Controllable Synthesis of Stereoregular Polyesters by Organocatalytic

Article pubs.acs.org/Macromolecules

Controllable Synthesis of Stereoregular Polyesters by Organocatalytic Alternating Copolymerizations of Cyclohexene Oxide and Norbornene Anhydrides Bing Han,† Li Zhang,† Binyuan Liu,*,† Xiaofang Dong,† Il Kim,*,‡ Zhongyu Duan,† and Patrick Theato§ †

Department of Polymer Science and Engineering, Hebei University of Technology, No. 8 Guangrong Road, Hongqiao District, Tianjin 300130, China ‡ The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Korea § Institute for Technical and Macromolecular, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany S Supporting Information *

ABSTRACT: A facile strategy has been demonstrated for the selective synthesis of highly stereoregular polyesters with cis-2,3-(exo, exo) or trans-2,3(exo, endo) repeating units by the organocatalysts mediated alternating copolymerization of cyclohexene oxide and norbornene anhydride (NA) stereoisomers. The geometrical structure of polyester can be tuned simply by modulating the type of NA isomers (endo- or exo-NA), monomer feed ratio, and reaction temperature. The cis- (>99%) and trans-polyesters (>99%) exhibit high glass transition temperature up to 129.8 and 115.9 °C, respectively. The resulting polyesters provide a versatile platform to incorporate various functional groups through the robust thiol−ene reaction of the pendant norbornenyl groups.

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INTRODUCTION Aliphatic polyesters as one of promising class of biodegradable polymer play an important role in biomedical, pharmaceutical, and environmental applications.1 The diversified aliphatic polyesters can be synthesized by carefully designing the monomer, polymer composition, and polymer microstructure to meet different requirements. Among these various parameters, the stereochemical structure is one of the most critical factors to tune their biodegradable and physical properties.2 For example, cis-poly(propylene fumarate) display notably lower glass transition temperature (Tg) than its trans counterpart.3 The biodegradable property of polylactide is affected by the microstructure such that the presence of D-lactyl unit sequence longer than 10 and a L-lactyl content lower than 0.3 in polylactide chain reduce the emzymatic degradability of rac-polylactide.4 It is therefore of remarkable interest to achieve desired features by controlling the stereochemical structure of polymer. Currently, three major strategies are available to synthesize aliphatic polyesters. The most two widely studied methods are step-growth polymerization of diols with diacids (or an acid derivative) and ring-opening polymerization (ROP) of cyclic esters. In general, the former method suffers from high energy cost for achieving high molecular weight (MW). In addition, it is difficult to introduce polar groups such as amine, hydroxyl, and carboxyl groups into polymer backbone because they intervene the polymerization procedure. Polyesters synthesized © XXXX American Chemical Society

by ROP have also limitations due to the availability of cyclic ester monomers. An alternative route involves copolymerization of epoxides and anhydrides, which was originally reported in the late 1960s.5 However, the difficulty of obtaining highMW polymers along with the undesirable side reactions prevented its development as a promising pathway for a long time.6 Coates and co-workers made a noticeable breakthrough by employing discrete zinc β-diketiminato complexes with withdrawing substituent on the ligand as catalysts for the copolymerization of alicyclic epoxides and anhydrides in 2007.3 After this, much attention has been paid to this pathway in recent years.7 Most studies have been concentrating on the optimization of transition metal catalysts and on the investigation of the effect of chemical structures of the cyclic anhydride and epoxides on the copolymerization and its final properties.7 To the best of our knowledge, no reports exist in the literature concerning the stereoregular copolymerization of epoxides with cyclic anhydride geometric isomers to yield the stereospecific polyester with functional group under organocatalyst via chain-growth polymerization. Only a few studies reported the copolymerization of epoxides with optical cyclic Received: March 15, 2015 Revised: April 30, 2015

