Highly Active Chiral Oxazolinyl Aminophenolate Magnesium Initiators

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Highly Active Chiral Oxazolinyl Aminophenolate Magnesium Initiators for Isoselective Ring-Opening Polymerization of racLactide: Dinuclearity Induced Enantiomorphic Site Control Jianwen Hu, Chao Kan, Haobing Wang,* and Haiyan Ma* Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China

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S Supporting Information *

ABSTRACT: The most important challenge in ring-opening polymerization of rac-lactide today is to obtain isotactic PLAs with high molecular weights. The goal of our work is to address this important challenge by developing novel catalysts that enable precise control over stereoselectivity and produce high molecular weight PLAs at the same time. Here we report some rare examples of isoselective magnesium complexes toward the ROP of rac-LA, which are supported by chiral oxazolinyl aminophenolate ligands. Preliminary kinetic investigations confirmed that isotactic PLAs could be achieved via an enantiomorphic site control mechanism by using these chiral magnesium complexes. A pair of racemic magnesium initiators proved to integrate hyperactivity (TOF up to 54 000 h−1 at 25 °C) with sufficient isoselectivity (Pm = 0.80) toward the ROP of rac-LA, leading to the formation of isotactic stereoblock PLA with high molecular weight (Mn = 461 kg mol−1) and a semicrystalline property (Tm = 164 °C). Detailed structural investigation on the magnesium lactate model complexes suggested that the high degree of enantiomorphic site control is derived from both the ligand chirality and the dinuclear feature of the magnesium active species.

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metals,54−58 and copper,59,60 have been extensively investigated, and some of them displayed excellent stereocontrol over the ROP of rac-LA. Among them, only a few complexes could afford isotactic PLAs from rac-LA with moderately high activities (TOF ≤ 2280 h−1).43,58 Recently we reported zinc complexes bearing chiral and achiral oxazolinyl/benzoxazolyl aminophenolate ligands that displayed high activities (e.g., TOF = 42 000−44 000 h−1) toward the ROP of rac-LA under the immortal melting conditions ([LA]0:[Zn]0:[iPrOH]0 = 10 000−20 000:1:100) but leading to PLAs with moderate molecular weights (15.0−18.5 kg mol−1).38 So far, isoselective metal catalysts capable of promoting large-scale ROP of rac-LA and producing PLAs with high molecular weights have rarely been reported.53,61 A survey of the literature further indicates that the reported catalysts showing high isoselectivity (Pm > 0.80) toward the ROP of rac-LA generally result in PLAs with molecular weights (Mn) typically less than 200 kg mol−1.61 Therefore, it is still a challenge and valuable goal to develop highly active catalysts that can produce isotactic stereoblock PLAs with high molecular weights. As a colorless and nontoxic metal element, magnesium, its discrete complexes have proven to be highly active initiators for the ROP of lactides. Likely due to the nature capable of having high coordination number, complexes of magnesium

INTRODUCTION Poly(lactic acid) (PLA) has attracted great attention due to its numerous environmental advantages, such as biocompatibility and biodegradability, over conventional polyolefin materials.1−4 It is sourced from biomass feedstocks such as corn starch and readily produced by the metal-catalyzed ring-opening polymerization (ROP) of various lactide (LA) monomers, such as S,S-lactide (L-LA), R,R-lactide (D-LA), R,S-lactide (mesoLA), and a mixture of S,S/R,R-lactides (rac-LA).5,6 Among these monomers, the stereoselective polymerization of rac-LA has been extensively explored in the past two decades since the stereochemistry of resultant PLAs could significantly affect their physical properties and the scope of application.7−9 Atactic or heterotactic PLAs are amorphous polymers suitable as packaging materials due to their short degradable periods,10 whereas isotactic or isotactic stereoblock PLAs from rac-LA are the most desired products and could serve as engineering plastics because of their superior mechanical and thermal performances.11 Thus, developing novel catalysts which could show high isoselectivity toward the ROP of rac-LA has become an attractive research goal recently.8 To date, most of the isoselective processes are based on Al− Salen complexes after Spassky’s breakthrough.12−27 In some cases, the isoselectivity (Pm) of aluminum complexes could exceed 0.98; however, these complexes generally suffer from low activities even at high temperatures (70−110 °C). Alternatively, initiators based on other metals, such as Zn,28−38 In,39−45 K,46−49 group IV metals,50−53 rare-earth © XXXX American Chemical Society

