Facile Synthesis of Aliphatic ω-Pentadecalactone Containing Diblock

Dec 21, 2018 - College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. •S Supporting Information...
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Facile Synthesis of Aliphatic ω‑Pentadecalactone Containing Diblock Copolyesters via Sequential ROP with L‑Lactide, ε‑Caprolactone, and δ‑Valerolactone Catalyzed by Cyclic Trimeric Phosphazene Base with Inherent Tribasic Characteristics Na Zhao,† Chuanli Ren,† Yong Shen,‡ Shaofeng Liu,*,† and Zhibo Li*,† Macromolecules Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/28/19. For personal use only.

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Key Laboratory of Biobased Polymer Materials, Shandong Provincial Education Department; College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China ‡ College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China S Supporting Information *

ABSTRACT: Degradable long-chain aliphatic polyesters are promising alternatives to polyolefins. The ring-opening polymerization (ROP) of ω-pentadecalactone (PDL) enables the synthesis of aliphatic polyesters with melting temperature close to low-density polyethylene (LDPE). However, this ROP reaction is of great challenge because of low ring strain of the PDL monomer. The occurrence of intra- and intermolecular transesterification reactions during ROP made it difficult to prepare well-defined block copolyesters for advanced properties. In this context, cyclic trimeric phosphazene base (CTPB) in combination with benzyl alcohol (BnOH) was proved to be an efficient initiator system for ROP of PDL, showing an excellent polymerization rate (TOF up to 600 h−1) at 80 °C. Well-defined diblock copolyesters, i.e., PPDL-b-PLLA, can be easily synthesized by sequential addition of PDL and L-lactide (L-LA). In contrast, sequential addition of PDL and ε-caprolactone (CL) or δvalerolactone (VL) only led to the random copolyesters under the same conditions. On the other hand, CTPB is an inherent tribasic superbase and can be partially neutralized to decrease its basicity for better control over active lactones such as CL and VL. Hence, we introduced 0.33−1.0 equiv of benzoic acid (PhCOOH) relative to CTPB and demonstrated that it can diminish the transesterification during ROP of CL or VL. Based on this strategy, PPDL-b-PCL or PPDL-b-PVL diblock copolyesters were successfully prepared by sequential addition of monomers of CL or VL after ROP of PDL. The block copolyester structures were confirmed by the 13C NMR spectroscopy and DSC characterizations.

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INTRODUCTION As one of the fastest growing industries in the world, the global plastics demand is growing steadily each year. Unfortunately, the source of most commercial plastics is petroleum, and almost half of the total amount of plastics is only used short term for packing applications.1 The use of biodegradable polymers that show competing properties with conventional non-biodegradable plastics, e.g., polyolefins, is believed as a potential way to solve the “white pollution” environmental problem.2−4 Aliphatic polyesters are one of the most attractive biodegradable polymers with good mechanical properties, biodegradability and biocompatibility.5−10 However, the applications of most aliphatic polyesters are quite limited due to their low melting points, which lead to the thermoplastic processing problems and undesired softening at elevated temperature. By increasing the amount of long, crystallizable methylene segments within aliphatic polyesters, melting points akin to linear polyethylene (PE) could be obtained.11 Long © XXXX American Chemical Society

chain polyesters have been successfully synthesized via stepgrowth polycondensation approaches and ring-opening polymerization (ROP) of macrolactones,12−15 of which the latter allowing the controlled synthesis of well-defined polyesters with high molecular weights has attracted much attention. Unlike the strained small and medium sized lactones, macrolactones with more than 12 ring members are relatively strain free; thus, the driving force for the ROP of these large rings is the entropy effect.16 The ROP of macrolactones, such as ω-pentadecalactone (PDL), has been extensively investigated during the past two decades.11 Metal-based catalysts, including potassium alkoxides,17 yttrium catalyst,15,18 and other metal complexs,19−25 can effectively catalyze the ROP of PDL. Considering the increasing demand of metal-free materials in biomedical and microelectronic application, organocatalysis has become a powerful alternative to metalReceived: December 21, 2018

