Highly Iso-Selective and Active Catalysts of Sodium and Potassium

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Highly Iso-Selective and Active Catalysts of Sodium and Potassium Monophenoxides Capped by a Crown Ether for the Ring-Opening Polymerization of rac-Lactide Jinjin Zhang, Jiao Xiong, Yangyang Sun, Ning Tang, and Jincai Wu* State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China S Supporting Information *

ABSTRACT: Sodium and potassium complexes supported by a bulky monophenoxy with one xanthenyl group at the orthoposition and 18-crown-6 or 15-crown-5 as an auxiliary ligand were synthesized and characterized. These complexes are highly iso-selective and active catalysts for the controlled ringopening polymerization of rac-lactide. The best isotacticity (Pm) achieved was 0.86, which is the highest iso-selectivity reported to date for an alkali-metal complex. In addition, the corresponding polymer exhibited a high Tm of 182 °C. Furthermore, the polymerization looks like an anti-Arrhenius reaction, which is slower at high temperatures than at low temperatures.

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INTRODUCTION Polylactide (PLA) derived from a renewable resource has gained considerable attention for its nontoxicity, biodegradability and biocompatibility and has been used in a wide range of applications in the medical, food, packaging, and agricultural fields as a replacement for oil-based materials.1 The ringopening polymerization (ROP) of lactides is an efficient method for synthesizing polylactide because of its advantages of a well-controlled molecular weight and low polydispersity (PDI).2 The physical and chemical properties of polylactide are highly dependent on its stereochemistry,3 hence, the stereocontrolled ROP of rac-lactide is currently a valuable and challenging research goal that can afford PLA with various stereo microstructures. Toward this end, numerous initiators, including metal complexes of aluminum,4 zinc,5 lanthanides,4d,6 indium,7 and magnesium,5d,8 have been reported to exhibit good selectivities.2,9 Compared to diverse heterotactic-selective systems, highly iso-selective initiators have rarely been reported. Among these initiators, only aluminum, indium, lanthanides, and zinc complexes have been extensively investigated because of their outstanding isotactic selectivities. No highly isoselective sodium or potassium metal complexes have been reported in this area until now. Sodium and potassium are innocuous, abundant elements in the human body and are suitable for the catalytic synthesis of polylactides for use in medical-related fields,10 thus exploring sodium or potassium complexes as catalysts for ring-opening polymerization of lactide is very valuable. Recent studies have shown that bulky ligand-supported sodium and potassium complexes can catalyze the ROP of lactide in a controlled manner with high activities.11 Usually, the selectivities of alkali-metal complexes are lower and rarely reported, for example, some simple sodium aryloxides reported © XXXX American Chemical Society

by Davidson group can iso-selectively catalyze the ROP of raclactide giving a modest isotactic-enriched PLA (Pm = 0.63),12 Kasperczyk reported lithium tert-butoxide can initiate the ROP of rac-lactide affording a heterotactic-enriched PLA (Pr = 0.76).13 The lower selectivity may result from the weak interaction between the incoming lactide and the active end of the polylactide or ligand in alkali-metal complex catalytic system, despite the electronic modulation of the ligand. To improve the iso-selectivity of alkali-metal complex, here, we used a strategy of embedding an active sodium/potassium phenoxide into a bent plane, with suitable stereohindrance at the rim for increasing the interaction between the incoming lactide and the active end of the polylactide or ligand.

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RESULTS AND DISCUSSION Synthesis of Sodium and Potassium Bulky Phenolates. A bulky ligand of 2-(1,3,6,8-tetra-tert-butyl-9-phenylxanthen-9-yl)-4,6-di-tert-butylphenol (HL) was synthesized by heating a mixture of benzyl aldehyde and 2,4-di-tertbutylphenol, resulting in a 17% yield. From its crystal structure (Figure S1, Supporting Information), the predominant feature of this monophenol is the bent planar xanthenyl group in the ortho position, with four bulky tert-butyl groups at its rim. This bent xanthenyl group like a half-cavity can embrace the hydroxyl group of phenol, and provide suitable stereohindrance for the hydroxy group. NaL and KL can be obtained in 90% and 93% yields, respectively, from the reactions of HL and NaN(SiMe3)2/potassium in toluene (Scheme 1). Although Received: September 26, 2014 Revised: October 22, 2014