Received: April 30, 2018 Revised: June 23, 2018

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DOI: 10.1021/acs.macromol.8b00924 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Illustration of Three Types of Highly Active Aminophenolate Magnesium Initiators with Diverse Stereoselectivities

Figure 1. Typical X-ray molecular structures of 1 (racemic mixture). Thermal ellipsoids represent the 30% probability surfaces. Hydrogen atoms are omitted for clarity.

Scheme 2. Synthesis of Complexes 1−6

supported by various ancillary ligands such as β-diketiminate, 62−67 trispyrazolyl/trisindazolylborate, 68,69 Schiff base,70−73 heteroscorpionate,74,75 aminophenolate,76−81 and others82 were often reported to exhibit high heteroselectivities. Highly isoselective magnesium complex for the ROP of rac-LA has not been obtained yet. Previously, we reported that magnesium complexes supported by claw-type tetradentate aminophenolate ligands (Scheme 1, I) exhibited extremely high activities (TOF up to 135 750 h−1) and afforded slightly

isotactic-bias PLAs (Pm = 0.54−0.60) with high molecular weights (Mn up to 531 kg mol−1) from rac-LA.79 Magnesium complexes supported by tridentate chiral pyrrolidinyl aminophenolate ligands (Scheme 1, II) showed relatively high heteroselectivity (Pr = 0.80) and moderate activities toward the ROP of rac-LA.30,76,77 Encouraged by the highly isoselective and active zinc complexes bearing chiral and achiral oxazolinyl/benzoxazolyl aminophenolate ligands,38 in this work, by introducing these chiral oxazolinyl aminophenolate B


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Macromolecules ligands, we report herein the isoselective magnesium initiators toward the ring-opening polymerization of rac-LA. Enantiomorphic site control mechanism has been confirmed via kinetic studies of the D-LA, L-LA, and rac-LA polymerization and the homonuclear decoupled 1H NMR spectra of the resultant PLAs. A racemic pair of the chiral oxazolinyl aminophenolate magnesium initiators could integrate high activity (TOF = 54 000 h−1) and isoselectivity (Pm = 0.80), leading to the formation of isotactic stereoblock PLA with high molecular weight (Mn = 461 kg mol−1). In addition, detailed structural investigation on the magnesium lactate model complexes suggests that the isoselectivity is attributed to the ligand chirality and the dinuclear nature of the magnesium active species, and the formation of stereoblock PLA is resulted from an enantiomorphic site control mechanism integrated with polymer chain exchange processes.