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DOI: 10.1021/acs.macromol.8b02690 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules based catalysts in ROP and other polymerizations.26−31 In regard to the challenging nonstrained macrolactone, however, only limited organocatalysts have been proved to be active in the ROP of PDL (Scheme 1), but with relatively low turnover

purchased from commercial suppliers and used without further purification unless otherwise noted. Instruments. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE NEO 400 MHz NMR spectrometer (400 MHz for 1H NMR and 100 MHz for 13C NMR). Chemical shifts were reported in δ (ppm), and the residual deuterated solvent peak was used as reference. Quantitative 13C NMR were recorded to determine the relative fractions of diad repeat unit sequences. The parameters used were as follows: 8.0% w/w polymer in CDCl3, temperature 25 °C, pulse width 90°, 18000 data points, relaxation delay 5.0 s, and the pulse program was zgig. Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDITOF MS) analyses were conducted on a Bruker Microflex MALDITOF MS spectrometer equipped with a 337 nm nitrogen laser. The polymer was dissolved in chloroform; trans-2-[3-(4-tert-butylphenyl)2-methyl-2-propenylidene]malononitrile (DCTB) was used as the matrix and CF3COOAg as the cationizing agent. The gel permeation chromatography (GPC) measurements were collected on a Wyatt OPTILAB rEX refractive index detector using chloroform as the eluent (flow rate: 1 mL/min, at 35 °C). The molecular weights and polydispersity (Đ) were calculated using polystyrene standards with narrow molecular weight distribution as references. The sample concentration used for GPC analysis was ca. 10 mg/mL. Differential scanning calorimetry (DSC) was performed using a TA differential scanning calorimeter DSC 25 that was calibrated using high purity indium at a heating rate of 10 °C/min. Melting points were determined from the second scan at a heating rate of 10 °C/min following a slow cooling rate of 10 °C/min to remove the influence of thermal history. General Procedures for Homopolymerization of PDL. Polymerizations were performed in 25 mL flame-dried Schlenk tubes interfaced to the dual-manifold Schlenk line. The reactor was charged with a predetermined amount of CTPB, initiator, and solvent and kept stirring for 10 min in the glovebox. The reactor was sealed, taken out of the glovebox, and immersed in the oil bath under the predetermined temperature. After equilibration at the desired polymerization temperature for 10 min, the polymerization was initiated by rapid addition of PDL solution (2.0 M in toluene) via a syringe. After a desired period of time, the polymerization was quenched by addition of benzoic acid solution (10 mg/mL in chloroform). The quenched mixture was then precipitated with 100 mL of methanol, filtered, washed by methanol to remove unreacted monomer, and dried in a vacuum oven at 30 °C to a constant weight. Preparation of PPDL-b-PLLA Copolyesters. 3.1 μL of BnOH (0.03 mmol), 0.15 mL of CTPB solution (0.2 M in toluene, containing 0.03 mmol of CTPB), and 1.5 mL of dry toluene were charged into a 25 mL flame-dried Schlenk tube and kept stirring for 10 min in the glovebox. The reactor was sealed, taken out of glovebox, and immersed in the oil bath at 80 °C. After equilibration at the polymerization temperature for 10 min, the polymerization was initiated by rapid addition of 1.5 mL of PDL solution (2.0 M in toluene, containing 3.0 mmol of PDL) via a syringe. After heating and stirring for 10 min, 3.0 mL of L-LA solution (1.0 M in THF, containing 3.0 mmol of L-LA) was added in a nitrogen flow. After 10 min at 80 °C, the polymerization was quenched by addition of 3 mL of benzoic acid solution (10 mg/mL in chloroform). The quenched mixture was then precipitated with 100 mL of methanol, filtered, washed by methanol to remove unreacted monomer, and dried in a vacuum oven at 30 °C to a constant weight. General Procedures for Homopolymerization of PDL or CL by CTPB with Benzoic Acid. Polymerizations were performed in 25 mL flame-dried Schlenk tubes interfaced to the dual-manifold Schlenk line. The reactor was charged with a predetermined amount of CTPB, initiator, and solvent and kept stirring for 10 min in the glovebox. The reactor was sealed, taken out of the glovebox, and immersed in the oil bath under the predetermined temperature. After equilibration at the desired polymerization temperature for 10 min, benzoic acid solution (0.04 M in toluene) was added in a nitrogen flow. Then the polymerization was initiated by rapid addition of monomer (CL or PDL) via a syringe. After a desired period of time, the polymerization

