A “Catalyst Switch” Strategy for the Sequential Metal-Free

Article pubs.acs.org/Macromolecules

A “Catalyst Switch” Strategy for the Sequential Metal-Free Polymerization of Epoxides and Cyclic Esters/Carbonate Junpeng Zhao,† David Pahovnik,† Yves Gnanou,‡ and Nikos Hadjichristidis*,† †

Physical Sciences and Engineering Division and ‡Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia S Supporting Information *

ABSTRACT: A “catalyst switch” strategy was used to synthesize well-defined polyether−polyester/polycarbonate block copolymers. Epoxides (ethylene oxide and/or 1,2butylene oxide) were first polymerized from a monoalcohol in the presence of a strong phosphazene base promoter (tBuP4). Then an excess of diphenyl phosphate (DPP) was introduced, followed by the addition and polymerization of a cyclic ester (ε-caprolactone or δ-valerolactone) or a cyclic carbonate (trimethylene carbonate), where DPP acted as both the neutralizer of phosphazenium alkoxide (polyether chain end) and the activator of cyclic ester/carbonate. This work has provided a one-pot sequential polymerization method for the metal-free synthesis of block copolymers from monomers which are suited for different types of organic catalysts.

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INTRODUCTION Ring-opening polymerization (ROP) of epoxides and cyclic esters share the common feature of maintaining an alkoxide/ hydroxyl end group as growing species. At first glance, their sequential ROP seems facile and the synthesis of the corresponding block copolymers straightforward. However, initiating/catalytic systems that work for epoxides,1,2 typically alkali alkoxides, provoke chain transfer reactions (i.e., transesterification on polymer) in the case of cyclic esters, and those that are appropriate for the latter, typically tin or aluminum alkoxides,3−6 are irrelevant for the former. On this basis, the synthesis of polyether−polyester block copolymers is possible but tedious as it generally requires multiple steps of synthesis, isolation and purification, etc.7 Because of the great efforts devoted into the development of organocatalytic polymerization methods in the past decade, a wide range of organic molecules are now available in polymer chemists’ toolbox.8−11 Similarly to their metallic counterparts, organic catalysts also need to be appropriately chosen for each specific monomer type to achieve the best compromise between polymerization rate and control. For example, tBuP4, one of the strongest phosphazene bases,12−14 is well suited for the polymerization of epoxides,15−23 but brings about extensive chain transfer reactions in the case of cyclic esters.24 On the other hand, strong organic acids, such as HCL· Et2O,25−27 (trifluoro)methanesulfonic acid,28−31 sulfonimide derivatives,32−35 and phosphoric acids, 36−39 have been employed to afford well-controlled ROP of cyclic esters/ carbonates via either a monomer activation mechanism or a monomer/chain end dual activation mechanism. The catalystmonomer suitability requirement has made it a challenge for polymer chemists to develop synthetic pathways toward sequential polymerization of different types of © XXXX American Chemical Society

monomers. Previously, we have used a relatively mild phosphazene base (t-BuP2) to sequentially polymerize ethylene oxide (EO) and cyclic esters.40 Because of the insufficient basicity of t-BuP2, EO appeared to be the only epoxide that can be involved by this method, and a long reaction time is needed to reach high conversion of EO. In the present study, we have combined the ideal base catalyst (t-BuP4) for epoxides and acid catalyst (diphenyl phosphate, DPP) for cyclic esters/carbonate into a novel “catalyst switch” strategy, namely, successively adding DPP and cyclic ester/carbonate after the polymerization of one or two epoxides, where DPP acted as both the neutralizer of phosphazenium alkoxide (the end group of living polyether chain) and the activator of cyclic ester/carbonate. Sequential ROP of two different types of monomers was successfully conducted in this manner, leading to well-defined polyether−polyester/polycarbonate block copolymers.