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RESULTS AND DISCUSSION Synthesis and Structures of Magnesium Complexes. The achiral benzoxazolyl- and chiral oxazolinyl-based aminophenol proligands L1−2H were reported in our previous work.38 L3−6H were prepared according to the similar procedure (see Supporting Information). Magnesium silylamido complexes 1−6 were synthesized by adding a stoichiometric amount of the corresponding proligand to a solution of Mg[N(SiMe3)2]2 in a 1:1 molar ratio in toluene and were isolated in yields of 38−58%. All new compounds were characterized by 1H and 13C{1H} NMR spectroscopy as well as elemental analysis. Similar to its zinc analogue, complex 1 bearing an achiral benzoxazolyl aminophenolate ligand was obtained as a racemate. The solid state structure of complex 1 determined by X-ray diffraction is depicted in Figure 1; both enantiomers are found in the centrosymmetric crystal structure. In spite of the inherent carbon chirality in L2−6H along with the newly formed stereogenic centers of magnesium atom and the skeleton N atom upon complexation, complexes 2−6 were obtained as mixtures of two diastereomers (for specific isomer ratios see Scheme 2), and the ratio of two isomers hardly changed after repeated recrystallization procedures. It was further proved that no interconversion of the two diastereomers could be observed even at an elevated temperature of 60 °C upon monitoring a C6D6 solution of complex 5. Among these complexes, complexes 2−4 showed high diastereoselectivity for isomer a (2a:2b = 10:1, 3a:3b = 14:1, and 4a:4b = 10:1). X-ray quality crystals of complex 3 were obtained from a saturated solution in toluene and nhexane. As shown in Figure 2, the molecule of 3a possesses the same configurations on the skeleton N and the magnesium center as those of 1a. Complexes 5 and 6, supported respectively by chiral ligands (R)-L5H and (S)-L6H having opposite configurations on the oxazolinyl ring, were synthesized with an identical diastereomer ratio of 4:1. The crystal structures of major isomers (R C 4 R N 1 R Mg 1 )-5a and (SC4SN1SMg1)-6a (5a, Figure 3, right; 6a, Figure 3, left) were detected. The structure of (RC4RN1RMg1)-5a can be brought into a perfect congruence with the mirror image of (SC4SN1SMg1)-6a, and their corresponding bond lengths and angles exhibit very limited differences. It is worth noting that the bond distances between Mg center and the oxazolinyl Nimino atom (Mg1−N2) in these magnesium complexes range from 2.107 to 2.123 Å. These bond lengths are not only shorter than those of Mg and pyrrolidinyl Namino atom (Mg− Namino = 2.172−2.252 Å) reported in our previous work,30 but

Figure 2. Typical X-ray molecular structure of 3a. Thermal ellipsoids represent the 30% probability surfaces. Hydrogen atoms are omitted for clarity.

also shorter than the other Mg−Nimino bonds (Mg−Nimino = 2.161−2.195 Å),70−72 which indicate a stronger chelation ability of the oxazolinyl-based aminophenolate ligands in this work. Isoselective Polymerization of rac-LA Initiated by Magnesium Complexes. The above magnesium complexes 1−6 were used to evaluate their catalytic performance for the ROP of rac-LA. As shown in Table 1 (also see Table S2, Supporting Information), these magnesium complexes could effectively initiate the ROP of rac-LA at ambient temperature either alone or in the presence of 2-propanol in a controlled manner. The activities of these magnesium complexes are obviously higher than those of the corresponding zinc complexes bearing the same type of ligands.38 For instance, complex 1 could polymerize rac-LA (toluene, [rac-LA]0/[Mg]0 = 500:1) at room temperature with 94% conversion within 15 min, while 86% conversion could be obtained in 110 min by its zinc counterpart (L1)ZnN(SiMe3)2 in toluene when a molar ratio of [rac-LA]0/[Zn]0 = 200:1 was adopted. Complex 2 supported by a chiral ligand also showed a significantly improvement on activity (TOF = 19 000 h−1) in comparison with its zinc analogue (TOF = 1140 h−1).38 In our previous work, it was found that both zinc complexes bearing achiral benzoxazolyl- and chiral oxazolinyl-based aminophenolate ligands L1−2 showed high isoselectivities (Pm = 0.87 and 0.83) toward the ROP of rac-LA. However, magnesium complex 1 bearing the achiral aminophenolate ligand only exhibited low isoselectivity toward rac-LA polymerization (Pm = 0.57, Table 1, runs 1 and 2), which was also lower than that of complex 2 bearing the chiral ligand (Pm = 0.63−0.64, Table 1, runs 3 and 4). It seems that the chirality of the ligand may exert some important role in controlling the stereoselectivity. We then focused our attention on screening magnesium complexes supported by chiral oxazolinyl aminophenolate ligands with various substituents. We conducted the polymerization by using complex 3 with the same chiral oxazolinyl moiety as complex 2 but a benzyl group on the skeleton N atom. Intriguingly, complex 3 showed significantly improved isoselectivity (Pm = 0.71−0.72, Table 1, runs 5 and 6) toward the ROP of rac-LA. Keeping the benzyl group on the skeleton N atom unchanged, complex 4 C