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NA) was prepared by thermal isomerization according to a reported procedure.10 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized from hexane prior to use. Toluene and tetrahydrofuran (THF) were refluxed and distilled over Na-benzophenone under nitrogen. Measurements. 1H and 13C 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) shifts 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). The glass transition temperatures (Tg) of polymers were determined at a heating rate of 10 °C/min on PerkinElmer Diamond differential scanning calorimetry (DSC) instrument. Thermogravimetric analyses (TGA) were performed on a TA Instruments SDT/Q 600 at a scanning speed of 10 °C/min (from room temperature to 400 °C) under a nitrogen atmosphere with a nitrogen purge rate of 80 mL/ min. The molecular weight of polymer was determined by using gel permeation chromatography (GPC) on a PL-GPC 220 instrument with a refractive index detector, calibrated with polystyrene standards. The columns used was MIXED-B 300 × 7.5 mm columns held at 40 °C, using THF as eluent at a flow rate of 1.0 mL/min. Wide-angle Xray diffraction (WAXD) measurements were performed on a BRUKER D8FOCUS X diffractometer using Cu Kα (λ = 1.5406 Å) source under tube voltage 40 kV and tube current 30 mA. All samples were scanned at the rate of 6° min−1 between 5° and 80° in the reflection mode. General Procedure for the Copolymerization of CHO and NA. All polymerizations were carried out in a 15 mL tube fitted with a three-way stopcock under an argon atmosphere free of moisture and oxygen. As a typical copolymerization of CHO and NA isomer, catalyst (Lewis base or quaternary onium salts), NA, and a Teflon-coated stirring bar were added into dried tube and stirred for 10 min. Then the solvent and CHO were added by using a hypodermic syringe at room temperature. The polymerizations proceeded in oil bath at predetermined temperatures. The reaction mixture after the removal of volatile fractions was added into excess amount of methanol, stirred for 4 h, and then filtered. This process was repeated three times to completely remove the catalyst, and the resulting polymer was obtained by vacuum-drying. Thiol−Ene Reaction. Polyester (1.05 g), THF (5.0 mL), and AIBN (5 mg, 0.03 mmol) were added to 15 mL Schlenk flask to yield a homogeneous solution and then bubbled with argon for 10 min. Mercaptoacetic acid (0.56 g, 6.0 mmol) was then added to the solution by using a hypodermic syringe, and the reaction mixture was transferred to oil bath controlled at 70 °C. After the reaction for 2 h at 70 °C, the polymerization was terminated by adding distilled water. The polymer precipitate obtained by filtering was dried under vacuum at 50 °C for 24 h.

anhydride monomers to provide enantiomerically enriched polyester.8 Compared to transition-metal-mediated polymerizations, organocatalysts offer new opportunities in precision polymer synthesis for enhancing the rate of polymerization and improving the selectivity to novel microstructures and topologies.9 Additionally, the organocatalytic polymerization processes provide advantages of avoiding the undesirable metal contaminants in the polymer, which can play detrimental roles in the performance of the final polymers to some extent and limit their application in some special fields such as microelectronics and biomedical field. The field of organocatalyzed polymerizations, however, is still in its infant development, especially in the stereospecific polymerization. In the present work, we report a facile access to prepare stereoregular polyester with tunable geometric structure through the organocatalytic copolymerization of cyclohexene oxide (CHO) with norbornene anhydride (NA) stereoisomers. As illustrated in Scheme 1, a variety of stereoregular and Scheme 1. Various Stereoregular Polyesters That Are Obtainable by the Alternating Copolymerization of NA Isomers with CHO

stereoirregular polyesters are accessible. The focus has been devoted to determine the influence of NA configuration on the copolymerization behavior under various conditions, in particular on the stereochemistry of resulting polyesters as well as their properties. The versatility of resulting polyesters was also evaluated by employing thiol−ene click reaction.

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RESULTS AND DISCUSSION Considering Lewis bases and quaternary onium salts have been extensively employed for the curing of epoxy−anhydride formulations and for the copolymerization of epoxides and cyclic anhydride as catalysts,8,11 various tertiary amines including DMAP, DBU, N-MeIm, and quaternary onium salts including TBACl and PPNCl were selected to screen their catalytic efficiency for the copolymerization of CHO and isomeric NAs at the molar ratio of CHO:NA:catalyst = 250:250:1 in toluene at 110 °C. It was found that N-MeIm, TBACl, and PPNCl can initiate the CHO/NA copolymerizations, and PPNCl afforded the polymer with the highest yield (Figure 1). Based on these screening tests of organocatalysts, PPNCl is selected as the model catalyst in this work. Table 1 summarizes copolymerization results in the presence of PPNCl as a catalyst. The copolymer yield is strongly dependent on the type of NA. At the identical copolymerization conditions, exoNA shows much better yield than endo-NA (compare entry 1 with entry 6 in Table 1), especially at higher NA-to-CHO feed

