CO2-Tuned Sequential Synthesis of Stereoblock Copolymers

Institute for Technical and Macromolecular, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany. Macromolecules , Article ASAP. DOI: 10...
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CO2‑Tuned Sequential Synthesis of Stereoblock Copolymers Comprising a Stereoregularity-Adjustable Polyester Block and an Atactic CO2‑Based Polycarbonate Block Bing Han,† Binyuan Liu,*,† Huining Ding,† Zhongyu Duan,† Xianhong Wang,*,‡ and Patrick Theato*,§ †

Department of Polymer Science and Engineering, Hebei University of Technology, No. 8 Guangrong Road, Hongqiao District, Tianjin 300130, China ‡ Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § Institute for Technical and Macromolecular, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany S Supporting Information *

ABSTRACT: Stereoblock copolymers composed of atactic CO2-based polycarbonate and various stereoregular polyester blocks with cis-2,3-(exo, exo), cis-2,3-(endo, endo), or partly trans-2,3-(exo, endo) repeating units were successfully synthesized via sequential ring-opening copolymerization (ROCOP) of cyclohexene oxide (CHO) with norbornene anhydride (NA) and then ROCOP of CHO with CO2 using a salcyCrCl/ PPNCl binary catalyst. The structure of the block copolymers was confirmed by NMR and GPC. Incorporation of carboxyl groups by the thiol−ene reaction of the pendant norbornenyl groups further confirmed the polymer structures. The geometric structure of polyester units in the block copolymers is tailored simply by varying the NA isomer and the monomer feed ratio of CHO to NA. Notably, CO2 suppressing the configuration transformation from cis-(exo, exo) to trans-(exo, endo) of polyester block has been revealed in the sequential ROCOP. Using this strategy, the stereospecific polyester with an dominant cis-2,3-(exo, exo)-poly(NA-alt-CHO) units, the thermodynamically less stable isomeric form, was obtained in the ROCOP of exoNA with an excess of CHO in the presence of CO2. The role of CO2 in restraining the geometric transformation from cis-(exo, exo) to trans-(exo, endo) was proposed in the ROCOP of exo-NA with an excess of CHO catalyzed by PPNCl.

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INTRODUCTION

So far, several strategies have been explored for the synthesis of polyester-block-polycarbonates by incorporating CO2-based polycarbonate and polyester linkages.13−19 One route involves copolymerization of epoxide/CO2 to generate a macroinitiator of a polycarbonate end-capped with a hydroxyl group, which allows for a following direct chain extension via lactide or lactone ring-opening polymerization (ROP). For example, Darensbourg and Lu13 reported the syntheses of polyester− polycarbonate AB diblock and ABA triblock copolymers with controllable lengths. They combined a cobalt(III) salen complex-catalyzed epoxide/CO2 copolymerization, yielding a hydroxyl end-capped polycarbonate in situ, with the chain extension by DBU (1,8-diazabicycloundec-7-ene)-catalyzed ROP of lactide. Williams et al.14 reported ABA triblock copolymers [polylactide-block-poly(cyclohexene carbonate)block-polylactide] using a dizinc catalyst for ROCOP of a dihydroxyl-capped poly(cyclohexene carbonate), followed by

Increasing attention has been paid to design and synthesize novel block copolymers owing to combination of specific properties of the constituent blocks and occurrence of unique properties via their microphase separations.1,2 In this context, block copolymers consisting of polyester and polycarbonate blocks are particularly desirable because they allow for a tailoring of the degradation rates of the individual blocks, polycarbonate or polyester, which will benefit substantially applications in drug delivery.3,4 Furthermore, development of an eco-friendly material from a renewable feedstock is a promising alternative in modern polymer science. From a sustainable viewpoint, aliphatic polycarbonates synthesized by the copolymerization of epoxides with CO2 have received particular interest over the past decades due to the possible large-scale application of abundant, nontoxic, low-cost, and renewable C1 resource CO2,5−10 which is of technological importance because CO2 is considered to be the major greenhouse gas responsible for global warming and the corresponding climate change.11,12 © XXXX American Chemical Society