Scheme 1. Representative Organocatalysts for the ROP of PDL

frequency (TOF).32−38 On the other hand, copolymerization of PDL with other lactones normally affords random copolyesters due to the fast transesterification.39,40 So far, only few examples, such as Zn/Ca complexes19,20 and Zn(C6F5)2-based Lewis pairs,25 could afford well-defined block copolyesters without apparent transesterification. Interestingly, by use of the “catalyst-switch” strategy, PPDL-b-PCL block copolyesters could be obtained via the strong superbase t-BuP4 and the weaker analogue t-BuP2 but required a long reaction time to achieve high conversions (>2 days).35 In our recent work, a novel cyclic trimeric phosphazene base (CTPB, Scheme 1) with moderate basicity (pKa = 33.3 in MeCN by NMR) was synthesized and exhibited high efficiency in the ROP of γ-butyrolactone (γ-BL), ε-caprolactone (CL), and rac-lactide (rac-LA).41−44 Besides, one-pot preparation of PBL-b-PLLA by sequential ROP of γ-BL and L-LA could be achieved in the presence of CTPB as a catalyst.45 Most recently, amphiphilic PEG-b-PBL diblocks can be prepared by using PEG-OH as macroinitiatior and CTPB as a catalyst.46 These results motivate us to employ organic CTPB as catalyst to conduct the ROP of PDL and the subsequent copolymerizations. Remarkably, CTPB with alcohol as initiator shows an extremely high activity for ROP of PDL at 80 °C with TOF as high as 600 h−1. The well-defined block copolyester PPDLb-PLLA can be easily synthesized by sequential polymerization catalyzed by CTPB. A less basic catalytic system formed by mixing CTPB with 1/3 to 1 equiv of benzoic acid could afford well-defined block copolyesters PPDL-b-PCL and PPDL-bPVL via sequential addition without inter-transesterification between the PPDL and PCL/PVL segments.

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EXPERIMENTAL SECTION

Materials. Tetrahydrofuran (THF) and toluene were purified by purging with dry nitrogen, followed by passing through columns of activated alumina. PDL was purchased from Energy Chemical Co, dissolved in toluene (2.0 M) and dried twice over CaH2 under a nitrogen atmosphere overnight. Benzyl alcohol (BnOH), CL, and VL were purchased from Aladdin Co. and stirred with CaH2 for 24 h, then distilled under reduced pressure, and stored over activated 4 Å molecular sieves in a glovebox. L-LA was purchased from TCI and recrystallized twice from toluene. CTPB was synthesized according to the procedure reported previously.41 All other chemicals were B

DOI: 10.1021/acs.macromol.8b02690 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. ROP of PDL Initiated by the CTPB/Alcohol Systema run

initiator

M/B/I

temp (°C)

time (min)

convb (%)

Mn,theor (kg mol−1)

Mn,GPCc (kg mol−1)

Đc

1 2 3 4 5 6 7 8 9d

BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH PEG 2000

100/1/1 100/1/1 100/1/1 100/1/1 100/1/1 20/1/1 50/1/1 200/1/1 50/1/1

RT RT RT 60 80 80 80 80 80

180 720 1080 10 10 2 5 30 60

21 61 82 56 >99 >99 >99 66 >99

5.1 14.8 19.8 13.5 23.8 4.9 12.6 35.0 14.0

13.5 18.6 20.3 34.7 7.1 14.5 44.1 18.4

2.08 2.21 1.76 1.80 2.08 2.01 1.78 1.74

a Conditions: CTPB 0.03 mmol; M/B/I = monomer/CTPB/initiator; [PDL]0 was 1.0 mol L−1; the base and initiator were mixed first in toluene, followed by addition of PDL. bDetermined by 1H NMR. cDetermined by GPC at 35 °C in chloroform relative to polystyrene standards. dPEG 2000 and CTPB were dissolved in THF first, and then PDL was added.

Figure 1. (a) GPC curves of PPDL samples with different molecular weights in Table 1, runs 5−8. (b) MALDI-TOF mass spectrum of PPDL produced by PDL/CTPB/BnOH = 20/1/1 for 2 min (Table 1, run 6).

to our previous work.41 A series of experiments were conducted to evaluate the polymerization behavior of CTPB as catalysts for the ROP of nonstrained macrolactone PDL. The ROP of PDL was first screened with 1 mol % CTPB and BnOH as initiator in toluene at room temperature (RT), achieving 21% conversion in 3 h (Table 1, run 1), 61% conversion in 12 h (Table 1, run 2), and 82% conversion in 18 h (Table 1, run 3). When the reaction temperature increased to 60 °C, the monomer conversion increased to 56% within 10 min (Table 1, run 4). With the further increase of reaction temperature to 80 °C, the current catalytic system CTPB/ BnOH showed extraordinary activity for ROP of PDL. High activity and conversion obtained at high temperature are probably because heating can significantly increase the solubility of PPDL polymer, the polymerization rate, and the equilibrium conversion. A quantitative conversion was achieved within 10 min, and the resulting PPDL had high molecular weights (Mn = 34.7 kg mol−1; Table 1, run 5). The MW distribution was relatively broad (Đ = 1.80) even at relatively low conversions (Table S1), indicating the existence of transesterification reactions, and this phenomenon was also observed for other catalytic systems.32,34,47 Note that the polymerization rate (TOF 600 h−1) at 80 °C by CTPB is much faster than previously reported organocatalysts32,34 and comparable to the most active metal-based catalysts.21,22,24 For example, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) in

