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Research Article Cite This: ACS Catal. 2019, 9, 1915−1922
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Development of Highly Enantioselective Catalysts for Asymmetric Copolymerization of meso-Epoxides and Cyclic Anhydrides: Subtle Modification Resulting in Superior Enantioselectivity Jie Li, Bai-Hao Ren, Shi-Yu Chen, Guang-Hui He, Ye Liu, Wei-Min Ren, Hui Zhou, and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Downloaded via EASTERN MICHIGAN UNIV on October 21, 2019 at 17:32:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: The asymmetric alternating copolymerization of meso-epoxides with cyclic anhydrides promoted by chiral catalysts or reagents is a powerful strategy for the synthesis of optically active polyesters with main-chain chirality. Herein, we show that, in conjunction with a nucleophilic cocatalyst, enantiopure dinuclear Al(III) complexes efficiently catalyze this asymmetric copolymerization, exhibiting high activity and achieving enantioselectivities up to 99% ee under mild conditions. Copolymer enantioselectivity and catalytic activity are revealed to be strongly affected by the axial linker, chiral diamine structure, and phenolate ortho-substituents. Density functional theory calculations confirm that the reactions corresponding to ring opening at (R)-C−O and (S)-C−O bonds of the Al-coordinated meso-epoxide during copolymerization exhibit significantly different Gibbs free energies of activation (Δ‡G). Enantiopure dinuclear Al(III) complex 3 bearing a hydrogenated binaphthol linker with the matched configuration is demonstrated to be the most active and enantioselective catalyst, featuring a broad substrate scope and allowing one to obtain a wide range of isotactic polyesters with a completely alternating structure and low polydispersity. Notably, most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures between 120 and 240 °C. Additionally, the mixing of selected isotactic (R)- and (S)polyesters in a 1:1 mass ratio afforded two crystalline stereocomplexes that exhibited enhanced thermal stability as well as new crystallization behavior and therefore significantly differed from the parent enantiopure polymers. KEYWORDS: asymmetric copolymerization, polyester, meso-epoxide, cyclic anhydride, dinuclear aluminum, stereocomplex
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mononuclear Cr−salan complexes.13 For example, copolymerization of maleic anhydride and (S)-phenyl glycidyl ether in the former case afforded polyesters with up to 99% head-to-tail linkages, which indicated the occurrence of a highly regioregular ring-opening step with >99% configuration retention at the methine carbon of the enantiopure epoxide incorporated into the polyester. Subsequently, Coates et al. succeeded in synthesizing highly tactic polyesters (96−97% head-to-tail linkages) via regioselective ring-opening alternating copolymerization of enantiopure propylene oxide and cyclic anhydrides using a chiral salen−Co(III)NO3 complex as a catalyst,14 demonstrating that, unlike their atactic analogs, both (R)- and (S)-(propylene succinate)s slowly crystallized from the melt after 1 week. In both cases, the degree of crystallinity gradually increased with time, and a very low melting polymorph (melting temperature (Tm) = 70 °C, melting enthalpy (ΔHm) = 48 J g−1) was observed after 19 days. Importantly, the stereoselective interaction between (R)-
INTRODUCTION Recently, the formation of degradable polyesters by alternating copolymerization of epoxides and cyclic anhydrides, both of which are readily available, has attracted increased attention,1 providing great opportunities for polymer functionalization to achieve extraordinary properties that cannot be realized via step-growth condensation of diols and diesters2 or ringopening polymerization of cyclic esters such as lactide, εcaprolactone, and β-lactones.3 Moreover, the presence of stereogenic centers in substituted epoxides allows one to synthesize polymers with controlled tacticity by stereospecific copolymerization.4 Stereoregular polymers show physicochemical properties superior to those of their irregular analogs because of the highly compact package structure of the former.5 Although a number of highly alternating copolymers have been prepared via this strategy using various metal-based catalysts (including Al,6 Zn,7 Cr,8 Co,9 Mn,10 and Fe11 complexes) and organocatalysts,12 the synthesis of stereoregular polyesters has been underexplored. In 2013, we discovered that binuclear Cr(III)−salan complexes bearing a binaphthol linker catalyze the copolymerization of terminal epoxides and cyclic anhydrides much more effectively than © 2019 American Chemical Society
Received: January 9, 2019 Revised: January 17, 2019 Published: January 17, 2019 1915
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis Chart 1. Structures of the Employed Dinuclear Al(III) Complexes
Table 1. Outcomes of Dinuclear Al(III) Complex-Catalyzed Asymmetric Alternating Copolymerization of COPO/PAa
entry
catalyst
time [h]
temp. [°C]
conv.b [%]
TOFc [h−1]
Mnd [kg mol−1]
Đd
eee [%]
specific rotation [deg]f
1 2 3 4 5 6g 7 8 9 10 11 12 13 14 15 16 17 18
(R,R,R,R)-1a (R,R,R,R)-1b (R,R,R,R)-1c (R,R,R,R)-1d (R,R,R,R)-1e (R,R,R,R)-1c (R,R,R,R,R)-2a (R,R,R,R,R)-2b (R,R,S,R,R)-2b (R,R,R,R,R)-3 (R,R,R,R,R)-4 (R,R,S,R,R)-4 (R,R,R,R)-5 (R,R,R,R,R,R)-6 (R,R,S,R,R,S)-6 (R,R,R,R)-7 (R,R,R,R)-8 (R,R)-9
2 2 1.7 2.5 9 24 5 2.5 48 1.5 18 48 4 42 43 60 48 62
25 25 25 25 25 0 25 25 25 25 25 25 25 25 25 25 25 25
62 78 79 92 99 62 98 33 54 93 58 51 93 53 64 76 32 70
78 98 116 92 28 6 49 33 3 155 8 3 58 3 4 3 2 3
6.7 7.1 7.3 8.6 8.7 7.3 8.5 6.9 7.3 8.3 8.8 8.3 8.0 5.3 6.3 5.3 5.9 4.8
1.17 1.15 1.13 1.11 1.12 1.12 1.11 1.14 1.23 1.11 1.11 1.14 1.13 1.14 1.30 1.89 1.69 2.17
56 (R,R) 90 (R,R) 91 (R,R) 85 (R,R) 37 (R,R) 98 (R,R) 97 (R,R) 90 (R,R) 27 (R,R) 99 (R,R) 81 (R,R) 55 (S,S) 65 (R,R) 10 (R,R) 7 (S,S) 26 (S,S) 1 (S,S) 24 (S,S)
118(−) 190(−) 198(−) 180(−) 78(−) 207(−) 204(−) 190(−) 57(−) 209(−) 171(−) 116(+) 137(−) 20(−) 15(+) 55(+) 1(+) 51(+)
a The reaction was performed in a 20 mL flask with a high vacuum valve in neat COPO (5.0 mL, 50 mmol) at a COPO/PA/catalyst/PPNCl molar ratio of 1000/250/1/2, except for entry 6. bCalculated using 1H NMR spectroscopy based on PA as the limiting reagent. cTurnover frequency (TOF) = mol of product (polyester)/mol of catalyst per hour. dDetermined using gel permeation chromatography in THF, calibrated with polystyrene. eMeasured by hydrolyzing the polymer, derivatizing the thus-produced diol with benzoyl chloride, and then determining the ee of the dibenzoate by HPLC. The (R,R)-product was the major enantiomer when the (R,R,R,R)-catalyst was used. fSpecific rotation was determined in chloroform at 20 °C (c = 1) using a polarimeter at λ = 589.3 nm. gThe reaction was carried out in toluene (COPO/toluene = 1:2 vol/vol).
complex, recrystallization was approximately 3 orders of magnitude faster than that for the parent enantiopure polymers. Recently, we employed bimetallic Al complexes to
and (S)-(propylene succinate) mixed in a 1:1 mass ratio resulted in the formation of a stereocomplex with improved crystallinity and increased Tm (up to 120 °C). For this 1916
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis
Figure 1. Gibbs free energy (ΔG) profiles corresponding to COPO ring opening at (R)-C−O and (S)-C−O bonds in toluene as a solvent catalyzed by complexes 1a−1e and 3. Energies are given in kcal mol−1.