Received: September 4, 2017 Revised: November 13, 2017

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DOI: 10.1021/acs.macromol.7b01905 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules yttrium complex initiated ROP of lactide. Later, they developed a switch process of ROP of lactone and ROCOP of epoxides and CO2 to yield triblock copolymer polycaprolactone-blockpoly(cyclohexene carbonate)-block-polycaprolactone,15−17 where the distinct catalytic cycles of ROP and ROCOP are controlled by the presence or absence of CO2. Using a similar strategy, Rieger and co-workers also prepared the polyesterblock-polycarbonates from mixtures of epoxide, lactone, and CO2 by alternating “on/off” cycles of CO2 and nitrogen flow.18 Additionally, Zhang and co-workers19 described a convenient method to synthesize multiblock copolymers with high efficiency from a one-pot/one-step polymerization of CO2, epoxide, and lactone by bridging two independent chain propagations via two cross-chain exchange reactions in one system under dual catalysts. Coates and co-workers20 reported the synthesis of high molecular mass polyester−polycarbonate diblock copolymers in one pot by terpolymerization of epoxides and cyclic anhydrides with CO2 using zinc β-diketiminato complexes as catalyst. The advantage of this method is the availability of structurally diverse polyesters from relatively cheap feedstocks produced on large scale in comparison with ROP of cyclic esters,3,21 opening the opportunity to produce versatile polyester-block-polycarbonate in a convenient way. Since this elegant work, several other homogeneous catalysts displaying similar selectivities have been developed.22−24 Recently, we reported the synthesis of polyester-block-polycarbonate via the terpolymerization of propylene oxide (PO), NA, and CO2 efficiently catalyzed by a single bifunctional salenCoX complex.25 A higher NA loading is unfavorable for the tepolymerization, which suggests that the length of polyester should be limited in the block of polyesterblock-polycarbonate copolymer. Luinstra and co-workers26 also noted that NA was not readily incorporated into the polymer chain in a PO/CO2/NA terpolymerization, while the presence of NA enhanced the selectivity of the CO2/PO coupling reaction from cyclic propylene carbonate to poly(propylene carbonate). However, as far as we know, little attention has been paid to the stereochemistry of the polyester segment in the synthesis of block copolymers consisting of polyester and polycarbonate using epoxides, cyclic anhydrides and CO2 as feedstocks. This is, however, of importance because the stereochemical structure of polyesters is profoundly related to their biodegradable and physical properties.27−33 For instance, cis-poly(propylene fumarate) displays a lower glass transition temperature compared to the trans-isomer.27 The biodegradability of poly(ethylene fumarate) with a trans (E)-configuration was found to be superior to that of the cis (Z)configured one.28 It is therefore of remarkable interest to synthesize the polyester-block-polycarbonate by incorporating CO2-based polycarbonate and polyester blocks with varying stereochemical structure. Herein, we report the synthesis of stereoblock copolymers comprised of a stereoregularity-tunable polyester and an atactic aliphatic polycarbonate blocks respectively via sequential ROCOP of CHO with NA and CHO with CO2 in one pot using a salcyCrCl/PPNCl binary catalytic system (Scheme 1). This synthesis possesses the inherent advantage of sequential immortal polymerization to prepare well-defined block copolymers without tapering of block structures. The other essential issue aims at elucidating the role of CO2 in tuning the stereostructure of the polyester block made with the ROCOP of CHO with NA in the synthesis of block copolymers or homopolyester itself. A novel strategy based using CO2 to

Scheme 1. Synthetic Route for the Stereoblock Polyesterblock-Polycarbonate Preparation

control the stereoregularity of ROCOP of CHO and exo-NA is explored.