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Figure 3. Typical X-ray molecular structures of 5a (left) and 6a (right). Thermal ellipsoids represent the 30% probability surfaces. Hydrogen atoms are omitted for clarity.

Table 1. ROP of rac-LA Initiated by Complexes 1−6a run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

cat. 1 1 2 2 3 3 4 4 5 5 5 5f 6 rac-Mgg rac-Mgg rac-Mgg rac-Mgf,g

feed ratio 500:1:0 500:1:1 500:1:0 500:1:1 500:1:0 500:1:1 500:1:0 500:1:1 500:1:0 500:1:1 1000:1:1 500:1:0 1000:1:1 1000:1:1 2000:1:1 5000:1:1 500:1:0

time

convb

TOF −1

Mn,cacldc

Mnd

(min)

(%)

(h )

(kg/mol)

(kg/mol)

15 6 3 1.5 10 3 6 3 5 2 2.5 180 2.5 1.5 3 5 120

94 92 97 95 93 94 95 92 95 92 90 92 91 94 93 90 91

1880 4600 9700 19000 2790 9400 4750 9200 5700 13800 21600 153 21840 37600 37200 54000 228

6.77 6.62 6.98 6.48 6.79 6.77 6.84 6.62 6.84 6.62 13.0 6.62 12.8 13.5 26.8 64.8 6.55

8.84 6.65 10.3 6.09 9.15 6.76 14.5 7.13 5.38 5.15 16.0 6.45 16.0 19.4 31.4 46.1 6.33

PDId

Pme

1.71 1.39 1.52 1.20 1.61 1.38 1.51 1.42 1.72 1.60 1.42 1.08 1.62 1.52 1.48 1.26 1.09

0.57 0.57 0.63 0.64 0.72 0.71 0.72 0.71 0.74 0.73 0.73 0.78 0.73 0.80 0.80 0.80 0.84

[rac-LA]0 = 1.0 M, toluene, 25 °C, feed ratio = [rac-LA]0/[Mg]0/[iPrOH]0 bDetermined by 1H NMR spectroscopy. cMn,cacld = ([rac-LA]0/[Mg]0) × 144.14 × conv (%). dDetermined by GPC with polystyrenes as standards. ePm is probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR spectroscopy. fAt −40 °C. grac-Mg is the racemic mixture of complexes 5 and 6 in 1:1 ratio. a

containing two phenyl groups on the oxazolinyl ring showed similar activity and isoselectivity (Pm = 0.71−0.72, Table 1, runs 7 and 8) as complex 3. Further decreasing the steric bulkiness of the chiral oxazolinyl moiety, complexes 5 and 6 with only one phenyl group on the oxazolinyl ring showed higher isoselectivities (Pm = 0.73−0.74, Table 1, runs 9−11) accompanied by very high activities (TOF up to 21 840 h−1). Moreover, the isoselectivity of complex 5 could be further increased to Pm = 0.78 at a lower temperature of −40 °C (Table 1, run 12). As noted above, most magnesium initiators reported in the literature only produced atactic or heterotactic polymers from the ROP of rac-LA. To probe the mechanism of isoselective polymerization enabled by complexes 3−6 in this work, the rates of polymerization of D-LA, L-LA, and rac-LA with 5 were determined (Figure 4). The polymerization reactions of complex 5 were found to be first-order in monomer concentration with disparate apparent rate constants of 0.144 ± 0.004 min−1 for D-LA and 0.041 ± 0.001 min−1 for L-LA

Figure 4. Plots of ln([LA]0/[LA]t) vs time for the ROP of D-LA, LLA, and rac-LA catalyzed by 5: kapp(D) = 0.144 ± 0.004 min−1, R = 0.997; kapp(L) = 0.041 ± 0.001 min−1, R = 0.999; (0 °C, toluene, [LA]0/[iPrOH]0/[5]0 = 750:1:1, [LA]0 = 0.33 M).