demonstrating that a subtle modification of these substituents results in marked enantioselectivity changes. Among biphenollinked dinuclear Al(III) complexes 1a−1e, enantiopure 1c with isopropyl groups at phenolate ortho-positions proved to be the most active and enantioselective catalyst, affording the corresponding polyester in 91% ee at 25 °C. In contrast, enantiopure dinuclear Al(III) complex 1b could efficiently catalyze asymmetric copolymerization at ambient temperature, furnishing the target polyester in 90% ee and exhibiting an activity (TOF = 98 h−1) lower than that of 1c (116 h−1) under identical conditions (Table 1, entries 2 and 3). When copolymerization was performed at 0 °C in toluene using (R,R,R,R)-1c/PPNCl as the catalyst, the resulting copolymer possessed high enantiopurity; that is, the R,R-product was obtained with enantioselectivities of up to 98% ee and exhibited a specific rotation of [α]20 D = −207 (c = 1 in CHCl3) (Table 1, entry 6). Biphenol-linked dinuclear Al(III) complexes 1a−1e with (R,R,R,R)-configuration possess two diastereoisomers with (R)- or (S)-biphenol stereochemistry, namely, (R,R,R,R,R)and (R,R,S,R,R)-conformers,16 with the former being better suited for the formation of isotactic fragment-enriched polyesters with (R,R)-configuration than the latter. At 25 °C (R,R,R,R,R)-2b and (R,R,S,R,R)-2b systems featured activities of 33 and 3 h−1, respectively, and afforded (R,R)-copolymers in 90 and 27% ee, respectively. Similarly to the case of biphenollinked dinuclear Al(III) complexes, substitution of orthoisopropyl groups on the phenolate ring for tert-butyl ones increased enantioselectivity at ambient temperature from 90% to 97% ee (entries 7−9), with the highest activity and enantioselectivity observed for enantiopure complex 3. In this case, high conversion was achieved already within 1.5 h at 25 °C, and the polyester obtained in 99% ee featured a low polydispersity index (Đ) of 1.11 (entry 10). Enantiopure dinuclear Al complexes 4−8 were synthesized to probe the effects of linker and chiral diamine backbone on catalytic activity and product enantioselectivity. Asymmetric COPO/PA copolymerization catalyzed by a system based on binaphthol linker-bearing (R,R,R,R,R)-4 furnished a completely alternating copolymer with (R,R)-configuration in 81% ee, while the (S,S)-configured polyester was obtained in
realize an unprecedented asymmetric copolymerization of cyclic anhydrides and achiral meso-epoxides affording isotactically enriched polyesters with a completely alternating nature and low polydispersity.15 Since the nucleophilic ring opening of meso-epoxides proceeds with configuration inversion at one of the two prochiral centers, successful asymmetrization-based alternating copolymerization of meso-epoxides and cyclic anhydrides promoted by a chiral catalyst can furnish enantiopure polyesters with main-chain chirality in repeated (R,R)- or (S,S)-trans-1,2-diol units. The highest enantiomeric excess of 91% ee was obtained for the copolymerization of cyclohexene oxide (CHO) and phthalic anhydride (PA) at 0 °C catalyzed by a biphenol-linked dinuclear Al complex with tert-butyl groups in phenolate ortho-positions in conjunction with bis(trihenylphosphine)iminium chloride (PPNCl) as a nucleophilic cocatalyst. Herein, we comprehensively study the effects of linker, chiral diamine structure, and phenolate ortho-substituents of the ligand on the activity and enantioselectivity of bimetallic Al complexes (Chart 1) for the asymmetric copolymerization of meso-epoxides and cyclic anhydrides. In particular, we elucidate the effect of steric hindrance created by phenolate orthosubstituents on enantioselectivity to ultimately develop privileged chiral catalysts for this asymmetric copolymerization. Moreover, we also characterize the crystallization and stereocomplexation behavior of selected stereoregular polyesters.
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RESULTS AND DISCUSSION Enantioselective Copolymerization of 3,4-Epoxytetrahydrofuran (COPO) and PA. Previously, we demonstrated that dinuclear Al complex 1d bearing tert-butyl groups at phenolate ortho-positions efficiently promotes asymmetric CHO/PA copolymerization to give enantioenriched polyesters, revealing that the highest ee of 91% was obtained at a low reaction temperature of 0 °C.15 Additionally, the phenolate ortho-substituents of the ligand were shown to strongly affect catalytic activity and product enantioselectivity, with neither small (methyl group) nor bulky (trimethylsilyl (TMS)) groups being beneficial for this asymmetric copolymerization. In the present work, we further explored the steric effects of phenolate ortho-substituents on product enantioselectivity, 1917
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis Table 2. Substrate Scope of the Asymmetric Copolymerization of meso-Epoxides with Anhydridesa
entry
reactants
temp. [°C]
time [h]
conv.b [%]
TOFc [h−1]
Mnd [kg mol−1]
Đd
1 2 3 4 5 6g 7 8 9g 10g 11
COPO/PA CHO/PA CPO/PA CBO/PA CEO/PA CDO/PA COPO/4,5-DMPA COPO/3,6-DMPA CDO/4,5-DMPA CDO/NA COPO/SA
25 25 25 25 25 25 25 25 50 50 25
1.5 1 15 13 10 8 11 11 12 10 24
93 93 28 85 96 81 39 30 70 99 44
155 232 5 16 24 25 9 7 14 25 5
8.3 6.5 2.3h 8.3 11.1 17.0 7.6h 6.5h 14.7 14.2 4.3h
1.16 1.15 1.21h 1.16 1.14 1.12 1.44h 1.17h 1.32 1.15 1.23h
eee [%] 99 98 83 98 98 98 90 87 96 95 93
(R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R)
specific rotationf [deg]
Tg/Tm [°C]
209(−) 32(−) i 37(+) 139(−) 113(−) i i 145(−) 475(−) i
−/235 150/− −/239 −/134 127/− −/185 −/180,205, 225 −/160 −/183 190/− −/121
The reaction was performed in excess epoxide (5.0 mL, 50 mmol) in a 20 mL flask with a high vacuum valve at an epoxide/anhydride/catalyst/ PPNCl molar ratio of 1000/250/1/2, except for entries 6, 9, and 10. bCalculated using 1H NMR spectroscopy based on anhydride as the limiting reagent. cTurnover frequency (TOF) = mol of product (polyester)/mol of catalyst per hour. dDetermined using gel permeation chromatography in THF, calibrated with polystyrene. eMeasured by hydrolyzing the polymer and analyzing the resulting diol by chiral GC or derivatizing this diol with benzoyl chloride and then determining the ee of the dibenzoate by HPLC. fSpecific rotation was determined in chloroform at 20 °C (c = 1) using a polarimeter at λ = 589.3 nm. gThe reaction was carried out in toluene solution at an epoxide/anhydride/toluene/catalyst/PPNCl molar ratio of 250/250/500/1/2. hThe polymer was heated to 250 °C, cooled to room temperature, and then analyzed by gel permeation chromatography in THF. Calibration was performed using polystyrene. iSpecific rotation could not be determined because of the poor solubility of the copolymer. a
ring opening of meso-epoxides. To simplify calculations, the polymer chain was replaced by 2-(methoxycarbonyl)benzoate, and COPO was chosen as a model meso-epoxide monomer. In this model, a COPO molecule coordinated to one Al(III) center and was subsequently ring-opened by 2(methoxycarbonyl)benzoate. The Gibbs free energies (Δ‡G) of the two transition states (TS-1 and TS-2) corresponding to ring opening at (R)-C−O and (S)-C−O bonds of coordinated COPO were different because of the chiral environment provided by the (R,R,R,R,R)-catalyst (Figure 1). For biphenollinked dinuclear Al(III) complexes 1a−1e, the transition state produced via attack at (S)-C−O was much lower in energy than that produced by attack at (R)-C−O in the epoxide; that is, the (R,R,R,R,R)-conformer catalyzed COPO/PA copolymerization at (S)-C−O in preference to that at (R)-C−O to yield polyesters with a predominantly (R,R)-configuration, in agreement with experimental findings. Moreover, the gap between the above two transition states increased in the order 1e < 1a < 1d < 1b ≈ 1c. At this point, it is worth noting that the highest enantioselectivity was congruent with the largest Gibbs free energy (Δ‡G = 7.8 kcal mol−1), which was observed for complex 3. Substrate Scope of Enantioselective Epoxide/Anhydride Copolymerization. With an active and enantioselective catalyst system ((R,R,R,R,R)-3/PPNCl) in hand, we evaluated the substrate scope of the epoxide/anhydride copolymerization, which was performed either in bulk or in toluene as a solvent. The relatively highly reactive PA was chosen as a model cyclic anhydride for catalytic activity and enantioselectivity testing. NMR spectroscopic analysis of the produced polyesters revealed that highly stereoregular structures were formed in all cases. Except for cyclopentene