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EXPERIMENTAL SECTION

Reagents and Methods. Unless otherwise stated, all reagents were purchased from commercial suppliers and used without further purification. All manipulations involving air- and/or water-sensitive compounds were carried out with the standard Schlenk and vacuum line techniques under an argon atmosphere. Cyclohexene oxide (CHO) was refluxed over CaH2 for 24 h and vacuum-distilled prior to use. Bis(triphenylphosphine)iminium chloride ([PPN]+ = [Ph3P−N PPh3]+) (PPNCl) was purchased from Acros and was dissolved in acetone and precipitated into an excess amount of ether. Afterward, precipitate was dried under vacuum prior to use. Toluene and THF were refluxed and distilled over Na-benzophenone under nitrogen before use. SalcyCrCl complex was synthesized according to a reported literature.34 endo-Norbornene anhydride (endo-NA) was recrystallized from acetone. exo-Norbornene anhydride (exo-NA) was prepared by thermal isomerization according to a previously reported procedure,35 with a purity greater than 98% as calculated by 1H NMR. The procedure of postpolymerization modification of copolymer by the AIBN-mediated thiol−ene reaction was performed according to our previous report.36 CO2 (>99.99%) and other reagents were used as received. 1 H and 13C{1H} NMR spectra were recorded on a Bruker-400 spectrometer at frequencies of 400 MHz (1H) and 100 MHz (13C), respectively. Their peak frequencies were referenced versus an internal standard (TMS) shift at 0 ppm for 1H NMR and against the solvent. Infrared (IR) spectra were obtained on a Bruker Vector 22 spectrometer at a resolution of 4 cm−1 (16 scans collected). Molecular weight determinations were performed using a PL-GPC 220 instrument with a refractive index detector. The columns used were MIXED-B 300 × 7.5 mm columns held at 35 °C, using THF as eluent at a flow rate of 1.0 mL/min. Sequential Ring-Opening Copolymerization of CHO with NA and CHO with CO2. The desired amount of NA, salcyCrCl, and the appropriate equivalent of PPNCl cocatalyst were dissolved in CHO, and the solution was added into a predried autoclave by syringe injection under an argon atmosphere. The autoclave was put into a bath of 110 °C and stirred until the NA consumption was completed. After the autoclave was cooled to 80 °C and slowly vented with argon, CHO was added and pressurized to the appropriate pressure with CO2 and then stirred at 80 °C for a fixed time. After the removal of volatile fractions, a minimum volume of CH2Cl2 was added to the reaction mixture before being poured into acidified methanol to precipitate the polymer. The crude copolymer was dissolved in a minimum volume of CH2Cl2 and then again precipitated into acidified methanol. This process was repeated three times, and the resulting copolymer was collected and dried under vacuum at 60 °C overnight.

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RESULTS AND DISCUSSION We and other researchers have found that stereoregular polyesters with well-defined cis-2,3-(exo, exo), trans-2,3-(exo, B

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Macromolecules Table 1. Synthesis of Polyester, Polycarbonate, and Polyester-block-Polycarbonatea time (min) entry 1 2 3 4 5 6 7 8 9 10 11

first step (molar ratio) (CHO/NA) 250/250 250/250 500/250 500/250 500/250 500/250 250/250 250/250

(endo-NA) (endo-NA) (endo-NA) (endo-NA) (exo-NA) (exo-NA) (exo-NA) (exo-NA)

250/250 (exo-NA) 250/250 (exo-NA)

first step

second step (molar ratio) (CHO/SalcyCrCl)

180 180 90 90 60 60 150 150

1000 750 750 1000 500 500 500

150 150

second step 180 180 180 180 135 90 135

yieldb (%) first step >99.0 100 ∼100 100 ∼100 100 >99.0 100 100 100

second step 48.7 51.6 42.4 46.5 64.6 35.0 43.0

Mnc (×103)

Mw/Mnc

13.7 19.2 15.6 21.0 18.9 21.6 17.7 23.7 11.2 19.6 21.3

1.32 1.49 1.50 1.56 1.56 1.58 1.27 1.42 1.25 1.40 1.30

SalcyCrCl (63.4 mg, 0.1 mmol), PPNCl (57.4 mg, 0.1 mmol), NA (4.1 g, 25 mmol), 5.0 mL of toluene as solvent; first block was synthesized from ROCOP of CHO and NA at 110 °C; second block was synthesized from ROCOP of CHO and CO2 at 80 °C, P(CO2) = 3.0 MPa. bCalculated by mass of isolated polymer. cDetermined by GPC. a