was quenched by addition of benzoic acid solution (10 mg/mL in chloroform). The quenched mixture was then precipitated with 100 mL of methanol, filtered, washed by methanol to remove unreacted monomer, and dried in a vacuum oven at 30 °C to a constant weight. General Procedures for the Preparation of Block Copolyesters of PDL and CL/VL Catalyzed by CTPB with Benzoic Acid. Polymerizations were performed in 25 mL flame-dried Schlenk tubes interfaced to the dual-manifold Schlenk line. The reactor was charged with a predetermined amount of CTPB, initiator, and solvent and kept stirring for 10 min in the glovebox. The reactor was sealed, taken out of glovebox, and immersed in the oil bath under the predetermined temperature. After equilibration at the desired polymerization temperature for 10 min, the polymerization was initiated by rapid addition of PDL solution (2.0 M in toluene) via a syringe. After 10 min, benzoic acid solution (0.04 M in toluene) was added in a nitrogen flow, followed by the addition of CL or VL. After a desired period of time, the polymerization was quenched by addition of benzoic acid solution (10 mg/mL in chloroform). The quenched mixture was then precipitated with 100 mL of methanol, filtered, washed by methanol to remove unreacted monomer, and dried in a vacuum oven at 30 °C to a constant weight.

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RESULTS AND DISCUSSION Homopolymerization of PDL Using the CTPB/Alcohol System. CTPB could be readily prepared in a multigram scale from commercially available hexachlorocyclotriphosphazene (HCCP) and an excess of iminotris(dimethylamino)phosphorene in toluene with convenient work-up according C

DOI: 10.1021/acs.macromol.8b02690 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Copolymerization of PDL/L-LA Using the CTPB/BnOH Systema run b

1 2c 3c 4c

PDL/LLA/B/I

[M]0 (mol/L)

temp (°C)

time (min)

CPDLd (%)

CL‑LAd (%)

Mn,theore

Mn,GPCf

Đf

50/100/1/1 (one-pot) 50/100/1/1 (PDL + LA) 10/100/1/1 (PDL + LA) 100/100/1/1 (PDL + LA)

1.0 1.0 1.0 1.0

20 80 80 80

10 10 + 10 10 + 10 10 + 10

0 >99 >99 >99

>99 79 90 80

14.5 23.5 15.5 35.5

13.9 28.6 14.8 37.8

1.45 1.69 1.82 1.73

Conditions: CTPB 0.03 mmol; [M]0 = [PDL]0 + [L-LA]0 = 1.0 mol L−1; the base and initiator were mixed first in toluene, followed by monomer addition. bOne-pot addition of PDL and L-LA. cSequential addition of PDL and L-LA. dDetermined by 1H NMR. eIn units of kg mol−1. f Determined by GPC at 35 °C in chloroform relative to PS standards; Mn,GPC is in units of kg mol−1. a

Figure 2. (a) GPC curves of PPDL (Table 1, run 7) and its block copolyesters with L-LA (Table 2, runs 1−4). (b) 13C NMR spectrum in CDCl3 of the block copolyester PPDL-b-PLLA (Table 2, run 2).

6) mainly exhibited molecular ion peaks corresponding to linear PPDL with BnO/H chain ends [Mn = 240.38n + 108.14 + 107.87 (Ag+) (g mol−1)]. Because of solubility issue and ionization difficulty, it is hard to obtained MALDI-TOF spectra of high molecular weight PPDL homopolymers. A small amount of cyclic species with lower molecular weights was also observed in the polymer with no chain ends [Mn = 240.38n + 107.87 (Ag+) (g mol−1)]. A poly(ethylene glycol) with molecular weight of 2.0 kg mol−1 (PEG 2000) was tested as the macroinitiator and provided high PDL conversion of 99% within 1 h (Table 1, run 9). 1H and 13C NMR analyses of the produced PEG-b-PPDL in CDCl3 (see the Supporting Information, Figures S1 and S2) gave the molecular weight of the copolyester as 17.5 kg mol−1, which was close to the value by GPC (18.4 kg mol−1). Copolymerization of PDL with L-LA Catalyzed by CTPB. ROP of PDL and L-LA was conducted by the one-pot addition approach of both monomers ([PDL + L-LA]0 = 1.0 M) in toluene at room temperature to investigate the copolymerization behavior. Premixed CTPB/BnOH solution was added to the mixture solution of PDL and L-LA and polymerized for 10 min. Monomer consumption was monitored individually by 1H NMR, and the result showed that L-LA was completely converted to polymer while no conversion of PDL was observed (Table 2, run 1). Even after prolonged reaction times of up to 2 h (Table S2, run 1) or polymerized at elevated temperature of 80 °C (Table S2, run 2), only PLLA homopolymer was obtained. These results proved that the PLLA growing chain showed no reactivity toward PDL, which was in agreement with the findings for the copolymerization of L-LA and PDL using aluminum Salen catalysts reported in the literature.47