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EXPERIMENTAL SECTION

Chemicals. ε-Caprolactone (CL; Alfa Aesar, 99%) was dried over calcium hydride and distilled under vacuum. N-Ethyldiisopropylamine (EDIPA; Alfa Aesar, 99%) was used as received. Tetrahydrofuran (THF; Fischer, HPLC grade) was dried successively by sodium and nbutyllithium. Trimethylene carbonate (TMC; TCI, 98%) was recrystallized twice from toluene (HPLC grade), dried extensively under vacuum and finally dissolved in purified toluene into a 0.1 g mL−1 solution. All other chemicals were purchased from Aldrich. Toluene (HPLC grade), EO (99.5%) and 1,2-butylene oxide (BO; 99%) were dried successively by calcium hydride and n-butyllithium. 3Phenyl-1-propanol (PPA; 98%) and δ-valerolactone (VL; 99%) were dried over calcium hydride and distilled twice under vacuum. t-BuP4 Received: April 21, 2014 Revised: May 21, 2014

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dx.doi.org/10.1021/ma500830v | Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Experimental Conditions and Molecular Characteristics of the Polymers sample

temperaturea (°C)

timea (h)

conversiona (%)

Mn,theorb (g mol−1)

Mn,NMRc (g mol−1)

Mn,SECd (g mol−1)

Mw/Mnd

PEO1 PEO1PCL1e PEO1PCL2 PEO1PCL3 PEO1PVL PEO2f PEO2PCLf PBO1g PBO1PVL PBO2g PBO2PVL PBO2PEO PBO2PEOPVL PBO2PEOPCL PBO2PEOPTMC1e PBO2PEOPTMC2