block-polycarbonate samples 2, 4, 6, 8, 10, and 11 as shown in Figure S1. To further exclude that the obtained copolymers from the sequential ROCOP are a blend of two homopolymers, we performed a postpolymerization modification of the copolymers composed of polyester and PCHC via the AIBNmediated, free-radical-mediated thiol−ene reaction with mercaptoacetic acid.36 We have previously demonstrated that polyesters with a low MW became soluble in methanol when polar carboxyl groups were attached onto the polymer side chains.36 These findings additionally support the fact that the copolymer via sequential copolymerization is not a mixture of polyester and polycarbonate but rather a true diblock copolymer. Note that if the resulting copolymer was a mixture of two homopolymers, the GPC profile and Mn of the COOHfunctionalized copolymer should have changed after washing with methanol due to the selective removal of the COOHfunctionalized polyester component in the mixture. However, the GPC curves of COOH-functionalized copolymer purified by methanol maintained the same profile compared to the unpurified COOH-functionalized copolymer. Further, Mn of the purified copolymer modified by mercaptoacetic acid increased from 19 300 g/mol for the parent copolymer to 21 500 g/mol (Figure 1A vs Figure 1B). For comparison, the GPC curves of a polyester and corresponding modified products are also provided in Figures 1C and 1D, respectively, in which the preparation conditions of the polyester of the copolymer were the same as that of the control polyester (Table S1), with the only exception that the COOH-functionalized polyester homopolymer was soluble in methanol. Further, for the grafted copolymer purified by methanol the vinyl signals of the norbornenyl ring disappeared in the NMR, while a signal due to −COOH proton at about 6.25 ppm appeared, as shown in Figure S2. In the 13C NMR spectrum of grafted copolymer also a new peak characteristic for the carboxylic carbon appeared at 173.3 ppm as well as a complete disappearance of the peak centered at 130−138 ppm characteristic for the vinyl carbon (Figure S3). More importantly, the intensity of signal assignable to the repeating cyclohexene carbonate unit and polyester in purified COOH-functionalized copolymer is in agreement with copolymer before modification. These results suggest that the bimodal GPC profile of the copolymer prepared via sequential ROCOP possibly arises from a chain transfer reaction in the

endo), or cis-2,3-(endo, endo) repeating units could be obtained via alternating copolymerization of CHO with NA stereoisomers, whose stereoselectivity is significantly dependent on the NA type, monomer feed ratio, and catalyst employed.22,27,31,36,37 With this knowledge in hand, block copolymers composed of a polyester block with variant stereochemical structure and a polycarbonate block were prepared via a ROCOP of CHO with NA isomer and variable molar ratio of CHO to NA at 110 °C under an argon gas atmosphere and then followed by a ROCOP of CHO with 3.0 MPa CO2 after a complete consumption of NA. Before the sequential ROCOP for the synthesis of the polyester-blockpolycarbonate, a control copolymerization of CHO and NA was carried out to obtain information on the NA consumption. Table 1 summarizes the sequential ROCOP results and the control of the copolymerization of CHO and NA using a salcyCrCl/PPNCl catalyst system. As shown in Table 1, the molecular weight (MW) increased for the copolymers from sequential ROCOP compared to the corresponding control polyesters (entries 2, 4, 6, 8, 10, and 11 vs entries 1, 3, 5, and 7, respectively). For clarity, we designed the control experiment of polycarbonate synthesized via copolymerization of CHO with CO2 in a second ROCOP step (entry 9, Table 1) to have a lower MW than that of the polyester obtained from the first ROCOP of CHO with NA (entry 7, Table 1). It was found that the molecular weights of copolymers prepared via sequential ROCOP of CHO with NA and CHO with CO2 (entries 10 and 11, Table 1) were higher than those of the corresponding parent polyester (entry 7, Table 1) and poly(cyclohexene carbonate)s (PCHC) (entry 9, Table 1) and increased with extending reaction time (entry 10 vs entry 11, Table 1), where the molecular weight of control polyester was 17 700 g/mol with a dispersity Mw/Mn of 1.27 (entry 7, Table 1) and that of control PCHC was 11 200 g/mol with Mw/Mn of 1.25 (entry 9, Table 1). Otherwise, if the obtained copolymers are a blend of polyester and PCHC, the molecular weight of copolymer obtained from sequential ROCOP should be no more than that of the polyester. This result indicates that the copolymer composed of polyester and polycarbonate has indeed a block structure; i.e., polyester-blockpolycarbonate was formed. Unsatisfactorily, the GPC curves show a slight bimodel profile (i.e., shoulder) for all polyesterC

