Making Various Degradable Polymers from Epoxides Using a

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Making Various Degradable Polymers from Epoxides Using a Versatile Dinuclear Chromium Catalyst Ye Liu,* Jia-Zhi Guo, Hui-Wen Lu, Hui-Bo Wang, and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024 Dalian, China S Supporting Information *

ABSTRACT: The environmental impact of industrially important plastics can be minimized by using renewable feedstocks and preparing biodegradable polymers. A significantly more meaningful strategy is the utilization of alternating copolymerization of epoxides and cyclic anhydrides or lactone or CO2 to provide degradable polyesters or polycarbonates. Herein, we report a versatile and efficient catalyst system based on dinuclear chromium complex bearing biphenol linking bridge for copolymerizing epoxides with cyclic anhydrides or CO2 or dihydrocoumarin (DHC), affording various degradable copolymers with perfectly alternating structure and narrow molecular weight distribution. Most of the polyesters from DHC are typical semicrystalline materials, being beneficial to the potential application of aliphatic polyesters as structural materials. Notably, this dinuclear chromium catalyst system was also discovered to be very efficient in the one-pot terpolymerization of cyclohexene oxide/phthalic anhydride/CO2, cyclohexene oxide/phthalic anhydride/DHC, or cyclohexene oxide/CO2/DHC, providing polyester-b-polycarbonate, polyester-b-polyester, or poly(ester-random-carbonate), respectively. This approach is valuable for the preparation of various novel materials with tailored property and functionality.

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INTRODUCTION Epoxides are versatile building blocks for organic and polymeric synthesis.1−5 Representative examples include the ring-opening of epoxides, which can afford the 1,2-difunctionalized chemicals through the delivery of water,6 azide,7 amine,8 cyanide,9 alcohol,10 thiol,11 carboxylic,12 halogen,13 or aryl14 nucleophiles to epoxides. Also, the ring expansion of epoxide through carbonylation in the presence of carbon monoxide (CO) provides lactones or anhydrides.15,16 The cycloaddition of epoxides with carbon dioxide (CO2) predominantly gives the corresponding five-membered cyclic carbonates,17 a type of nontoxic polar reagent. Indeed, epoxides as important fundamental and industrial chemicals, such as ethylene oxide and propylene oxide, were widely applied to the production of polyethers, commonly used as foams, sealants, surfactants, elastomers, and biomedical components.2 Recently, numerous efforts were paid to develop degradable epoxide-based materials with tunable properties by the alternating copolymerization with CO2 or cyclic anhydrides, affording degradable polycarbonates or polyesters.18−21 In some cases, the regio- and/or stereoselective copolymers were obtained by the use of singlesite metal catalysts with well-defined structure.22,23 Aliphatic polyesters, a promising class of biodegradable polymers, have been widely used in pharmaceutical, environmental, and commodity materials, such as packaging applications because of their excellent barrier properties.24 Condensation polymerization of diols with diacids or diesters and ring-opening polymerization of cyclic esters are muchstudied methods for polyesters synthesis. The former usually © XXXX American Chemical Society

suffers from high energy cost, requiring high temperature or vacuum condition to remove water or alcohol byproduct for achieving high molecular weight.25 The latter is generally limited to the availability of structurally diverse monomers, mainly focusing on 4-, 6-, and 7-membered cyclic esters, including lactide and ε-caprolactone.26−29 Indeed, in 1960, Fischer reported a different approach for the formation of polyesters by the ring-opening copolymerization of epoxide and cyclic anhydride first.30 This process allows for polymer functionality to be easily adjusted due to the wide availability of cyclic anhydrides and epoxides. However, the poor catalytic activity, the low molecular weight products, and the undesirable side reaction of epoxide homopolymerization significantly hindered its development. The first well-controlled polymerization of propylene oxide with phthalic anhydride was reported by Inoue’s group in 1985 using an aluminum porphyrin complex.31 It was not until 2007 that Coates and co-workers discovered that discrete 2-cyano-β-diketiminatozinc acetate catalyst, which had previously been used for epoxide/CO2 copolymerziation,32 was also effective for copolymerizating various epoxides with cyclic anhydrides, affording high molecular weight copolymers with perfectly alternating structure and narrow molecular weight distributions.33 Subsequently, various metal complexes have been applied to catalyze this copolymerization, including magnesium,34 zinc,35 Received: September 20, 2017 Revised: December 23, 2017

