Stereoselective Ring-Opening Polymerization of rac-Lactides

Sep 26, 2013 - These complexes were characterized and used as catalysts in rac-lactide or l-lactide polymerization. The X-ray diffraction analysis sho...
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Stereoselective Ring-Opening Polymerization of rac-Lactides Catalyzed by Aluminum Hemi-Salen Complexes Bo Gao, Ranlong Duan, Xuan Pang,* Xiang Li, Zhi Qu, Zhaohui Tang, Xiuli Zhuang, and Xuesi Chen Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China S Supporting Information *

ABSTRACT: A series of unreported aluminum complexes supported by asymmetrically O,N,N,O-quadridentate hemisalen ligands were synthesized by binaphthyl-imine derivatives. These complexes were characterized and used as catalysts in rac-lactide or L-lactide polymerization. The X-ray diffraction analysis showed that molecular structures of (S)-3 and (rac)-4 were mononuclear coordination compounds with fivecoordinated aluminum atoms in the solid state. Using 2propanol as cocatalyst, complex (S)-6 revealed the highest activity among these aluminum coordination compounds toward the ring-opening polymerization of L-lactide, and complex (S)-2 displayed the highest stereospecificity for the ring-opening polymerization of rac-lactide, affording partially isotactic polylactide with a Pm of 0.64. The polymerization kinetics using (S)-6 as a catalyst were investigated at great lengths. The kinetics of the polymerization consequences proved that the polymerization was first-order in monomer as well as catalyst. There was a linear dependence between the rac-lactide conversion and the number-average molecular weight of the macromolecules. The PDI values of these macromolecules were in a narrow range (1.04−1.09).

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INTRODUCTION Biodegradable and biocompatible polyesters have found a wide range of applications, such as sutures, bone fracture fixation devices, drug controlled release carriers, tissue engineering scaffolds, and green plastics for wrapping materials, disposal containers, and fibers.1 In particular, polylactide (PLA), which is derived from lactic acid as a renewable resource, has become one of the promisingly biodegradable and biocompatible polyesters.2 Polylactide is generally prepared via the ringopening polymerization (ROP) of lactide initiated via coordination compounds, such as a number of alkoxides of Sn,3 Al,4,13,14 Zn,5 Mg,6 Fe,7 Ti,8 In,9 and rare-earth metals,10 organo-catalysts,11 or enzymes.12 Aluminum-based catalysts have been widely used in ROP and are among the efficient initiators in the synthesis of PLA due to their high degrees of polymerization control and effectiveness in controlling polymer tacticities by modification of the ancillary ligands.4,13,14 In recent decades, massive trials have been engaged in the choice of suitable auxiliary ligands for enhancing the properties of the compounds in polymerization. Many efforts have been made to obtain PLA with high tacticity via ring-opening polymerization of racemic lactide by stereoselective catalysts with suitable ligands. Some remarkable studies have tried to illustrate the connection between the aluminum-based complexes supported by salen-type Schiff base and stereospecificity13,14a−c (Figure 1). Among them, Spassky discovered that an aluminum alkoxide initiator bearing a salen-type Schiff base ligand derived from R© XXXX American Chemical Society

Figure 1. Catalysts explored for stereoselective ROP of lactides.

(+)-1,1′-binaphthyl-2,2′-diamine could give in a highly stereocontrolled polymerization of rac-lactide isotactic and crystalline PLA with a higher Tm than optically pure poly-L-lactic acid.13a Received: July 19, 2013

A

dx.doi.org/10.1021/om400714q | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Synthetic Route for Aluminum Complexes

Coates reported that rac-(SalBinap)AlOiPr supported by a salen-type Schiff base ligand could yield predominantly isotactic PLAs.13b Our research team had investigated a number of aluminum compounds bearing salen Schiff base ligands. 14 These compounds were demonstrated to be high-efficiency singlesite living catalysts toward the well-dominated ROP of raclactide. Furthermore, these aluminum compounds can afford valid control of the PLA microstructures, facilitating the generation of an isotactic inclination in the polymerization of rac-lactide. However, as far as we are aware, few studies on hemi-salen-type Schiff bases bearing an asymmetrical binaphthyl moiety (Scheme 1 in this work) including their metal coordination compounds have been investigated in the polymerization of rac-lactide. Encouraged by the effective utilization of previously reported aluminum coordination compounds supported by a Schiff base,14 we deem that aluminum coordination compounds with a hemi-salen-type Schiff base are possibly effective catalysts for valid control of the ROP of rac-lactide. So we are intrigued to investigate the catalysis of aluminum complexes based on a hemi-salen binaphthyl moiety. Herein, the rudimentary results of the preparation and characterization of hemi-salen aluminum complexes based on asymmetrical O,N,N,O-quadridentate ligands are reported, when serving as catalysts to polymerize rac-lactide in a controlled strategy under moderate situations to attain partially isotactic polylactide.

pounds (S)-La, (S)-Lb, and (S)-Lc in moderate yields (56.1− 74.2%) according to the literature.15 Synthesis of Ligands (S)-L1−(S)-L6, (rac)-L2, (rac)-L4, and (rac)-L5. As shown in Scheme 1, ligands (S)-L1, (S)-L2, and (S)-L4 were readily prepared in good yields (80.1−83.7%) by combining the corresponding compounds and modified salicylaldehydes with a catalytic amount of p-toluenesulfonic acid in toluene under refluxing conditions according to the literature.15c Ligands (S)-L3, (S)-L5, and (S)-L6 were easily synthesized in high yields (80.4−90.7%) by combining the appropriate compounds and modified salicylaldehydes in absolute ethanol (see Scheme 1) according to the literature.15d Synthesis of Aluminum Complexes (S)-1−(S)-6, (rac)2, (rac)-4, and (rac)-5. Aluminum complexes (S)-1−(S)-6 were synthesized easily in high yields under mild conditions by combining 1.0 equivalent of trimethylaluminum and the corresponding ligands under an inert atmosphere and were isolated as yellow or orange precipitates from toluene in 80.4− 94.3% yield (Scheme 1). The syntheses of complexes (rac)-2, (rac)-4, and (rac)-5 were analogously carried out according to the procedure of complexes (S)-2, (S)-4, and (S)-5 using ligands (S)-L2, (S)-L4, and (S)-L5 (1 equiv, 1.00 mmol) and AlMe3 (1 equiv, 1.00 M in toluene, 1.00 mL, 1.00 mmol). All aluminum complexes were characterized by 1H and 13C NMR spectroscopy and elemental analysis. The 1H and 13C NMR spectra of (S)-1−(S)-6, (rac)-2, (rac)-4, and (rac)-5 showed similar resonances in the regions of −1.11 to −1.10 ppm for the methyl protons of the Al-CH3 group in the 1H NMR spectra and −11.14 to −10.11 ppm in the 13C NMR spectra. This revealed that there was only one environment for the ligands and methyl groups bonded to the aluminum atoms. Moreover, the 1H NMR spectra of (S)-1−(S)-6, (rac)-2, (rac)4, and (rac)-5 showed sharp signals, indicating there is no fluctuation in these aluminum atoms’ coordination environment. For example, as shown in Figure 2, the methyl proton on