A

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ROP of rac-Lactide Catalyzed by KL and NaL. The ROP of rac-lactide with KL and NaL as catalysts was systematically examined. Overall, KL efficiently catalyzed the immortal ROP of rac-LA with 1, 5, or 10 equiv of BnOH as a co-initiator under controllable conditions, resulting in the expected molecular weights and low molecular weight distributions (PDI = 1.03− 1.13, Table 1, entries 1−9, Figure S10). KL is a highly active catalyst; the polymerization was accomplished in 2 and 10 min for an initial [rac-lactide]0:[KL]0:[BnOH]0 ratio of 200:1:10 at 0 and −60 °C (entries 3 and 5), respectively. When the ratio of [rac-lactide]0:[KL]0:[BnOH]0 was 200:1:10, modest isotacticenriched PLA (Pm = 0.63) was prepared in THF at 0 °C (entry 3). In addition, the iso-selectivity increased to Pm = 0.64 and 0.70 when the temperature was decreased to −30 °C and −60 °C, respectively (entries 4−5). When CH2Cl2 was used as a solvent instead of THF, the Pm slightly decreased to 0.66 (entry 9). Therefore, THF was the better reaction solvent. For NaL, a similar iso-selectivity of Pm = 0.71 was obtained at −60 °C (entry 10). According to the monomer activation mechanism assumed for most alkali-metal phenoxides,11a,g the lactide is activated after coordinating with the K or Na atom. Consequently, the BnOH can attack the carbonyl group to initiate the ROP reaction. 1H NMR analysis of the polymer confirmed this mechanism, as the polymer chain was capped with one benzyl ester and had a hydroxy on one end (Figure S11). Synthesis and Structures of Complexes 1 and 2. Inspired by the aforementioned modest iso-selectivities, complexes 1 and 2 were designed in which the second auxiliary ligand of the crown is a flexible cap for the half-cavity of the bent planar xanthenyl group (Scheme 1). Their components were confirmed by the elemental analysis, NMR spectra, DOSY experiments in CDCl3, and crystal structures (Figures 2, 3, S12−S19). The molecular structure of complex 1, which was crystallized in the monoclinic chiral space group P21, shows that K1 is coordinated to one oxygen atom of the phenoxy group and to six oxygen atoms of the 18-crown-6. The chirality of this crystal originates from the asymmetric xanthenyl plane; K1 does not lie in the perpendicular bisected plane of the xanthene,

Scheme 1. Synthesis of the Sodium and Potassium Complexes

NaL and KL crystals were not obtained, a [KL·BnOH] crystal complex was obtained when a toluene solution of KL and BnOH at a ratio of 1:1.5 was cooled. However, in solution, [KL•BnOH] can dissociate to KL and BnOH, as shown by the chemical shift of BnOH, which is the same as that of free BnOH, and the different diffusion coefficients of BnOH and KL in CDCl3 (Figure S8). Meanwhile, the pure [KL•BnOH] solid cannot be obtained because BnOH is partially or completely removed under vacuum. In the solid structure of [KL•BnOH] (Figures 1 and S9), K1 is coordinated with the O1 of the phenoxy, the O3 of the BnOH and one benzene ring of the xanthenyl group, resulting in an asymmetric environment around K1 for the chiral bent xanthenyl plane; because of the hydrogen interactions of O3−H−O1A and O3A−H−O1, a dimer structure can occur in the solid state. This crystal structure may inform us that lactide and BnOH can coordinate to the K atom during the ROP process.

Figure 1. Molecular structure of [KL•BnOH] (the methyl groups of six tert-butyl and most of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å) and angles (o): K1−O1 2.5649(14), K1−O3 2.6476(14), K1−O3A 3.3088(15), O3−O1A 2.5491(18), O3−O1A 2.5491(18), K1 to the plane of C30−C35−C34 2.9944 (10), O1−K1−O3 114.88(5), C1−O1−K1 120.15(10), and O3−H3OH−O1A 164.3. Symmetry code of A: 1x, -y, 1-z. B

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Table 1. Lactide Polymerization Catalyzed by NaL, KL, 1, and 2a

entry

cat.