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DOI: 10.1021/acs.macromol.7b02042 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Chromium catalysts for the synthesis of epoxide-based copolymers. All used complexes are racemic, only if explained specially, for I, with equimolar R,R,R,R and S,S,S,S configurations, and for II, equimolar R,R and S,S configurations.

chromium,36 cobalt,37 and manganese38 complexes, many of which exhibited remarkably higher activity with the addition of a nucleophilic cocatalyst. Except for the synthesis of epoxidebased polyesters, much attention was paid to the copolymerization of epoxides with CO2 for the formation of degradable polycarbonates, an environmentally benign approach, due to the economic and environmental benefits arising from the utilization of renewable source.2,4,5,39,40 Notably, bimetallic or dinuclear metal catalysts were frequently discovered to show superior performance compared to the corresponding mononuclear analogues during the homopolymerization of epoxides or copolymerization with cyclic anhydrides or CO2, probably due to an improved facility to accommodate the monomers and lower barriers for the attack of carboxylate or alkoxide at one metal center onto the bound monomer molecules at the other. 41−44 In this contribution, we report a versatile dinuclear chromium catalyst I, which showed excellent activity for epoxide/anhydride, epoxide/CO2, and epoxide/dihydrocoumarin (DHC) copolymerizations to selectively afford the corresponding copolymers with more than 99% alternating structure (Figure 1). Notably, this catalyst was also proven to be very effective for the epoxide/CO2/anhydride, epoxide/anhydride/DHC, and epoxide/CO2/DHC terpolymerizations.

improved significantly by the adjustment of electronic effects on the ligand substituents and axial group.47 On the contrary, AlIII−Salen analogues have shown nearly reverse results, which has high activity for the copolymerization of epoxide with anhydride and neglected activity for the epoxide/CO 2 copolymerization.48−50 Interestingly, CrIII−Salen analogues were active for both epoxide/anhydride18,36 and epoxide/ CO2copolymerizations.5,51 As a result, our study mainly focused on chromium complexes for copolymerization reactions of epoxides with various monomers. For a comparison purpose, both dinuclear I and mononuclear II chromium complexes were all applied to the copolymerization of cyclohexene oxide (CHO) and succinic anhydride (SA), with nucleophilic PPN-Y (PPN = bis(triphenylphosphine)iminium, Y = Cl, NO3, N3) as cocatalyst (Table S1 in the Supporting Information). An excess of epoxide can keep the high solubility of SA during the copolymerization, thus making the reaction system homogeneous. It was found that Ia/PPN-Cl could initiate CHO/SA copolymerization with a reactivity of 12 h−1 at a molar ratio of Ia/PPN-Cl/SA/CHO = 1/2/500/1000 at 60 °C (Table S1, entry 2), affording the copolymer with 84% ester unit content, being indicative of the successive insertion of a certain amount of CHO. There are negligible differences in the reactivity and selectivity for substitution of the axial group X with NO3 or N3 as well as Y anion of PPN-Y cocatalyst (Table S1, entries 2−4). The copolymerization could be operated at ambient temperature, and the resultant polyesters showed a 93% alternating structure (Table S1, entry 5). An increase in reaction temperature resulted in an enhancement of the rate, but it has a negative effect on ester unit selectivity (Table S1, entries 6 and 7). When catalyst loading with anhydride was decreased to 0.1%, no obvious change in catalyst activity and selectivity was observed (Table S1, entry 8). Both activity and selectivity were reduced when the mononuclear CrIII complex IIb was used for this copolymerization (Table S1, entry 9). Especially, nearly

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RESULTS AND DISCUSSION Copolymerization Epoxides with Cyclic Anhydrides. Mononuclear trivalent MIII−Salen (M = Co, Cr, Al) complexes have demonstrated to be effective for copolymerizing epoxides with CO25 or anhydride,18 and the metal ion in these complexes has a dramatic effect on the copolymerization process. For example, the CoIII−Salen complex (Salen = N,N′-bis(3,5-ditert-butylsalicylidene)-1,2-cyclohexanediamine) showed excellent activities for copolymerization of epoxides and CO2;45 nevertheless, no or low activity was observed in epoxide/ anhydride copolymerization,46 although the activity can be B