40 RT RT 40 RT 40 40 40 RT 40 RT 40 RT 40 RT 40

24 0.017 8 8 3.5 24 240 72 48 48 4 48 4 8 0.017 48

>99 >99 37 83 96 >99 99 96 85 >99 96

3600 10400 6100 9300 9400 3000 3700 1600 − 4000 12600 6200 13600 13700 − 14400

3600 10200 5700 9000 9300 3100 3600 1400 − 4100 18800 6 300 14300 14400 − 14600

3500 9200 6000 8700 8400 2800 3300 1300 − 3800 17400 6200 12300 15100 − 13 200

1.04 1.72 1.06 1.10 1.11 1.05 1.05 1.05 − 1.04 1.17 1.04 1.13 1.09 h 1.07

a

Polymerization temperature, time and monomer conversion for the polyether precursor (in the case of homopolymers) or the second block (in case of diblock copolymers) or the third block (in the case of triblock terpolymers). bTheoretical number-average molecular weight calculated from feed and monomer conversion. cNumber-average molecular weight calculated from 1H NMR spectra using integrals of the characteristic signals. d Number-average molecular weight and polydispersity obtained from SEC analysis (THF, 35 °C, PEO standards). eSynthesized by sequential ROP of the monomers with t-BuP4 as a single catalyst. fTHF was used as solvent for these two experiments, while others were conducted in toluene. g [BO]0 = 1.4 M for PBO1 and 5.5 M for PBO2. hBimodal distribution. mixture was withdrawn in an argon flow and injected into a mixture of 5 mL of THF and a few drops of AcOH. A few drops of this solution was diluted with THF for SEC measurement, the rest was poured into diethyl ether to precipitate PEO. The white powder was then collected, dried in vacuum and used for 1H NMR measurements. Theoretical number-average molecular weight (Mn,theor, assuming complete EO conversion) = 3600 g mol−1. Mn,SEC = 3500 g mol−1, Mw/Mn = 1.04. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 3.77−3.49 (−CH2CH2O−), 3.48−3.44 (PhCH2CH2CH2O−PEO), 2.70−2.66 (PhCH2CH2CH2O−PEO), 1.93−1.87 (PhCH2CH2CH2O−PEO); Mn,NMR = 3600 g mol−1. Poly(ethylene oxide)-b-poly(ε-caprolactone) (PEO1PCL3, Table 1). About 5.5 mL of reaction mixture containing living PEO (ca. 0.15 mmol PEO−OH + PEO−O−, 0.03 mmol PEO−O−) was withdrawn with a syringe in an argon flow and injected into another flame-dried reaction flask preheated at 40 °C. Then, 0.45 mL of DPP solution (0.18 mmol DPP) was added into this flask, upon which the brownish yellow color of PEO solution disappeared immediately indicating the neutralization of the alkoxide PEO chain ends. Then, 15 min later, 1.0 mL of CL (9.0 mmol) was added and the reaction mixture was stirred at 40 °C. Aliquots were withdrawn (0.1 mL each) in an argon flow in different time intervals. Each aliquot was injected to a mixture of 1.5 mL of CDCl3 and two drops of EDIPA. This solution was used for 1H NMR measurement to determine the conversion of CL. 0.15 mL of such CDCl3 solution was diluted with 1.5 mL of THF for SEC analysis. After the withdrawal of last aliquot at 8 h, the reaction was quenched by addition of ca. 1 mL of EDIPA. Then the solution was poured into a cold (−20 °C) mixture of methanol and diethyl ether (1/1, v/v) to precipitate PEO-b-PCL diblock copolymer. The white powder was then collected, dried in vacuum and used for SEC and 1H NMR analysis. Convn(CL) = 83%; Mn,theor(PCL) = 5700 g mol−1, Mn,theor(PEO-b-PCL) = 9300 g mol−1. Mn,SEC = 8700 g mol−1, Mw/Mn = 1.10. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 4.23−4.20 (−PEO−CH2CH2OCO− PCL), 4.19−3.91 (−OCOCH 2 CH 2CH2 CH2 CH 2−), 3.77−3.49 (−CH2CH2O−), 3.48−3.44 (PhCH2CH2CH2O−PEO−), 2.70−2.66 (PhCH 2 CH 2 CH 2 O−PEO−), 2.43−2.17 (−OCOCH 2 CH 2 CH 2 CH2CH2−), 1.93−1.88 (PhCH2CH2CH2O−PEO−), 1.77−1.56 (−OCOCH2CH2CH2CH2CH2−), 1.43−1.33 (−OCOCH2CH2CH2CH2CH2−); Mn,NMR(PCL) = 5400 g mol−1, Mn,NMR(PEO-b-PCL) = 9000 g mol−1.

(0.8 M in n-hexane) and acetic acid (AcOH) were used as received. DPP (99%) was first dissolved in toluene (HPLC grade) followed by slow cryo-evaporation of toluene on the vacuum line, and then dissolved in purified toluene to prepare a 0.4 M solution. Instrumentation. Size exclusion chromatography (SEC) coupled with successively connected UV and RI detectors was conducted in THF at 35 °C using two 7.8 mm × 300 mm (5 μm) Styragel columns (Styragel HR 2 and Styragel HR 4) at a flow rate of 1.0 mL min−1. Calibration was done with a series of poly(ethylene oxide) (PEO) standards (Fluka) to obtain accurate number-average molecular weights of the PEOs, apparent number-average molecular weights (Mn,SEC) of other (co)polymers and polydispersities (Mw/Mn). Nuclear magnetic resonance (NMR) measurements were carried out at room temperature using a Bruker AVANCEDIII 600 spectrometer operating at 600 MHz; CDCl3 (Aldrich) was used as solvent. 1H NMR spectra were used to calculate the molecular weight (Mn,NMR) of the polyether precursors (the first block) using the integrals of the characteristic signals from the end group and polyether main body, as well as the block copolymers using Mn,NMR of the first block and the integrals of the characteristic signals from all the blocks. Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI−TOF MS) measurements were performed on a Bruker Ultraflex III MALDI−TOF mass spectrometer (Bruker Daltonik, Bremen, Germany). Samples were dissolved in THF (10 mg mL−1) and mixed with a solution of sodium trifluoroacetate in THF (10 mg mL−1) in a volume ratio of 5:1. This solution was then mixed with a solution of matrix, 2,5-dihdroxybenzoic acid (DHB) for polyether homopolymers or 2,4,6-trihydroxyacetophenone (THAP) for copolymer, in THF (20 mg mL−1) in a volume ratio of 1:20. Then, 0.4 μL of the final solution was spotted on the target plate (dried-droplet method). The reflective positive ion mode was used to acquire the mass spectra of the samples. The calibration was done externally with the poly(methyl methacrylate) standards using the nearest neighbor positions. Polymer Synthesis. PEO−Polyester Diblock Copolymers. For PEO precursors, typical procedure of PEO1 (Table 1): 0.15 mL of PPA (1.1 mmol), 0.28 mL of t-BuP4 solution (0.22 mmol of t-BuP4) and 40 mL of clean toluene were charged into a reaction flask. 4.5 mL of EO (90 mmol) was slowly condensed into the flask at −20 °C. The flask was then sealed by a stopcock, and temperature was slowly elevated to 40 °C. After being heated with stirring for 24 h, the flask was cooled down to room temperature. Then 5 mL of the reaction B