[LA]0/[Cat]0/[BnOH]0

t (min)

convn (%)b

Mn,cal. (g/mol)c

Mn,obs. (g/mol)d

Mw/Mn

Pme

1 2 3f 4g 5 6 7 8 9h 10 11 12 13 14g 15 16 17 18 19 20g 21 22 23 24 25

KL KL KL KL KL KL KL KL KL NaL 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2

200/1/1 200/1/5 200/1/10 200/1/10 200/1/10 250/1/10 300/1/10 350/1/10 200/1/10 200/1/10 200/1/1 400/1/1 200/1/5 200/1/10 200/1/10 250/1/10 300/1/10 350/1/10 400/1/10 200/1/10 100/1/10 150/1/10 200/1/10 250/1/10 300/1/10

10 10 2 5 10 10 10 15 30 10 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

83 99 99 78 98 96 95 88 87 87 74 76 99 64 99 99 99 99 98 69 98 97 99 98 99

23 900 5800 3000 2400 2900 3600 4200 4500 2600 2600 21 400 43 900 5800 2000 3000 3700 4400 5100 5800 2100 1500 2200 3000 3600 4400

24 000 5900 2800 2200 2900 3500 4200 4600 2700 2900 3100 7000 4800 1900 2800 3400 4100 5300 6100 2100 1700 2500 2800 3300 4500

1.04 1.06 1.03 1.13 1.10 1.08 1.05 1.08 1.09 1.07 1.04 1.08 1.08 1.16 1.03 1.06 1.10 1.03 1.06 1.08 1.13 1.10 1.03 1.07 1.08

0.69 0.69 0.63 0.64 0.70 0.70 0.70 0.69 0.66 0.71 0.82 0.87 0.81 0.74 0.79 0.82 0.82 0.84 0.86 0.72 0.78 0.78 0.80 0.81 0.81

Reactions were performed in 10 mL of THF, [cat.]0 = 1.0 mM, at −60 °C. bDetermined by 1H NMR spectroscopy. cCalculated from the molecular weight of rac-LA × [LA]0/[BnOH]0 × conversion yield + MBnOH. dMeasured by GPC in THF calibrated with standard polystyrene samples and corrected using the Mark−Houwink factor of 0.58.14 eDetermined by analysis of all of the tetrad signals in the methine region of the homonucleardecoupled 1H NMR spectrum. f0 °C. g−30 °C. hIn 10 mL of CH2Cl2.

a

of 18-crown-6 on the K atom in complex 1. It is note that there are two slightly different isomers in one asymmetric unit with different Cipso−O−Na (Cipso is the ipso-carbon of C1 or C61) angles of 150.95(13)° and 154.58(14)° for C1−O1−Na1 and C61−O8−Na2, respectively for the solid structure of complex 2. DFT calculations at the B3LYP 6-31G* level showed that the isomer containing Na2 is more stable than the other one, with a decreased energy value of 1.09 kcal/mol. This energy difference is rather small, which indicates that the angle of Cipso−O−Na varies smoothly in solution. Because 15-crown-5 coordinates to the Na atom, 15-crown-5 should also move quickly and should be synchronized with the vibration of Cipso−O−Na. The oscillation of 15-crown-5 may potentially affect the catalytic behavior of complex 2. ROP of rac-Lactide Catalyzed by Complexes 1 and 2. Complexes 1 and 2 were systematically tested for the ROP of rac-LA. For complex 1, iso-selectivities of Pm = 0.81, 0.87, and 0.82 were achieved when the ratios of [rac-lactide]0:[1]0: [BnOH]0 were 200:1:1, 400:1:1, and 200:1:5, respectively, in THF at −60 °C (Table 1, entries 11−13). The enhancement of iso-selectivity may be attributed to the increased interaction between the lactide and the active end of the polylactide in a rather crowded environment for the coordination of 18-crown6. The catalysis behavior of complex 1 differs from that of KL:

Figure 2. Molecular structure of complex 1. Selected bond lengths (Å) and angles (deg): K1−O1 2.934(3), K1−O8 2.747(3), K1−O3 2.828(4), K1−O4 3.043(4), K1−O5 3.053(3), K1−O6 3.006(3), K1− O7 2.767(3), and C1−O1−K1 144.7(3).