DOI: 10.1021/acs.macromol.7b02042 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Substrate Scope for the Copolymerization of Epoxides with Anhydridea

entry

epoxide

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16g 17h

CHO

CPO COPO CEO CBO CDO PO GO SO Cl-SO ECH (S)-GO (R)-GO

anhydride

time (h)

convb (%)

TOFc (h−1)

polyesterd (%)

Mne (kg/mol)

PDIe

SA GA DGA PA MA SA

24 3 2 1.5 1 12 3 12 12 12 24 8 24 24 24 2.5 9

94 97 93 >99 22 53 95 99 94 99 75 98 78 53 91 97 98

19 161 232 333 110 22 158 41 39 41 16 61 16 11 19 194 54

77 >99 72 >99 15 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99

8.7 5.3 1.9 16.2 6.1 4.4 −f 3.4 4.6 5.2 5.3 12.8 4.3 6.2 21.2 12.6 13.8

1.24 1.55 1.26 1.20 1.71 1.04 −f 1.65 1.08 1.31 1.17 1.18 1.23 1.15 1.65 1.15 1.21

Reaction was performed in a flame-dried vial equipped with a stir bar, Ib/PPN-NO3/epoxide/anhydride = 1/2/500/1000 (molar ratio) at 80 °C (entries 1−10) or 30 °C (entries 11−17). Entries 1−9, performed in neat epoxides. Entry 10, performed in toluene solution (epoxide/toluene = 1:1, mass ratio). Entries 11−17 performed in toluene solution (epoxide/toluene = 1:2, volume ratio). bConversion of anhydride, determined by 1H NMR spectroscopy. cTurnover frequency (TOF) = mol of polymer (polyester)/mol of catalyst per hour. dSelectivity for polyesters over polyethers, determined by 1H NMR spectroscopy. eDetermined by gel permeation chromatography in THF, calibrated with polystyrene. fNot determined because of its low solubility. g(R,R,R,R)-Ib was used, and enantioselectivity was determined by HPLC with 98% ee (S,S). h(R,R,R,R)-Ib was used, and enantioselectivity was determined by HPLC with 84% ee (R,R). a

complete loss in activity was observed for the mononuclear CrIII IIb mediated propylene oxide/SA copolymerization, demonstrating the importance and superior performance of dinuclear structure (Table S1, entries 10 and 11). The wide availability and structure diversity for both epoxide and cyclic anhydride monomers give the resultant polyesters with various structure, excellent functionality, and tunable property. With the highly active catalyst system established, we turned our attention to examine the scope of substrates. Since CHO has relatively high reactivity, it was chosen as a model monomer for copolymerizing with various anhydrides. Significantly different from SA, glutaric anhydride (GA), diglycolic anhydride (DGA), phthalic anhydride (PA), and maleic anhydride (MA) exhibited higher activities for copolymerizing with CHO (Table 1, entries 1 versus 2−5). For example, the copolymerization activity for PA/CHO was up to 333 h−1, and the resulting polymer had perfect alternating structure (Table 1, entry 4). However, a large amount of polyether structure up to 85% was formed in the MA/CHO copolymerization (Table 1, entry 5). This result also demonstrates that the solubility, ring strain, substituent pattern, and functional group of the cycle anhydrides have huge effects on copolymerization activity and polymer selectivity.48,52 Using SA as the model anhydride, its copolymerization with various meso-epoxides, such as cyclopentene oxide (CPO), 3,4epoxytetrahydrofuran (COPO), 1,2-epoxy-4-cyclohexene (CEO), cis-2,3-epoxybutane (CBO), and 1,4-dihydronaphthalene oxide (CDO), was tested at 80 °C (Table 1, entries 6−10). All resultant copolymers possess perfectly alternating structure. Moreover, the terminal epoxides were also tested for copolymerizing with SA in toluene solution at 30 °C. Previously, the dinuclear metal complexes of this ligand