dx.doi.org/10.1021/ma500830v | Macromolecules XXXX, XXX, XXX−XXX

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Poly(ethylene oxide)-b-poly(δ-valerolactone) (PEO1PVL, Table 1). This synthesis followed a similar procedure as described above using 0.84 mL of VL (9.0 mmol). Polymerization of VL was quenched after stirring the reaction mixture at room temperature for 3.5 h. The product was precipitated in a cold (−20 °C) mixture of methanol and diethyl ether (1/1, v/v) and dried in vacuum. Convn(VL) = 96%; Mn,theor(PVL) = 5800 g mol−1, Mn,theor(PEO-b-PVL) = 9400 g mol−1. Mn,SEC = 8 400 g mol−1, Mw/Mn = 1.11. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 4.24−4.21 (−PEO−CH2CH2OCO−PVL), 4.21−3.93 (−OCOCH2CH2CH2CH2−), 3.77−3.49 (−CH2CH2O−), 3.48−3.44 (PhCH2CH2CH2O− PEO−), 2.70−2.66 (PhCH 2 CH 2 CH 2 O−PEO−), 2.46−2.21 (−OCOCH 2 CH 2 CH 2 CH 2 −), 1.93−1.88 (PhCH 2 CH 2 CH 2 O− PEO−), 1.88−1.50 (−OCOCH2CH2CH2CH2−); Mn,NMR(PVL) = 5700 g mol−1, Mn,NMR(PEO-b-PVL) = 9300 g mol−1. Block Copolymers Based on Poly(1,2-butylene oxide) (PBO). For PBO precursors, typical procedure of PBO2 (Table 1): 0.24 mL of PPA (1.76 mmol), 9 mL of toluene, and 8.5 mL of BO (97.7 mmol) were charged into a reaction flask. Then, 0.44 mL of t-BuP4 solution (0.35 mmol of t-BuP4) was added to the reaction mixture which was then heated at 40 °C and stirred for 48 h. A 0.1 mL aliquot was withdrawn to determine the conversion of BO. Then 46 mL of toluene was added to dilute the living PBO solution, and 6.4 mL of the diluted solution was withdrawn and quenched with AcOH, followed by removal of the solvent under vacuum. The product was analyzed by SEC and 1H NMR measurements. Convn(BO) > 99%; Mn,theor = 4000 g mol−1. Mn,SEC = 3800 g mol−1, Mw/Mn = 1.04. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 3.77−3.10 (−CH2CH(CH2CH3)O−), 2.70−2.64 (PhCH2CH2CH2O−PBO) and protons from [t-BuP4H]+, 1.92−1.85 (PhCH2CH2CH2O−PBO), 1.73−1.33 (−CH2CH(CH2CH3)O−), 1.06−0.77 (−CH2CH(CH2CH3)O−); Mn,NMR = 4100 g mol−1. Poly(1,2-butylene oxide)-b-poly(δ-valerolactone) (PBO2PVL, Table 1). About 6.4 mL of reaction mixture containing living PBO (ca. 0.18 mmol PBO−OH + PBO−O−, 0.035 mmol PBO−O−) was withdrawn with a syringe in an argon flow and injected into another flame-dried reaction flask. A 0.54 mL aliquot of DPP solution (0.22 mmol DPP) was added to this flask, upon which the brownish yellow color of PBO solution disappeared immediately. Then 15 min later, 1.5 mL of VL (16.2 mmol) was added and the reaction mixture was stirred at room temperature. Aliquots were withdrawn from time to time for SEC and 1H NMR analysis. After 4 h, the reaction was quenched by addition of ca. 1 mL of EDIPA. Then the solution was poured into cold (−20 °C) methanol to precipitate PBO-b-PVL diblock copolymer. The white sticky solid was then collected, dried in vacuum and analyzed by SEC and 1H NMR measurements. Convn(VL) = 95%; Mn,theor(PVL) = 8600 g mol−1, Mn,theor(PBO-b-PVL) = 12 600 g mol−1. Mn,SEC = 17 400 g mol−1, Mw/Mn = 1.17. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 4.96−4.88 (−PBO−CH2 CH(CH 2 CH 3 )OCO−PVL), 4.22−3.92 (−OCOCH2CH2CH2CH2−), 3.73−3.15 (−CH2CH(CH2CH3)O−), 2.70−2.65 (PhCH2CH2CH2O−PBO−), 2.47−2.20 (−OCOCH 2 CH 2 CH 2 CH 2 −), 1.92−1.85 (PhCH 2 CH 2 CH 2 O− PBO−), 1.83−1.63 (−OCOCH 2 CH 2 CH 2 CH 2 −), 1.63−1.41 (−CH2CH(CH2CH3)O−), 1.05−0.77 (−CH2CH(CH2CH3)O−); Mn,NMR(PVL) = 14 700 g mol−1, Mn,NMR(PBO-b-PVL) = 18 800 g mol−1. Poly(1,2-butylene oxide)-b-poly(ethylene oxide) (PBO2PEO, Table 1). First, 3.5 mL of EO (70 mmol) was condensed into the rest of the living PBO solution (ca. 1.4 mmol PBO−OH + PBO−O−, 0.28 mmol PBO−O−). The solution was stirred and heated at 40 °C for 48 h to ensure complete consumption of EO. Then 6.4 mL of the solution was withdrawn and quenched with AcOH, followed by removal of the solvent under vacuum. The product was analyzed by SEC and 1H NMR measurements. Mn,theor(assuming complete EO conversion) = 6200 g mol−1. Mn,SEC = 6200 g mol−1, Mw/Mn = 1.04. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 3.77−3.15 (−CH2CH(CH2CH3)O− and −CH2CH2O−), 2.70−2.63 (PhCH2CH2CH2O−PBO) and protons from [t-BuP4H]+, 1.92−1.85 (PhCH 2 CH 2 CH 2 O−PBO), 1.72−1.35 (−CH 2 CH-