and the benzene ring of phenoxide is slightly twisted, as shown by the curve of O1−C1−C4−C11 (Figure S19). The molecular structure of complex 2 (Figure 3) is similar to that of complex 1, except that the cap of 15-crown-5 on Na1 is smaller than that C

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Figure 3. Molecular structures of the two isomers of complex 2. Selected bond lengths (Å) and angles (deg): Na1−O1 2.2768(16), Na1−O3 2.5754(19), Na1−O4 2.6349(19), Na1−O5 2.468(2), Na1−O6 2.516(2), Na1−O7 2.482(2), Na2−O8 2.2698(17), Na2−O10 2.479(2), Na2−O11 2.648(2), Na2−O12 2.446(2), Na2−O13 2.528(2), Na2−O14 2.519(2), C1−O1−Na1 150.95(13), and C61−O8−Na2 154.58(14).

Figure 4. Methine region of the (a) 1H NMR spectrum and (b) homonuclear decoupled 1H NMR spectrum of the PLA generated using complex 1 (Table 1, entry 19).

when the amount of BnOH relative to complex 1 is only one equivalent (as shown in entries 11−12), the molecular weights of the final polymers are smaller than expected, which may result from a cyclization side reaction. Normally, the use of alcohol as a co-initiator can improve control of polymerization in sodium/potassium phenolate systems.5d,11a,b However, the coordination of BnOH with the K/Na atom in complexes 1 and 2 is slightly difficult because of the crowded circumstances around the K/Na atom. To overcome this problem, the amount of BnOH was increased to five or ten equivalents of the catalyst; fortunately, this side reaction can be suppressed and decreases to an acceptable level (Table 1, entries 14−25, Figures S20 and S21). The high iso-selectivities are also maintained in the presence of ten equivalents of BnOH (Pm = 0.79−0.86, entries 14−19). The 1H NMR and decoupled 1H NMR spectra of the methine region of a typical sample derived from entry 19 in Table 1 are shown in Figures 4 and S22 (Pm = 0.86), and they corroborate the high isotacticity of the poly(racLA).15 In addition, this polymer had a high Tm of 182 °C (Figure 5), which is significantly higher than that of the optically pure poly(L-LA) and agrees well with the highly isotactic polymer structure.16 For complex 2, similar isoselectivities can be achieved for ROP of rac-lactide (Table 1, entries 20−25). It is to note that because the solubility of rac-

Figure 5. DSC thermograms of the second heating cycles of the poly(rac-LA) from entry 19 of Table 1 (A) and the PLA sample prepared from poly(L-LA) (B).

lactide is low in THF at low temperature and a high [BnOH]: [Cat.] ratio is necessary for a controllable polymerization D

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Figure 6. MALDI−TOF spectrum of poly(rac-LA) prepared by the ROP of rac-LA (Table 1, entry 19).

reaction, the molecular weight of the final polymer cannot reach a high value in this system. It is interesting to find that the polymerization reaction was faster at −60 °C than that at −30 °C for complexes 1 and 2 (entries 14 and 15; entries 20 and 23). To ensure that this result was accurate, we conducted similar experiments more than ten times. In fact, the rate of polymerization decreased sharply at room temperature, and very few polymers were obtained under the same conditions (except temperature); but this phenomena was not discovered in KL and NaL catalytic systems. A possible explanation for this result is that the crown on potassium, as the cap of the half-cavity, can sway quickly, which can be inferred from the crystal structure of complex 2. We speculate that the rapid oscillations of the crown will clog the entrance to the half-cavity and inhibit the transport of lactide and BnOH to the potassium center. The oscillation of the crown may become slower at lower temperatures, thereby resulting in accelerated polymerization. The oscillation of the crown is a reaction that is competitive with the polymerization of lactide. The two normal reactions combine to make the polymerization reaction similar to an anti-Arrhenius reaction.17 But the real mechanism may be more complex than a simple competitive rate of oscillation of the crown ether, much temperature-depended dynamic equilibria and other factors may also have effects on this reaction. Therefore, the real mechanism for this anti-Arrhenius-like reaction is not very clear at this stage; more definitive data will be necessary to support our speculation. End-group analysis by 1H NMR spectroscopy revealed that the polymer chains were end-capped by one benzyl ester and one hydroxyl group (Figure S23). The MALDI−TOF mass spectra of the final polymers (Figures 6 and S24) confirmed it further by the series of peaks at 144.04m + 108 + 23, which were assigned to m(C6H8O4) + BnOH + Na+. Therefore, the polymerization potentially proceeds via a monomer-activated mechanism as KL, NaL, and most other alkali-metal phenoxides.11 Some intermolecular transesterification may occur during this process, especially for the catalytic system

of complex 2, as a cluster of weak peaks with a difference in molecular mass of ∼72 Da were observed in the MALDI−TOF mass spectra. However, this process is not problematic because of the controllable molecular weights and narrow PDIs of the final polymers.

