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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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00113 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019
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ACS Catalysis
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, Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China 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 of 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
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 ring-opening polymerization of cyclic esters such as lactide, -caprolactone, and -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 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 analogues, both (R)- and (S)-(propylene succinate)s slowly crystallized from the melt after one 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)- 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 complex, recrystallization was approximately three orders of magnitude faster than for the parent enantiopure polymers. Recently, we have employed bimetallic Al complexes to 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
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Chart 1. Structures of the employed dinuclear Al(III) complexes. (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 tertbutyl groups in phenolate ortho-positions in conjunction with bis(trihenylphosphine)iminium chloride (PPNCl) as a nucleophilic co-catalyst. 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 ortho-substituents 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. RESULTS AND DISCUSSION Enantioselective copolymerization of 3,4epoxytetrahydrofuran (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, demonstrating that a subtle modification of these substituents results in marked
enantioselectivity changes. Among biphenol-linked dinuclear Al(III) complexes 1a–e, 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, i.e., the R,Rproduct was obtained with enantioselectivities of up to 98% ee 20 and exhibited a specific rotation of [𝛼]D = −207 (c = 1 in CHCl3) (Table 1, entry 6). Biphenol-linked dinuclear Al(III) complexes 1a–e 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 biphenol-linked dinuclear Al(III) complexes, substitution of ortho-isopropyl 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 (Ɖ) 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
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ACS Catalysis
Table 1. Outcomes of dinuclear Al(III) complex–catalyzed asymmetric alternating copolymerization of COPO/PA.a O
O
O
O
+
Catalyst
O
PPNCl
O
O
COPO
O O
O
n
PA
Entry
Catalyst
Time [h]
Temp. [°C]
Conv.b [%]
TOFc [h−1]
M nd [kg mol−1]
Ɖd
eee [%]
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)
Specific rotation [°]f 118(−) 190(−) 198(−) 180(−) 78(−) 207(−) 204(−) 190(−) 57(−) 209(−) 171(−) 116(+) 137(−) 20(−) 15(+) 55(+) 1(+) 51(+)
aThe
reaction was performed in a 20-mL autoclave 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 = 589.3 nm. gThe reaction was carried out in toluene (COPO/toluene = 1:2,vol/vol).
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 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, e.g., (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 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 density functional theory (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–e and 3 were studied to explore the steric effect of phenolate ortho-substituents on the asymmetric ringopening 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 ringopening 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 biphenol-linked dinuclear Al(III) complexes 1a–e, 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, i.e., 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.
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S
O
R O
R
1=
Al
2=
-10
-9.6 C-2
-15.6
R-1 -17.3 R-2
G = 3.7 kcal/mol
-20
0 -5
0 GS
-7.8 C-1 C-2 -10.7
-10
-16.8 R-1
-15 -20
G = 5.0 kcal/mol
Complex 1a
O
TS-2 TS-1
-18.3 R-2
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-15.6 R-2
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-18.6 R-2
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8.5 8.0
5 0 -5
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-9.6 C-1 -12.7 C-2
-10
-16.3 R-1 -18.4 R-2
-15 -20
G = 3.6 kcal/mol
Complex 1d
Gibbs free energy(kcal/mol)
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Gibbs free energy(kcal/mol)
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ACS Catalysis
7.4 TS-2
5 0
0.5
-5 -10
TS-1
0 GS
-14.1 C-1 -15.1 C-2
-15 -20
Complex 1e
-19.4
R-2
-22.8 R-1
G = 7.8 kcal/mol Complex 3
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–e and 3. Energies are given in kcal mol−1.