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RESULT AND DISCUSSION Synthesis of Compounds (S)-La, (S)-Lb, (S)-Lc and (rac)La, (rac)-Lb, (rac)-Lc. Heating (S)-(−)-1,1′-binaphthyl-2,2′diamine or (rac)-1,1′-binaphthyl-2,2′-diamine with corresponding modified 2-bromoanisole in toluene solution afforded the corresponding (S)-(−)-1,1′-binaphthyl-2,2′-diamine comB

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set to 80 °C, and the reaction time was extended to 16 h (see Figure 3). Crystals of (S)-3 and (rac)-4 suitable for an X-ray structure determination were obtained from a THF and hexane solution. The molecular structures of (S)-3 and (rac)-4 are depicted in Figure 4 and Figure 5, respectively, together with selected bond lengths and angles. The crystallographic data are summarized in the Supporting Information, Table S1. X-ray structural analysis reveals that (S)-3 and (rac)-4 are single-nuclear complexes with both of the aluminum atoms in distorted square pyramidal environments defined by O1, N1, N2, and O2 from quadridentate ligands and C35 from Al-CH3. For the moieties from salicylaldehydes in quadridentate ligands, the Al1−O2 bond lengths (1.783(3) Å in (S)-3, 1.775(2) Å in (rac)-4, respectively) were close to the corresponding one (1.8035(15) Å (av)) in compound 2 in the reference (see Figure S3 in the Supporting Information).14b Both the bond lengths of Al1−N2 were 2.001(2) Å in (S)-3 and (rac)-4, and they were similar to the corresponding one (2.0303(18) Å (av)) observed in compound 2 in ref 14b. For the moieties from anisoles in quadridentate ligands, the Al1−O1 bond lengths (2.104(2) Å in (S)-3, 2.137(2) Å in (rac)-4, respectively) were longer than the corresponding one (1.8035(15) Å (av)) in compound 2 in ref 14b. This may be because the Al1−O1 bond lengths in (S)-3 and (rac)-4 are coordination bonds, while the corresponding bond lengths in compound 2 are sigma bonds. Both the bond lengths of Al1− N1 were 1.876(3) Å in (S)-3 and (rac)-4, shorter than the

Figure 2. 1H NMR spectra of ligand (S)-L2 and complex (S)-2.

the anisole section of (S)-2 is a sharp singlet at 4.11 ppm, while 3.50 ppm in (S)-L2, indicating that oxygen and nitrogen are coordinated to the aluminum. Complex (S)-5 had steric hindrance, and it was found that the ligand (S)-L5 and AlMe3 were substantially converted to complex 5a at 60 °C in 10 h. However, complex (S)-5 was converted gradually by complex 5a along with a methyl on the aluminum atom, the methyl group could be eliminated when the reaction temperature was

Figure 3. Stacked 1H NMR (400 MHz, CDCl3) spectra of the mixture (bottom) of ligand (S)-L5 and trimethylaluminum in toluene at 10 h at 60 °C (complex 5a is the main ingredient), the mixture (middle) of ligand (S)-L5 and trimethylaluminum in toluene at 12 h at 80 °C (the mole ratio of complex (S)-5/complex 5a is approximately equal to 3), and the mixture (top) of ligand (S)-L5 and trimethylaluminum in toluene at 16 h at 80 °C (complex (S)-5 is the main ingredient). C

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angles are 117.43(16)°, 113.89(16)°, 93.57(11)°, and 91.19(11)°, respectively. The amount of distortion can be measured by the structural index parameter τ.16 The τ value ranges from 0 (perfectly square pyramidal) to 1 (perfectly trigonal bipyramidal). For complex (S)-3, the τ value is 0.44, which indicates it is close to square pyramidal geometry in the solid state, similar to salen(tBu)AlOSiMe3 in ref 16. The τ value, 0.49, of complex (rac)-4 was deduced by the same process. Obviously, in both complexes, these aluminum centers are in distorted square pyramidal environments. Polymerization of L-LA and rac-LA. All aluminum complexes were investigated as catalysts for the ring-opening polymerization of L-LA and rac-LA. The polymerizations were carried out in toluene, and the representative polymerization results are summarized in Table 1. These aluminum complexes showed moderate to high activities (81.6−93.0% conversion) with the cocatalysis 2-propanol at 70 °C. It is worth noting that the activities of these complexes decreased with the increase in substituent size on the benzene rings, while electron-withdrawing substituents raising the polymerization rate. Complex (S)-6 showed the highest activity (kapp = 0.0915 h−1, kp = 18.3 L mol−1 h−1, Table 1, entry 6, Figure 8) in these complexes. It was postulated that higher electronegativity of the substituent on the ligand would result in a weaker aluminum alkoxide bond and hence higher reaction rate. For example, for the ringopening polymerization of L-LA, (S)-6 attained a higher kapp value (Table 1, entry 8) compared to complex (S)-1 (Table 1, entry 1) under the same polymerization conditions. We have investigated poly(rac-LA) (Table 1, entry 19) with a homonuclear decoupled 1H NMR spectrum of the methine region18 (Figure 6). The Pm value19 (0.64) indicated that these polymer chains were partially isotactic. It is worth noting that the Pm selectivities increased from 0.50 to 0.57 with the increase in the size of the substituents on salicylaldehyde parts from hydrogen atoms to tert-butyls on the benzene ring (Table 1, entries 9, 10) and from 0.50 to 0.55 with an increase in the size of the substituents at anisole groups from hydrogen atoms to tert-butyls (Table 1, entries 9, 13). The tacticity of the poly(racLA) was significantly influenced by the reaction temperature. Upon lowering the temperature from 70 °C to 40 °C, the Pm value increased from 0.57 to 0.64 (Table 1, entries 10, 18, 19). The kinetics of polymerization of L-LA with different temperature and monomer/initiator ratios were investigated in toluene using (S)-6. In all cases, the polymerization followed first-order kinetics in the concentration of lactide (Figure 8). The first-order kinetics implied that the concentration of the active species remained unchanged, and the growing polymer chains remained alive during the entire polymerization. The molecular weight of the resultant polymer increased linearly with an increase in the monomer conversion, and the PDI values of these polymers were kept in a narrow band (1.04− 1.09). This suggested the living feature of the catalytic systems (Figure 9). To determine the order in initiator, kapp was plotted versus the concentration of (S)-6 (see Figure 8 and Figure 10). From this plot, kapp increased linearly with the (S)-6 concentration, manifesting that the order in catalyst was firstorder. Therefore, the polymerization of L-LA using (S)-6 followed a kinetic equation of the form −d[LA]/dt = kp[LA][Al]. The initiation mechanism was clarified by end-group analysis of the oligomers of L-LA, which were prepared via the polymerization of the L-LA at small monomer to initiator ratio ([L-LA]/[(S)-6]/[2-propanol] = 50:1:1) (Figure 11, Table 1,