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CONCLUSIONS In summary, highly iso-selective and highly active sodium and potassium monophenolate catalysts for the ROP of rac-lactide are reported in this study; these catalysts can be used to obtain an isotactic PLA with a Pm as high as 0.86, which is the highest recorded value for an alkali-metal-based system and is comparable to those observed for zinc-, indium-, and lanthanide-based systems. Because sodium and potassium are innocuous, abundant elements in the human body and are suitable for the catalytic synthesis of medical polylactide, it is valuable to improve the stereocontrollability of this system by adjusting the phenol ligands and to understand the origin of iso-selectivity and reaction mechanisms. Further efforts in these directions are underway in our laboratory.

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EXPERIMENTAL SECTION

General Considerations. All syntheses and manipulations of airand moisture-sensitive materials were performed under a dry nitrogen atmosphere or in a glovebox using standard Schlenk techniques. The 1 H NMR and 13C NMR spectra were recorded on a JEOL ECS-400 MHz spectrometer. The 1H NMR chemical shifts are reported in ppm with the residual protons in the deuterated solvent (δ 7.26 ppm of CDCl3) as the reference. The 13C NMR chemical shifts are reported in ppm with the residual 13C in the solvent (δ 77.0 ppm of CDCl3) as the reference. The elemental compositions of the complexes were measured using an Elemental Vario EL series CHN analyzer with the samples under a nitrogen atmosphere. The molecular weights (Mn and Mw) and the molecular mass distributions (Mw/Mn) of the polymer samples were measured by gel permeation chromatography (GPC) at 25 °C using THF as the solvent, an eluent flow rate of 1 mL/min, and narrow polystyrene standards as reference samples. The measurements were performed using a Waters 1525 binary system that was equipped with a Waters 2414 RI detector using two Styragel columns (102−106 kg/mol). Each reported value is the average of two E