55% ee when (R,R,S,R,R)-4 was employed. This result indicates the strong chirality induction ability of the axial linker. A change of the chiral diamine backbone from diaminocyclohexane to 1,2-diaminobenzene (complex 5) considerably decreased both enantioselectivity (65% ee) and catalytic activity (58 h−1) at ambient temperature. Unfortunately, complex 6 with four chiral carbon centers and two chiral binaphthols could not efficiently catalyze the asymmetric COPO/PA copolymerization; for example, (R,R)- and (S,S)configured copolymers were obtained in only 10% ee (for (R,R,R,R,R,R)-6) and 7% ee (for (R,R,S,R,R,S)-6), respectively (Table 1, entries 14 and 15). Rigid linker-containing (R,R,R,R)-7 exhibited low catalytic activity (3 h−1) and poor enantioselectivity ((S,S)-configuration, 26% ee), behaving similarly to mononuclear Al(III) complex 9 (entry 18). Interestingly, a nearly complete loss of activity and enantioselectivity was observed for the catalyst system based on (R,R,R,R)-8 with a p-terphenyl linker. These screening experiments imply that linker stereochemistry plays an important role in determining the enantioselectivity of dinuclear Al complex-catalyzed asymmetric COPO/PA copolymerization. Origin of Epoxide Ring-Opening Enantioselectivity Revealed by Density Functional Theory (DFT) Calculations. To gain further insights into the effect of steric hindrance on the chiral induction ability of biphenol-linked dinuclear Al(III) complexes, polymer chain growth processes were simulated by DFT calculations. Since catalysts with (R,R,R,R,R)- or (S,S,S,S,S)-configuration have been shown to be best suited for enantioselectivity control, (R,R,R,R,R)Al(III) complexes 1a−1e and 3 were studied to explore the steric effect of phenolate ortho-substituents on the asymmetric 1918
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis oxide (CPO), numerous meso-epoxides such as CHO, cis-2,3epoxybutane (CBO), 1,2-epoxy-4-cyclohexene (CEO), and 1,4-dihydronaphthalene oxide (CDO) exhibited good reactivity at ambient temperature, affording (R,R)-polyesters in more than 98% ee. Moreover, several cyclic anhydrides such as 4,5-dimethylphthalic anhydride (4,5-DMPA), 3,6-dimethylphthalic anhydride (3,6-DMPA), and succinic anhydride (SA) were employed for asymmetric copolymerization using COPO as a model meso-epoxide monomer. As expected, the produced copolymers showed perfectly alternating structures and high ee values of 87−93% (Table 2, entries 7, 8, and 11). The enantioselective copolymerization of CDO with 4,5-DMPA or naphthyl anhydride (NA) could be performed in toluene at an elevated temperature of 50 °C to give the corresponding polyesters with high enantioselectivities of 96% and 95% ee, respectively (Table 2, entries 9 and 10). It should be noted that the resulting CDO/NA copolymer ((R,R)-configuration, 95% ee) featured a high specific rotation of [α]20 D = −475 (c = 1 in CHCl3) and possessed a high glass transition temperature (Tg) of 190 °C. Importantly, the resulting isotactic polyesters were mostly semicrystalline materials with melting temperatures of 120− 240 °C. For example, a fairly sharp and high-temperature endothermic peak (ΔHm = 20.68 J g−1) was observed at 236 °C for isotactic COPO/PA copolymers with 99% ee, while the DSC thermogram of the atactic copolymer featured a peak corresponding to Tg = 105 °C (Figure 2, top). Figure 2
representative example provided by poly(cyclopentene phthalate) (CPO/PA copolymer). Isotactic fragment-enriched (65% ee) poly(cyclopentene phthalate) featured an endothermic melting peak at 221 °C (ΔHm = 23.63 J g−1), while the copolymer with 83% ee featured Tm = 239 °C and ΔHm = 41.33 J g−1 (Figure 3). Although diffraction peaks at 2θ =
Figure 3. DSC thermograms (top) and WAXD profiles (bottom) of isotactic fragment-enriched) poly(cyclopentenephthalate)s with ee values of (A) 83% and (B) 65%.
14.9°, 19.7°, and 24.5° were observed in both cases, their intensity was higher in the latter case; that is, the 83% ee polymer featured a higher degree of crystallinity. Macromolecular stereocomplexation is the stereoselective interaction between two polymers with opposite enantiomeric configurations proceeding via the formation of an interlocked ordered assembly. Usually, such stereocomplexes exhibit significantly improved thermal properties (both Tm and crystallinity) and feature crystallization behaviors significantly different from those of constituent enantiomeric polymers. Recently, a variety of newly discovered CO2-based polycarbonate stereocomplexes derived from meso-epoxides have been shown to exhibit unique crystallization behaviors.17 With these isotactic polyesters with different configurations in hand, we aimed to explore the interaction between oppositely configured copolymers derived from meso-epoxides. Unfortunately, in most cases, no cocrystallization was observed in blends containing equivalent amounts of these enantiomeric polymers with identical chemical compositions and opposite configurations. To our delight, we discovered two polyester stereocomplexes containing isotactic crystallizable poly(CDO-altPA) and amorphous poly(CDO-alt-NA). A 1:1 mixture of (R)poly(CDO-alt-PA) (Mn = 7.0 kg mol−1, Đ = 1.12) and (S)poly(CDO-alt-PA) (Mn = 8.5 kg mol−1, Đ = 1.14) was prepared by dissolving equal amounts of each enantiopure copolymer in dichloromethane and slowly evaporating the solvent to afford a dichloromethane-insoluble powder. The thus-obtained product featured a single endothermic peak at
Figure 2. DSC thermograms (top) and WAXD profiles (bottom) of (A) polyester prepared from COPO/PA in 99% ee and (B) atactic polyester prepared from COPO/PA.
(bottom) shows the wide-angle X-ray diffraction (WAXD) profiles of atactic and (R)-configured COPO/PA copolymers. No diffraction peaks were observed for the atactic copolymer, in agreement with its amorphous nature. In contrast, sharp diffraction peaks at 2θ = 15.5°, 17.5°, 23.3°, 28.4°, and 31.1° were observed for the isotactic (R)-copolymer, which demonstrated that this polymer is a typical semicrystalline material. The crystallization ability of these stereoregular polyesters greatly increased with increasing isotacticity, with a 1919
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis Tm ≈ 210 °C with ΔHm = 30.33 J g−1, and these values were significantly higher than those obtained for (R)- or (S)poly(CDO-alt-PA) with 98% ee (Tm = 185 °C, ΔHm = 26.46 J g−1; Figure 4, top). Furthermore, the WAXD patterns of
Figure 5. DSC thermograms (top) and WAXD profiles (bottom) of various poly(CDO-alt-NA)s: (A) enantiopure isotactic polymer (95% ee) and (B) 1:1 (w/w) mixture of (R)- and (S)-poly(CDO-alt-NA)s.
Figure 4. DSC thermograms (top) and WAXD profiles (bottom) of various poly(CDO-alt-PA)s. (A) Atactic, (B) enantiopure isotactic polymer (98% ee), and (C) 1:1 (w/w) mixture of (R)- and (S)poly(CDO-alt-PA)s.
values of up to 99%. A subtle modification of phenolate orthosubstituents significantly changed both enantioselectivity and catalyst activity, as observed for enantiopure biphenol-linked Al(III) complexes 1a−1e. DFT calculations performed on the above complexes confirmed that the two transition states (TS1 and TS-2) corresponding to ring opening at (R)-C−O and (S)-C−O bonds of coordinated meso-epoxides during copolymerization featured markedly different Gibbs free energies (ΔΔG) because of the steric effects of phenolate orthosubstituents. Moreover, both catalytic activity and product enantioselectivity were strongly affected by the linker and chiral diamine backbones of binuclear Al(III) complexes. Most of the produced isotactic polyesters were typical semicrystalline materials with melting temperatures of 120− 240 °C. The crystalline stereocomplex obtained by mixing (R)and (S)-isotactic crystallizable poly(CDO-alt-PA) in a 1:1 mass ratio exhibited an increased melting temperature of 210 °C and featured a crystallization behavior significantly different from those of constituent enantiomers. Additionally, although highly isotactic poly(CDO-alt-NA) with 95% ee was a typical amorphous material with a high Tg of 190 °C, mixing of amorphous (R)- and (S)-polymers in equivalent amounts afforded a crystalline stereocomplexed polyester with a high melting temperature of 334 °C. Thus, this study presents a first-time report on the formation of crystalline stereocomplexed polyesters from opposite enantiomers of amorphous polymers and is therefore of high novelty and practical significance.