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Mg]0:[iPrOH]0 = 1000:1:1 was adopted, remarkably, rac-Mg showed both higher activities (1.5 min, TOF: 37 600 h−1, Table 1, run 14) and higher isoselectivities (Pm = 0.80) than those of the single component (Table 1, runs 11 and 13). At a lower reaction temperature of −40 °C, a moderate activity could still be maintained (TOF: 228 h−1); meanwhile, the isoselectivity of rac-Mg was further increased to 0.84 (Table 1, run 17). In the homonuclear decoupled 1H NMR spectrum (Figure S21) of the resulted PLA, the close intensity ratio of rmm:mmr:mrm to 1:1:1 as well as the relatively small rmr signal indicates that the polymer main chains are essentially stereoblocks (e.g., -RRRRRRSSSSSS-) embedded with a small amount of single insertion errors. Previously, Coates and coworkers demonstrated that rac-(SalBinap)AlOiPr showed high stereocontrol toward the ROP of rac-LA through a polymer chain exchange mechanism,13,16 leading to the formation of stereoblock PLA with high melting point (187 °C). The stereocontrol of our magnesium complexes should be similar to Coates’ process. Moreover, it is worth noting that the mixture of 5 and 6 was able to polymerize up to 5000 equiv of rac-LA with the corresponding TOF value of 54 000 h−1, producing isotactic PLA (Pm = 0.80) with Mn as high as 461 kg mol−1, narrow PDI (1.26), and relatively high melting point (164 °C, see Figure S22). To the best of our knowledge, the TOF value of 54 000 h−1 is the highest activity reported for an isoselective initiator toward the ROP of rac-LA, and the Mn of 461 kg mol−1 is also the highest molecular weight obtained for the semicrystalline isotactic PLA (Pm ≥ 0.80) from the ROP of rac-LA. Previously, we reported that magnesium complexes supported by tridentate chiral pyrrolidinyl aminophenolate ligands (Table 2, II) showed heteroselectivity (Pr = 0.80) toward the ROP of rac-LA.30 The distinct stereoselectivity switch for rac-LA polymerization using pyrrolidinyl and oxazolinyl aminophenolate ligated magnesium complexes promoted us to have some insight into the structure of the active species. We then conducted NMR tube reactions of magnesium complexes II and 5 with (R)-tert-butyl lactate in C6D6 to monitor the structures of magnesium lactate model species in solution (Table 2). However, due to the existence of diastereomers, all the spectra are hardly assignable. According to the literature,33 diffusion-ordered NMR spectroscopy (DOSY) could detect the hydrodynamic radius of complex species in solution. The reaction mixtures were therefore subjected to DOSY NMR spectroscopic studies. A comparison

(Figure 4). The 3.5-fold difference between D-LA and L-LA indicated that enantiomorphic site control should be the major contributor to the isoselectivity. Meanwhile, a kinetic plot with two approximate linear stages and a short curved transition was obtained for rac-LA polymerization, which also indicates that the favored monomer is polymerized first at a faster rate than the disfavored monomer (Figure S17). To further confirm the mechanism, the homonuclear decoupled 1H NMR spectrum of a typical polymer sample produced by complex 5 was analyzed (Figure S18), which shows that the ratio of rmr:rmm:mmr:mrm is about 1:1:1:2. Clearly, this indicates the formation of sequences -RRRRSSRRRR-/-SSSSRRSSSS- with single insertion stereoerrors along the polymer chain and is in consistent with an enantiomorphic site control mechanism. Moreover, in a typical polymerization run catalyzed by complex 5, the tacticities of polymer samples with different conversions were detected. The plot of Pm vs conversion showed that the highest Pm values (Pm = 0.74) appeared at 95% conversions, and the lowest value (Pm = 0.65) appeared at ca. 50% conversion (Figure 5). This is a clear evidence for the formation of a tapered stereoblock polymer chain via an enantiomorphic site control mechanism.