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removed under vacuum, and the faint-yellow solid residue was washed with dry hexane. Yield: 0.87 g (88%). A single crystal suitable for structural characterization was obtained from slow cooling of a warm toluene solution. Anal. Calcd for C61H89KO8: C 74.05, H 9.07. Found: C 73.78, H 9.42. 1H NMR (400 MHz, CDCl3): δ7.54 (d, J = 2.6 Hz, Ar−H, 0.18H), 7.37 (d, J = 2.4 Hz, Ar−H, 1.25H), 7.30−7.28 (m, Ar− H, 1.82H), 7.23 (d, J = 2.6 Hz, Ar−H, 0.15H), 7.16−7.04 (m, Ar−H, 3.34H), 6.89 (d, J = 2.4 Hz, Ar−H, 0.49H), 6.87 (d, J = 2.4 Hz, Ar−H, 0.87H), 6.77 (d, J = 3.2 Hz, 0.46H), 6.53 (m, Ar−H, 0.93H), 6.44 (m, Ar−H, 0.91H), 6.30 (d, J = 2.2 Hz, Ar−H, 0.12H), 6.25 (d, J = 2.2 Hz, Ar−H, 0.45H), 6.16 (d, J = 2.6 Hz, Ar−H, 0.13H), 6.02 (d, J = 3.2 Hz, Ar−H, 0.45H), 5.86 (d, J = 2.0 Hz, Ar−H, 0.11H), 3.67 (s,−CH2− 24H), 1.58 (s, tBu−H, 1.2H), 1.49 (s, tBu−H, 9.16H), 1.47 (m, tBu− H, 7.98H), 1.34 (s, 3.92H), 1.22 (m, tBu−H, 5.44H), 1.20 (s, tBu−H, 7.72H), 1.18 (s, tBu−H, 8.47H), 1.10 (s, tBu−H, 4.14H), 1.07 (s, t Bu−H, 4.02H), 1.05 (s, tBu−H, 1.1H), 0.95 (s, tBu−H, 0.85H).13C NMR (100 MHz, CDCl3) δ 150.1, 150.0, 150.0, 149.1, 149.0, 146.6, 146.4, 146.0, 144.3, 143.2, 142.9, 140.6, 137.8, 136.6, 136.2, 135.9, 135.8, 129.4, 128.3, 127.3, 127.2, 126.5, 125.7, 123.5, 123.2, 122.6, 77.3, 77.0, 76.7, 70.0, 54.6, 53.5, 52.6, 39.8, 35.1, 34.8, 34.1, 34.1, 31.4, 31.4, 31.3, 31.2, 31.1, 29.6, 29.1, 28.1. This spectrum was explained in the Supporting Information. Synthesis of Complex 2. A solution of HL (0.69 g, 1.00 mmol) with 1.1 mmol of NaN(SiMe3)2 (1.00 mol/L in hexane) and 5.5 mmol of 15-crown-5 was stirred in toluene (50.0 mL) at room temperature under an N2 atmosphere for 12 h. The solvent was removed under vacuum, and the faint-yellow solid residue was washed with dry hexane. Yield: 0.79 g (85%). A single crystal suitable for structural characterization was obtained from slow cooling of a warm toluene solution. Anal. Calcd for C59H85NaO7: C 76.25, H 9.22. Found: C 76.05, H 9.47. 1H NMR (400 MHz, CDCl3) δ7.54 (d, J = 2.6 Hz, Ar− H, 0.14H), 7.37 (d, J = 2.4 Hz, Ar−H, 0.42H), 7.33−7.27 (m, Ar−H, 1.59H), 7.26 (d, J = 2.6 Hz, Ar−H, 0.22H), 7.14−7.04 (m, Ar−H, 3.52H), 6.89 (d, J = 2.4 Hz, Ar−H, 0.78H), 6.87 (d, J = 2.4 Hz, Ar−H, 0.23H), 6.78 (d, J = 3.2 Hz, Ar−H, 0.82H), 6.52 (m, Ar−H, 1.52H), 6.46−6.42 (m, Ar−H, 0.23H), 6.30 (d, J = 2.2 Hz, Ar−H, 0.10H), 6.25 (d, J = 2.2 Hz, Ar−H, 0.10H), 6.16 (d, J = 2.6 Hz, Ar−H, 0.11H), 6.01 (d, J = 3.2 Hz, Ar−H, 0.74H), 5.86 (d, J = 2.0 Hz, Ar−H, 0.10H), 3.70 (s,−CH2− 20H), 1.58 (s, tBu−H, 0.99H), 1.49 (d, tBu−H, 3.13H), 1.47−1.45 (m, tBu−H, 13.67H), 1.34 (s, tBu−H, 1.06H), 1.22 (s, t Bu−H, 8.28H), 1.20 (s, tBu−H, 13.54H), 1.18 (s, tBu−H, 2.40H), 1.10 (s, tBu−H, 1.22H), 1.07 (s, tBu−H, 6.69H), 1.06 (s, tBu−H, 1.37H), 0.95 (s, tBu−H, 1.02H). 13C NMR (100 MHz, CDCl3) δ 150.2, 150.1, 149.2, 149.0, 147.6, 146.5, 146.1, 144.4, 143.3, 143.0, 140.7, 137.9, 136.7, 136.1, 135.9, 135.0, 130.6, 129.5, 129.0, 128.8, 128.7, 128.4, 128.2, 128.1, 127.9, 127.6, 127.4, 127.3, 126.6, 125.9, 125.8, 125.3, 123.6, 123.6, 123.3, 123.3, 122.7, 122.2, 77.3, 77.0, 76.7, 70.6, 63.2, 54.7, 53.6, 52.7, 41.7, 39.8, 35.4, 35.2, 35.2, 35.2, 34.9, 34.3, 34.2, 31.7, 31.5, 31.5, 31.4, 31.3, 31.3, 31.2, 31.18, 29.7, 29.2, 28.6, 28.2, 22.6, 14.1. This spectrum was explained in the Supporting Information. General Procedure for Polymerization of rac-lactide. A typical polymerization procedure is illustrated by the synthesis of PLA([LA]0/[cat.]0/[BnOH]0 = 400:1:10; Table 1, entry 19). A solution of 1 (0.0099 g, 0.010 mmol) and BnOH (1.00 mL, 0.10 M in toluene) in THF (10 mL) was added to a solution of rac-lactide (0.58 g) in THF under rapid stirring at −60 °C. The reaction mixture was stirred for 5 min and then quenched by the addition of water (0.5 mL). The polymer was precipitated by adding hexane (20 mL). A white crystalline solid was obtained by recrystallization from a CH2Cl2/ hexane mixed solvent and dried under vacuum. Crystallographic Studies. The data were collected using a SuperNova (Dual) X-ray diffractometer equipped with a graphitemonochromated Cu/Mo Kα radiation source (λ = 1.54184/0.71073 Å). The structures were solved by direct methods using the Siemens SHELXTL PLUS program.18 Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. All hydrogen atoms were placed by geometrical considerations and were added to the structure-factor calculations. The crystal data and refinement results are summarized in Table S1.