Table 2. Substrate scope of the asymmetric copolymerization of meso-epoxides with anhydrides.a O O R R
S
O
O
O
*
(R,R,R,R,R)-Complex 3/PPNCl
+
O O
O
R
R R
O
*
R
R
R S R
R
O O R R
O S
Reactants
1 2 3 4 5 6g
COPO/PA CHO/PA CPO/PA CBO/PA CEO/PA CDO/PA COPO/4,5DMPA COPO/3,6DMPA CDO/4,5DMPA CDO/NA COPO/SA
7 8 9g 10g 11
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O O
=
=
R
O
O
O COPO
Entry
O
CHO
CPO
CBO
CEO
CDO
PA
Temp. [°C] 25 25 25 25 25 25
Time [h] 1.5 1 15 13 10 8
[%] 93 93 28 85 96 81
[h−1] 155 232 5 16 24 25
Mn [kg mol−1] 8.3 6.5 2.3h 8.3 11.1 17.0
25
11
39
9
25
11
30
50
12
50 25
10 24
4,5-DMPA
3,6-DMPA
SA
NA
1.16 1.15 1.21h 1.16 1.14 1.12
eee [%] 99 (R,R) 98 (R,R) 83 (R,R) 98 (R,R) 98 (R,R) 98 (R,R)
Specific rotationf [°] 209(−) 32(−) –i 37(+) 139(−) 113(−)
7.6h
1.44h
90 (R,R)
–i
7
6.5h
1.17h
87 (R,R)
–i
–/160
70
14
14.7
1.32
96 (R,R)
145(−)
–/183
99 44
25 5
14.2 4.3h
1.15 1.23h
95 (R,R) 93 (R,R)
475(−) –i
190/– –/121
Conv.b
TOFc
d
Ɖd
aThe
Tg/Tm [°C] –/235 150/– –/239 –/134 127/– –/185 –/180,205, 225
reaction was performed in excess epoxide (5.0 mL, 50 mmol) in a 20-mL autoclave 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. e Measured 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 = 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.
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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 oxide (CPO), numerous mesoepoxides such as CHO, cis-2,3-epoxybutane (CBO), 1,2-epoxy4-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 (3,6DMPA), and succinic anhydride (SA) were employed for asymmetric copolymerization using COPO as a model mesoepoxide 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 20 high specific rotation of [𝛼]D = −475 (c = 1 in CHCl3) and possessed a high glass transition temperature (Tg) of 190 °C.
thermogram of the atactic copolymer featured a peak corresponding to Tg = 105 °C (Figure 2, top). Figure 2 (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 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 = 14.9°, 19.7°, and 24.5° were observed in both cases, their intensity was higher in the latter case, i.e., the 83% ee polymer featured a higher degree of crystallinity.
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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.
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
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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%.
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 co-crystallization was observed in blends containing equivalent amounts of these
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ACS Catalysis enantiomeric polymers with identical chemical compositions and opposite configurations.
15.0°, 19.5°, 24.3°, 28.3°, and 30.8° were observed (Figure 5, bottom). On the contrary, the highly isotactic poly(CDO-altNA) with 95% ee was demonstrated to be a typical amorphous material with a high Tg of 190 °C.
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Figure 4. DSC thermograms (top) and WAXD profiles (bottom) of various poly(CDO-alt-PA)s. (A) atactic, (B) enantiopure isotactic polymer (98% ee), (C) 1:1 (w/w) mixture of (R)- and (S)poly(CDO-alt-PA)s.
To our delight, we discovered two polyester stereocomplexes containing isotactic crystallizable poly(CDO-alt-PA) and amorphous poly(CDO-alt-NA). A 1:1 mixture of (R)poly(CDO-alt-PA) (Mn = 7.0 kg mol−1, Ɖ = 1.12) and (S)poly(CDO-alt-PA) (Mn = 8.5 kg mol−1, Ɖ = 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 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 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 X-ray 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, Ɖ = 1.15) and (S)-poly(CDO-alt-NA) (Mn = 9.3 kg mol−1, Ɖ = 1.17) featured a sharp endothermic melting peak at 334 °C (Figure 5, top). Moreover, WAXD analysis provided further evidence of co-crystallization in this polymer blend, for which eight diffraction peaks with 2θ = 8.5°, 11.5°, 14.3°,
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Figure 5. DSC thermograms (top) and WAXD profiles (bottom) of various poly(CDO-alt-NA)s: (A) enantiopure isotactic polymer (95% ee), (B) 1:1 (w/w) mixture of (R)- and (S)- poly(CDO-altNA)s.
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 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–e. DFT calculations performed on the above complexes confirmed that the two transition states (TS-1 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-
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ACS Catalysis time report on the formation of crystalline stereocomplexed polyesters from opposite enantiomers of amorphous polymers and is therefore of high novelty and practical significance.
15222−15231.
(7) (a) Jeske, R. C.; DiCiccio, A. M.; Coates, G. W., Alternating
ASSOCIATED CONTENT Experimental procedures, characterizations, synthetic details, equipment specifications, NMR, statistical analysis.This material is available free of charge via the Internet at http://pubs.acs.org. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.xxxxxxx. AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Notes The authors declare no competing financial interests. (8)
ACKNOWLEDGMENT 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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