(CH2CH3)O−), 1.05−0.77 (−CH2CH(CH2CH3)O−); Mn,NMR(PEO) = 2200 g mol−1, Mn,NMR(PBO-b-PEO) = 6300 g mol−1. Poly(1,2-butylene oxide)-b-poly(ethylene oxide)-b-poly(δ-valerolactone) (PBO2PEOPVL, Table 1). About 5.0 mL of reaction mixture containing living PBO-b-PEO (ca. 0.14 mmol PBO-b-PEO−OH + PBO-b-PEO−O−, 0.028 mmol PBO-b-PEO−O−) was withdrawn with a syringe in an argon flow and injected into another flame-dried reaction flask. Then 0.45 mL of DPP solution (0.18 mmol DPP) was added to this flask, upon which the brownish yellow color of the solution disappeared immediately. After 15 min, 1.0 mL of VL (10.8 mmol) was added, and the reaction mixture was stirred at room temperature. Aliquots were withdrawn from time to time for SEC and 1 H NMR analysis. After 4 h, the reaction was quenched by addition of ca. 1 mL of EDIPA. The product was precipitated in cold (−20 °C) methanol, collected and dried in vacuum. Convn(VL) = 96%; Mn,theor(PVL) = 7400 g mol−1, Mn,theor(PBO-b-PEO-b-PVL) = 13 600 g mol−1. Mn,SEC = 12 300 g mol−1, Mw/Mn = 1.13. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 4.24−4.21 (−PEO−CH 2 CH 2 OCO−PVL), 4.21−3.93 (−OCOCH2CH2CH2CH2−), 3.77−3.15 (−CH2CH(CH2CH3)O− and −CH2CH2O−), 2.70−2.66 (PhCH2CH2CH2O−PBO−), 2.46− 2.21 (−OCOCH2CH2CH2CH2−), 1.92−1.85 (PhCH2CH2CH2O− PBO−), 1.83−1.63 (−OCOCH 2 CH 2 CH 2 CH 2 −), 1.63−1.41 (−CH2CH(CH2CH3)O−), 1.05−0.78 (−CH2CH(CH2CH3)O−); Mn,NMR(PVL) = 8000 g mol−1, Mn,NMR(PBO-b-PEO-b-PVL) = 14 300 g mol−1. Poly(1,2-butylene oxide)-b-poly(ethylene oxide)-b-poly(ε-caprolactone) (PBO2PEOPCL, Table 1). This was synthesized in a similar manner using 1.2 mL of CL (10.8 mmol). Polymerization of CL was quenched after stirring the reaction mixture at 40 °C for 8 h. The product was precipitated in cold (−20 °C) methanol, collected and dried in vacuum. Convn(CL) = 85%; Mn,theor(PCL) = 7500 g mol−1, Mn,theor(PBO-b-PEO-b-PCL) = 13 700 g mol−1. Mn,SEC = 15 100 g mol−1, Mw/Mn = 1.09. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20− 7.15 (aromatic protons on the end group), 4.24−4.21 (−PEO− CH2CH2OCO−PCL), 4.20−3.91 (−OCOCH2CH2CH2CH2CH2−), 3.77−3.15 (−CH2CH(CH2CH3)O− and −CH2CH2O−), 2.70−2.66 (PhCH 2 CH 2 CH 2 O−PBO−), 2.43−2.17 (−OCOCH 2 CH 2 CH 2 CH2CH2−), 1.92−1.85 (PhCH2CH2CH2O−PBO−), 1.78−1.62 (−OCOCH2CH2CH2CH2CH2−), 1.62−1.42 (−CH2CH(CH2CH3)O−), 1.42−1.33 (−OCOCH 2 CH 2 CH 2 CH 2 CH 2 −), 1.05−0.78 (−CH 2 CH(CH 2 CH 3 )O−); M n,NMR (PCL) = 8100 g mol −1 , Mn,NMR(PBO-b-PEO-b-PCL) = 14 400 g mol−1. Poly(1,2-butylene oxide)-b-poly(ethylene oxide)-b-poly(trimethylene carbonate) (PBO2PEOPTMC2, Table 1). This was also synthesized in a similar manner using the “catalyst switch” strategy: About 5.0 mL of reaction mixture containing living PBO-bPEO (ca. 0.14 mmol PBO-b-PEO−OH + PBO-b-PEO−O−, 0.028 mmol PBO-b-PEO−O−) was withdrawn and injected into another flame-dried reaction flask. 0.45 mL of DPP solution (0.18 mmol DPP) was introduced and the solution was heated to 40 °C. After 15 min, 12 mL of preheated (40 °C) toluene solution of TMC (11.7 mmol TMC) was added. The reaction mixture was stirred at 40 °C for 48 h, during which aliquots were withdrawn from time to time for SEC and 1H NMR analysis. The reaction was quenched by addition of ca. 1 mL of EDIPA. The product was precipitated in cold (−20 °C) methanol, collected and dried in vacuum. Convn(TMC) = 96%; Mn,theor(PTMC) = 8 200 g mol−1, Mn,theor(PBO-b-PEO-b-PTMC) = 14 400 g mol−1. Mn,SEC = 13 200 g mol−1, Mw/Mn = 1.07. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.20−7.15 (aromatic protons on the end group), 4.38−4.08 (−PEO−CH2CH2OCOO−PTMC and −OCOOCH2CH2CH2−), 3.78−3.15 (−CH2CH(CH2CH3)O− and −CH2CH2O−), 2.70−2.66 (PhCH2CH2CH2O−PBO−), 2.18−1.84 (−OCOOCH2CH2CH2− and PhCH2CH2CH2O−PBO−), 1.70−1.40 (−CH2CH(CH2CH3)O−), 1.05−0.78 (−CH2CH(CH2CH3)O−); Mn,NMR(PTMC) = 8300 g mol−1, Mn,NMR(PBO-b-PEO-b-PTMC) = 14 600 g mol−1. C