independent measurements and was corrected using a factor of 0.58 for polylactide according to the literature. Calorimetric measurements were conducted using a Sapphire DSC apparatus manufactured by PerkinElmer Instruments. Polymer samples (5 to 7 mg) were placed in aluminum pans and heated/cooled at a rate of 10 °C min−1 under a nitrogen atmosphere. The previous thermal history of the samples was erased by heating them to 250 °C before cooling them to −20 °C. Measurements were performed between 50 and 230 °C. The melting temperatures were evaluated as the maxima of the melting endotherms, and the heats of fusion were calculated from the peak areas. The MALDI−TOF mass spectroscopic data were obtained using α-cyano-4-hydroxycinnamic acid as the matrix in a Bruker Daltonics, Inc., BIFLEX III MALDI−TOF mass spectrometer. Materials. Toluene, THF, and hexane were dried by refluxing with sodium benzophenone ketyl. CH2Cl2 was distilled from P2O5; rac-LA was purchased from Daigang BIO Engineer Ltd. of China and was recrystallized from toluene three times. CDCl3 was purchased from J&K Scientific, Ltd., in Beijing and was dried over activated molecular sieves. KN(SiMe3)2 and NaN(SiMe3)2 were purchased from J&K Scientific, Ltd., and were used as received. Potassium, sodium, 18crown-6, and 15-crown-5 were purchased from local companies and were used as received. Synthesis of 9-(3,5-Di-tert-butyl-2-hydroxyphenyl)-2,4,5,7tetra-tert-butyl-9-phenylxanthene (HL). A melt mixture of 2,4di-tert-butylphenol (0.19 mol, 40.00 g) and concentrated HCl (5.00 mL) was stirred for 1 h. Benzaldehyde (0.045 mol, 5.00 mL) was subsequently added, and the reaction was stirred for an additional 60 h at 190 °C. After the residue was washed with ethanol or methanol, a pure white powder was obtained. Yield: 4.20 g (17%). Anal. Calcd for C49H66O2: C, 85.66; H, 9.68. Found: C, 85.40; H, 9.65. 1H NMR (400 MHz, CDCl3): δ 7.38 (d, J = 2.4 Hz, Ar−H, 2H), 7.30 (d, J = 2.2 Hz, Ar−H, 1H), 7.08 (m, Ar−H, 3H), 6.87 (d, J = 2.4 Hz, Ar−H, 2H), 6.47−6.41 (dd, J = 7.8, J = 2.0 Hz, Ar−H, 2H), 6.26 (d, J = 2.2 Hz, Ar−H, 1H), 5.04 (s, O−H, 1H), 1.49 (s, tBu−H, 18H), 1.34 (s, tBu− H, 9H), 1.18 (s, tBu−H, 18H), 1.10 (s, tBu−H, 9H). 13C NMR (100 MHz, CDCl3): δ 150.4, 150.2, 146.9, 144.5, 140.8, 136.8, 136.5, 129.7, 128.6, 127.6, 127.4, 126.8, 125.9, 123.8, 123.4, 122.9, 52.9, 35.4, 35.1, 34.4, 34.3, 31.5, 31.4, 29.9. Synthesis of Complex KL. A solution of HL (0.69 g, 1.00 mmol) and KN(SiMe3)2 (1.10 mmol, 1.00 mol/L in hexane) was stirred in toluene (50.0 mL) at 90 °C under an N2 atmosphere for 12 h, resulting in a pale-yellow solution. The solvent was removed under vacuum, and the yellow solid residue was washed with dry hexane. Yield: 0.65 g (90%). Anal. Calcd for C49H65KO2: C, 81.16; H, 9.03. Found: C 81.06, H 9.41. 