Figure 5. Plot of Pm vs conversion for polymerization of rac-LA with complex 5 (25 °C, toluene, [LA]0/[iPrOH]0/[5]0 = 500:1:1, [LA]0 = 1.0 M).

We then conducted the polymerization of rac-LA by using a racemic catalyst pair (rac-Mg), namely, a mixture of complexes 5 and 6 in a 1:1 molar ratio. When a ratio of [rac-LA]0:[rac-

Table 2. DOSY Measurements (298 K) and X-Ray Crystallographic Data for Magnesium Complexes and Lactate Model Speciesa X-ray complex

selectivity (Pm)

II 1 5 II + (R)-tert-butyl lactate 1 + (R)-tert-butyl lactate 5 + (R)-tert-butyl lactate 5 + (R)-tert-butyl lactate

0.20 0.57 0.73

solvent

Dtb (×10−10 m2/s)

C6D6 C6D6 C6D6 d8-THF

4.47 3.55 3.52 6.09

rHc (Å)

a (Å)

b (Å)

rX‑rayd (Å)

7.73 7.56 7.62

5.48 5.15 5.37

6.90 6.65 6.78

7.76 9.62 9.83 7.12

a NMR tube reactions were prepared in situ by mixing complex II or 5 with (R)-tert-butyl lactate in a molar ratio of 1:1. Concentrations of all complexes and TMS were 10.0 mmol/L. bDt, translational diffusion coefficients, measured by DOSY. crH, calculated hydrodynamic radius from Dt. d Calculated according to rX‑ray = (a2b)1/3, where a and b are respectively the longest and shortest semiaxes of the prolate ellipsoid formed by the complex and are determined from the solid-state structures.

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Scheme 3. Proposed Mechanism for the Formation of Stereoblock PLA (Isopropoxyl Ester Ends of Polymer Chains Are Omitted for Clarity)

absence of a ligand chirality. Thus, we would conclude that both the ligand chirality and the dinuclear feature of the magnesium species are likely the source of the observed isostereocontrol in this system. Based on the above-mentioned results, a mechanism is proposed for the formation of stereoblock PLA from rac-LA when using racemic catalytic system rac-Mg. As shown in Scheme 3, initially, the predominant ring-opening event is the selective coordination−insertion of favored D-LA or L-LA monomer with the dimeric active species generated from (R)-5 and (S)-6 in the presence of iPrOH. Once the disfavored LA enantiomer is inserted, polymeryl exchange processes would occur and the polymer chains with a “wrong” active end will be redistributed to a matched active species via a dissociation− reassembly procedure. The propagation of the favored LA monomers will then resume, generating a stereoblock structure finally. Alternatively, since we cannot rule out the formation of heteronuclear dimeric active species from a mixture of (R)-5 and (S)-6, both enantiomorphic site-controlled -LLLL- and -DDDD- sequence might propagate simultaneously in the heteronuclear species. When the disfavored LAs are enchained, an intramolecular polymer exchange process realized simply by dissociation−recoordination of the polymer chain to the metal centers would resume the propagation with the matched active site. This process would also lead to the formation of stereoblock PLAs and is suggested to play an important role upon taking into account the high selectivity in the formation of stereoblocks.