were proven to be very efficient in the homopolymerization of terminal epoxides.41 However, the copolymerization of SA with various terminal epoxides, including propylene oxide (PO), phenyl glycidyl ether (GO), styrene oxide (SO), 4-chlorostyrene oxide (Cl-SO), and epichlorohydrin (ECH), selectively provided the corresponding polyesters with perfectly alternating structures (Table 1, entries 11−15). Furthermore, we investigated the regioselectivity of the ring-opening copolymerization by the use of enantiopure epoxides. The enantioselectivity was determined by hydrolyzing the polyesters and analyzing the resulting diols by chiral HPLC method. Enantiopure (R,R,R,R)-Ib mediated copolymerization of (S)GO with SA could proceed smoothly at 30 °C, and nearly complete conversion of SA was achieved within 2.5 h; the resultant polymer has an enantioselectivity of 98%, confirming that the catalyst predominantly induced the ring-opening at the methylene carbon of (S)-GO and retained the stereochemistry at the methine carbon of epoxide incorporated into the polyester (Table 1, entry 16). However, the copolymerization of unmatched (R)-GO with SA using enantiopure (R,R,R,R)-Ib gives the polymer with an enantioselectivity of 84%, which means that at least 8% ring-opening process occurred at methine carbon atom (Table 1, entry 17, and Figures S50− S52). Moreover, the coplymerization of the matched (S)-GO showed a higher activity in comparison with the unmatched configuration epoxide (194 h−1 versus 54 h−1). This result is similar to the mononuclear CoIII−Salen mediated PO/SA copolymerization37as well as the copolymerization CO2 with epoxides.45 Copolymerization of Epoxides with 3,4-Dihydrocoumarin. The use of a lactone as a comonomer instead of cyclic anhydride for the copolymerization with epoxides gives another C

DOI: 10.1021/acs.macromol.7b02042 Macromolecules XXXX, XXX, XXX−XXX

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endothermic peak was found at 138 °C for poly(COPO-altDHC) sample from differential scanning calorimetry (DSC) analysis, and sharp diffraction peaks at 2θ values of 16.1°, 20.6°, and 24.6° were observed in the wide-angle X-ray diffraction (WAXD) spectroscopy. These results demonstrate that poly(COPO-alt-DHC) is a typical semicrystalline material. But for the poly(CDO-alt-DHC) copolymer, it was amorphous with a Tg of 63 °C (Figure S41). However, for the mononuclear complex IIa mediated CPO/DHC copolymerization, the reactivity was only 6 h−1 (Table 2, entry 11), which was similar to the results from Coates’ group.53 Copolymerization of Epoxides with CO2. The dinuclear CrIII−Salen catalyst was also employed for the copolymerization of epoxides with CO2 (Table 3). Using CHO as the model monomer, a neglected activity was observed in the absence of cocatalyst (Table 3, entry 1). With the addition of PPN-Cl cocatalyst, the activity was increased to 483 h−1 at a molar ratio of Ia/PPN-Cl/CHO = 1/2/2000 at 80 °C. Neither cyclic carbonates nor polyether structure was detected in the copolymerization process (Table 3, entry 2). Notably, the dinuclear catalyst was proved to be efficient at low loadings, without sacrificing polymer selectivity and molecular weight control (Table 3, entries 5−7). A TOF of 390 h−1 was obtained in this dinuclear system even with extremely low catalyst loading of 0.002% ([Ia]:[CHO] = 1:50000). Terpolymerization of CHO/PA/CO2, CHO/PA/DHC, and CHO/CO2/DHC. Terpolymerization is a powerful method to prepare novel polymers with tunable property and functionality by controlling the polymer composition and sequence. Various block, random, or gradient structures can be formed through the one-pot terpolymerization process, dependent on the competition polymerization rates of several monomers. Because of the versatility of the dinuclear CrIII catalyst for copolymerizing epoxides with different monomers such as CO2, cyclic anhydrides, and DHC, it gives the possibility for the synthesis of various terpolymers by the introduction of these monomers, in which metal alkoxide intermediates concern every catalytic cycle of the repeat units.54−60 As a consequence, we were of great interest to perform the one-pot terpolymerization of three monomers, without sequential monomer addition. In the CHO/PA/CO2, CHO/PA/DHC, or CHO/CO2/ DHC terpolymerization, the insertion of PA, CO2, or DHC to the common shared metal alkoxide intermediate is rapid, but epoxide ring-opening and further insertion into the formed metal carboxylate, carbonate, or phenoxide intermediates is a rate-determining step. At first, CO2 was introduced to CHO/ PA copolymerization (Scheme 1, cycles I and II); the terpolymerization was performed in neat CHO at 80 °C and 1.0 MPa CO2 pressure with catalyst Ia/PPN-Cl/PA/CHO = 1/ 2/500/1000 molar ratio. We discovered that PA was converted to the polyesters within 2 h, while CO2 pressure remained constant until PA was consumed completely. Then the polymerization entered the second phase, leading to the formation of polyester-b-polycarbonates terpolymers (Figure 2 and Table S2). It was notable that no formation of polyether unit by the successive CHO insertion was observed in the resultant terpolymers (Figure S42). Especially, the polymers resulted from various time points all have narrow molecular weight distributions (99 >99 >99 >99 >99 >99