stereoregular polyesters featured peaks at 2θ = 13.9°, 16.7°, 18.6°, 20.6°, 22.° 26.0°, 28.3°, 30.0°, and 34.8°, demonstrating that isotactic poly(CDO-alt-PA) is a typical semicrystalline polymer. Conversely, the 1:1 mixture of (R)- and (S)poly(CDO-alt-PA) featured peaks at 2θ = 9.8°, 18.6°, 21.0°, and 27.0° (Figure 4, bottom). Thus, the results of both DSC (Differential scanning calorimetry) and WAXD (wide-angle Xray diffraction) analyses illustrated that the stereoselective interaction between two polyesters with opposite enantiomeric configurations affords a crystalline stereocomplex. Since not much is known about the formation of crystalline stereocomplexes from amorphous polymers upon stereocomplexation,17b,18 we were delighted to discover that the DSC trace of a 1:1 mixture of (R)-poly(CDO-alt-NA) (Mn = 8.2 kg mol−1, Đ = 1.15) and (S)-poly(CDO-alt-NA) (Mn = 9.3 kg mol−1, Đ = 1.17) featured a sharp endothermic melting peak at 334 °C (Figure 5, top). Moreover, WAXD analysis provided further evidence of cocrystallization in this polymer blend, for which eight diffraction peaks with 2θ = 8.5°, 11.5°, 14.3°, 15.0°, 19.5°, 24.3°, 28.3°, and 30.8° were observed (Figure 5, bottom). In contrast, the highly isotactic poly(CDO-alt-NA) with 95% ee was demonstrated to be a typical amorphous material with a high Tg of 190 °C.
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CONCLUSION In summary, we demonstrated that, in conjunction with PPNCl, hydrogenated binaphthol-linked dinuclear Al(III) complexes can effectively catalyze the enantioselective copolymerization of meso-epoxides with cyclic anhydrides to afford polyesters with a completely alternating structure and ee
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b00113. 1920
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
Research Article
ACS Catalysis
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Chin. J. Polym. Sci. 2014, 32, 768−777. (c) Saini, P. K.; Romain, C.; Zhu, Y.; Williams, C. K. Di-magnesium and zinc catalysts for the copolymerization of phthalic anhydride and cyclohexene oxide. Polym. Chem. 2014, 5, 6068−6075. (d) Thevenon, A.; Garden, J. A.; White, A. J. P.; Williams, C. K. Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymerization of Epoxides and Carbon Dioxide or Anhydrides. Inorg. Chem. 2015, 54, 11906−11915. (e) Garden, J. A.; Saini, P. K.; Williams, C. K. Greater than the Sum of Its Parts: A Heterodinuclear Polymerization Catalyst. J. Am. Chem. Soc. 2015, 137, 15078−15081. (f) Abbina, S.; Chidara, V. K.; Du, G. Ring-Opening Copolymerization of Styrene Oxide and Cyclic Anhydrides by using Highly Effective Zinc Amido−Oxazolinate Catalysts. ChemCatChem 2017, 9, 1343−1348. (8) (a) Huijser, S.; HosseiniNejad, E.; Sablong, R.; de Jong, C.; Koning, C. E.; Duchateau, R. Ring-Opening Co- and Terpolymerization of an Alicyclic Oxirane with Carboxylic Acid Anhydrides and CO2 in the Presence of Chromium Porphyrinato and Salen Catalysts. Macromolecules 2011, 44, 1132−1139. (b) DiCiccio, A. M.; Coates, G. W. Ring-Opening Copolymerization of Maleic Anhydride with Epoxides: A Chain-Growth Approach to Unsaturated Polyesters. J. Am. Chem. Soc. 2011, 133, 10724−10727. (c) Darensbourg, D. J.; Poland, R. R.; Escobedo, C. Kinetic Studies of the Alternating Copolymerization of Cyclic Acid Anhydrides and Epoxides, and the Terpolymerization of Cyclic Acid Anhydrides, Epoxides, and CO2 Catalyzed by (salen)CrIIICl. Macromolecules 2012, 45, 2242−2248. (d) Si, G.; Zhang, L.; Han, B.; Duan, Z.; Li, B.; Dong, J.; Li, X.; Liu, B. Novel chromium complexes with a [OSSO]-type bis(phenolato) dianionic ligand mediate the alternating ring-opening copolymerization of epoxides and phthalic anhydride. Polym. Chem. 2015, 6, 6372− 6377. (9) (a) Hosseini Nejad, E.; Paoniasari, A.; Koning, C. E.; Duchateau, R. Semi-aromatic polyesters by alternating ring-opening copolymerisation of styrene oxide and anhydrides. Polym. Chem. 2012, 3, 1308− 1313. (b) Bernard, A.; Chatterjee, C.; Chisholm, M. H. The influence of the metal (Al, Cr and Co) and the substituents of the porphyrin in controlling the reactions involved in the copolymerization of propylene oxide and cyclic anhydrides by porphyrin metal(III) complexes. Polymer 2013, 54, 2639−2646. (c) DiCiccio, A. M.; Longo, J. M.; Rodríguez-Calero, G. G.; Coates, G. W. Development of Highly Active and Regioselective Catalysts for the Copolymerization of Epoxides with Cyclic Anhydrides: An Unanticipated Effect of Electronic Variation. J. Am. Chem. Soc. 2016, 138, 7107−7113. (10) (a) Robert, C.; de Montigny, F.; Thomas, C. M. Tandem synthesis of alternating polyesters from renewable resources. Nat. Commun. 2011, 2, 586. (b) Liu, D.; Zhang, Z.; Zhang, X.; Lü, X. Alternating Ring-Opening Copolymerization of Cyclohexene Oxide and Maleic Anhydride with Diallyl-Modified Manganese(III)−Salen Catalysts. Aust. J. Chem. 2016, 69, 47−55. (11) Sanford, M. J.; Peña Carrodeguas, L.; Van Zee, N. J.; Kleij, A. W.; Coates, G. W. Alternating Copolymerization of Propylene Oxide and Cyclohexene Oxide with Tricyclic Anhydrides: Access to Partially Renewable Aliphatic Polyesters with High Glass Transition Temperatures. Macromolecules 2016, 49, 6394−6400. (12) (a) Han, B.; Zhang, L.; Liu, B.; Dong, X.; Kim, I.; Duan, Z.; Theato, P. Controllable Synthesis of Stereoregular Polyesters by Organocatalytic Alternating Copolymerizations of Cyclohexene Oxide and Norbornene Anhydrides. Macromolecules 2015, 48, 3431−3437. (b) Li, H.; Zhao, J.; Zhang, G. Self-Buffering Organocatalysis Tailoring Alternating Polyester. ACS Macro Lett. 2017, 6, 1094− 1098. (c) Hošt’álek, Z.; Trhlíková, O.; Walterová, Z.; Martinez, T.; Peruch, F.; Cramail, H.; Merna, J. Alternating copolymerization of epoxides with anhydrides initiated by organic bases. Eur. Polym. J. 2017, 88, 433−447. (d) Li, H.; Luo, H.; Zhao, J.; Zhang, G. WellDefined and Structurally Diverse Aromatic Alternating Polyesters Synthesized by Simple Phosphazene Catalysis. Macromolecules 2018, 51, 2247−2257. (e) Hu, L.-F.; Zhang, C.-J.; Wu, H.-L.; Yang, J.-L.; Liu, B.; Duan, H.-Y.; Zhang, X.-H. Highly Active Organic Lewis Pairs for the Copolymerization of Epoxides with Cyclic Anhydrides: MetalFree Access to Well-Defined Aliphatic Polyesters. Macromolecules
Experimental procedures, characterizations, synthetic details, equipment specifications, NMR, and statistical analysis (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. ORCID
Wei-Min Ren: 0000-0003-4425-1453 Xiao-Bing Lu: 0000-0001-7030-6724 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, Grant 21690073), and Program for Changjiang Scholars and Innovative Research Team in University (IRT-17R14).