Figure 4. Perspective view of (S)-3 with thermal ellipsoids drawn at the 30% probability level. Hydrogens and uncoordinated solvent are omitted for clarity. Selected bond lengths (Å) and angles (deg): Al1− O1 2.104(2), Al1−C35 1.956(4), Al1−N2 2.001(2), Al1−N1 1.876(3), Al1−O2 1.783(3), O2−Al1−C35 113.89(16), N1−Al1− C35 117.43(16), N1−Al1−N2 93.57(11), O2−Al1−N2 91.19(11). τ = 0.44.

Figure 5. Perspective view of (rac)-4 with thermal ellipsoids drawn at the 30% probability level. Hydrogens and uncoordinated solvent are omitted for clarity. Selected bond lengths (Å) and angles (deg): Al1− O1 2.137(2), Al1−C35 1.955(3), Al1−N2 2.001(2), Al1−N1 1.876(3), Al1−O2 1.775(2), O2−Al1−C35 117.45(12), N1−Al1− C35 121.29(13), N1−Al1−N2 94.96(10), O2−Al1−N2 91.69(10). τ = 0.49.

corresponding bond length (2.0303(18) Å (av)) in compound 2, perhaps because the bond lengths of Al1−N1 in (S)-3 and (rac)-4 are sigma bonds, while the corresponding bond lengths in compound 2 are coordination bonds. In complex (S)-3, the N1−Al1−C35, O2−Al1−C35, N1−Al1−N2, and O2−Al1−N2 D

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Table 1. Polymerization Data of LA with Complexes (S)-1−(S)-6, (rac)-2, (rac)-4, and (rac)-5 in Toluenea entry

complex

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

(S)-1 (S)-2 (S)-3 (S)-4 (S)-5 (S)-6 (S)-6 (S)-6 (S)-1 (S)-2 (S)-3 (S)-4 (S)-5 (S)-6 (rac)-2 (rac)-4 (rac)-5 (S)-2 (S)-2 (rac)-2

monomer

temp (°C)

t (h)

[LA]0/[Al]

convb (%)

Mn(calcd)c × 10−4

MnGPCd × 10−4

PDId

kappe

Pm

70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 55 40 40

36 46 32 32 45 29 18 13 36 46 32 32 45 24 48 32 45 48 48 48

100 100 100 100 100 100 75 50 100 100 100 100 100 100 100 100 100 100 100 100

85.2 82.9 85.0 87.1 84.7 93.0 88.0 90.0 84.0 80.5 82.9 84.0 85.0 84.7 81.5 83.8 83.5 63.3 47.6 44.1

1.23 1.20 1.23 1.26 1.22 1.34 1.27 1.30 1.21 1.16 1.20 1.21 1.23 1.22 1.17 1.21 1.20 0.92 0.69 0.64

2.04 2.01 2.08 2.14 2.13 2.34 2.14 2.25 2.29 2.11 2.24 2.22 2.13 2.21 2.24 2.20 2.22 1.60 1.17 1.08

1.09 1.04 1.08 1.09 1.08 1.09 1.06 1.05 1.08 1.09 1.12 1.12 1.11 1.09 1.10 1.11 1.09 1.08 1.05 1.07

0.0514 0.0412 0.0581 0.0614 0.0447 0.0915 0.1121 0.1840 n.a. n.a. n.a. n.a. n.a. 0.0889 n.a. n.a. n.a. n.a. n.a. n.a.

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.50 0.57 0.50 0.52 0.55 0.50 0.55 0.50 0.52 0.62 0.64 0.59

L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA

All polymerizations were carried out in toluene solution, [LA]0 = 0.5 mol L−1, [2-propanol]/[Al] = 1. bMeasured by 1H NMR. cCalculated from the molecular weight of LA × [LA]0/[Al] × conversion + Mw2‑propanol. dObtained from GPC analysis and calibrated against polystyrene standard. The true value of Mn(calcd) could be calculated according to the formula Mn = 0.58MnGPC.20 ekapp (h−1): calculated from the relationship kp = kapp/[Al].

a

entry 8). The 1H NMR spectra of the PLA oligomers revealed that the integral ratio of the two peaks at 1.24 ppm (the methyl protons from the isopropoxycarbonyl end group) and 4.35 ppm (the methine proton adjacent to the hydroxyl end group) approximates 6:1, signifying that the aggregating chains were end-capped with an isopropyl ester and a hydroxyl group;21 in other words, the alkyl aluminum compound has been converted into an isopropoxy aluminum species at the origin of the aggregation, so the actual initiator is the isopropoxy aluminum species (Figure 12). The ring-opening occurred via a so-called coordination insertion mechanism.22