combination with BnOH as initiator catalyzed ROP of PDL only with a TOF of 7.4 h−1 (1% loading of catalyst, 100 °C, 5 h, [PDL] = 2.5 mol L−1, 37% conversion).32 1-tert-Butyl-4,4,4t r i s ( d i m e t h y l a m i n o ) - 2 , 2 -b i s [ t r i s ( d i m e t h y l a m i n o ) phosphoranylid-enamino]-2Λ5,4Λ5-catenadi(phosphazene) (tBuP4), one of the most basic compounds, achieved 91% conversion of PDL with a TOF of 91 h−1 (1% loading of catalyst, 80 °C, 1 h, [PDL] = 0.7 mol L−1, BnOH as initiator).34 The molecule structure of CTPB determined by Xray diffraction is a big sphere with a 1.4 nm diameter and high symmetry.41 In the presence of BnOH as initiator, CTPB as catalyst is basic to react with BnOH and initiate the ROP reactions. Besides, the bulky and symmetric counterion could facilitate the formation of a loose ion pair, e.g., [BnO]−[CTPBH]+, and lead to high activity. Therefore, it is not a surprise that CTPB has moderate basicity (pKa = 33.3 in MeCN) but exhibits high activity toward ROP of cyclic esters, including ω-PDL in this work and also LA, ε-CL, γ-BL, and δVL in our previous reports.31,41−43 To prepare PPDLs with different molecular weights, the ratio of CTPB/BnOH was fixed at 1/1, and the ROP with 20− 200 equiv of PDL was conducted at 80 °C (Table 1, runs 5− 8). A faster polymerization was observed when 5 mol % of CTPB was used (Table 1, run 6), but the distribution (Đ = 2.08) became broader than that obtained by the 1 mol % CTPB loading (Table 1 run 5, Đ = 1.80), indicating the enhanced inter/intrachain transesterifications with the increased catalyst concentrations. With the PDL/BnOH ratio changed from 20/1 to 200/1, the molecular weights of the resulting polymers increased accordingly from 7.1 to 44.1 kg mol−1, and the GPC curves are shown in Figure 1a. The MALDI-TOF MS analysis of the resulting PPDL with relatively low molecular weight (Figure 1b and Table 1, run D

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Macromolecules Table 3. Copolymerization of PDL/CL Using the CTPB/BnOH Systema run

PDL/CL/B/I

[M]0 (mol/L)

temp (°C)

time (min)

convb (%)

Tm (°C)

Mn,theor (kg mol−1)

Mn,GPCc (kg mol−1)

Đc

1 2 3

100/100/1/1 (one-pot) 100/100/1/1 (CL + PDL) 100/100/1/1 (PDL + CL)

1.0 1.0 1.0

20 20 80

30 5 + 30 10 + 5

81 >99 >99

74.8 72.8 80.8

28.8 35.5 35.5

36.4 42.1 47.5

2.01 1.91 2.17

Conditions: CTPB 0.03 mmol; [M]0 = [PDL]0 + [CL]0 = 1.0 mol L−1; the base and initiator were mixed first in toluene, followed by the addition of monomers. bTotal conversion of PDL and CL determined by 1H NMR. cDetermined by GPC at 35 °C in chloroform relative to PS standards. a

Figure 3. (a) 13C NMR spectrum in CDCl3 of the random copolyester (black spectrum, Table 3, run 1) and block copolyester (red spectrum, Table 4, run 8) of PDL and CL. (b) GPC curves of PPDL (Table 1, run 7) and its block copolyester with CL (Table 4, run 8).