EXPERIMENTAL SECTION

Reagents and Methods. Unless otherwise special, 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 Schlenkand vacuum line techniques under an argon atmosphere. Cyclohexene oxide was distilled over CaH2. endo-Norbornene anhydride was recrystallized from acetone and dried under vacuum at 60 °C over 24 h. Bis(triphenylphosphine)iminium chloride ([PPN]+[Ph3P−N PPh3]+) (PPNCl) (Acros) was dissolved in acetone and precipitated by an excess amount of ether, and then the precipitate was dried under vacuum. Triphenylphosphine (PPh 3 ), dimethylaminopyridine (DMAP), and tetrabutylammonium chloride (TBACl) were recrystallized from ethanol prior to use. N-Methylimidazole (N-MeIm) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were purchased from Sigma-Aldrich and used directly. endo-Norbornene anhydride (endoNA) was recrystallized from acetone. exo-Norbornene anhydride (exoB

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Figure 1. Effect of organocatalyst type on the conversion of CHO in the CHO/NA copolymerization in toluene. Conditions: CHO/NA/ organocatalyst = 250:250:1 (molar ratio), 110 °C, 1 h.

Figure 2. 1H NMR spectra of resulting polyesters under (A) exo-NA/ CHO = 1:1, 110 °C (entry 1, Table 1), (B) exo-NA/CHO = 1:1.5, 110 °C (entry 2, Table 1), and (C) endo-NA/CHO = 1:1, 110 °C (entry 7, Table 1).

ratios (entries 1 and 2 in Figure 1). The faster drop in activity for the copolymerization with endo-NA might be due to the greater steric hindrance of endo-norbornenyl ring in comparison with the exo-type ring.12 The higher yields of polymer are recorded at higher temperature (entries 2 and 3; 7, 11, and 12, Table 1) and at higher [CHO]/[NA] ratio (entries 2 and 4, Table 1). The IR spectra of the products showed characteristic absorption peaks corresponding to CO stretching of ester groups at 1732 cm−1. The presence of negligible signal assignable to the repeating oxy(1,2-cyclohene) unit (δ = 3.65 ppm) in the 1H NMR spectra (Figure 2) indicated that the resulting copolymers featured an highly alternating structure, even at high [CHO]/[NA] ratios. The modulation of NA conformation resulted in copolymers with various geometrical structures. As can be seen from Figure 2A, the CHO/exo-NA copolymers prepared at 1:1 feed ratio showed signals of olefinic protons of the norbornenyl ring at 6.18 ppm, 2,3-position methine protons in the NB ring centered at 2.60 ppm as a single peak, and 1,4-position methine protons in the NB ring at 3.02 ppm. On the basis of 1H NMR data from the model compounds of isomeric 5-norbornene-2, 3-dicarboxylic acid dialkyl ester,11a,12 the existence of only one peak at δ = 2.60 ppm indicates that two ester groups on the C2

and C3 positions place on the same side as the methylene bridge (C7) of NA ring, suggesting that 2,3-(exo, exo) ester unit, namely cis geometric polyester (>99%), was achieved. The presence of cis-norbornene-2,3-(exo, exo) diesters substituted group was also confirmed by 13C NMR spectroscopy as a single peak of vinyl carbon was located at 137.9 ppm, and only one ester carbon signal appeared at 172.6 ppm in the spectrum of above-mentioned polyesters (Figure S1C in the Supporting Information). In comparison, the copolymerization of CHO/ endo-NA at 1:1 ratio resulted in trans-enchained polyester (>99%), as evidenced from the fact that there are two separate single peak at 2.63 and 3.34 ppm with the intensity ratio 1:1 for the methine protons at 2,3-positions in the NB ring (Figure 2C). Because of the nonequivalent chemical surroundings, the protons at 2,3-positions at upfield (δ = 2.63 ppm) are on the exo position and the other at downfield (δ = 3.34 ppm) linked to ester substituent on the opposite side of the methylene bridge (endo-type ester substituted).11a,b Additionally, two separate single peaks at 6.25 and 6.02 ppm with the intensity ratio 1:1 represent the vinyl protons in the NB ring. The 13C NMR data further support this conclusion, where olefinic carbons in the norbornenyl ring resonances display two signals and the ester carbon split from one peak for cis-polyester into