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first step and second step. Indeed, this chain transfer is a generally known phenomenon for the metal complex-mediated copolymerization of epoxides with anhydride or with CO2.23,27,38−42 To conclude so far, although the GPC profile showed a shoulder, the polyester-block-polycarbonates were successfully synthesized via a sequential ROCOP of CHO with NA and then CHO with CO2 with their block structure confirmed. With polyester-block-polycarbonate in hand, a more detailed insight into the stereostructure of the blocks was provided by NMR spectroscopy. Figure 2A shows the 1H NMR spectrum of polyester-block-polycarbonate with the polyester block prepared at [CHO]/[endo-NA]/[salcyCrCl]/ [PPNCl] = 250/250/1/1 for 180 min and the polycarbonate block obtained at [CHO]/ [salcyCrCl]/[PPNCl] = 1000/1/1 (entry 2, Table 1). The olefinic protons of the NA ring and the protons H1, H2, H3, and H4 showed broad signals in the region of 6.02−6.37 ppm and centered at 3.00−3.50 ppm, respectively, being very similar to the peaks found for controlled poly(endo-NA-alt-CHO) (Figure S4). To gain further insights into the stereochemical information on the polyester linkage in the polyester-blockpolycarbonate, the block copolymer was subjected to a reduction with LiAlH4 similar to previous reports.30,37 The reduced product of 5-norbornene-2,3-dimethanol was found to possess 98% of cis-2,3-(endo, endo) conformer (Figure S5). The 13 C NMR spectrum also clearly showed the characteristic resonances for PCHC as illustrated by the presence of peaks in the region of 152−155 ppm. According to the integrated area of carbonate carbon signals indicative of syndiotactic (δ 153.1 ppm, 153.3 ppm) and isotactic (δ 153.7 ppm) PCHC,43,44 it

Figure 1. GPC curves of original copolymer from sequential ROCOP (A), copolymer after postpolymerization modification with mercaptoacetic acid (B), polyester (C), and polyester modified by mercaptoacetic acid, where the preparation conditions of polyester in the copolymer is full are the same as that of the control polyester. Reaction conditions: (A) first step: [exo-NA]/[CHO]/ [salcyCrCl]/[PPNCl] = 100/100/1/1, exo-NA: (1.64 g, 10 mmol), 2.0 mL of toluene as solvent, 110 °C, 40 min, second step: [CHO]/[salcyCrCl]/[PPNCl] = 1000/1/1, 80 °C, 180 min, PE/PCHC = 16/84, 100% cis-(exo, exo); (B) 70 °C, 70 min, mercaptoacetic acid (0.22 g, 2.4 mmol), yield 97.0%; (C) [exo-NA]/[CHO]/[salcyCrCl]/[PPNCl] = 100/100/1/1, exo-NA: (1.64 g, 10 mmol), 2.0 mL of toluene as solvent, 110 °C, 40 min; (D) (exo, exo)-cis PE (0.65g, 2.5 mmol), 70 °C, 70 min, mercaptoacetic acid (0.28 g, 3 mmol), yield 98.5%.

Figure 2. 1H NMR spectra of polyester-block-polycarbonate of entries 2, 4, 6, and 8 in Table 1. D

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Figure 3. 13C NMR spectra of polyester-block-polycarbonate of entries 2, 4, 6, and 8 in Table 1.

Scheme 2. CO2-Controlled Stereoselective ROCOP of CHO and exo-NA

Interestingly, the polyester-block-polycarbonate copolymer with a polyester block that was prepared with equivalent amount of CHO and exo-NA (entry 8, Table 1) exhibited a single peak at δ = 6.18 ppm assigned to the olefinic proton of the NA ring and signals at 2.60 and 3.02 ppm assigned to the 2,3-position methine protons and the 1,4-position methine protons in the 1H NMR spectrum (Figure 2D), indicating that the polyester linkage with only cis-2,3-(exo, exo) ester units. The preference of a 2,3-(exo, exo)-cis-stereostructure of the polyester block was evidently confirmed by 13C NMR spectroscopy due to the presence of a single peak at 137.9 ppm assigned to the vinyl carbon and a appeared at 172.5 ppm representative for one ester carbon in the structure (Figure 3D). Consequently, a polyester-block-polycarbonate with 100% cis-(exo, exo)-polyester block was obtained in this case. This finding is in contrast to a previous observation in which a configuration transformation from cis-(exo, exo)-type to trans(exo, endo)-type occurred when the copolymerization of CHO with exo-NA in equimolar ratio was conducted after NA consumption and then addition of an another amount of CHO using PPNCl as sole catalyst or metal complex-based binary catalyst system.36,37 The case suggests that CO2 plays an