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fonic acid were obtained from Aldrich and applied without further purification. Synthesis and Characterization of Compounds (S)-La, (S)-Lb, (S)-Lc and (rac)-La, (rac)-Lb, (rac)-Lc. Compound (S)-La. Upon stirring a solution of Pd(OAc)2 (0.045 g, 0.20 mmol, 5 equiv) and BINAP (0.25 g, 0.40 mmol, 10 equiv) in toluene in a Schlenk flask under argon, 2-bromoanisole (4.0 mmol, 1 equiv) and S-(−)-1,1′binaphthyl-2,2′-diamine (1.137 g, 4.00 mmol, 1 equiv) were added. The solution was stirred for 5 min, and subsequently sodium tertbutyloxysodium (0.576 g, 6.00 mmol, 1.5 equiv) was added. The mixture was allowed to stir at room temperature for 10 min. The Schlenk flask was heated to 70 °C by immersion in an oil bath. After 5 h the mixture was cooled to room temperature, taken up in diethyl ether (40 mL), and washed with brine. The resulting solution was dried over anhydrous potassium carbonate, filtered, and concentrated. The crude product was purified by flash chromatography on silica gel using 10:1 hexane−ethyl acetate containing 2% NEt3 as the eluant, affording 0.241 g of a colorless solid of the product in 61.7% isolated yield. 1H NMR (400 MHz, CDCl3, δ, ppm): 7.92−7.80 (m, 4H, ArH), 7.72 (d, J = 9.0 Hz, 1H, ArH), 7.39−7.09 (m, 8H, ArH), 6.95−6.83 (m, 2H, ArH), 6.78−6.75 (m, 1H, ArH), 5.70 (bs, 1H, NH), 3.68 (bs, 2H, NH2), 3.52 (s, 3H, OCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 149.79, 142.50, 139.93, 133.90, 133.71, 132.30, 129.51, 129.39, 128.94, 128.39, 128.07, 128.00, 127.24, 126.73, 124.59, 124.04, 123.25, 122.37, 121.30, 120.62, 118.23, 118.05, 117.48, 117.18, 112.52, 110.89 (26C, ArC), 55.38 (1C, OCH3). Anal. Calcd for C27H22N2O (%): C, 83.05; H, 5.68; N, 7.17. Found: C, 83.09; H, 5.71; N, 7.20. HRMS (m/ z): calcd for C27H22N2O 390.48; found 390.20 [M + H]+. Compound (S)-Lb. The synthesis of (S)-Lb was carried out according to the procedure of ligand (S)-La, using 2-bromo-5fluoroanisole (0.820 g, 4.00 mmol) and S-(−)-1,1′-binaphthyl-2,2′diamine (1.137 g, 4.00 mmol, 1 equiv). The product was isolated as a colorless powder. Yield: 0.303 g, 74.2%. 1H NMR (400 MHz, CDCl3, δ, ppm): 7.90−7.78 (m, 4H, ArH), 7.55 (d, J = 9.0 Hz, 1H, ArH), 7.35 − 7.11 (m, 8H, ArH), 6.65−6.50 (m, 2H, ArH), 5.56 (bs, 1H, NH), 3.69 (bs, 2H, NH2), 3.54 (s, 3H, OCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 152.33, 143.70, 139.98, 133.95, 133.76, 132.41, 129.56, 129.50, 129.00, 128.45, 128.18, 128.07, 127.33, 126.82, 124.59, 124.44, 123.55, 122.43, 121.37, 120.68, 118.31, 118.09, 117.60, 117.21, 112.55, 110.97 (26C, ArC), 55.46 (1C, OCH3). Anal. Calcd for

EXPERIMENTAL SECTION

General Considerations. All operations refer to air- and moisturesensitive comounds and were performed in an atmosphere of dry and decontaminated inert gases employing Schlenk line techniques. Toluene, tetrahydrofuran, and hexane were dried using Na−K alloy and distilled under nitrogen. Elemental analyses were accomplished with a Varian EL microanalyzer. 1H NMR, 1H COSY, 13C NMR, and 1 H−13C HMQC spectra were obtained on a Bruker AV 300 or 400 M apparatus at 25 °C in CDCl3 for compounds and macromolecules. The morphology conversions were confirmed via the integral signals at 1.65 ppm for the LA morphon and 1.59 ppm for PLA in CDCl3. Pm values were computed from different tetrad intensities measured via homonuclear decoupled 1H NMR. Gel permeation chromatography (GPC) measurements were conducted with a Waters 515 GPC with CHCl3 as the eluant (flow rate: 1 mL min−1, at 35 °C). The molecular weight was adjusted through a PS standard. Crystallographic data were gathered on a Bruker APEX CCD diffractometer using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) at 187.5 K, and calculations were performed using the SHELXTL-97 crystallographic software package.23 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced in calculated positions with the displacement factors of the host carbon atoms. Ligands were prepared in accord with the literature.15 AlMe3, 2-propanol, rac-1,1′-binaphthyl2,2′-diamine, S-(−)-1,1′-binaphthyl-2,2′-diamine, palladium(II) acetate, rac-BINAP, tert-butyloxysodium, 2-bromoanisole, 2-bromo-4,6di-tert-butylanisole, 2-bromo-5-fluoroanisole, salicylaldehyde, 3,5-ditert-butylsalicylaldehyde, 3,5-dichlorosalicylaldehyde, and p-toluenesulE

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Figure 6. Homonuclear decoupled 1H NMR spectrum of the methine region of poly(rac-LA) ((a) by complex (S)-2 at 70 °C, Pm = 0.57, Table 1, entry 13; (b) by complex (S)-2 at 40 °C, Pm = 0.64, Table 1, entry 19; (c) by complex (rac)-2 at 40 °C, Pm = 0.59, Table 1, entry 20, 400 MHz, CDCl3). C27H21FN2O (%): C, 79.39; H, 5.18; N, 6.86. Found: C, 79.36; H, 5.15; N, 6.79. HRMS (m/z): calcd for C27H21FN2O 408.47; found 408.20 [M + H]+. Compound (S)-Lc. The synthesis of (S)-Lc was carried out according to the procedure of ligand (S)-La, using 2-bromo-4,6-ditert-butylanisole (1.197 g, 4.00 mmol). The product was isolated as a colorless powder. Yield: 0.229 g, 56.1%. 1H NMR (400 MHz, CDCl3, δ, ppm): 7.91−7.74 (m, 4H, ArH), 7.62 (d, J = 9.0 Hz, 1H, ArH), 7.32−7.06 (m, 8H, ArH), 6.97 (s, 1H, ArH), 5.53 (bs, 1H, NH), 3.75 (bs, 2H, NH2), 3.43 (s, 3H, OCH3), 1.33 (s, 9H, C(CH3)3), 1.29 (s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3, δ, ppm): 149.42, 145.59, 142.87, 142.38, 140.67, 134.67, 134.06, 133.80, 129.70, 129.13, 128.87, 128.50, 128.21, 128.06, 127.00, 126.83, 126.77, 124.26, 123.92, 122.82, 120.43, 118.28, 117.18, 116.51, 115.39, 112.18, 106.42 (26C, ArC), 59.88 (1C, OCH3), 35.14 (1C, C(CH3)3), 34.58 (1C, C(CH3)3), 31.28 (3C, C(CH3)3), 30.69 (3C, C(CH3)3). Anal. Calcd for C35H38N2O (%): C, 83.63; H, 7.62; N, 5.57. Found: C, 83.64; H, 7.57; N, 5.52. HRMS (m/z): calcd for C35H38N2O: 502.69; Found: 502.30 [M + H]+.