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REFERENCES
(1) (a) Longo, J. M.; Sanford, M. J.; Coates, G. W. Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure−Property Relationships. Chem. Rev. 2016, 116, 15167−15197. (b) Paul, S.; Zhu, Y.; Romain, C.; Brooks, R.; Saini, P. K.; Williams, C. K. Ring-opening copolymerization (ROCOP): synthesis and properties of polyesters and polycarbonates. Chem. Commun. 2015, 51, 6459−6479. (2) Odian, G. Principles of Polymerization, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2004. (3) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Organocatalytic Ring-Opening Polymerization. Chem. Rev. 2007, 107, 5813−5840. (4) (a) 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. (b) Lu, X.-B.; Ren, W.-M.; Wu, G.-P. CO2 Copolymers from Epoxides: Catalyst Activity, Product Selectivity, and Stereochemistry Control. Acc. Chem. Res. 2012, 45, 1721−1735. (5) Schultz, J. M. Polymer Crystallization: The Development of Crystalline Order in Thermoplastic Polymers; Oxford University Press: New York, 2001. (6) (a) Aida, T.; Inoue, S. Catalytic Reaction on Both Sides of a Metalloporphyrin Plane. Alternating Copolymerization of Phthalic Anhydride and Epoxypropane with an Aluminum PorphyrinQuaternary Salt System. J. Am. Chem. Soc. 1985, 107, 1358−1364. (b) Hosseini Nejad, E.; van Melis, C. G. W.; Vermeer, T. J.; Koning, C. E.; Duchateau, R. Alternating Ring-Opening Polymerization of Cyclohexene Oxide and Anhydrides: Effect of Catalyst, Cocatalyst, and Anhydride Structure. Macromolecules 2012, 45, 1770−1776. (c) Van Zee, N. J.; Sanford, M. J.; Coates, G. W. Electronic Effects of Aluminum Complexes in the Copolymerization of Propylene Oxide with Tricyclic Anhydrides: Access to Well-Defined, Functionalizable Aliphatic Polyesters. J. Am. Chem. Soc. 2016, 138, 2755−2761. (d) Fieser, M. E.; Sanford, M. J.; Mitchell, L. A.; Dunbar, C. R.; Mandal, M.; Van Zee, N. J.; Urness, D. M.; Cramer, C. J.; Coates, G. W.; Tolman, W. B. Mechanistic Insights into the Alternating Copolymerization of Epoxides and Cyclic Anhydrides Using a (Salph) AlCl and Iminium Salt Catalytic System. J. Am. Chem. Soc. 2017, 139, 15222−15231. (7) (a) Jeske, R. C.; DiCiccio, A. M.; Coates, G. W. Alternating copolymerization of Epoxides and cyclic anhydrides: An improved route to aliphatic polyesters. J. Am. Chem. Soc. 2007, 129, 11330− 11331. (b) Wu, L.-Y.; Fan, D.-d.; Lü, X.-q.; Lu, R. Ring-opening copolymerization of cyclohexene oxide and maleic anhydride catalyzed by mononuclear [Zn(L)(H2O)] or binuclear [Zn2(L)(OAc)2(H2O)] complex based on the Salen-type Schiff-base ligand. 1921
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ACS Catalysis 2018, 51, 3126−3134. (f) Ji, H.-Y.; Wang, B.; Pan, L.; Li, Y.-S. Lewis pairs for ring-opening alternating copolymerization of cyclic anhydrides and epoxides. Green Chem. 2018, 20, 641−648. (13) Liu, J.; Bao, Y.-Y.; Liu, Y.; Ren, W.-M.; Lu, X.-B. Binuclear chromium−salan complex catalyzed alternating copolymerization of epoxides and cyclic anhydrides. Polym. Chem. 2013, 4, 1439−1444. (14) Longo, J. M.; DiCiccio, A. M.; Coates, G. W. Poly(propylene succinate): A New Polymer Stereocomplex. J. Am. Chem. Soc. 2014, 136, 15897−15900. (15) Li, J.; Liu, Y.; Ren, W.-M.; Lu, X.-B. Asymmetric Alternating Copolymerization of Meso-epoxides and Cyclic Anhydrides: Efficient Access to Enantiopure Polyesters. J. Am. Chem. Soc. 2016, 138, 11493−11496. (16) Liu, Y.; Ren, W.-M.; Liu, C.; Fu, S.; Wang, M.; He, K.-K.; Li, G.-G.; Zhang, R.; Lu, X.-B. Mechanistic Understanding of Dinuclear Cobalt(III) Complex Mediated Highly Enantioselective Copolymerization of meso-Epoxides with CO2. Macromolecules 2014, 47, 7775− 7788. (17) (a) Wu, G.-P.; Jiang, S. D.; Lu, X.-B.; Ren, W.-M.; Yan, S. K. Stereoregular poly(cyclohexene carbonate)s: Unique crystallization behavior. Chin. J. Polym. Sci. 2012, 30, 487−492. (b) Liu, Y.; Ren, W.M.; Wang, M.; Liu, C.; Lu, X.-B. Crystalline Stereocomplexed Polycarbonates: Hydrogen-Bond-Driven Interlocked Orderly Assembly of the Opposite Enantiomers. Angew. Chem., Int. Ed. 2015, 54, 2241−2244. (c) Liu, Y.; Wang, M.; Ren, W.-M.; Xu, Y.-C.; Lu, X.-B. Crystalline Hetero-Stereocomplexed Polycarbonates Produced from Amorphous Opposite Enantiomers Having Different Chemical Structures. Angew. Chem., Int. Ed. 2015, 54, 7042−7046. (d) Lv, X.B. Stereoregular CO2 Copolymers: from Amorphous to Crystalline Materials. Acta Polym. Sin. 2016, 9, 1166−1178. (18) (a) Auriemma, F.; De Rosa, C.; Di Caprio, M. R.; Di Girolamo, R.; Ellis, W. C.; Coates, G. W. Stereocomplexed Poly(Limonene Carbonate): A Unique Example of the Cocrystallization of Amorphous Enantiomeric Polymers. Angew. Chem., Int. Ed. 2015, 54, 1215−1218. (b) Ren, W.-M.; Yue, T.-J.; Zhang, X.; Gu, G.-G.; Liu, Y.; Lu, X.-B. Stereoregular CO2 Copolymers from Epoxides with an Electron-Withdrawing Group: Crystallization and Unexpected Stereocomplexation. Macromolecules 2017, 50, 7062−7069.