dx.doi.org/10.1021/ma500830v | Macromolecules XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Well-controlled ROP of CL and VL with DPP as a catalyst has already been demonstrated previously.36,37 To test the catalytic efficiency of DPP for the ROP of epoxides, a controlled experiment was done to polymerize EO in toluene with PPA as initiator and DPP as catalyst ([EO]0 = 2 M, [EO]0/[PPA]0/ [DPP] = 80/1/1). After the reaction mixture was heated at 40 °C for 48 h, no polymer was formed, as indicated by SEC and 1 H NMR analysis. ROP of epoxides catalyzed by extremely strong organic or inorganic Brønsted acids (much stronger than DPP) have been known for decades.41−44 However, due to the mechanistic complications, extensive occurrence of side reactions (e.g., transesterification scrambling reactions and the formation of 1,4-dioxane byproduct), and the fact that most of the systems do not work for substituted epoxides, preparation of well-defined polyether-based materials seems to be very limited with this method. Therefore, in the present study, we have chosen the well-controlled t-BuP4-catalyzed anionic ROP for the epoxides. EO was polymerized in toluene with PPA as initiator in the presence of 0.2 equiv of t-BuP4 (PEO1 in Table 1). The polymerization was allowed to proceed at 40 °C for 24 h to ensure complete consumption of EO. The aliquot withdrawn at the end of the polymerization was analyzed by SEC, NMR and MALDI−TOF MS. As PEO standards were used for calibration, the molecular weight of PEO samples determined by SEC (Mn,SEC, Table 1) can be considered accurate. The obtained PEO1 (before isolation) shows a Mn,SEC of 3500 g mol−1, which fits well with the theoretical molecular weight calculated from the feed ratio (Mn,theor = 3600 g mol−1), and a low polydispersity (Mw/Mn = 1.04). 1H NMR spectrum of isolated PEO1 shows all the characteristic signals of both PEO and PPA moieties (Figure S1, Supporting Information). The molecular weight calculated from the peak integrals (Mn,NMR = 3600 g mol−1) agrees well with Mn,theor and Mn,SEC. MALDI− TOF MS also confirms the expected structure and molecular weight (Figure S2). Portions of the solution containing the living PEO1 precursor were withdrawn and used for the sequential ROP of the cyclic esters. To the first portion, CL was added directly (Table 1, PEO1PCL1). Nearly quantitative conversion of CL was reached in 1 min, however, the product has a high polydispersity and a seemingly bimodal distribution as shown by SEC analysis (Table 1 and Figure 1), which is probably due to the insufficient proton transfer rate (compared to the chain growth) and the extensive occurrence of chain transfer reaction simultaneously with the chain growth. This experiment demonstrates that using t-BuP4 as a single catalyst for the sequential ROP of epoxide and cyclic ester is not an ideal choice. To the other portions of the living PEO1 solution, an excess of DPP (1.2 equiv with regard to PPA) was added for the neutralization of phosphazenium alkoxide (living PEO chain end), followed by the addition of CL or VL to form PEO-bPCL and PEO-b-PVL diblock copolymers (Scheme 1; [CL]0 = [VL]0 = 1.3 M). Aliquots were withdrawn from the reaction mixture for 1H NMR and SEC analysis. Linear kinetic plots are obtained for both CL and VL (Figure 2). VL shows a much faster polymerization rate and reaches a conversion of 96% in 3.5 h (PEO1PVL in Table 1). The conversion of CL reaches only 37% in 8 h at room temperature (PEO1PCL2 in Table 1). An elevated temperature (40 °C, PEO1PCL3 in Table 1)

