well-defined PTMC even at ambient temperature. For the Brønsted acid-catalyzed ROP of cyclic esters leading to well-defined polyesters, an activated monomer mechanism was previously proposed.45 In order to confirm the catalytic role of DPP in the ring-opening process for the ROP of TMC using PPA, the titration experiment for TMC with DPP was carried out in CDCl3 at room temperature. The chemical shifts for the carbonyl carbon of the TMC were downfield with the increasing molar ratio of DPP and TMC ([DPP]/[TMC]), and the chemical shift reached the constant value of 148.85 ppm for [DPP]/[TMC] ≥ 2, as shown in Figure 4. The NMR titration experiment indicated that DPP activated the carbonate group of

[DPP]0 ratio of 50/1/1 (Table 1, run 3). The polymerization homogeneously proceeded and was quenched by an immobiTable 1. Ring-Opening Polymerization of Trimethylene Carbonate (TMC) Using Diphenyl Phosphate (DPP) and 3Phenyl-1-propanol (PPA)a run

[TMC]0/ [PPA]0/ [DPP]0

time (h)

convn (%)b

Mn,calcd (g mol−1)c

Mn,NMR (g mol−1)b

Mw/Mnd

1 2 3 4e

20/1/1 30/1/1 50/1/1 100/1/1

6 16 36 24

91.1 99.6 97.9 95.1

1990 3190 5130 9850

2100 3000 5220 9640

1.10 1.09 1.09 1.13

a

Temperature, room temperature; solvent, toluene; [TMC]0, 1.0 mol·L−1. bDetermined by 1H NMR in CDCl3 cCalculated from ([TMC]0/[PPA]0) × convn × (MW of TMC) + (MW of PPA). d Determined by SEC in CHCl3 using PSt standards. eTemperature, 70 °C; [TMC]0, 0.5 mol·L−1.

lized base, and the obtained polymer was purified by reprecipitation using CH2Cl2 as the good solvent and cold methanol as the poor solvent. The conversion of TMC was 97.9% for the polymerization time of 36 h even at room temperature, which was directly determined by the 1H NMR spectra of aliquots of the polymerization mixtures in CDCl3. The number average molecular weight (Mn,NMR) of the obtained polymer estimated from the 1H NMR measurement agreed fairly well with that (Mn,calcd) calculated from the initial molar ratio of [TMC]0/[PPA]0 and monomer conversions; the Mn,NMR and Mn,calcd values were 5220 g mol−1 and 5130 g mol−1, respectively. The polydispersity index (Mw/Mn) of 1.09 estimated by the SEC measurement was relatively low. We confirmed that the obtained polymer was determined to be poly(trimethylene carbonate) (PTMC) because of the appearance of the peaks at 1.97−2.11 and 4.13−4.32 ppm due to the polymer main chain and at 1.97, 2.71, 4.13, and 7.16− 7.37 ppm due to the initiator residue in the 1H NMR spectrum, as shown in Figure 1. The 1H NMR measurement also strongly

Figure 1. 1H NMR spectrum of poly(trimethylene carbonate) (PTMC) initiated from 3-phenyl-1-propanol (PPA) in CDCl3 (run 3). 1775

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Figure 2. (a) MALDI−TOF MS spectrum in reflector mode of the obtained poly(trimethylene carbonate) (PTMC) (Table 1, run 3), (b) expanded spectrum, and (c) calculated molecular weights.

Figure 4. Chemical shifts of carbonyl carbon in the 13C NMR spectrum observed by titration of trimethylene carbonate (TMC) with diphenyl phosphate (DPP) in CDCl3.

polymerization characteristics for the DPP-catalyzed ROP of lactones, such as δ-VL and ε-CL. Synthesis of Well-Defined Polycarbonates with Functional Groups in Chain-End and Side-Chain. To expand the intrinsic advantage of the DPP-catalyzed ROP of TMC, we first synthesized the end-functionalized PTMC. Scheme 3 represents the synthetic procedure for the end-functionalization of polycarbonates, and Table 2 lists the polymerization results. The DPP-catalyzed ROP of TMC was carried out using functional initiators (FI), such as 6-azido-1-hexanol (1a), propargyl alcohol (1b), and N-(2-hydroxyethyl)maleimide (1c) at room temperature. The Mn,NMRs of 5070 g mol−1 for PTMC initiated from 1a, 5080 g mol−1 from 1b, and 5130 g mol−1 from 1c well agreed with the Mn,calcds of 5190, 5010, and 5010 g mol−1, respectively (Table 2, runs 5, 6, and 7,

Figure 3. SEC traces of poly(trimethylene carbonate)s (PTMC) obtained with the molar ratio of trimethylene carbonate (TMC) and 3-phenyl-1-propanol (PPA) ([TMC]0/[PPA]0) of (a) 20, (b) 30, (c) 50, and (d) 100 (eluent, CHCl3; flow rate, 0.8 mL·min−1).