The method described for (S)-La, (S)-Lb, and (S)-Lc was used for the synthesis of (rac)-La, (rac)-Lb, and (rac)-Lc, respectively. Synthesis and Characterization of Ligands. Ligand (S)-L1. A mixture of (S)-La (0.390 g, 1.00 mmol), salicylaldehyde (0.122 g, 1.00 mmol), and a catalytic amount of p-toluenesulfonic acid in toluene (30 mL) was refluxed for 8 h. After solvent evaporation at reduced pressure, the crude product was purified by flash chromatography on silica gel using 15:1 hexane−ethyl acetate containing 2% NEt3 as the eluant, affording 0.411 g of the yellow solid of the product in 82.9% isolated yield. 1H NMR (300 MHz, CDCl3, δ, ppm): 12.10 (bs, 1H, OH), 8.57 (s, 1H, NCH), 8.06 (d, J = 8.8 Hz, 1H, ArH), 8.00−7.89 (m, 3H, ArH), 7.85 (d, J = 8.0 Hz, 1H, ArH), 7.74 (d, J = 9.0 Hz, 1H, ArH), 7.62 (d, J = 8.8 Hz, 1H, ArH), 7.56−7.38 (m, 2H, ArH), 7.35− 7.11 (m, 7H, ArH), 7.01 (d, J = 8.3 Hz, 1H, ArH), 6.87−6.52 (m, 3H, ArH), 5.66 (bs, 1H, NH), 3.50 (s, 3H, OCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 162.30 (1C, NCH), 160.81, 149.00, 144.50, 139.09, 133.76, 133.41, 132.73, 132.65, 132.57, 132.07, 129.84, 129.33, 128.98, 128.11, 127.15, 126.43, 126.35, 125.97, 124.62, 123.29, 120.63, 120.61, 120.21, 119.18, 118.69, 118.46, 117.63, 117.08, 116.43,113.20, F

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Figure 7. Kinetics of the ROP of L-LA by (S)-1−(S)-5 with 2propanol at 70 °C in toluene with [LA]0 = 0.5 mol L−1, [Al]0 = 5 × 10−3 mol L−1, [LA]0/[Al]0 = 100. (■) (S)-1, kapp = 0.0514 h−1; (●) (S)-2, kapp = 0.0412 h−1; (◇) (S)-3, kapp = 0.0581 h−1; (□) (S)-4, kapp = 0.0614 h−1; (○) (S)-5, kapp = 0.0447 h−1.

Figure 10. kapp versus the concentration of (S)-6/2-propanol initiator for the L-LA polymerization at 70 °C in toluene ([L-LA]0 = 0.5 mol L−1, kp =18.3 L mol−1 h−1).

Figure 8. Kinetics of the ROP of L-LA and rac-LA by (S)-6 with 2propanol at 70 °C in toluene with [LA]0 = 0.5 mol L−1; kp = kapp/ [Al]0. (●) L-LA, [LA]0/[Al]0 = 100, kapp = 0.0915 h−1, kp = 18.3 L mol−1 h−1; (■) L-LA, [LA]0/[Al]0 = 75, kapp = 0.1221 h−1; (▲) L-LA, [LA]0/[Al]0 = 50, kapp = 0.1840 h−1; (○) rac-LA, [LA]0/[Al]0 = 100, kapp = 0.0889 h−1.

Figure 11. 1H NMR spectrum of a polymer sample obtained from the (S)-6/2-propanol system with [LA]0/[(S)-6]0/[2-propanol]0 = 50:1:1 (in CDCl3, Table 1, entry 8).

Figure 12. Proposed mechanism for the ROP of lactide initiated by aluminum complexes and 2-propanol.

Figure 9. Plot of PLA Mn and polydispersity (Mw/Mn) as a function of L-LA conversion using complex (S)-6/2-propanol, [M]0/[Al]0 = 100, at 70 °C in toluene.

110.57, 109.41 (32C, ArC), 55.31 (1C, OCH3). Anal. Calcd for C34H26N2O2 (%): C, 82.57; H, 5.30; N, 5.66. Found: C, 82.54; H, G