Figure 1. SEC traces of the products (PEO1 and PEO1PCL1 in Table 1) from sequential ROP of EO and CL using t-BuP4 as a single catalyst.

speeds up the polymerization and the conversion reaches 83% in 8 h. Molecular weight of the diblock copolymer shows a linear dependence on the conversion of both CL and VL (Figure S3) with a slightly increased polydispersity at high conversion. Figure 3 shows the SEC traces of the isolated products of PEO1 and the diblock copolymers derived from it (PEO1PCL2, PEO1PCL3 and PEO1PVL in Table 1). Figure 4 shows the 1H NMR spectrum of a representative PEO1-based diblock copolymer (PEO1PCL3 in Table 1), presenting all the characteristic signals and fitting integrals from the main bodies of the two blocks, from the end groups and from the monomeric units linking the two blocks. As can be seen in Table 1, all the products have low polydispersities, and the molecular weights calculated from 1H NMR spectra (Mn,NMR) fit well with the theoretical values (Mn,theor, Table 1). MALDI− TOF MS measurement was performed on a PEO-b-PCL diblock copolymer with a short PCL block (PEO1PCL2 in Table 1), which further confirms the well-defined PEO-b-PCL structure (Figure 5). Such results demonstrate the effectiveness of the “catalyst switch” strategy in the sequential block copolymerization of EO and cyclic esters for the preparation of well-defined diblock copolymers based on PEO and polyesters. In the present “catalyst switch” system, the ROP of the cyclic esters were actually performed in the presence of both DPP and its deprotonated form (phosphazenium diphenyl phosphate salt) with the ratio of the two being 1/0.2 (mol/mol). Then a question was raised if the existence of the latter had any impact on the polymerization. To answer such a question, two controlled experiments were designed and carried out. In the first one, CL was polymerized with PPA as initiator and 1.0 equiv of DPP as catalyst in toluene at 40 °C. The second one followed the same procedure as the “catalyst switch” strategy; namely, PPA was first mixed with 0.2 equiv of t-BuP4, then 1.2 equiv of DPP and a desired amount of CL were successively added. In this way, the polymerizations of CL were conducted with the same monomer concentration and [CL]0/[PPA]0/ [DPP] ratio with or without the presence of phosphazenium diphenyl phosphate salt (0.2 equiv of DPP and PPA). Kinetic study shows a lower polymerization rate for the second controlled experiment (Figure S4), for which the reason is not yet clear, but it seems that the presence of phosphazenium diphenyl phosphate salt has slowed down the DPP-catalyzed D

dx.doi.org/10.1021/ma500830v | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. General Reaction Scheme toward the Synthesis of Polyether−Polyester Diblock Copolymers via Sequential MetalFree ROP of Epoxides and Cyclic Esters Using the “Catalyst Switch” Strategy

Figure 4. 1H NMR spectrum of a representative PEO-based diblock copolymer (PEO1PCL3 in Table 1) prepared using the “catalyst switch” strategy after precipitation and drying.

Figure 2. Kinetic plots of CL (PEO1PCL2 and PEO1PCL3 in Table 1) and VL (PEO1PVL in Table 1) in their sequential ROP with EO using the “catalyst switch” strategy.

polyether does not have an impact on the acid-catalyzed ROP of cyclic esters (Figure S4).45 BO was polymerized similarly to EO ([BO]0 = 1.4 M, PBO1 in Table 1). After heating at 40 °C for 72 h, a BO conversion of 41% was reached. The “catalyst switch” strategy was applied with a portion of living PBO1 solution to polymerize VL aiming at a PBO-b-PVL diblock copolymer. However, nearly no VL polymerization was observed after 48 h as demonstrated by SEC (Figure S6) and 1H NMR analysis. The presence of residual BO monomer seems to be the reason for the nonoccurrence of the DPP-catalyzed ROP of VL. Because of the basicity from its cyclic ether nature, the epoxide monomer most probably formed hydrogen bonds with DPP or was even added to the P−OH bond,46 thus preventing the ROP of the cyclic ester. As a consequence of a similar effect, the “catalyst switch” strategy applied in THF aiming at a PEO-b-PCL diblock copolymer led to a CL conversion of