TMC. Thus, we proposed a polymerization mechanism that PPA reacted with the activated TMC to afford the ring-opened carbonate adduct of TMC and PPA, and the process was repeated to produce PTMC, as shown in Scheme 2. The DPPcatalyzed ROP of TMC proceeded through the activated monomer mechanism to produce the PTMC possessing the 3phenylpropoxy group as the α-chain end and the hydroxyl group as the ω-chain end, which is very similar to the 1776

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Scheme 2. Activated Monomer Mechanism for the Diphenyl Phosphate (DPP) Catalyzed ROP of Trimethylene Carbonate (TMC) Using 3-Phenyl-1-propanol (PPA)

confirmed and the peaks due to the initiator residue appeared at 1.41, 1.65, 3.28, and 4.05 ppm for 2a, at 2.55, and 4.73 ppm for 2b, and at 3.84, 4.05, and 6.74 ppm for 2c (Figure S3, Supporting Information). Furthermore, in order to synthesize well-defined polycarbonates with functional groups in the side-chains, the DPPcatalyzed ROP was carried out using trimethylene carbonates having functional groups, such as 5,5-dimethyl-1,3-dioxan-2-one (3a), 5,5-dibromomethyl-1,3-dioxan-2-one (3b), 5-benzyloxy1,3-dioxan-2-one (3c), 5-methyl-5-allyloxycarbonyl-1,3-dioxan2-one (3d), and 5-methyl-5-propargyloxycarbonyl-1,3-dioxan2-one (3e), as shown in Scheme 3. All the polymerizations proceeded in a well-controlled manner to afford the corresponding polycarbonates, and all results are listed in Table 3. In order to accelerate the polymerization rate, the

Scheme 3. Synthesis of End-Functionalized Poly(trimethylene carbonate)s (2) by Ring-Opening Polymerization of Trimethylene Carbonate (TMC) Using Functional Initiators (1) and Poly(trimethylene carbonate)s with Functional Groups in Side Chain (4) by Ring-Opening Polymerization of Functional Trimethylene Carbonates (3)

Table 3. Ring-Opening Polymerization of 5,5-Dimethyl-1,3dioxan-2-one (3a), 5,5-Dibromomethyl-1,3-dioxan-2-one (3b), 5-Benzyloxy-1,3-dioxan-2-one (3c), 5-Methyl-5allyloxycarbonyl-1,3-dioxan-2-one (3d), and 5-Methyl-5propargyloxycarbonyl-1,3-dioxan-2-one (3e) Using Diphenyl Phosphate (DPP) and 3-Phenyl-1-propanol (PPA)a

Table 2. Synthesis of End-Functionalized Poly(trimethylene carbonate)s (PTMCs) by the DPP-Catalyzed ROP of Trimethylene Carbonate (TMC) Using 6-Azido-1-hexanol (1a), Propargyl Alcohol (1b), and N-(2Hydroxyethyl)maleimide (1c)a run

functional initiator (1)

convn (%)b

Mn,calcd (g mol−1)c

Mn,NMR (g mol−1)b

Mw/Mnd

5 6 7

1a 1b 1c

98.9 97.0 94.3

5190 5010 5010

5070 5080 5130

1.08 1.10 1.10

run

monomer

time (h)

convn (%)b

Mn,calcd (g mol−1)c

Mn,NMR (g mol−1)b

Mw/Mnd

8 9 10e 11 12

3a 3b 3c 3d 3e

5 6 14 10 14

89.0 83.3 90.0 90.4 89.2

5930 12100 9510 9180 8970

6450 12700 10200 9380 9030

1.19 1.20 1.20 1.15 1.13

Temperature, 100 °C; solvent, toluene; [3]0, 1.0 mol·L−1; [3]0/ [PPA]0/[DPP]0, 50/1/1. bDetermined by 1H NMR in CDCl3 c Calculated from ([3]0/[PPA]0) × convn × (MW of 3) + (MW of PPA). dDetermined by SEC in CHCl3 using PSt standards. e[3c]0, 0.5 mol·L−1. a