can be concluded that the PCHC block in the block copolymer has an atactic structure. This result suggests that the obtained polyester-block-polycarbonate is composed of a cis-2,3-(endo, endo)-polyester block and an atactic PCHC block. To achieve polyester-block-polycarbonate copolymers with diverse stereostructural polyester blocks, the monomers feed ratio and NA type in the first step ROCOP of CHO with NA were varied. As estimated by the peak intensity of olefinic protons in Figure 2B, the polyester block in polyester-blockpolycarbonate obtained at [CHO]/[endo-NA]/[salcyCrCl]/ [PPNCl] = 500/250/1/1 (entry 4, Table 1) is composed of 62% cis-(endo, endo)-polyester and 38% trans-(exo, endo)polyester. In comparison, when using exo-NA instead of endoNA at otherwise identical conditions in first stage (entry 6, Table 1), the resulting polyester-block-polycarbonate showed apparently a very different stereoselectivity, with the polyester block containing 35% cis-(exo, exo) and 65% trans-(exo, endo) conformation in the polyester-block-polycarbonate copolymers, as illustrated in Figure 2C. These results are also supported by the 13C NMR spectra (Figures 3B and 3C). Likewise, the signals of the PCHC segment in the block copolymer are indicative for an atactic structure. E

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Figure 4. 1H NMR spectra of poly(exo-NA-alt-CHO) under various conditions catalyzed by PPNCl (A: [exo-NA]/[CHO]/[PPNCl] = 50/50/1, 0.1 MPa Ar, 110 °C, 3.0 h; B: [exo-NA]/[CHO]/ [PPNCl] = 50/100/1, 3.0 MPa CO2, 110 °C, 1.0 h; C: [exo-NA]/[CHO]/[PPNCl] = 50/100/1, 3.0 MPa CO2, 110 °C, 5.0 h; D: [exo-NA]/[CHO]/[PPNCl] = 50/100/1, 3.0 MPa CO2, 110 °C, 5.0 h, then CO2 was replaced by argon and stirred for 2 h; E: [exo-NA]/[CHO]/[PPNCl] = 50/100/1, 3.0 MPa Ar, 110 °C, 5.0 h).

important role in suppressing the cis-(exo, exo)−trans-(exo, endo) transformation of polyester. To address the role of CO2 in restraining the configuration transformation, a PPNCl-catalyzed copolymerization of exo-NA with excess CHO was performed at 110 °C in toluene under different atmospheres (Scheme 2) and analyzed by 1H NMR (Figure 4) and IR (Figure 5) spectroscopy, respectively. The observation was made that the chemical shifts assigned to H1, H2, H3, and H4 and olefic resonances (H5, H6) of a polyester obtained under 3.0 MPa CO2 with an excess of CHO are almost consistent with that of a polyester obtained at an equimolar ratio of CHO to exo-NA at 0.1 MPa argon, which obviously differ from that of a polyester derived from the copolymerization with excess CHO under 3.0 MPa argon (Figure 4). Two separated single peaks at 2.63 and 3.34 ppm with an intensity ratio of 1:1 for the methine protons at 2,3positions are assigned to the trans-conformer and appeared for a polyester obtained with an excess of CHO under 3.0 MPa argon, suggesting an occurrence of a cis−trans transformation from cis-configuration of exo-NA to 2,3-(exo, endo)-trans-ester units configurations during the copolymerization under 3.0 MPa argon. On the contrary, a polyester obtained under 3.0 MPa CO2 with an excess of CHO possessed 90% (exo, exo)-cisconfiguration stereostructure (i.e., 10% trans-ester units), which

Figure 5. IR spectra of poly(exo-NA-alt-CHO) under various conditions catalyzed by PPNCl (A: [exo-NA]/[CHO]/[PPNCl] = 50/100/1, 3.0 MPa CO2, 110 °C, 1.0 h; B: [exo-NA]/[CHO]/ [PPNCl] = 50/100/1, 3.0 MPa CO2, 110 °C, 5.0 h; C: [exo-NA]/ [CHO]/[PPNCl] = 50/100/1, 3.0 MPa Ar, 110 °C, 5.0 h).

F

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Scheme 3. Proposed the Mechanism of Copolymerization of exo-NA with Excess CHO Catalyzed by PPNCl in High Pressure of CO2