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DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
pubs.acs.org/acscatalysis
Development of Highly Enantioselective Catalysts for Asymmetric Copolymerization of meso-Epoxides and Cyclic Anhydrides: Subtle Modification Resulting in Superior Enantioselectivity Jie Li, Bai-Hao Ren, Shi-Yu Chen, Guang-Hui He, Ye Liu, Wei-Min Ren, Hui Zhou, and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Downloaded via EASTERN MICHIGAN UNIV on October 21, 2019 at 17:32:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The asymmetric alternating copolymerization of meso-epoxides with cyclic anhydrides promoted by chiral catalysts or reagents is a powerful strategy for the synthesis of optically active polyesters with main-chain chirality. Herein, we show that, in conjunction with a nucleophilic cocatalyst, enantiopure dinuclear Al(III) complexes efficiently catalyze this asymmetric copolymerization, exhibiting high activity and achieving enantioselectivities up to 99% ee under mild conditions. Copolymer enantioselectivity and catalytic activity are revealed to be strongly affected by the axial linker, chiral diamine structure, and phenolate ortho-substituents. Density functional theory calculations confirm that the reactions corresponding to ring opening at (R)-C−O and (S)-C−O bonds of the Al-coordinated meso-epoxide during copolymerization exhibit significantly different Gibbs free energies of activation (Δ‡G). Enantiopure dinuclear Al(III) complex 3 bearing a hydrogenated binaphthol linker with the matched configuration is demonstrated to be the most active and enantioselective catalyst, featuring a broad substrate scope and allowing one to obtain a wide range of isotactic polyesters with a completely alternating structure and low polydispersity. Notably, most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures between 120 and 240 °C. Additionally, the mixing of selected isotactic (R)- and (S)polyesters in a 1:1 mass ratio afforded two crystalline stereocomplexes that exhibited enhanced thermal stability as well as new crystallization behavior and therefore significantly differed from the parent enantiopure polymers. KEYWORDS: asymmetric copolymerization, polyester, meso-epoxide, cyclic anhydride, dinuclear aluminum, stereocomplex
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mononuclear Cr−salan complexes.13 For example, copolymerization of maleic anhydride and (S)-phenyl glycidyl ether in the former case afforded polyesters with up to 99% head-to-tail linkages, which indicated the occurrence of a highly regioregular ring-opening step with >99% configuration retention at the methine carbon of the enantiopure epoxide incorporated into the polyester. Subsequently, Coates et al. succeeded in synthesizing highly tactic polyesters (96−97% head-to-tail linkages) via regioselective ring-opening alternating copolymerization of enantiopure propylene oxide and cyclic anhydrides using a chiral salen−Co(III)NO3 complex as a catalyst,14 demonstrating that, unlike their atactic analogs, both (R)- and (S)-(propylene succinate)s slowly crystallized from the melt after 1 week. In both cases, the degree of crystallinity gradually increased with time, and a very low melting polymorph (melting temperature (Tm) = 70 °C, melting enthalpy (ΔHm) = 48 J g−1) was observed after 19 days. Importantly, the stereoselective interaction between (R)-
INTRODUCTION Recently, the formation of degradable polyesters by alternating copolymerization of epoxides and cyclic anhydrides, both of which are readily available, has attracted increased attention,1 providing great opportunities for polymer functionalization to achieve extraordinary properties that cannot be realized via step-growth condensation of diols and diesters2 or ringopening polymerization of cyclic esters such as lactide, εcaprolactone, and β-lactones.3 Moreover, the presence of stereogenic centers in substituted epoxides allows one to synthesize polymers with controlled tacticity by stereospecific copolymerization.4 Stereoregular polymers show physicochemical properties superior to those of their irregular analogs because of the highly compact package structure of the former.5 Although a number of highly alternating copolymers have been prepared via this strategy using various metal-based catalysts (including Al,6 Zn,7 Cr,8 Co,9 Mn,10 and Fe11 complexes) and organocatalysts,12 the synthesis of stereoregular polyesters has been underexplored. In 2013, we discovered that binuclear Cr(III)−salan complexes bearing a binaphthol linker catalyze the copolymerization of terminal epoxides and cyclic anhydrides much more effectively than © 2019 American Chemical Society
Received: January 9, 2019 Revised: January 17, 2019 Published: January 17, 2019 1915
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis Chart 1. Structures of the Employed Dinuclear Al(III) Complexes
Table 1. Outcomes of Dinuclear Al(III) Complex-Catalyzed Asymmetric Alternating Copolymerization of COPO/PAa
entry
catalyst
time [h]
temp. [°C]
conv.b [%]
TOFc [h−1]
Mnd [kg mol−1]
Đd
eee [%]
specific rotation [deg]f
1 2 3 4 5 6g 7 8 9 10 11 12 13 14 15 16 17 18
(R,R,R,R)-1a (R,R,R,R)-1b (R,R,R,R)-1c (R,R,R,R)-1d (R,R,R,R)-1e (R,R,R,R)-1c (R,R,R,R,R)-2a (R,R,R,R,R)-2b (R,R,S,R,R)-2b (R,R,R,R,R)-3 (R,R,R,R,R)-4 (R,R,S,R,R)-4 (R,R,R,R)-5 (R,R,R,R,R,R)-6 (R,R,S,R,R,S)-6 (R,R,R,R)-7 (R,R,R,R)-8 (R,R)-9
2 2 1.7 2.5 9 24 5 2.5 48 1.5 18 48 4 42 43 60 48 62
25 25 25 25 25 0 25 25 25 25 25 25 25 25 25 25 25 25
62 78 79 92 99 62 98 33 54 93 58 51 93 53 64 76 32 70
78 98 116 92 28 6 49 33 3 155 8 3 58 3 4 3 2 3
6.7 7.1 7.3 8.6 8.7 7.3 8.5 6.9 7.3 8.3 8.8 8.3 8.0 5.3 6.3 5.3 5.9 4.8
1.17 1.15 1.13 1.11 1.12 1.12 1.11 1.14 1.23 1.11 1.11 1.14 1.13 1.14 1.30 1.89 1.69 2.17
56 (R,R) 90 (R,R) 91 (R,R) 85 (R,R) 37 (R,R) 98 (R,R) 97 (R,R) 90 (R,R) 27 (R,R) 99 (R,R) 81 (R,R) 55 (S,S) 65 (R,R) 10 (R,R) 7 (S,S) 26 (S,S) 1 (S,S) 24 (S,S)
118(−) 190(−) 198(−) 180(−) 78(−) 207(−) 204(−) 190(−) 57(−) 209(−) 171(−) 116(+) 137(−) 20(−) 15(+) 55(+) 1(+) 51(+)
a The reaction was performed in a 20 mL flask with a high vacuum valve in neat COPO (5.0 mL, 50 mmol) at a COPO/PA/catalyst/PPNCl molar ratio of 1000/250/1/2, except for entry 6. bCalculated using 1H NMR spectroscopy based on PA as the limiting reagent. cTurnover frequency (TOF) = mol of product (polyester)/mol of catalyst per hour. dDetermined using gel permeation chromatography in THF, calibrated with polystyrene. eMeasured by hydrolyzing the polymer, derivatizing the thus-produced diol with benzoyl chloride, and then determining the ee of the dibenzoate by HPLC. The (R,R)-product was the major enantiomer when the (R,R,R,R)-catalyst was used. fSpecific rotation was determined in chloroform at 20 °C (c = 1) using a polarimeter at λ = 589.3 nm. gThe reaction was carried out in toluene (COPO/toluene = 1:2 vol/vol).
complex, recrystallization was approximately 3 orders of magnitude faster than that for the parent enantiopure polymers. Recently, we employed bimetallic Al complexes to
and (S)-(propylene succinate) mixed in a 1:1 mass ratio resulted in the formation of a stereocomplex with improved crystallinity and increased Tm (up to 120 °C). For this 1916
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis
Figure 1. Gibbs free energy (ΔG) profiles corresponding to COPO ring opening at (R)-C−O and (S)-C−O bonds in toluene as a solvent catalyzed by complexes 1a−1e and 3. Energies are given in kcal mol−1.
demonstrating that a subtle modification of these substituents results in marked enantioselectivity changes. Among biphenollinked dinuclear Al(III) complexes 1a−1e, enantiopure 1c with isopropyl groups at phenolate ortho-positions proved to be the most active and enantioselective catalyst, affording the corresponding polyester in 91% ee at 25 °C. In contrast, enantiopure dinuclear Al(III) complex 1b could efficiently catalyze asymmetric copolymerization at ambient temperature, furnishing the target polyester in 90% ee and exhibiting an activity (TOF = 98 h−1) lower than that of 1c (116 h−1) under identical conditions (Table 1, entries 2 and 3). When copolymerization was performed at 0 °C in toluene using (R,R,R,R)-1c/PPNCl as the catalyst, the resulting copolymer possessed high enantiopurity; that is, the R,R-product was obtained with enantioselectivities of up to 98% ee and exhibited a specific rotation of [α]20 D = −207 (c = 1 in CHCl3) (Table 1, entry 6). Biphenol-linked dinuclear Al(III) complexes 1a−1e with (R,R,R,R)-configuration possess two diastereoisomers with (R)- or (S)-biphenol stereochemistry, namely, (R,R,R,R,R)and (R,R,S,R,R)-conformers,16 with the former being better suited for the formation of isotactic fragment-enriched polyesters with (R,R)-configuration than the latter. At 25 °C (R,R,R,R,R)-2b and (R,R,S,R,R)-2b systems featured activities of 33 and 3 h−1, respectively, and afforded (R,R)-copolymers in 90 and 27% ee, respectively. Similarly to the case of biphenollinked dinuclear Al(III) complexes, substitution of orthoisopropyl groups on the phenolate ring for tert-butyl ones increased enantioselectivity at ambient temperature from 90% to 97% ee (entries 7−9), with the highest activity and enantioselectivity observed for enantiopure complex 3. In this case, high conversion was achieved already within 1.5 h at 25 °C, and the polyester obtained in 99% ee featured a low polydispersity index (Đ) of 1.11 (entry 10). Enantiopure dinuclear Al complexes 4−8 were synthesized to probe the effects of linker and chiral diamine backbone on catalytic activity and product enantioselectivity. Asymmetric COPO/PA copolymerization catalyzed by a system based on binaphthol linker-bearing (R,R,R,R,R)-4 furnished a completely alternating copolymer with (R,R)-configuration in 81% ee, while the (S,S)-configured polyester was obtained in
realize an unprecedented asymmetric copolymerization of cyclic anhydrides and achiral meso-epoxides affording isotactically enriched polyesters with a completely alternating nature and low polydispersity.15 Since the nucleophilic ring opening of meso-epoxides proceeds with configuration inversion at one of the two prochiral centers, successful asymmetrization-based alternating copolymerization of meso-epoxides and cyclic anhydrides promoted by a chiral catalyst can furnish enantiopure polyesters with main-chain chirality in repeated (R,R)- or (S,S)-trans-1,2-diol units. The highest enantiomeric excess of 91% ee was obtained for the copolymerization of cyclohexene oxide (CHO) and phthalic anhydride (PA) at 0 °C catalyzed by a biphenol-linked dinuclear Al complex with tert-butyl groups in phenolate ortho-positions in conjunction with bis(trihenylphosphine)iminium chloride (PPNCl) as a nucleophilic cocatalyst. Herein, we comprehensively study the effects of linker, chiral diamine structure, and phenolate ortho-substituents of the ligand on the activity and enantioselectivity of bimetallic Al complexes (Chart 1) for the asymmetric copolymerization of meso-epoxides and cyclic anhydrides. In particular, we elucidate the effect of steric hindrance created by phenolate orthosubstituents on enantioselectivity to ultimately develop privileged chiral catalysts for this asymmetric copolymerization. Moreover, we also characterize the crystallization and stereocomplexation behavior of selected stereoregular polyesters.