12.2 18.4 5.1 10.3 17.2 18.7 26.7

2.50 1.11 1.12 1.14 1.12 1.16 1.16

a

Reaction was performed in neat CHO (30 mmol) in 20 mL autoclave under a 2.0 MPa CO2 pressure. bTurnover frequency (TOF) = mole of polymer (polycarbonate)/mol of cat. per hour. cDetermined by 1H NMR spectroscopy. dDetermined by gel permeation chromatography in THF, calibrated with polystyrene.

Scheme 1. Copolymerization Paths of Epoxide with Various Monomers

poly(CHO-alt-DHC) makes the reaction heterogeneous, and the PDI becomes broader in comparison with poly(PA-altCHO). The junction units between poly(PA-alt-CHO) and poly(CHO-alt-DHC) segment make their proton signals (Hb′ and Hc′) appear at ∼5.0 ppm, which leads to the integral areas of ∼5.0 ppm (Hb′ and Hc′ for junction unit and Hc for the poly(CHO-alt-DHC)) and ∼4.2 ppm (Hb for the poly(CHOalt-DHC)) unequally (Figure S45). Especially, the relative amount of integral area at ∼4.2 ppm increased, while the concentration of DHC monomer decreased with the polymerization time, demonstrating that the successful chain extension took place (Figure S46). Also, the crystallization and melting behavior of the resultant polymer was studied by DSC in a nitrogen flow (Figure S48). A quite high melting endothermic peak at 149 °C and a glass-transition temperature at 80 °C were observed, assigning to the crystalline poly(CHO-alt-DHC) segment and amorphous poly(PA-alt-CHO) segment, respectively. Interestingly, the terpolymerization of CHO/DHC/CO2 (Scheme 1, cycles I and III) showed a significantly different result. Although the terpolymerization was performed for 24 h, it is very difficult to separate a purified polymer. The reactivity of terpolymerization decreased significantly in comparison with individual CHO/CO2 or CHO/DHC copolymerizations. The 1 H NMR spectra of the reaction mixtures showed that the

Figure 2. Reaction profile of PA conversion (black square), ester unit content (red dot), and carbonate unit content (green triangle) with reaction time during CHO/PA/CO2 terpolymerization.

Cl/PA/DHC/CHO = 1/2/250/250/2000 molar ratio (Table 4). Similar to CHO/PA/CO2terpolymerization, PA insertion to the metal alkoxide intermediate is fast in comparison with DHC; nearly all PA monomer was converted to the desired poly(PA-alt-CHO)within 0.33 h (Figure S44). The relatively low nucleophilicity of phenoxide makes the second copolymerization process very sluggish. Also, the crystallinity of the E

DOI: 10.1021/acs.macromol.7b02042 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 4. CHO/PA/CO2 Terpolymerization Mediated by Dinuclear Catalyst Ia

entry

time (h)

PA convb (%)

CHO/PA contentc (%)

CHO/DHC contentc (%)

Mnd

PDId

1 2 3 4 5 6

0.33 1 2 3 4 5

97 >99 >99 >99 >99 >99

100 100 79 67 58 51

0 0 21 33 42 49

4.96 4.98 5.90 6.51 6.88 7.06

1.18 1.19 1.26 1.27 1.30 1.31

Reaction was performed in neat CHO (20 mmol) in a flame-dried vial equipped with a stir bar at 80 °C. Ia/PPN-Cl/PA/DHC/CHO = 1/2/250/ 250/2000, molar ratio. bConversion of PA, determined by 1H NMR spectroscopy. cDetermined by 1H NMR spectroscopy. dDetermined by gel permeation chromatography in THF, calibrated with polystyrene. a

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, grants 21690073, 21504011 and 51473027), China Postdoctoral Science Foundation (grant 2015M571294), Liaoning Provincial Natural Science Foundation of China (grant 2015020234) and Program for Chang Jiang Scholars and Innovative Research Team in University (IRT-17R14). X.-B.L. gratefully acknowledges the Chang Jiang Scholars Program (T2011056) from the Ministry of Education of China.

conversion is low, and the signals are assigned to the polyester, polycarbonate, and their junction units (Figure S49), indicating a random terpolymerization (Scheme 1, cycles I and III).