of the hydrodynamic radii determined from X-ray structure of II (6.90 Å) and DOSY experiment (7.76 Å) demonstrates that the lactate model species (LII)Mg[(R)-OCHMeCOOtBu)] generated from complex II remains monomeric in solution. By contrast, the hydrodynamic radius (9.83 Å) of lactate model species (L5)Mg[(R)-OCHMeCOOtBu)] obtained from complex 5 is substantially larger than the X-ray radius (6.78 Å) of complex 5 (Table 2), indicating the generation of dimeric species in solution. On the basis of their ligand structures, we suggest that the smaller steric bulkiness around the imino nitrogen donor of oxazolinyl in comparison to the amino nitrogen donor of pyrrolinyl is the structural feature leading to an aggregation of the lactate species (L5)Mg[(R)-OCHMeCOOtBu)]. Here then comes the question whether the dimeric nature of the active species generated from complex 5 leads to the isoselective control in the ROP of rac-LA. To answer this, the dynamic radius of lactate species (L5)Mg[(R)-OCHMeCOOtBu)] generated in THF-d8 was obtained to be 7.12 Å, similar to the X-ray radius of 5 (Table 2), which accordingly indicates the monomeric nature of the species in THF. Verified by synthetic scale polymerizations, complexes 1−6 did show no stereoselectivity in THF (Pm = 0.48−0.55, Table S2). These results imply that the dinuclear nature of the generated active species should be responsible for the isoselective control in the polymerization of rac-LA, and the THF molecule takes part into competitive coordination with the metal center to form the nonselective mononuclear species. Moreover, since the lactate species generated from complex 1 and (R)-tert-butyl lactate also possesses a dinuclear structure in C6D6 (Table 2), the low isoselectiviy of complex 1 could be attributed to the F