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RESULTS AND DISCUSSION Enantioselective Copolymerization of 3,4-Epoxytetrahydrofuran (COPO) and PA. Previously, we demonstrated that dinuclear Al complex 1d bearing tert-butyl groups at phenolate ortho-positions efficiently promotes asymmetric CHO/PA copolymerization to give enantioenriched polyesters, revealing that the highest ee of 91% was obtained at a low reaction temperature of 0 °C.15 Additionally, the phenolate ortho-substituents of the ligand were shown to strongly affect catalytic activity and product enantioselectivity, with neither small (methyl group) nor bulky (trimethylsilyl (TMS)) groups being beneficial for this asymmetric copolymerization. In the present work, we further explored the steric effects of phenolate ortho-substituents on product enantioselectivity, 1917
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ACS Catalysis Table 2. Substrate Scope of the Asymmetric Copolymerization of meso-Epoxides with Anhydridesa
entry
reactants
temp. [°C]
time [h]
conv.b [%]
TOFc [h−1]
Mnd [kg mol−1]
Đd
1 2 3 4 5 6g 7 8 9g 10g 11
COPO/PA CHO/PA CPO/PA CBO/PA CEO/PA CDO/PA COPO/4,5-DMPA COPO/3,6-DMPA CDO/4,5-DMPA CDO/NA COPO/SA
25 25 25 25 25 25 25 25 50 50 25
1.5 1 15 13 10 8 11 11 12 10 24
93 93 28 85 96 81 39 30 70 99 44
155 232 5 16 24 25 9 7 14 25 5
8.3 6.5 2.3h 8.3 11.1 17.0 7.6h 6.5h 14.7 14.2 4.3h
1.16 1.15 1.21h 1.16 1.14 1.12 1.44h 1.17h 1.32 1.15 1.23h
eee [%] 99 98 83 98 98 98 90 87 96 95 93
(R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R) (R,R)
specific rotationf [deg]
Tg/Tm [°C]
209(−) 32(−) i 37(+) 139(−) 113(−) i i 145(−) 475(−) i
−/235 150/− −/239 −/134 127/− −/185 −/180,205, 225 −/160 −/183 190/− −/121
The reaction was performed in excess epoxide (5.0 mL, 50 mmol) in a 20 mL flask with a high vacuum valve at an epoxide/anhydride/catalyst/ PPNCl molar ratio of 1000/250/1/2, except for entries 6, 9, and 10. bCalculated using 1H NMR spectroscopy based on anhydride as the limiting reagent. cTurnover frequency (TOF) = mol of product (polyester)/mol of catalyst per hour. dDetermined using gel permeation chromatography in THF, calibrated with polystyrene. eMeasured by hydrolyzing the polymer and analyzing the resulting diol by chiral GC or derivatizing this diol with benzoyl chloride and then determining the ee of the dibenzoate by HPLC. fSpecific rotation was determined in chloroform at 20 °C (c = 1) using a polarimeter at λ = 589.3 nm. gThe reaction was carried out in toluene solution at an epoxide/anhydride/toluene/catalyst/PPNCl molar ratio of 250/250/500/1/2. hThe polymer was heated to 250 °C, cooled to room temperature, and then analyzed by gel permeation chromatography in THF. Calibration was performed using polystyrene. iSpecific rotation could not be determined because of the poor solubility of the copolymer. a
ring opening of meso-epoxides. To simplify calculations, the polymer chain was replaced by 2-(methoxycarbonyl)benzoate, and COPO was chosen as a model meso-epoxide monomer. In this model, a COPO molecule coordinated to one Al(III) center and was subsequently ring-opened by 2(methoxycarbonyl)benzoate. The Gibbs free energies (Δ‡G) of the two transition states (TS-1 and TS-2) corresponding to ring opening at (R)-C−O and (S)-C−O bonds of coordinated COPO were different because of the chiral environment provided by the (R,R,R,R,R)-catalyst (Figure 1). For biphenollinked dinuclear Al(III) complexes 1a−1e, the transition state produced via attack at (S)-C−O was much lower in energy than that produced by attack at (R)-C−O in the epoxide; that is, the (R,R,R,R,R)-conformer catalyzed COPO/PA copolymerization at (S)-C−O in preference to that at (R)-C−O to yield polyesters with a predominantly (R,R)-configuration, in agreement with experimental findings. Moreover, the gap between the above two transition states increased in the order 1e < 1a < 1d < 1b ≈ 1c. At this point, it is worth noting that the highest enantioselectivity was congruent with the largest Gibbs free energy (Δ‡G = 7.8 kcal mol−1), which was observed for complex 3. Substrate Scope of Enantioselective Epoxide/Anhydride Copolymerization. With an active and enantioselective catalyst system ((R,R,R,R,R)-3/PPNCl) in hand, we evaluated the substrate scope of the epoxide/anhydride copolymerization, which was performed either in bulk or in toluene as a solvent. The relatively highly reactive PA was chosen as a model cyclic anhydride for catalytic activity and enantioselectivity testing. NMR spectroscopic analysis of the produced polyesters revealed that highly stereoregular structures were formed in all cases. Except for cyclopentene
55% ee when (R,R,S,R,R)-4 was employed. This result indicates the strong chirality induction ability of the axial linker. A change of the chiral diamine backbone from diaminocyclohexane to 1,2-diaminobenzene (complex 5) considerably decreased both enantioselectivity (65% ee) and catalytic activity (58 h−1) at ambient temperature. Unfortunately, complex 6 with four chiral carbon centers and two chiral binaphthols could not efficiently catalyze the asymmetric COPO/PA copolymerization; for example, (R,R)- and (S,S)configured copolymers were obtained in only 10% ee (for (R,R,R,R,R,R)-6) and 7% ee (for (R,R,S,R,R,S)-6), respectively (Table 1, entries 14 and 15). Rigid linker-containing (R,R,R,R)-7 exhibited low catalytic activity (3 h−1) and poor enantioselectivity ((S,S)-configuration, 26% ee), behaving similarly to mononuclear Al(III) complex 9 (entry 18). Interestingly, a nearly complete loss of activity and enantioselectivity was observed for the catalyst system based on (R,R,R,R)-8 with a p-terphenyl linker. These screening experiments imply that linker stereochemistry plays an important role in determining the enantioselectivity of dinuclear Al complex-catalyzed asymmetric COPO/PA copolymerization. Origin of Epoxide Ring-Opening Enantioselectivity Revealed by Density Functional Theory (DFT) Calculations. To gain further insights into the effect of steric hindrance on the chiral induction ability of biphenol-linked dinuclear Al(III) complexes, polymer chain growth processes were simulated by DFT calculations. Since catalysts with (R,R,R,R,R)- or (S,S,S,S,S)-configuration have been shown to be best suited for enantioselectivity control, (R,R,R,R,R)Al(III) complexes 1a−1e and 3 were studied to explore the steric effect of phenolate ortho-substituents on the asymmetric 1918
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ACS Catalysis oxide (CPO), numerous meso-epoxides such as CHO, cis-2,3epoxybutane (CBO), 1,2-epoxy-4-cyclohexene (CEO), and 1,4-dihydronaphthalene oxide (CDO) exhibited good reactivity at ambient temperature, affording (R,R)-polyesters in more than 98% ee. Moreover, several cyclic anhydrides such as 4,5-dimethylphthalic anhydride (4,5-DMPA), 3,6-dimethylphthalic anhydride (3,6-DMPA), and succinic anhydride (SA) were employed for asymmetric copolymerization using COPO as a model meso-epoxide monomer. As expected, the produced copolymers showed perfectly alternating structures and high ee values of 87−93% (Table 2, entries 7, 8, and 11). The enantioselective copolymerization of CDO with 4,5-DMPA or naphthyl anhydride (NA) could be performed in toluene at an elevated temperature of 50 °C to give the corresponding polyesters with high enantioselectivities of 96% and 95% ee, respectively (Table 2, entries 9 and 10). It should be noted that the resulting CDO/NA copolymer ((R,R)-configuration, 95% ee) featured a high specific rotation of [α]20 D = −475 (c = 1 in CHCl3) and possessed a high glass transition temperature (Tg) of 190 °C. Importantly, the resulting isotactic polyesters were mostly semicrystalline materials with melting temperatures of 120− 240 °C. For example, a fairly sharp and high-temperature endothermic peak (ΔHm = 20.68 J g−1) was observed at 236 °C for isotactic COPO/PA copolymers with 99% ee, while the DSC thermogram of the atactic copolymer featured a peak corresponding to Tg = 105 °C (Figure 2, top). Figure 2
representative example provided by poly(cyclopentene phthalate) (CPO/PA copolymer). Isotactic fragment-enriched (65% ee) poly(cyclopentene phthalate) featured an endothermic melting peak at 221 °C (ΔHm = 23.63 J g−1), while the copolymer with 83% ee featured Tm = 239 °C and ΔHm = 41.33 J g−1 (Figure 3). Although diffraction peaks at 2θ =
Figure 3. DSC thermograms (top) and WAXD profiles (bottom) of isotactic fragment-enriched) poly(cyclopentenephthalate)s with ee values of (A) 83% and (B) 65%.