Table 1. Copolymerization of CHO and NA Stereoisomer under Organocatalysta entry

[Cat.]/[CHO]/[NA] (molar ratio)

solvent (mL)

temp (°C)

t (h)

yieldb (%)

stereoselectivityc (%)

Mnd (×10−3)

PDId

Tge (°C)

1 2 3 4 5 6 7 8 9 10 11 12

1/50/50 (exo) 1/75/50 (exo) 1/75/50 (exo) 1/100/50 (exo) 1/50/100 (exo) 1/50/50 (endo) 1/50/50 (endo) 1//100/50 (endo) 1//100/50 (endo) 1//50/100(endo) 1/50/50 (endo) 1/50/50 (endo)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.0 1.0 1.0 1.0

110 110 90 110 110 110 110 110 110 110 90 60

1 3 5 3 2.5 1 3 4 4 3 5 48

64 95 84 95 94 31 84 95 96 54 28 16

cis (>99) trans (>99) trans (>99) trans (>99) cis (>92) trans (>99) trans (>99) trans (>99) trans (>99) − − −

4.4 9.3 8.4 8.0 7.9 4.1 5.9 6.9 8.5 5.1 3.5 3.0

1.6 2.0 2.6 2.3 1.2 1.8 2.7 2.5 3.0 1.5 1.9 1.2

113.7 112.5 112.6 115.9 129.8 103.9 114.9 111.8 112.6 116.7 111.4 112.8

a

Cat. = 0.02 mmol. bCalculated by mass of isolated polymer. cAccording to 1H NMR: cis (%) = A2.52−2.62/(A6.02 + A6.17 + A6.25); trans (%) = (A6.02 + A6.25)/(A6.02 + A6.17 + A6.25). dNumber-average molecular weight (Mn) andpolydispersity index (PDI) determined by GPC. eDetermined by DSC. “−”: not detected. C

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Figure 3. 1H NMR spectra of the mixture under exo-NA/CHO/PPNCl = 50/75/1 molar ratio at 110 °C after (A) 0.5, (B) 1.0, and (C) 1.5 h.

three peaks due to the presence of nonequivalent exo and endo environments (Figure S1A). Compared to configuration-reserved (exo, exo)-polyester from exo-NA, the cis−trans transformation was observed from (endo, endo)-configurations of endo-NA to (exo, endo)-ester units configurations on 2,3-positions in CHO/endo-NA copolymerizations. These results show that the tendency of the endo-isomer to exclusively generate trans-5-norbornene-2,3(endo, exo)-diester unit in the copolymer chain is plausibly caused by the fact that the exo-isomer of norbornene derivative is known to be thermodynamically more stable than endoisomer,10,12a,b,14 and the trans-conformation can diminish the flagpole interactions in comparison with the cis-conformation, where the ester units lie in staggered position and have the longest distance and less crowed space.11a,14 On the other hand, the cis-5-norbornene-2, 3-(endo, endo)-diester unit is unstable due to the van der Waals repulsive force between the bulky steric ester substituents.12a This hypothesis is further evidenced by the conformational energy of model compounds of cis-5norbornene-2,3-endo,endo-dicarboxylic acid dicyclohexyl ester (cis-2,3-endo, endo), cis-5-norbornene-2,3-exo,exo-dicarboxylic acid dicyclohexyl ester (cis-2,3-exo, exo), and trans-5-norbornene-2,3-exo,endo-dicarboxylic acid dicyclohexyl ester (transexo, endo) at their lowest energy conformations, where the cis2,3-(endo, endo) structure has the highest conformational energy and the (exo, endo)-trans-analogue with the lowest conformational energy. The ring-opening of endo-type NA to form trans product releases more energy than that of exoisomer.15The higher stability of the trans isomer as well as the higher steric bulk of the cyclohexyl group may thus be considered as driving force for the cis-(endo, endo)−trans-(endo, exo) transformation. Another interesting result is that the stereostructure of copolymer is also dependent on the monomers feed ratio. When the copolymerization performed in excess CHO ([CHO]/[exo-NA] = 1.5 or 2.0), the 1H NMR spectra presented essentially four different peaks of equal area in the 2.40−3.60 ppm region corresponding to H2 and H3 protons