was indicated according to the 1H NMR spectrum (Figure 4C). Noteworthy, in the 1H NMR spectrum of the block copolymer, derived from copolymerization of exo-NA with excess CHO under 3.0 MPa CO2 for 5.0 h, after releasing CO2 and further heating under argon for 2.0 h, the intensity of peaks characteristic of trans-type polyester increased to 55% and cistype declined from 90% to 45% (Figure 4D). This result clearly demonstrates that the CO2 substantially restricts the cis-(exo, exo)−trans-(exo, endo) transformation during the copolymerization of exo-NA with excess CHO. In addition, it was found that the reaction of exo-NA with CHO proceeded faster than that of CO2 with CHO under 3.0 MPa CO2, which is evidenced by the finding for exo-NA conversion of 51% into polyester within 1.0 h, while no cyclohexene carbonate (CHC) is formed. Even prolonging reaction time to 5.0 h, only a few quantities of cis-CHC were found in the copolymerization of excess CHO with NA in the presence of CO2 as indicated by the appearance of a weak absorption in the IR spectrum around 1803 cm−1 with a highenergy shoulder (Figure 5B) along with the observation of proton signal in the 1H NMR spectrum at 4.66 ppm of the CH2 group adjacent to the cyclic carbonate ring (Figures 4C and 4D).45 It has been proposed that the [PPN]+ alkoxides (species A, Scheme 3) serve as a strong Lewis base to deprotonate the αposition of a carbonyl group to generate a carbanion that promotes the cis−trans rearrangement by epimerization in the presence of an excess of epoxide.31,36,37 However, due to its the weaker basicity, the carboxylate anion (species B, Scheme 3) cannot abstract the proton at the α-position of carbonyl group to form the respective carbanion, and consequently, substituents at the exo-position cannot freely rotate to the endoposition, which would be necessary to yield the thermodynamically more stable trans-configuration polyesters in the presence of a cyclic anhydride.31,36,37 The similar plausible reason could account for the explanation that no cis−trans transformation occurred at 3.0 MPa pressure of CO2, where CO2 easily inserts into the PPN−OR bond to form the less basic [PPN]+ carbonate (species C, Scheme 3). The assumption of CO2 insertion into PPN−OR is evidenced by formation of the cyclic cyclohexene carbonate during the copolymerization of exo-NA with an excess of CHO in 3.0 MPa CO2. Also, it has been reported in the literature that the coupling reaction of CO2 with

epoxides catalyzed by PPNX salt results in cyclic carbonates by a fast insertion step of CO2 into PPN−OR.46 Accordingly, we proposed a mechanism of the copolymerization of exo-NA with an excess of CHO under CO2 atmosphere, as shown in Scheme 3. As demonstrated by the terpolymerization of epoxide, anhydride, and CO2 in one pot,20,23,24 both insertion of NA and CO2 into PPN−OR bond are a fast step (where the epoxide insertion is the rate-determining step), but the insertion of NA into PPN−OR bond is much faster than the CO2 insertion into PPN−OR. Therefore, circle 1 in Scheme 3 is preferred in the presence of NA during the copolymerization of CHO with NA, where the active species are species A and B until NA is fully consumed under high pressure of CO2. After NA is completely consumed, circle 2 is taking over and less basic [PPN]+ carbonates (species C) are formed with the remaining epoxide. Apparently, when conducted under an argon atmosphere instead of CO2 atmosphere, only circle 1 is active, and the presence of species A or species B depends on the molar ratio of CHO to NA. Based on previous reports,31,36 species A are formed in an excess of CHO; otherwise, species B are preferred.

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CONCLUSION

In summary, stereoblock copolymers consisiting of a polyester block with isomeric structure and an atactic polycarbonate block have been conveniently synthesized via a sequential ROCOP of CHO with NA and then ROCOP of CHO with CO2 using a salcyCrCl/PPNCl binary catalyst. The stereoregularity of the polyester block could be tuned simply by changing the NA isomer and the molar ratio of CHO to NA. Remarkably, the role of CO2 in restricting the cis-(exo, exo)−trans-(exo, endo) configurational transformation has been revealed in the sequential ROCOP and copolymerization of NA with CHO. The formation of a [PPN]+ carbonate anion is responsible for suppressing the cis-(exo, exo)−trans-(exo, endo) transformation during the copolymerization of CHO with exo-NA under high pressure of CO2. The dependence of the biodegradability of synthesized block copolymer on the stereostructure is underway. G

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01905. Table S1 and Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.L.). *E-mail: [email protected] (X.W.). *E-mail: [email protected] (P.T.). ORCID

Binyuan Liu: 0000-0003-2204-8489 Xianhong Wang: 0000-0002-4228-705X Patrick Theato: 0000-0002-4562-9254 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51373046 and 51473045) and the National Natural Science Foundation of Hebei Province (B2014202013).

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01905 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01905 Macromolecules XXXX, XXX, XXX−XXX