dx.doi.org/10.1021/om400714q | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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133.99, 133.63, 133.23, 132.41, 130.23, 129.59, 129.48, 128.75, 128.70, 128.44, 128.23, 127.41, 126.64, 126.34, 123.99, 122.66, 122.76, 120.32, 118.17, 117.79, 117.46, 116.45, 116.35 (32C, ArC), 59.92 (OCH3), 35.10 (1C, C (CH3)3), 34.56 (1C, C (CH3)3), 31.43 (3C, C(CH3)3), 30.87 (3C, C(CH3)3). Anal. Calcd for C42H40Cl2N2O2 (%): C, 74.66; H, 5.97; N, 4.15. Found: C, 74.66; H, 5.97; N, 4.15. HRMS (m/z): calcd for C42H40Cl2N2O2 675.69; found 675.20 [M + H]+. Ligand (S)-L6. The synthesis of ligand (S)-L6 was carried out according to the procedure of ligand (S)-L3, using (S)-Lb (0.408 g, 1.00 mmol) and 3,5-dichlorosalicylaldehyde (0.390 g, 1.00 mmol). The product was isolated as a red powder. Yield: 0.536 g, 92.2%. 1H NMR (400 MHz, CDCl3, δ, ppm): 12.94 (bs, 1H, OH), 8.38 (s, 1H, NCH), 8.04 (d, J = 8.8 Hz, 1H, ArH), 7.95 (d, J = 8.1 Hz, 1H, ArH), 7.85 (d, J = 9.0 Hz, 1H, ArH), 7.79 (d, J = 8.0 Hz, 1H, ArH), 7.57−7.45 (m, 3H, ArH), 7.40−7.12 (m, 6H, ArH), 6.93−6.90 (m, 2H, ArH), 6.47−6.42 (m, 2H, ArH), 5.33 (bs, 1H, NH), 3.47 (s, 3H, OCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 160.94 (1C, NCH), 159.53, 155.53, 151.02, 143.89, 139.90, 133.81, 133.45, 133.01, 132.37, 130.11, 129.50, 129.44, 129.11, 128.11, 127.93, 127.39, 126.67, 126.62, 126.46, 124.30, 123.00, 122.80, 122.52, 120.25, 119.25, 119.16, 118.08, 117.70, 106.34, 106.12, 99.45, 99.18 (32C, ArC), 55.53 (OCH3). Anal. Calcd for C34H23Cl2FN2O2 (%): C, 70.23; H, 3.99; N, 4.82. Found: C, 70.05; H, 3.87; N, 4.70. HRMS (m/z): calcd for C34H23Cl2FN2O2 581.11; found 581.10 [M + H]+. Synthesis and Characterization of Complexes. Complex (S)-1. A mixture of (S)-L1 (0.495 g, 1.00 mmol) and AlMe3 (1.00 M in toluene, 1.00 mL, 1.00 mmol) in 20 mL of toluene was stirred for 10 h at 60 °C under an argon atmosphere and concentrated to 2 mL to give a yellow powder, from which the mother liquor was decanted. The product was washed with 1 mL of hexane and dried under vacuum. The product was isolated as a yellow solid. Yield: 0.504 g, 94.3%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.18 (s, 1H, NCH), 8.07 (d, J = 8.8 Hz, 1H, ArH), 6.49−8.00 (m, 19H, ArH), 4.12 (s, 3H, OCH3), −1.11 (s, 3H, AlCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 167.90 (1C, NCH), 147.18, 143.86, 140.96, 137.84, 36.39, 134.37, 133.59, 129.94, 129.02, 128.93, 128.21, 128.00, 127.70, 127.49, 127.33, 126.70, 125.94, 125.81, 125.51, 125.28, 124.70, 123.53, 122.24, 122.02, 120.69, 119.32, 118.20, 117.38, 116.41, 110.96, 109.53, 108.37 (32C, ArC), 55.25 (1C, OCH 3 ), −10.74 (1C, AlCH3 ). Anal. Calcd for C35H27AlN2O2 (%): C, 78.64; H, 5.09; N, 5.24. Found: C, 78.67; H, 5.14; N, 5.12. Complex (S)-2. Complex (S)-2 was prepared with a similar method to that for (S)-1, using (S)-L2 (0.607 g, 1.00 mmol) and AlMe3 (1.00 mmol). The product was isolated as a yellow solid. Yield: 0.545 g, 84.3%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.13 (s, 1H, NCH), 7.96 (d, J = 8.5 Hz, 1H, ArH), 7.90 (d, J = 8.2 Hz, 1H, ArH), 7.70 (d, J = 8.0 Hz, 1H, ArH), 7.64 (d, J = 8.9 Hz, 1H, ArH), 7.52−7.12 (m, 8H, ArH), 7.12−6.96 (m, 1H, ArH), 6.84−6.68 (m, 4H, ArH), 6.60 (t, J = 7.0 Hz, 1H, ArH), 4.11 (s, 3H, OCH3), 1.46 (s, 9H, C(CH3)3), 1.21 (s, 9H, C(CH3)3), −1.05 (s, 3H, AlCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 168.46 (1C, NCH), 161.40, 147.25, 147.07, 144.24, 143.72, 139.99, 138.68, 134.21, 133.17, 132.18, 131.51, 130.04, 129.54, 128.99, 128.85, 128.67, 128.08, 127.93, 127.55, 127.42, 126.52, 125.97, 125.55, 125.29, 125.19, 123.23, 123.01, 122.22, 118.54, 117.94, 115.97, 108.01 (32C, ArC), 55.51 (1C, OCH3), 35.22 (1C, C(CH3)3), 33.93 (1C, C(CH3)3), 31.10 (3C, C(CH3)3), 29.38 (3C, C(CH3)3), −10.60 (1C, AlCH3). Anal. Calcd for C43H43AlN2O2 (%): C, 79.85; H, 6.70; N, 4.33. Found: C, 79.81; H, 6.64; N, 4.25. Complex (rac)-2. The method described for (S)-2 was used for the synthesis of (rac)-2 from ligand (rac)-L2. Complex (S)-3. Complex (S)-3 was prepared with a similar method to that for (S)-1, using (S)-L3 (0.563 g, 1.00 mmol) and AlMe3 (1.00 mmol). The product was isolated as an orange solid. Yield: 0.547 g, 90.7%. Crystals of (S)-3 suitable for X-ray structural determination were grown in a THF−hexane mixed solution. 1H NMR (300 MHz, CDCl3, δ, ppm): 8.12 (s, 1H, NCH), 8.00 (d, J = 8.6 Hz, 1H, ArH), 7.93 (d, J = 8.3 Hz, 1H, ArH), 7.77−7.68 (m, 2H, ArH), 7.67 (s, 1H, ArH), 7.56−6.99 (m, 8H, ArH), 6.8−6.88 (m, 4H, ArH), 6.71 (d, J = 8.6 Hz, 1H, ArH), 4.25 (s, 3H, OCH3), −1.11 (s, 3H, AlCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 166.36 (1C, NCH), 158.21,