(Figure 2B and Figure S2), apparently differing from the product obtained at [CHO]/[exo-NA] = 1.0, where only two peaks at 2.63 and 3.34 ppm. The 1H NMR spectra of copolymers are consistent with the 1H NMR spectrum of copolymer obtained from CHO/endo-NA copolymerization at [CHO]/[endo-NA] = 1.0 (Figure 2C), implying the formation of trans-polyesters (>99%). It is worthy to note that the 1H NMR spectra of copolymers obtained in excess endo-NA ([CHO]/[endo-NA] = 0.5, entry 10 in Table 1) and exo-NA ([CHO]/[exo-NA] = 0.5, entry 5 in Table 1) display somewhat broad signals in the 6.00−6.30 ppm region, corresponding to olefins protons of norbornene ring (Figure S3C). The intensity of proton signals representing the exo-position (H2) are weaker than those representing the endo-position (H3), suggesting that the stereoregularity of copolymer obtained in excess endo-NA is different from that of cis-(exo, exo)- or trans-(exo, endo)polyester mentioned above. However, highly cis-polyester (entry 5, Table 1) was obtained in the copolymerization with excess exo-NA. These results clearly show that the use of excess NA in the copolymerization plays a negative role in achieving high yield of copolymer, and this behavior may be attributed to a higher viscosity of reaction system in the presence of excess NA, which retarded the monomer molecules to diffuse to the active sites, thus resulting in the lower yield. The CHO/exo-NA ([CHO]/[exo-NA] = 1.5) copolymerization procedure was traced by NMR spectroscopy in order to gain further insights into the transformation of stereostructure (Figure 3). In the early period of polymerization only one signal of olefinic proton at around 6.18 ppm, indicating the formation of polyester with (exo, exo)-cis structure. In the later stage of polymerization when most of the exo-NA is consumed, the signal splits into two peaks (Figure 3C), suggesting the transformation of microstructure of polyester from cis to trans(exo, endo) structure. On the basis of the facts described above, a reaction pathway is proposed as described in Scheme 2. The alkoxide anion formed in excess CHO is suggested to serve as Lewis base and to eliminate the proton at α-position of carbonyl group to generate corresponding carbanion. As a D

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Macromolecules Scheme 2. Proposed Mechanism of Cis−Trans Isomerization of Polyester Chain in Excess CHO

kJ/mol. The high activation energy of cis−trans transformation indicates the transformation from cis- to trans-isomer is favorable at high temperature (110 °C). Indeed, the full isomerization took only 2 min at 110 °C. As expected, the geometrical structure is also tunable by controlling reaction temperature. Figure 5 shows the 1H NMR

result, the sp3-type trigonal-pyramidal carbanion and the low activation barrier for the inversion of geometry are both contributed to the exo−endo mutual isomerization. The similar situation was also observed in the stereoselective synthesis of exo-5-norbornene-2-carboxylic acid from endo-rich methyl 5norbornene-2-carboxylate under basic conditions as reported by Ogino et al.11b In order to further address the isomerization, the copolymerization was conducted in two steps. Initially, copolymerization was performed at [CHO]/[exo-NA] = 1.0 at 110 °C. After all the monomers were completely consumed, additional CHO was introduced into the mixture and stirred at different temperatures. The whole process was traced by NMR spectroscopy (Figure S4). It is worthwhile noting that polyester with cis-selective structure was predominantly formed before the addition of excess amount of CHO, while the signals at around 6.18 ppm assigned to cis-type vinylic protons gradually decreased with a concomitant increase in the vinylic proton signal at 6.25 and 6.02 ppm assigned to the trans-type polyester after the CHO addition. The temperature dependence of cis− trans isomerization rates in the presence of excess CHO is depicted in Figure 4. Accordingly, the rate at the corresponding temperature was used to calculate the corresponding propagation rate constants k, subsequently to determine the activation energies via Arrhenius plots (Figure S5). The calculated activation energy for the transformation is 214.7