polymerizations were carried out at 100 °C, resulting in monomer conversions of 83.3−90.4% for the polymerization time of 5−14 h though with a slight increase in the polydispersity indices, Mw/Mn = 1.13−1.20, compared to the results for the polymerization at room temperature (Table 1). The Mn,NMR of 6450, 12700, 10200, 9380, and 9030 g mol−1 for the polymers of 4a, 4b, 4c, 4d, and 4e, respectively, fairly agreed with the Mn,calcd of 5930, 12100, 9510, 9180, and 8970 g mol−1, respectively. In addition, the polymer structures were confirmed as poly(trimetylene carbonate)s with functional groups in the side-chains without any undesired reactions, such as backbiting, decarboxylation, and transesterification reactions,

a

Temperature, room temperature; solvent, toluene; time, 36 h ; [TMC]0, 1.0 mol·L−1; [TMC]0/[1]0/[DPP]0, 50/1/1. bDetermined by 1H NMR spectra in CDCl3. . cCalculated from ([TMC]0/[1]0) × convn × (MW of TMC) + (MW of 1). dDetermined by SEC in CHCl3 using PSt standards.

respectively), and their polydispersities were narrow for the Mw/Mn of 1.08−1.10. In addition, the polymer structures were confirmed as end-functionalized poly(trimethylene carbonate)s (2) possessing azido, ethynyl, and maleimide groups with quantitative end-functionalization, based on the 1H NMR measurements. The peaks due to the polymer main chain were 1777

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Scheme 4. Synthesis of Macrocyclic Poly(trimethylene carbonate) (cyclic-PTMC) by Click Reaction of α,ω-End-Functionalized Poly(trimethylene carbonate) (N3−PTMC−CCH)

Figure 5. 1H NMR spectra of 2a (upper), N3−PTMC−CCH (middle) and cyclic-PTMC (lower) in CDCl3.

Figure 6. MALDI−TOF MS spectra of (a) N3−PTMC−CCH and (b) cyclic-PTMC.

by the 1H NMR measurements; the peaks due to the polymer main chain were confirmed and the peaks due to side chains appeared at 1.00 ppm for 4a, at 3.54 ppm for 4b, at 5,31 and

7.23−7.37 ppm for 4c, at 1.27, 4.64, 5.28, and 5.87 ppm for 4d, and at 1.30, 2.54, and 4.72 ppm for 4e, as shown in Figure S4, Supporting Information, respectively. In addition, all SEC 1778

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weight region when compared to that of N3−PTMC−CCH. Furthermore, there is no peak in the molecular weight region higher than that of N3−PTMC−CCH, strongly supporting the fact that the intermolecular click reaction did not proceed, and the intramolecular click cyclization selectively proceeded to afford the cyclic polycarbonate, cyclic-PTMC. To determine the difference in the physical property between linear and cyclic polycarbonates, the viscosity of the polymers was compared before and after the click reaction. The intrinsic viscosities [η] in THF were 14.6 and 4.2 mL·g−1 for N3−PTMC−CCH and the cyclic-PTMC, respectively, while there was no difference in the molecular weight between the linear and cyclic polycarbonates, such as the Mn,NMRs of 5320 (Mw/Mn, 1.09) and 5340 g mol−1 (1.11), respectively, which agreed with the well-known fact that the macrocyclic structure leads to a decrease in the hydrodynamic volume compared to the linear structure, and subsequent decrease in viscosity of the solution state.48−50 Thus, the macrocyclic PTMC was successfully synthesized by esterification of the azido-functionalized PTMC and subsequent intramolecular click cyclization. Diblock Copolymerization of Trimethylene Carbonate with δ-Valerolactone, ε-Caprolactone, and L-Lactide. We previously reported that DPP efficiently catalyzed the controlled/living ROP of ε-CL or δ-VL leading to well-defined polyesters. Thus, we synthesized diblock copolymers consisting of a polycarbonate segment and PCL or PVL segments by diblock ROPs without quenching, as shown in Scheme 5. After the DPP-catalyzed ROP of TMC using PPA, ε-CL or δ-VL were continuously polymerized as the second monomer. The SEC trace of the first ROP shifted to the higher molecular weight region while maintaining a narrow polydispersity; the Mn,NMR values increased from 4920 (Mw/Mn, 1.11) to 10500 g mol−1 (1.12) for the TMC/ε-CL system and from 4990 (1.09) to 9760 g mol−1 (1.14) for the TMC/δ-VL system, as shown in Figure 8, parts a and b, respectively. For the 1H NMR measurement, the peaks due to the PPA residue and the PTMC main chain were observed along with the appearance of the peaks due to the second polymer segment at 1.59−1.78, 2.34, and 4.24 ppm for the TMC/δ-VL system and at 1.34−1.45, 1.56−1.70, 2.31, and 4.24 ppm for the TMC/ε-CL system, as shown in Figures S9a and S10a, Supporting Information, respectively. In addition, only two peaks due to the carbonyl carbons of PTMC and PVL appeared at 154.9 and 173.2 ppm, respectively, for the TMC/δ-VL system, and those of PTMC and PCL at 154.9 and 173.5 ppm, respectively, for the TMC/εCL system, as shown in the 13C NMR spectra (Figures S9b and S10b, Supporting Information, respectively). These results strongly indicated that the polymer structures were confirmed as the diblock copolymers, poly(trimethylene carbonate)-blockpoly(δ-valerolactone) (PTMC-b-PVL) and poly(trimethylene carbonate)-block-poly(ε-caprolactone) (PTMC-b-PCL), which were synthesized by the DPP-catalyzed diblock ROPs of TMC and ε-CL or δ-VL, respectively. We aimed to synthesize diblock copolymers consisting of polycarbonate and polylactide segments. Unfortunately, the catalytic activity of DPP was insufficient for the ROP of LLA, and we thus tuned the catalytic performance of DPP using nucleophilic organocatalysts. Peruch et al. previously reported that the Brønsted acid of HX (X = Cl, OSO2CH3, OSO2CF3) coupled with DMAP effectively controlled the living ROP of LLA through the dual activation of the monomer and initiator/ chain end.51 Thus, we used their technique that the catalytic property of DPP was tuned using DMAP; the ROP of TMC