1H NMR (400 MHz, CDCl3): δ 7.37 (d, J = 2.6 Hz, Ar−H, 2H), 7.30 (d, J = 2.4 Hz, Ar−H, 1H), 7.11−7.05 (m, Ar−H, 3H), 6.87 (d, J = 2.6 Hz, Ar−H, 2H), 6.44 (dd, J = 7.8, J = 2.0 Hz, Ar−H, 2H), 6.25 (d, J = 2.4 Hz, Ar−H, 1H), 1.49 (s, tBu−H, 18H), 1.34 (s, tBu−H, 9H), 1.18 (s, tBu−H, 18H), 1.10 (s, tBu−H, 9H). 13C NMR (100 MHz, CDCl3): δ 150.4, 150.2, 146.9, 144.5, 140.8, 136.8, 136.5, 129.7, 128.6, 127.6, 127.4, 126.8, 126.0, 123.8, 123.4, 122.9, 52.9, 35.4, 35.1, 34.4, 34.3, 31.5, 31.4, 29.9. Synthesis of Complex NaL. A solution of HL (0.69 g, 1.00 mmol) and 1.1 mmol of NaN(SiMe3)2 (1.00 mol/L in hexane) was stirred in toluene (50.0 mL) at room temperature under an N2 atmosphere for 12 h. The solvent was removed under vacuum, and the faint yellow solid residue was washed with dry hexane. Yield: 0.66 g (93%). Anal. Calcd for C49H65NaO2: C 83.00; H, 9.24. Found: C 82.94, H 9.47. 1H NMR (400 MHz, CDCl3) δ 7.37 (d, J = 2.4 Hz, Ar− H, 2H), 7.30 (d, J = 2.4 Hz, Ar−H, 1H), 7.12−7.06 (m, Ar−H, 3H), 6.87 (d, J = 2.4 Hz, Ar−H, 2H), 6.44 (dd, J = 7.8, J = 2.0 Hz, Ar−H, 2H), 6.25 (d, J = 2.4 Hz, Ar−H, 1H), 1.49 (s, tBu−H, 18H), 1.34 (s, t Bu−H, 9H), 1.18 (s, tBu−H, 18H), 1.10 (s, tBu−H, 9H). 13C NMR (100 MHz, CDCl3): δ 150.2, 150.1, 146.8, 144.4, 140.7, 136.7, 136.3, 129.5, 128.5, 127.5, 127.3, 126.6, 125.8, 123.7, 123.3, 122.8, 52.7, 35.3, 35.0, 34.3, 34.2, 31.3, 31.2, 29.7. Synthesis of Complex 1. A solution of HL (0.69 g, 1.00 mmol), 1.1 mmol of KN(SiMe3)2 (1.00 mol/L in hexane), and 5.5 mmol of 18-crown-6 was stirred in toluene (50.0 mL) at room temperature under an N2 atmosphere for 12 h. The solvent was subsequently F

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra of the complexes, the polymerization studies (Figures S1−S24), and CIF files that provide the crystallographic data for HL, [KL·BnOH], complex 1, and complex 2 with CCDC reference numbers of 1017620, 1017621, 1017622, and 1017623, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.W.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (No. 21271092, 21071069, and 21171078), the Science Foundation of Gansu Province of China (1308RJ2A121), the project for the National Basic Science Personnel Training Fund (J1103307), and the ScGrid of the Supercomputing Center of the Chinese Academy of Sciences are gratefully acknowledged.

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ABBREVIATIONS ROP, ring-opening polymerization; PDI, polydispersity; PLA, polylactide; DOSY, diffusion-ordered spectroscopy; NOE, nuclear Overhauser effect; BnOH, benzyl alcohol

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REFERENCES

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dx.doi.org/10.1021/ma502000y | Macromolecules XXXX, XXX, XXX−XXX