Figure 5. 1H NMR spectra of polyesters obtained by CHO/endo-NA ([CHO]/[endo-NA] = 1.0) copolymerization in toluene using PPNCl as catalyst at different temperatures: (A) 110 °C (entry 7, Table 1), (B) 90 °C (entry 11), and (C) 60 °C (entry 12).

spectra of polyesters produced at 110, 90, and 60 °C in the CHO/endo-NA ([CHO]/[endo-NA] = 1.0) copolymerizations. The chemical shifts assigned to H5, H6 and H1, H2, H3, and H4 resonances varied with the reaction temperature, especially at low temperature (60 and 90 °C) where the signals of H2 and H3 corresponding to exo structure were not observed and those of H5 and H6 were broadened. The 1H NMR spectrum of copolymer obtained at 90 °C was similar to that of copolymer obtained at excess endo-NA. According to the anionic pathway (Scheme 2), the easier dissociating ion formed at higher temperature, preferably offering the trans-polyester. As indicated from the energy analyses, if the energy difference between the conformations is large enough to restrict the free rotation among the different conformations, the conformers can be isolated. At low temperature, the insufficient energy to surmount the unfavorable energy barriers between the conformers results in polyester with the mixture of conformations. Table 1 shows that the number-average molecular weight (Mn) values of polyesters are ranging between 3000 and 9000, which are lower than the theoretical expected Mn and the dispersity index values (Mw/Mn) of polyesters were ranging between 1.57 and 3.02. The discrepancy of Mn values is most likely due to the presence of protic impurity traces, such as diacid resulting from hydrolyzed anhydride that can act as chain transfer agents.7a,16 Slow initiation followed by fast propagation might be another factor. The morphology of resulting polyesters was investigated by using WAXD (Figure S6). Both trans- and cis-polyesters show the similar diffraction patterns, showing two broad halos. This indicates that resulting trans- or cis-polyester was amorphous while with short-range ordering structure. The Tg of resulting polyesters is dependent on the geometrical structure. The transpolyester exhibits Tg value at 115.9 °C (entry 4 in Table 1 and

Figure 4. Variation of the peak intensity at 6.25 and 6.02 ppm that are assigned to the trans-type polyester at different temperatures. E

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processable thermoplastics possessing a Tg of 129.8 and 115.9 °C, respectively. Diverse functional polymers were easily designed through the robust thiol−ene click reaction via reactive norborneyl rings, which provides a versatile modification platform to afford various functional polymers. Efforts are underway to investigate the detailed copolymerization mechanism to figure out the effect of experimental parameters on copolymerization behaviors and stereoselective control.

Figure S7), while polyester consisting of 2,3-(exo, exo) ester units shows Tg value at 129.8 °C (entry 5 in Table 1 and Figure S7A). However, both the trans- and cis-polyesters have similar thermal decomposition temperatures at around 320 °C (Figure S8). The results of thermal analyses show that the polyesters produced in this study are kinds of good thermoplastics. Given the highly reactive pendant norbornenyl group,17 properties of the resulting polyesters can be easily adjusted through postpolymerization modification. As a concrete demonstration of the robust and facile nature of modification for the obtained polyesters, a free-radical-mediated the thiol− ene reaction17d,18 was conducted to introduce polar carboxyl groups onto polymer side chain by reaction with mercaptoacetic acid under AIBN initiation, allowing for greater versatility in the regulation of polymer functionality such as hydrophilicity, rate of biodegradation, etc. The disappearance or diminishment of vinyl signals of the norbornenyl ring with particularly appearance of a signal due to −COOH proton at about 12.55 ppm (Figures S9 and S10) and the appearance of the characteristic absorption peak due to the O−H stretching vibration of carboxyl group at around 3560 cm−1 in the IR spectra (Figures S11 and S12) both evidence that the thiol−ene reaction successfully occurred. In addition, Mn of polyester was increased from 6890 to 8900 by grafting mercaptoacetic acid moieties as shown in Figure 6. It is well-known that ethanol is a

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S16. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.macromol.5b00555.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (B.L.). *E-mail [email protected] (I.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51373046 and 51473045), Natural Science Foundation of Hebei Province (B2014202013), Highlevel Excellent Talents in University of Hebei Province, Changjiang Scholars and Innovative Research Team in University (IRT13060), and New Century Excellent Talents in University (NCET-10-0125). I.K. thanks the Fusion Research Program for Green Technologies through the National Research Foundation of Korea (2012M3C1A1054502).