traces showed monomodal shape, as shown in Figure S5, Supporting Information. These results for the synthesis of the poly(trimethylene carbonate)s with functional groups in the side-chain and chainend indicated that DPP performed as the ROP catalyst and was inactive for the functional groups in the initiators and monomers to afford well-defined polycarbonates. Synthesis of Macrocyclic Polycarbonate. To demonstrate one of the applications for the end-functionalized polycarbonates prepared by the DPP-catalyzed ROP of TMC, we synthesized a macrocyclic PTMC (cyclic-PTMC) by the intramolecular click reaction of the α,ω-end-functionalized PTMC, as shown in Scheme 4.46,47 We confirmed the structure of the α-end-functionalized poly(trimethylene carbonate) with the azido group (2a; Mn,NMR = 5070 g mol−1, Mw/Mn = 1.08) based on the MALDI−TOF MS spectrum, as shown in Figure S6, Supporting Information, and carried out the esterification of 2a by 5-hexynoic acid using 1-ethyl-3-(3dimethylaminopropyl)carbodiimide (EDC) and DMAP to produce the end-functionalized poly(trimethylene carbonate) having the azido group at the α-chain-end and the ethynyl group at the ω-chain-end, N3−PTMC−CCH. The signal due to the ethynyl proton was confirmed at 1.98 ppm in the 1H NMR spectrum of N3−PTMC−CCH, as shown in Figure 5b, and two series of peaks corresponding to N3−PTMC−C CH (○) and its N2-eliminated polymer (□) are observed in the MALDI−TOF MS spectrum, as shown in Figure 6a, which was caused by decomposition of the azido group during the ionization process of the MALDI−TOF MS measurement.48,49 In addition, the click cyclization of N3−PTMC−CCH was carried out using copper(I) bromide and pentamethyldiethylenetriamine (PMDETA) in CH2Cl2 at room temperature for 40 h. After purification using preparative SEC, we confirmed the structure of the obtained polymer by a 1H NMR measurement such that the peak at 1.98 ppm due to the ethynyl proton disappeared and that at 7.31 ppm due to the triazole proton appeared in the 1H NMR spectrum, as shown in Figure 5c. The MALDI−TOF MS spectrum of the obtained polymer showed one series of peaks (●), which was completely the same as that of N3−PTMC−CCH, as shown in Figure 6b. Furthermore, the absorption peak at 2100 cm−1 due to the azido group disappeared in the IR spectrum after the click reaction, as shown Figure S7. Figure 7 shows the SEC traces of both the N3−PTMC−C CH and resultant polymer. The SEC trace of the product displayed a monodisperse sharp peak in the lower molecular

Figure 7. SEC traces of N3−PTMC−CCH (dashed line) and cyclicPTMC (solid line) (eluent, CHCl3; flow rate, 0.8 mL·min−1). 1779