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CONCLUDING REMARKS We have demonstrated that the dinuclear CrIII complexes bearing biphenol linking bridge in conjunction with nucleophilic PPN-Y cocatalyst were efficient and versatile catalyst systems for the copolymerization of epoxides with cyclic anhydrides, CO2 or DHC, affording various biodegradable polyesters or polycarbonates with perfectly alternating structure and narrow molecular weight distribution. Especially, most of the polyesters from DHC are typical semicrystalline materials, which can be expected to expand the potential application of aliphatic polyesters as structural materials. By taking insight into the mechanism of various copolymerization processes, in which alkoxide intermediates concern each catalytic cycle for the formation of ester or carbonate units, the dinuclear chromium catalyst was succeeded in applying into the one-pot terpolymerization of CHO/PA/CO2, CHO/PA/DHC, or CHO/CO2/DHC, affording polyester-b-polycarbonate, polyester-b-polyester or poly(ester-random-carbonate). Benefiting from the versatile catalyst system, this one-pot terpolymerization not only simplifies the preparation of block polymers but also is expected to be valuable for the preparation of materials with tailored property and functionality.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02042. General experimental procedures and characterizations polymers (PDF)

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REFERENCES

(1) Wang, C.; Luo, L.; Yamamoto, H. Metal-Catalyzed Directed Regio- and Enantioselective Ring-Opening of Epoxides. Acc. Chem. Res. 2016, 49, 193−204. (2) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Stereoselective Epoxide Polymerization and Copolymerization. Chem. Rev. 2014, 114, 8129−8152. (3) Herzberger, J.; Niederer, K.; Pohlit, H.; Seiwert, J.; Worm, M.; Wurm, F. R.; Frey, H. Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation. Chem. Rev. 2016, 116, 2170−2243. (4) Paul, S.; Zhu, Y.; Romain, C.; Brooks, R.; Saini, P. K.; Williams, C. K. Catalysts for CO2/epoxide Ring-Opening Copolymerization. Chem. Commun. 2015, 51, 6459−6479. (5) Darensbourg, D. J. Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2. Chem. Rev. 2007, 107, 2388−2410. (6) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Asymmetric Catalysis with Water: Efficient Kinetic Resolution of Terminal Epoxides by Means of Catalytic Hydrolysis. Science 1997, 277, 936−938. (7) Nugent, W. A. Chiral Lewis Acid Catalysis. Enantioselective Addition of Azide to meso-Epoxides. J. Am. Chem. Soc. 1992, 114, 2768−2769. (8) Arai, K.; Lucarini, S.; Salter, M. M.; Ohta, K.; Yamashita, Y.; Kobayashi, S. The Development of Scalemic Multidentate Niobium Complexes as Catalysts for the Highly Stereoselective Ring Opening of meso-Epoxides and meso-Aziridines. J. Am. Chem. Soc. 2007, 129, 8103−8111. (9) Cole, B. M.; Shimizu, K. D.; Krueger, C. A.; Harrity, J. P. A; Snapper, M. L.; Hoveyda, A. H. Discovery of Chiral Catalysts through Ligand Diversity: Ti-Catalyzed Enantioselective Addition of TMSCN to meso-Epoxides. Angew. Chem., Int. Ed. Engl. 1996, 35, 1668−1671. (10) Schneider, C.; Sreekanth, A. R.; Mai, E. Scandium−BipyridineCatalyzed Enantioselective Addition of Alcohols and Amines to mesoEpoxides. Angew. Chem., Int. Ed. 2004, 43, 5691−5694.

AUTHOR INFORMATION

Corresponding Authors

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

Xiao-Bing Lu: 0000-0001-7030-6724 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.macromol.7b02042 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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DOI: 10.1021/acs.macromol.7b02042 Macromolecules XXXX, XXX, XXX−XXX