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(5) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Controlled Ring-Opening Polymerization of Lactide and Glycolide. Chem. Rev. 2004, 104, 6147−6176. (6) Thomas, C. M. Stereocontrolled Ring-Opening Polymerization of Cyclic Esters: Synthesis of New Polyester Microstructures. Chem. Soc. Rev. 2010, 39, 165−173. (7) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Biocompatible Initiators for Lactide Polymerization. Polym. Rev. 2008, 48, 11−63. (8) Stanford, M. J.; Dove, A. P. Stereocontrolled Ring-Opening Polymerization of Lactide. Chem. Soc. Rev. 2010, 39, 486−494. (9) Dijkstra, P. J.; Du, H.; Feijen, J. Single Site Catalysts for Stereoselective Ring-Opening Polymerization of Lactides. Polym. Chem. 2011, 2, 520−527. (10) Tsuji, H. Poly(lactide) Stereocomplexes: Formation, Structure, Properties, Degradation, and Applications. Macromol. Biosci. 2005, 5, 569−597. (11) Fukushima, K.; Kimura, Y. Stereocomplexed Polylactides (NeoPLA) as High-Performance Bio-based Polymers: Their Formation, Properties, and Application. Polym. Int. 2006, 55, 626−642. (12) Spassky, N.; Wisniewski, M.; Pluta, C.; LeBorgne, A. Highly Stereoelective Polymerization of rac-(D,L)-Lactide with a Chiral Schiff’s Base/Aluminium Alkoxide Initiator. Macromol. Chem. Phys. 1996, 197, 2627−2637. (13) Ovitt, T. M.; Coates, G. W. Stereoselective Ring-Opening Polymerization of rac-Lactide with a Single-Site, Racemic Aluminum Alkoxide Catalyst: Synthesis of Stereoblock Poly(lactic acid). J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4686−4692. (14) Radano, C. P.; Baker, G. L.; Smith, M. R., III Stereoselective Polymerization of a Racemic Monomer with a Racemic Catalyst: Direct Preparation of the Polylactic Acid Stereocomplex from Racemic Lactide. J. Am. Chem. Soc. 2000, 122, 1552−1553. (15) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. Stereoselective Ring-Opening Polymerization of Racemic Lactide Using AluminumAchiral Ligand Complexes: Exploration of a Chain-End Control Mechanism. J. Am. Chem. Soc. 2002, 124, 5938−5939. (16) Ovitt, T. M.; Coates, G. W. Stereochemistry of Lactide Polymerization with Chiral Catalysts: New Opportunities for Stereocontrol Using Polymer Exchange Mechanisms. J. Am. Chem. Soc. 2002, 124, 1316−1326. (17) Zhong, Z.; Dijkstra, P. J.; Feijen, J. [(salen)Al]-Mediated, Controlled and Stereoselective Ring-Opening Polymerization of Lactide in Solution and without Solvent: Synthesis of Highly Isotactic Polylactide Stereocopolymers from Racemic d,l-Lactide. Angew. Chem. 2002, 114, 4692−4695. (18) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Controlled and Stereoselective Polymerization of Lactide: Kinetics, Selectivity, and Microstructures. J. Am. Chem. Soc. 2003, 125, 11291−11298. (19) Tang, Z.; Chen, X.; Pang, X.; Yang, Y.; Zhang, X.; Jing, X. Stereoselective Polymerization of rac-Lactide Using a Monoethylaluminum Schiff Base Complex. Biomacromolecules 2004, 5, 965−970. (20) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White, A. J. P.; Williams, D. J. Remarkable Stereocontrol in the Polymerization of Racemic Lactide Using Aluminum Initiators Supported by Tetradentate Aminophenoxide Ligands. J. Am. Chem. Soc. 2004, 126, 2688−2689. (21) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Stereoselective Ring-Opening Polymerization of a Racemic Lactide by Using Achiral Salen− and Homosalen−Aluminum Complexes. Chem. - Eur. J. 2007, 13, 4433−4451. (22) Du, H.; Pang, X.; Yu, H.; Zhuang, X.; Chen, X.; Cui, D.; Wang, X.; Jing, X. Polymerization of rac-Lactide Using Schiff Base Aluminum Catalysts: Structure, Activity, and Stereoselectivity. Macromolecules 2007, 40, 1904−1913. (23) Chen, H.-L.; Dutta, S.; Huang, P.-Y.; Lin, C.-C. Preparation and Characterization of Aluminum Alkoxides Coordinated on SalenType Ligands: Highly Stereoselective Ring-Opening Polymerization of rac-Lactide. Organometallics 2012, 31, 2016−2025. (24) Maudoux, N.; Roisnel, T.; Dorcet, V.; Carpentier, J.-F.; Sarazin, Y. Chiral (1,2)-Diphenylethylene-Salen Complexes of Triel Metals:

CONCLUSIONS We reported herein some rare examples of isoselective magnesium complexes, which combined both high activity and distinct isoselectivity toward the ROP of rac-LA at ambient temperature. The kinetic studies for the polymerizations of D-LA, L-LA, and rac-LA showed that an enantiomorphic-site control mechanism should be responsible for the isoselectivity. In the presence of a racemic version of chiral oxazolinyl aminophenolate magnesium complexes, semicrystalline stereoblock PLAs with record high molecular weights (Mn up to 461 kg mol−1) could be produced with high activities through polymeryl exchange processes. Both the ligand chirality and the dinuclear nature of the active species are suggested to exert significant effects on the isoselective control of these magnesium complexes in the ROP of rac-LA and will impact the design of future magnesium-based catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00924. Polymerization data; Figures S1−S22 and Tables S1, S2 (PDF) Detailed experimental protocols (PDF) X-ray data of 1, 3, 5, and 6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(H.W.) E-mail [email protected]. *(H.M.) E-mail [email protected]. ORCID

Haobing Wang: 0000-0002-9104-1726 Haiyan Ma: 0000-0003-0810-2493 Author Contributions

J.H. and C.K. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is subsidized by the National Natural Science Foundation of China (NNSFC, 21474028) and the Fundamental Research Funds for the Central Universities (WD1113011). All the financial support is gratefully acknowledged. H. Ma also thanks the very kind donation of a Braun glovebox by the AvH Foundation.

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