5.30; N, 5.60. HRMS (m/z): calcd for C34H26N2O2 494.58; found 495.2 [M + H]+. Ligand (S)-L2. The synthesis of ligand (S)-L2 was carried out according to the procedure of ligand (S)-L1, using (S)-La (0.390 g, 1.00 mmol) and 3,5-di-tert-butylsalicylaldehyde (0.234 g, 1.00 mmol). The product was isolated as a yellow powder. Yield: 0.486 g, 80.1%. 1H NMR (300 MHz, CDCl3, δ, ppm): 12.82 (bs, 1H, OH), 8.55 (s, 1H, NCH), 8.04 (d, J = 8.8 Hz, 1H, ArH), 7.96 (d, J = 8.0 Hz, 1H, ArH), 7.84−6.58 (m, 16H, ArH), 5.67 (bs, 1H, NH), 3.50 (s, 3H, OCH3), 1.28 (s, 9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3, δ, ppm): 163.04 (1C, NCH), 158.26, 148.89, 144.56, 139.71, 138.97, 136.67, 133.99, 133.57, 132.77, 132.46, 129.72, 128.78, 128.16, 127.98, 127.56, 127.12, 126.46, 126.33, 125.77, 124.69, 123.19, 120.67, 120.36, 118.95, 118.13, 117.89, 116.25, 110.53 (32C, ArC), 55.33 (1C, OCH3), 34.87 (2C, C(CH3)3), 33.99 (2C, C(CH3)3), 31.40 (3C, C(CH3)3), 29.16 (3C, C(CH3)3). Anal. Calcd for C42H42N2O2 (%): C, 83.13; H, 6.98; N, 4.62. Found: C, 83.09; H, 6.91; N, 4.60. HRMS (m/z): calcd for C42H42N2O2 606.80; found 607.30 [M + H]+. Ligand (S)-L3. (S)-La (0.390 g, 1.00 mmol) was dissolved in 5 mL of absolute EtOH, and a solution of 3,5-dichlorosalicylaldehyde (0.390 g, 1.00 mmol) in 5 mL of absolute EtOH was added. The solution turned red, and a powder started to precipitate. After 2 h under stirring, the red solid was removed by filtration, washed twice with EtOH, and dried under vacuum. Yield: 0.521 g, 92.5%. 1H NMR (300 MHz, CDCl3, δ, ppm): 12.90 (bs, 1H, OH), 8.37 (s, 1H, NCH), 8.09 (d, J = 8.8 Hz, 1H, ArH), 7.99 (d, J = 8.1 Hz, 1H, ArH), 7.93 (d, J = 8.9 Hz, 1H, ArH), 7.87 (d, J = 8.1 Hz, 1H, ArH), 7.74 (d, J = 9.0 Hz, 1H, ArH), 7.61−7.47 (m, 2H, ArH), 7.46−7.13 (m, 6H, ArH), 7.00− 6.95 (m, 2H, ArH), 6.85−6.62 (m, 3H, ArH), 5.60 (bs, 1H, NH), 3.54 (s, 3H, OCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 161.08 (1C, NCH), 155.45, 148.97, 143.88, 139.18, 133.70, 133.40, 132.90, 132.37, 132.26, 130.02, 129.54, 129.39, 129.30, 128.21, 128.00,127.88, 127.38, 126.57, 126.36, 124.75, 124.50, 123.42, 122.68, 122.40, 120.80, 120.68, 120.22, 119.45, 118.89, 118.05, 116.19, 110.58 (32C, ArC), 55.37 (1C, OCH3). Anal. Calcd for C34H24Cl2N2O2 (%): C, 72.47; H, 4.29; N, 4.97. Found: C, 72.53; H, 4.35; N, 5.07. HRMS (m/z): calcd for C34H24Cl2N2O2 563.47; found 563.10 [M + H]+. Ligand (S)-L4. The synthesis of ligand (S)-L4 was carried out according to the procedure of ligand (S)-L1, using (S)-Lb (0.408 g, 1.00 mmol) and salicylaldehyde (0.122 g, 1.00 mmol). The product was isolated as a yellow powder. Yield: 0.429 g, 83.7%. 1H NMR (400 MHz, CDCl3, δ, ppm): 12.17 (bs, 1H, OH), 8.61 (s, 1H, NCH), 8.08 (d, J = 8.8 Hz, 1H, ArH), 7.98 (d, J = 8.1 Hz, 1H, ArH), 7.91 (d, J = 8.9 Hz, 1H, ArH), 7.85 (d, J = 7.9 Hz, 1H, ArH), 7.64 (d, J = 8.8 Hz, 1H, ArH), 7.58 (t, J = 11.3 Hz, 1H, ArH), 7.50 (t, J = 7.4 Hz, 1H, ArH), 7.47−7.43 (m, 1H, ArH), 7.39−7.13 (m, 6H, ArH), 7.02 (d, J = 8.3 Hz, 1H, ArH), 6.80 (t, J = 7.9 Hz, 2H, ArH), 6.48−6.42 (m, 2H, ArH), 5.42 (s, 1H, NH), 3.48 (s, 3H, OCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 162.31 (1C, NCH), 160.84, 159.33, 156.94, 150.98, 144.62, 139.83, 133.81, 133.48, 132.83, 132.73, 132.05, 129.92, 129.10, 128.28, 128.14, 127.95, 127.15, 126.50, 126.45, 126.03, 124.50, 123.13, 119.21, 119.14, 119.05, 118.57, 117.74, 117.15, 106.27, 106.05, 99.35, 99.09 (32C, ArC), 55.48 (1C, OCH3). Anal. Calcd for C34H25FN2O2 (%): C, 79.67; H, 4.92; N, 5.47. Found: C, 79.61; H, 4.89; N, 5.42. HRMS (m/z): calcd for C34H25FN2O2 512.57; found 513.2 [M + H]+. Ligand (S)-L5. The synthesis of ligand (S)-L5 was carried out according to the procedure of ligand (S)-L3, using (S)-Lc (0.502 g, 1.00 mmol) and 3,5-dichlorosalicylaldehyde (0.390 g, 1.00 mmol). The product was isolated as a red powder. Yield: 0.610 g, 90.3%. 1H NMR (400 MHz, CDCl3, δ, ppm): 13.13 (bs, 1H, OH), 8.51 (s, 1H, NCH), 8.02 (d, J = 8.8 Hz, 1H, ArH), 7.88−7.95 (m, 2H, ArH), 7.79 (d, J = 8.0 Hz, 1H, ArH), 7.64 (d, J = 9.0 Hz, 1H, ArH), 7.57 (d, J = 8.8 Hz, 1H, ArH), 7.49 (t, J = 7.4 Hz, 1H, ArH), 7.43 (d, J = 8.4 Hz, 1H, ArH), 7.38−7.28 (m, 2H, ArH), 7.29 (s, 1H, ArH), 7.18−7.23 (m, 1H, ArH), 7.12−6.96 (m, 3 H, ArH), 6.86 (d, J = 8.3 Hz, 1H), 5.24 (bs, 1H, NH), 3.39 (s, 3H, OCH3), 1.28 (s, 9H, C(CH3)3), 1.23 (s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3, δ, ppm): 160.12 (1C, NCH), 155.70, 149.19, 146.09, 143.38, 142.18, 140.08, 134.63, H