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Scheme 5. One-Pot Synthesis of Diblock Copolymers by Ring-Opening Polymerizations of Trimethylene Carbonate (TMC) with δ-Valerolactone (δ-VL), ε-Caprolactone (ε-CL), and L-Lactide (LLA) Using Diphenyl Phosphate (DPP) and the DPP/4Dimethylaminopyridine (DMAP) Catalyst System

Figure 8. (a) SEC traces of first sequence of poly(trimethylene carbonate) (PTMC; solid line) and poly(trimethylene carbonate)-block-poly(δvalerolactone) (PTMC-b-PVL; dashed line), (b) SEC traces of first sequence of PTMC (solid line) and poly(trimethylene carbonate)-block-poly(εcaprolactone) (PTMC-b-PCL; dashed line), and (c) SEC traces of first sequence of PTMC (solid line) and poly(trimethylene carbonate)-blockpoly(L-lactide) (PTMC-b-PLLA; dashed line) (eluent, CHCl3; flow rate, 0.8 mL·min−1).

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was carried out for [TMC]0[PPA]0/[DPP]0 = 50/1/1, DPP and DMAP were then added to the first polymerization mixture without quenching, and the second ROP of LLA in CH2Cl2 was carried out for [TMC]0[PPA]0/[DPP]0/[DMAP]0 = 50/1/3/6 to afford poly(trimethylene carbonate)-block-poly(L-lactide) (PTMC-b-PLLA), as shown in Scheme 3. A monomodal SEC trace of the TMC obtained by the first polymerization shifts to the higher molecular weight region while maintaining the monodispersity after the second ROP of LLA, as shown in Figure 8c; the Mn,NMR value increased from 5440 to 11000 g mol−1, and the polydispersity maintained the Mw/Mn of 1.10. The structure of PTMC-b-PLLA was confirmed by the 1H NMR and 13C NMR measurements; in the 1H NMR spectrum (Figure S11a, Supporting Information), the peaks due to the PPA residue and the PTMC main chain were observed as well as the peaks due to PLLA at 1.57 and 5.11−5.21 ppm. In addition, the peaks due to PTMC appeared at 28.0, 64.2, and 154.9 ppm and the peaks due to PLLA were also confirmed at 16.6, 69.0, and 169.6 ppm, in the 13C NMR spectrum (Figure S11b, Supporting Information). These results indicated that DPP possesses an excellent catalytic performance for controlling the living ROPs of the cyclic carbonate, lactone, and lactide to produce homopolymers and diblock copolymers.

CONCLUSIONS

In this study, we succeeded in the synthesis of various poly(trimethylene carbonate)s (PTMCs) with well-defined structures by the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) using diphenyl phosphate (DPP) as the organocatalyst. DPP activated the carbonate group of TMC, an alcohol molecule reacted as the initiator with the activated TMC to afford the ring-opened carbonate adduct of TMC and the alcohol, and the process was repeated to produce PTMC without any undesired reactions, such as backbiting, decarboxylation, and transesterification reactions; i.e., the DPP-catalyzed ROP of TMC proceeded through the activated monomer mechanism to produce the PTMC possessing the 3-phenylpropoxy group as the α-chain end and the hydroxyl group as the ω-chain end. In addition, DPP performed as the ROP catalyst and was inactive for the functional groups in the initiators and monomers to afford welldefined poly(trimethylene carbonate)s with functional groups in the side-chain and chain-end. For the ROP of cyclic monomers, DPP is the versatile organocatalyst for cyclic carbonates as well as lactones of δ-VL and ε-CL and for the Llactide (LLA) associated with the nucleophilic organocatalyst of 4-dimethylaminopyridine, whose catalytic property produced 1780

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well-defined diblock copolymers consisting of PTMC with polyesters of δ-VL, ε-CL, and LLA.

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ASSOCIATED CONTENT

S Supporting Information *

Results of the kinetic measurements and postpolymerizations of TMC and all spectra of the polymers. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the MEXT (Japan) program “Strategic Molecular and Materials Chemistry through Innovative Coupling Reactions” of Hokkaido University. The JSPS Fellowship for Young Scientists (K.M.) is also gratefully acknowledged. Dr. F. Peruch (University of Bordeaux) is acknowledged for his advice and help for the polymerization procedure of LLA using DMAP, and Dr. H. Kaga (National Institute of Advanced Industrial Science and Technology Hokkaido) is acknowledged for his help with the polymerization equipment.

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