respectively. Similarly, the two peaks at δ = 64.40 and 68.99 ppm in methine region referred to PDL*−PDL and LA*−LA segments. Besides, the DSC curve of the PPD-b-PLA copolyester (Table 2, run 2) is given in Figure S7 and shows two melting points at 92.0 and 140.8 °C assigned to the PPDLblock and PLA-block, respectively. These results indicated the formation of PPDL-b-PLLA diblock copolyester. The relatively low Tm of PLLA block is probably due to the partial epimerization on the α protons of LA during the ROP reactions, indicated by the homonulcear decoupled 1H NMR of PLLA obtained under similar conditions (Figure S21). Varying the PDL/L-LA ratios from 10/100 to 50/100 to 100/ 100 (Table 2, runs 2−4), the molecular weights of the block copolyester increased from 14.8 to 37.8 kg mol−1 accordingly (Figure 2a, purple, green, and blue curves), which made it possible to prepare PPDL-b-PLLAs with controllable molecular weights and monomer compositions. The GPC curve for the block copolyester obtained in Table 2, run 3, with a short PDL-block (10 units of PDL) and a relatively long LA-block (71 units of LA) is shown in Figure S9 (black curve), in comparison with the GPC curve (Figure S9, red curve) for PDL homopolymer obtained in the first sequential polymerization in the same experiment. These two curves are well separated, and no PDL homopolymer tailing was observed, indicating all the PDL chains were active for ROP of L-LA and formed PPDL-b-PLLA diblock copolyesters under these conditions. Copolymerization of PDL with CL Catalyzed by CTPB. With CTPB as the catalyst, the copolymerization of PDL and CL via one-pot addition reached 81% conversion in 30 min at room temperature (Table 3, run 1). It is worth noting that ω-

Based on these results, copolymerization of PDL with L-LA was conducted via sequential addition of PDL and L-LA to the CTPB/BnOH solution (Table 2, runs 2−4) to prepare block copolyester. PDL was allowed to achieve complete monomer conversion in toluene at 80 °C, after which THF solution of LLA was successively added at 80 °C. The second monomer LLA also reached high conversion, and the molecular weight of the resultant block polymer was increased from 14.5 to 28.6 kg mol−1 according to GPC analysis (Figure 2a, black and green curves). The resultant copolyester was analyzed by 1H NMR (Figure S4). The integrations of benzyl protons of PhCH2O−, CH2−O of PDL-block, and CH−O of LA-block are ca. 2.00, 100, and 131, respectively. These results suggest that the PDLblock contained 50 units of PDL monomer and the LA-block had 66 units of LA monomer. The second value was relatively lower than the theoretical one (79 = 100 × 79%; Table 2, run 2, and Figure S3), probably due to formation of a small amount of PLA homopolymer with low molecular weights, which could be removed during precipitation. The DOSY NMR experiment of precipitated polymer (Figure S6) showed only one logarithm of diffusion coefficient (log D) in the vertical axis. Based on the linear correlation between log D and the molecular weights (log Mw), the result suggests the formation of a PPDL-b-PLA block copolyester and the absence of PPDL or PLA homopolymers. The monomer sequencing in the polymer chain was characterized by the carbonyl region (165−175 ppm) and the methine region (60−70 ppm) in 13C NMR spectroscopy.25,33 There only two prominent carbonyl diad resonances were present at δ = 174.01 and 169.60 ppm (Figure 2b), which correspond to PDL*−PDL and LA*−LA segments (asterisk represents the observed carbonyl carbon), E

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Macromolecules Table 4. Polymerizations Using the CTPB/Benzoic Acid Systema run

M2

PDL/M2/B/I

A/B

temp (°C)

time (min)

convb (%)

Mn,theor (kg mol−1)

Mn,GPCc (kg mol−1)

Đc

1 2 3 4 5 6 7 8 9 10

CL CL CL CL

0/100/1/1 0/100/1/1 0/100/1/1 0/100/1/1 100/0/1/1 100/0/1/1 100/0/1/1 50/100/1/1 50/100/1/1 50/100/1/1

1/3 2/3 3/3 3/3 1/3 2/3 3/3 1/3 3/3 1/3

80 80 80 110 80 80 110 80 110 80

60 60 60 240 240 240 240 10 + 60 10 + 240 10 + 60

96 34 11 99 17 5 13 97 81 93

11.1

17.4

1.56

11.5

17.1

1.79

23.2 21.4 21.4

28.6 27.5 26.9

1.77 1.81 1.75

CL CL VL

a Conditions: CTPB 0.03 mmol; A/B = PhCOOH/CTPB; [M]0 = [PDL]0 + [CL]0 or [VL]0 = 1.0 mol L−1; the base and initiator were mixed first in toluene, followed by the addition of monomers. bTotal conversion of PDL and the second monomer determined by 1H NMR. cDetermined by GPC at 35 °C in chloroform relative to PS standards.