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146.75, 146.64, 143.51, 143.21, 135.14, 134.18, 133.08, 132.44, 131.39, 129.86, 129.68, 128.93, 128.12, 127.96, 127.66, 127.39, 127.32, 126.81, 125.98, 125.73, 125.61, 125.19, 124.16, 123.38, 123.29, 121.74, 120.99, 120.02, 118.21, 116.81, 108.39 (32C, ArC), 55.44 (1C, OCH3), −10.88 (1C, AlCH3). Anal. Calcd for C35H25AlCl2N2O2 (%): C, 69.66; H, 4.18; N, 4.64. Found: C, 69.58; H, 4.11; N, 4.60. CCDC number: 914984. Complex (S)-4. Complex (S)-4 was prepared with a similar method to that for (S)-1, using (S)-L4 (0.513 g, 1.00 mmol) and AlMe3 (1.00 mmol). The product was isolated as a yellow solid. Yield: 0.489 g, 88.5%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.22 (s, 1H, NCH), 8.10 (d, J = 8.8 Hz, 1H, ArH), 8.01 (d, J = 8.8 Hz, 1H, ArH), 7.97− 7.85 (m, 2H, ArH), 7.80 (d, J = 8.8 Hz, 1H, ArH), 7.68−6.51 (m, 13H, ArH), 6.44 (d, J = 8.8 Hz, 1H, ArH), 4.12 (s, 3H, OCH3), −1.11 (s, 3H, AlCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 162.22 (1C, N CH), 160.76, 159.26, 156.00, 150.35, 144.01, 139.43, 133.54, 133.23, 132.71, 132.60, 131.77, 129.99, 129.00, 127.97, 127.87, 127.69, 127.02, 126.22, 126.10, 125.89, 125.00, 122.93, 119.01, 118.89, 118.77, 118.45, 117.49, 117.02, 106.11, 105.89, 99.02, 98.87 (32C, ArC), 55.42 (1C, OCH3), −10.11 (1C, AlCH3). Anal. Calcd for C35H26AlFN2O2 (%): C, 76.08; H, 4.74; N, 5.07. Found: C, 76.02; H, 4.73; N, 5.00. Complex (rac)-4. The method described for (S)-4 was used for the synthesis of (rac)-4 from ligand (rac)-L4. Crystals of (rac)-4 suitable for X-ray structural determination were grown in a THF−hexane mixed solution. CCDC number: 914985. Complex (S)-5. Complex (S)-5 was prepared with a similar method to that for (S)-1, using (S)-L5 (0.676 g, 1.00 mmol) and AlMe3 (1.00 mmol) at 80 °C for 16 h. The product was isolated as a yellow solid. Yield: 0.520 g, 80.4%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.03 (s, 1H, NCH), 7.96 (d, J = 8.5 Hz, 1H, ArH), 7.89 (d, J = 8.2 Hz, 1H, ArH), 7.77 (d, J = 8.6 Hz, 2H, ArH), 7.50 (d, J = 8.9 Hz, 1H, ArH), 7.42 − 7.23 (m, 6H, ArH), 7.03 (t, J = 7.5 Hz, 1H, ArH), 6.91 (s, 1H, ArH), 6.78 (d, J = 8.4 Hz, 1H, ArH), 6.61 (s, 1H, ArH), 6.44 (s, 1H, ArH), 4.25 (s, 3H, OCH3), 1.41 (s, 9H, C(CH3)3), 1.06 (s, 9H, C(CH3)3), −1.10 (s, 3H, AlCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 166.03 (1C, NCH), 157.63, 148.18, 147.86, 146.97, 146.79, 144.75, 143.06, 138.69, 134.80, 134.07, 132.85, 132.14 130.81, 129.68, 129.40, 129.01, 128.90, 128.50, 127.95, 127.78, 127.39, 126.73, 126.60, 126.36, 125.63, 125.12, 123.49, 121.64, 120.76, 119.70, 114.15, 113.48 (32C, ArC), 66.51 (1C, OCH3), 35.41(1C, C (CH3)3), 34.52 (1C, C (CH3)3), 31.77 (3C, C(CH3)3), 30.95 (3C, C(CH3)3), −10.99 (1H, AlCH3). Anal. Calcd for C43H41AlCl2N2O2 (%): C, 72.16; H, 5.77; N, 3.91. Found: C, 72.20; H, 5.83; N, 3.99. Complex (rac)-5. The method described for (S)-5 was used for the synthesis of (rac)-5 from ligand (rac)-L5. Complex (S)-6. Complex (S)-6 was prepared with a similar method to that for (S)-1, using (S)-L6 (0.581 g, 1.00 mmol) and AlMe3 (1.00 mmol). The product was isolated as an orange solid. Yield: 0.559 g, 90.1%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.07 (s, 1H, NCH), 8.00 (d, J = 8.5 Hz, 1H, ArH), 7.94 (d, J = 8.1 Hz, 1H, ArH), 7.67− 7.73 (m, 2H, ArH), 7.50−7.17 (m, 7H), 7.04 (t, J = 7.6 Hz, 1H), 6.97 (s, 1H), 6.77−6.62 (m, 3H), 6.55 (t, J = 8.6 Hz, 1H), 4.23 (s, 3H, OCH3), −1.11 (s, 3H, AlCH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 166.73 (1C, NCH), 158.18, 156.27, 153.90, 147.00, 143.21, 139.65, 135.32, 134.26, 133.11, 132.57, 131.45, 129.79, 129.12, 128.11, 127.76, 127.41, 127.13, 126.94, 126.12, 125.81, 125.28, 124.07, 123.54, 121.77, 121.19, 119.98, 118.28, 118.20, 109.07, 108.86, 97.93, 97.64 (32C, ArC), 55.92 (1C, OCH3), −11.14 (1H, AlCH3). Anal. Calcd for C43H41AlCl2N2O2 (%): C, 67.64; H, 3.89; N, 4.51. Found: C, 67.20; H, 3.83; N, 4.42. General Procedure for Lactide Polymerization. In a typical polymerization experiment, aluminum complex (30 μmol) and the required amount of lactides in toluene (60 mL) were loaded in a flame-dried vessel containing a magnetic bar. The vessel was placed in an oil bath thermostated at 70 °C. The solution was stirred for about 10 min, when the catalyst was activated completely by 2-propanol, and subsequently the required amount of lactides was added. After a certain reaction time, the polymer was isolated by precipitation with cold methanol. The precipitate was collected and dried under vacuum at 35 °C for 36 h.

Article

CONCLUSIONS In conclusion, we report a number of new aluminum complexes bearing 2-bromoanisole, modified salicylaldehyde, and binaphthyl diamine that serve as single-site, living catalysts for the polymerization of L-LA and rac-LA. Electron-withdrawing substituents dramatically raise the polymerization rate. Microstructural analysis of the polymers catalyzed by these complexes reveals that the asymmetrically O,N,N,O-quadridentate ligands have the ability to control the tacticity of the polymer, and this ability varies according to ligand size and number. The bulky substituents on the ligand affect the tacticities of the polymers. In addition, kinetic analysis indicates that polymerizations of lactide by complex (S)-6 are first-order in both lactide monomer and catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data and refinements for complexes (S)3 and (rac)-4 in CIF format and two-dimensional NMR spectra of complex (S)-2 are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21204082, 51173183, 21004061, and 51021003) and the Ministry of Science and Technology of China (No. 2011AA02A202).

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REFERENCES

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