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REFERENCES

(1) (a) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147. (b) Coulembier, O.; Degée, P.; Hedrick, J. L.; Dubois, P. Prog. Polym. Sci. 2006, 31, 723. (c) Tian, H.-Y.; Tang, Z.-H.; Zhuang, X.-L.; Chen, X.-S.; Jing, X.-B. Prog. Polym. Sci. 2012, 37, 237. (2) (a) Zhang, L.; Nederberg, F.; Messman, J. M.; Pratt, R. C.; Hedrick, J. L.; Wade, C. G. J. Am. Chem. Soc. 2007, 129, 12610. (b) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000, 12, 1841. (c) Becker, J. M.; Pounder, R. J.; Dove, A. P. Macromol. Rapid Commun. 2010, 31, 1923. (d) Rasal, R. M.; Janorkar, A. V.; Hirt, D. E. Prog. Polym. Sci. 2010, 35, 338. (3) Jeske, R. C.; DiCiccio, A. M.; Coates, G. W. J. Am. Chem. Soc. 2007, 129, 11330. (4) Tsuji, H.; Miyauchi, S. Biomacromolecules 2001, 2, 597−604. (5) (a) Inoue, S.; Kitamura, K.; Tsuruta, T. Makromol. Chem. 1969, 126, 250. (b) Fischer, R. F. J. Polym. Sci. 1960, 44, 155. (c) Tsuruta, T.; Matsuura, K.; Inoue, S. Makromol. Chem. 1964, 75, 211. (6) (a) Aida, T.; Inoue, S. J. Am. Chem. Soc. 1985, 107, 1358. (b) Maeda, Y.; Nakayama, A.; Kawasaki, N.; Hayashi, K.; Aiba, S.; Yamamoto, N. Polymer 1997, 38, 4719. (c) Kuran, W.; Niestochowski, A. Polym. Bull. 1980, 2, 411. (d) Chen, X.-H.; Zhang, Y.-F.; Shen, Z.-Q. Makromol. Chem. 1992, 193, 2989. (e) Takeuchi, D.; Aida, T.; Endo, T. Macromol. Rapid Commun. 1999, 20, 646. (7) (a) Huijser, S.; HosseiniNejad, E.; Sablong, R.; de Jong, C.; Koning, C. E.; Duchateau, R. Macromolecules 2011, 44, 1132. (b) DiCiccio, A. M.; Coates, G. W. J. Am. Chem. Soc. 2011, 133, 10724. (c) Nejad, E. H.; Paoniasari, A.; Koning, C. E.; Duchateau, R. Polym. Chem. 2012, 3, 1308. (d) Nejad, E. H.; van Melis, C. G. W.; Vermeer, T. J.; Koning, C. E.; Duchateau, R. Macromolecules 2012, 45, 1770. (e) Darensbourg, D. J.; Poland, R. R.; Escobedo, C. Macromolecules 2012, 45, 2242. (f) Liu, J.; Bao, Y.-Y.; Liu, Y.; Ren,

Figure 6. GPC curves of cis-polyester before (A) and after (B) thiol− ene reaction.

representative antisolvent of polyesters, used for precipitation before modification. However, it is interesting to note that the modified polyester was soluble in ethanol after the incorporation of carboxyl group into backbones, indicating the improvement of polarity of polyester (Figure S13). Hydroxyl and amine groups were also introduced into polyester side chains instead of thioglycolic acid by 2-mercaptoethanol and cysteamine (Figures S14−S16), thereby dramatically widening the potential application window for these polyesters.

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CONCLUSIONS In summary, we have demonstrated a facile synthesis of highly stereoregular polyesters with cis-2, 3-(exo, exo) or trans-2, 3(exo, endo) repeating units in a highly alternating manner in CHO/NA stereoisomers copolymerization in the presence of an organocatalyst. The geometrical structure of polyester is tunable simply by the type of NA (endo- or exo-NA), by [CHO]/[NA] feed ratio, and by reaction temperature. The cis(>99%) and trans-polyesters (>99%) are amorphous and meltF

DOI: 10.1021/acs.macromol.5b00555 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b00555 Macromolecules XXXX, XXX, XXX−XXX