14.9°, 19.7°, and 24.5° were observed in both cases, their intensity was higher in the latter case; that is, the 83% ee polymer featured a higher degree of crystallinity. Macromolecular stereocomplexation is the stereoselective interaction between two polymers with opposite enantiomeric configurations proceeding via the formation of an interlocked ordered assembly. Usually, such stereocomplexes exhibit significantly improved thermal properties (both Tm and crystallinity) and feature crystallization behaviors significantly different from those of constituent enantiomeric polymers. Recently, a variety of newly discovered CO2-based polycarbonate stereocomplexes derived from meso-epoxides have been shown to exhibit unique crystallization behaviors.17 With these isotactic polyesters with different configurations in hand, we aimed to explore the interaction between oppositely configured copolymers derived from meso-epoxides. Unfortunately, in most cases, no cocrystallization was observed in blends containing equivalent amounts of these enantiomeric polymers with identical chemical compositions and opposite configurations. To our delight, we discovered two polyester stereocomplexes containing isotactic crystallizable poly(CDO-altPA) and amorphous poly(CDO-alt-NA). A 1:1 mixture of (R)poly(CDO-alt-PA) (Mn = 7.0 kg mol−1, Đ = 1.12) and (S)poly(CDO-alt-PA) (Mn = 8.5 kg mol−1, Đ = 1.14) was prepared by dissolving equal amounts of each enantiopure copolymer in dichloromethane and slowly evaporating the solvent to afford a dichloromethane-insoluble powder. The thus-obtained product featured a single endothermic peak at
Figure 2. DSC thermograms (top) and WAXD profiles (bottom) of (A) polyester prepared from COPO/PA in 99% ee and (B) atactic polyester prepared from COPO/PA.
(bottom) shows the wide-angle X-ray diffraction (WAXD) profiles of atactic and (R)-configured COPO/PA copolymers. No diffraction peaks were observed for the atactic copolymer, in agreement with its amorphous nature. In contrast, sharp diffraction peaks at 2θ = 15.5°, 17.5°, 23.3°, 28.4°, and 31.1° were observed for the isotactic (R)-copolymer, which demonstrated that this polymer is a typical semicrystalline material. The crystallization ability of these stereoregular polyesters greatly increased with increasing isotacticity, with a 1919
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis Tm ≈ 210 °C with ΔHm = 30.33 J g−1, and these values were significantly higher than those obtained for (R)- or (S)poly(CDO-alt-PA) with 98% ee (Tm = 185 °C, ΔHm = 26.46 J g−1; Figure 4, top). Furthermore, the WAXD patterns of
Figure 5. DSC thermograms (top) and WAXD profiles (bottom) of various poly(CDO-alt-NA)s: (A) enantiopure isotactic polymer (95% ee) and (B) 1:1 (w/w) mixture of (R)- and (S)-poly(CDO-alt-NA)s.
Figure 4. DSC thermograms (top) and WAXD profiles (bottom) of various poly(CDO-alt-PA)s. (A) Atactic, (B) enantiopure isotactic polymer (98% ee), and (C) 1:1 (w/w) mixture of (R)- and (S)poly(CDO-alt-PA)s.
values of up to 99%. A subtle modification of phenolate orthosubstituents significantly changed both enantioselectivity and catalyst activity, as observed for enantiopure biphenol-linked Al(III) complexes 1a−1e. DFT calculations performed on the above complexes confirmed that the two transition states (TS1 and TS-2) corresponding to ring opening at (R)-C−O and (S)-C−O bonds of coordinated meso-epoxides during copolymerization featured markedly different Gibbs free energies (ΔΔG) because of the steric effects of phenolate orthosubstituents. Moreover, both catalytic activity and product enantioselectivity were strongly affected by the linker and chiral diamine backbones of binuclear Al(III) complexes. Most of the produced isotactic polyesters were typical semicrystalline materials with melting temperatures of 120− 240 °C. The crystalline stereocomplex obtained by mixing (R)and (S)-isotactic crystallizable poly(CDO-alt-PA) in a 1:1 mass ratio exhibited an increased melting temperature of 210 °C and featured a crystallization behavior significantly different from those of constituent enantiomers. Additionally, although highly isotactic poly(CDO-alt-NA) with 95% ee was a typical amorphous material with a high Tg of 190 °C, mixing of amorphous (R)- and (S)-polymers in equivalent amounts afforded a crystalline stereocomplexed polyester with a high melting temperature of 334 °C. Thus, this study presents a first-time report on the formation of crystalline stereocomplexed polyesters from opposite enantiomers of amorphous polymers and is therefore of high novelty and practical significance.
stereoregular polyesters featured peaks at 2θ = 13.9°, 16.7°, 18.6°, 20.6°, 22.° 26.0°, 28.3°, 30.0°, and 34.8°, demonstrating that isotactic poly(CDO-alt-PA) is a typical semicrystalline polymer. Conversely, the 1:1 mixture of (R)- and (S)poly(CDO-alt-PA) featured peaks at 2θ = 9.8°, 18.6°, 21.0°, and 27.0° (Figure 4, bottom). Thus, the results of both DSC (Differential scanning calorimetry) and WAXD (wide-angle Xray diffraction) analyses illustrated that the stereoselective interaction between two polyesters with opposite enantiomeric configurations affords a crystalline stereocomplex. Since not much is known about the formation of crystalline stereocomplexes from amorphous polymers upon stereocomplexation,17b,18 we were delighted to discover that the DSC trace of a 1:1 mixture of (R)-poly(CDO-alt-NA) (Mn = 8.2 kg mol−1, Đ = 1.15) and (S)-poly(CDO-alt-NA) (Mn = 9.3 kg mol−1, Đ = 1.17) featured a sharp endothermic melting peak at 334 °C (Figure 5, top). Moreover, WAXD analysis provided further evidence of cocrystallization in this polymer blend, for which eight diffraction peaks with 2θ = 8.5°, 11.5°, 14.3°, 15.0°, 19.5°, 24.3°, 28.3°, and 30.8° were observed (Figure 5, bottom). In contrast, the highly isotactic poly(CDO-alt-NA) with 95% ee was demonstrated to be a typical amorphous material with a high Tg of 190 °C.
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CONCLUSION In summary, we demonstrated that, in conjunction with PPNCl, hydrogenated binaphthol-linked dinuclear Al(III) complexes can effectively catalyze the enantioselective copolymerization of meso-epoxides with cyclic anhydrides to afford polyesters with a completely alternating structure and ee
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b00113. 1920
DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922
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ACS Catalysis
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Experimental procedures, characterizations, synthetic details, equipment specifications, NMR, and statistical analysis (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. ORCID
Wei-Min Ren: 0000-0003-4425-1453 Xiao-Bing Lu: 0000-0001-7030-6724 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, Grant 21690073), and Program for Changjiang Scholars and Innovative Research Team in University (IRT-17R14).
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DOI: 10.1021/acscatal.9b00113 ACS Catal. 2019, 9, 1915−1922