Figure 4. 1H NMR spectra of CTPB and BnOH with different amounts of PhCOOH in C6D6: (a) BnOH; (b) PhCOOH/CTPB/BnOH = 3/3/3; (c) PhCOOH/CTPB/BnOH = 2/3/3; (d) PhCOOH/CTPB/BnOH = 1/3/3; (e) CTPB/BnOH = 1/1.

PDL as the first block was performed at 80 °C (Table 3, run 3) considering its poor solubility at RT. Subsequent addition of CL monomer at this temperature also produced random copolyesters (Figure S12). Apparently, sequential ROP of PDL and CL, regardless of feeding sequences, was not successful in producing block copolyesters using the CTPB/BnOH catalytic system, which was in contrast to the case of copolymerization of PDL and L-LA. The diad contents of these two copolyester samples (Table 3, runs 2 and 3) were also determined by quantitative 13C NMR (Figures S11 and S12) and given in Table S3. The calculated diad distributions for random copolyesters having the same compositions are also shown in parentheses in Table S3 for comparison. The copolyester obtained in Table 3, run 2, showed excellent agreement between the experimental and the calculated values, while the copolyester prepared in Table 3, run 3, exhibited a significant deviation. This phenomenon is probably because of different reaction time in the second sequential polymerizations (30 min in Table 3, run 2, versus 5 min in Table 3, run 3). Then a question was raised on how to reduce or avoid transesterification reactions in the copolymerization of PDL and CL and thus to produce block copolyesters. It is known that weak phosphazene bases lead to better control over the

methylene protons of PPDL and PCL recorded by the 400 MHz 1H NMR spectrometer are observed as overlapped peaks, so the monomer conversions were given as the total conversions of the two monomers. The monomer sequencing of the resulting polymer was analyzed by the quantitative 13C NMR experiment (Figure 3a, black spectrum, and Figure S10), which exhibited four peaks at 173.98, 173.91, 173.60, and 173.52 ppm in the carbonyl region corresponding to PDL*− PDL, PDL*−CL, CL*−PDL, and CL*−CL linkages. The integrations of these four signals are 0.25, 0.23, 0.23, and 0.29, which do well match the calculated diad distribution for strict random copolymer with the same composition (Table S3, in parentheses). These results suggest that the PDL and CL units are randomly distributed in the copolyester chain. Moreover, the random characteristics were confirmed by the existence of only one melting point (Tm) at 74.8 °C (Figure 5d), which was between the Tm of PPDL (>90 °C) and PCL (ca. 60 °C). Sequential polymerization was then tried to synthesize a block copolyester of PDL and CL. We first performed ROP of CL at room temperature, which went to complete within 5 min. PDL solution was then introduced to the PCL polymerization solution at room temperature, while only a random copolyester was obtained (Table 3, run 2). On the other hand, the ROP of F

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Macromolecules Scheme 2. Synthesis of PPDL-r-PCL and PPDL-b-PCL Diblock Copolyesters

indicating that the basicity of CTPB decreased with the increment of PhCOOH. On the basis of these promising results, we tried the sequential copolymerization of PDL and CL with 1/3 to 1 equiv of benzoic acid introduced before the CL feeding (Table 4, runs 8 and 9), as shown in Scheme 2. With 1/3 amount of benzoic acid, the molecular weights increased from 14.5 to 28.6 kg mol−1 after the copolymerization of CL at 80 °C (Table 4, run 8), as GPC traces exhibited in Figure 3b. The well-defined blocky structure of the resultant copolyester was confirmed by the 13C NMR spectroscopy (Figure 3a, red spectrum), which showed only two major peaks at carbonyl regions corresponding to the sequential PDL*−PDL (174.0 ppm) and CL*−CL (173.5 ppm) linkages.25 As compared to

ROP of more active lactone such as LA, CL, and VL because of the suppressed transesterification.41,48 However, it was reported that the use of the weaker base t-BuP2 instead of the strong superbase t-BuP4 could not initiate ROP of PDL and seriously slowed down the ROP reactions of CL.34,35 Therefore, it is of great importance to develop a new catalytic system with suitable basicity and structure to achieve the synthesis of well-defined PPDL-b-PCL block copolyester in a reasonable time scale. Previously, we have reported the structure of CTPB confirmed by X-ray diffraction as a spherical molecule with a core comprising a nonplanar six-membered ring.41 Because of the nonplanar nature of the central ring, every CTPB molecule can be divided into three separated conjugated {[(Me2N)2P N]2PN} fractions, which is unlike t-BuP4 and other branched phosphazenes. 49 Therefore, CTPB could be considered as a triacid base with three basic sites, while the branched phosphazenes are usually used as monoacidic bases. Inspired by Hedrick’s work,50 an interesting way of reducing the basicity of CTPB via introducing different amount of acid to neutralize fractional basic site was proposed. Benzoic acid (PhCOOH) is a good candidate because of its good solubility in toluene. Hence, the ROP of CL was first tried using CTPB combined with benzoic acid (Table 4, run 1; CTPB/PhCOOH = 3/1). The trial turned out to be effective in that CL could be converted to polymer within 1 h at 80 °C under this condition. When the benzoic acid was increased to 2/3 and 1 equiv to CTPB (Table 4, runs 2 and 3), the polymerization rates decreased drastically, and the conversions of CL within 1 h were only 34% and 11%, respectively. Higher temperature favored the polymerization, and a 99% conversion could be achieved within 4 h with 1 equiv of benzoic acid at 110 °C (Table 4, run 4). We also tried the ROP of PDL with CTPB and benzoic acid (Table 4, runs 5−7), but it only led to low monomer conversion below 20%, even with higher temperature and longer reaction time. 1H NMR analyses of CTPB and BnOH with different amounts of PhCOOH in C6D6 were conducted to study the fractional neutralization of CTPB basic site (Figure 4). With the ratios of PhCOOH/CTPB/BnOH increased from 1/3/3 to 2/3/3 to 3/3/3, the chemical shift of −CH2− in BnOH shifted from 5.27 to 5.23 to 5.13 ppm,

Figure 5. DSC thermograms of the second heating run for different PPDL/PCL copolyesters: (a) block copolyester, PDL + CL sequential polymerization by CTPB/PhCOOH in Table 4, run 8; (b) random copolyester, PDL + CL sequential polymerization by CTPB in Table 3, run 3; (c) random copolyester, CL + PDL sequential polymerization by CTPB in Table 3, run 2; (d) random copolyester, PDL + CL one-pot addition polymerization by CTPB in Table 3, run 1. G

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the spectrum for PPDL-r-PCL (Table 3, run 1; Figure 3a, black spectrum), the two resonances at 173.9 and 173.6 ppm for the sequential PDL*−PCL and PDL−PCL* were absent in the spectrum of block copolyester (Figure 3a, red spectrum). The copolymerization with 1 equiv of benzoic acid at 110 °C (Table 4, run 9) afforded well-defined block copolyester as well, as confirmed by 13C NMR analysis (Figure S15). The DSC curve of the copolyester showed two distinct melting points at 57.4 and 90.1 °C assigned to PCL and PPDL block, respectively (Figure 5a, green curve), further validating the structure of the block copolyester. Under the same condition, the copolyester of PDL and VL could be conveniently prepared (Table 4, run 10), and the blocky character was confirmed via 13C NMR spectroscopy (Figure S17), DSC analysis (Figure S18), and GPC measurement (Figure S19).

CONCLUSIONS CTPB was employed as an efficient catalyst for the homopolymerization of PDL and copolymerization with LLA, CL, and VL. For the homopolymerization of PDL by CTPB/BnOH, quantitative conversion was obtained, and the TOF was as high as 600 h−1 at 80 °C, which is much faster as compared to previously reported organocatalyst systems. With the CTPB/BnOH catalytic system, block copolyesters PPDLb-PLLA were successfully synthesized via sequential polymerization of PDL and L-LA and comfirmed by 13C NMR and DSC analyses. On the contrary, under similar conditions, only random copolyesters of PDL and CL were prepared because of the interchain transesterification reactions. Therefore, a less basic CTPB/benzoic acid complex was designed and allowed the synthesis of well-defined block copolyesters PPDL-b-PCL and PPDL-b-PVL via sequential addition of monomers. The block structures were confirmed by 13C NMR and DSC measurements. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02690.

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Additional polymerization data, NMR spectra of polymer materials (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(S.L.) E-mail [email protected]. *(Z.L.) E-mail [email protected]. ORCID

Shaofeng Liu: 0000-0002-1230-0946 Zhibo Li: 0000-0001-9512-1507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by NSFC (Nos. 21704048, 21434008, 21871157, and 21704049), Major Program of Shandong Province Natural Science Foundation (No. ZR2017ZB0105), Shandong Province Natural Science Foundation (Nos. ZR2017PB008 and ZR2018JL007), and the Taishan Scholars Program (No. tsqn20161031) is gratefully acknowledged. H

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