Suppressing Cyclic Polymerization for Isoselective Synthesis of High

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Suppressing Cyclic Polymerization for Isoselective Synthesis of HighMolecular-Weight Linear Polylactide Catalyzed by Sodium/Potassium Sulfonamidate Complexes Changjuan Chen,†,‡ Yaqin Cui,† Xiaoyang Mao,† Xiaobo Pan,† and Jincai Wu*,† †

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou University, Lanzhou 730000, People’s Republic of China ‡ College of Chemistry and Pharmaceutical Engineering, Huanghuai University, Zhumadian 463000, People’s Republic of China S Supporting Information *

ABSTRACT: A new sodium/potassium crown ether complex system with a series of bichelating sulfonamides as ligands was developed for the ring-opening polymerization (ROP) of rac-lactide. In this system, the side reaction of cyclic polymerization can be suppressed very well because of very different ROP rates initiated by BnOH and sulfonamide anion. The synthesis of high molecular weight linear polylactide with molecular weight high up to 107 kg/mol was successful. The best isoselectivity also can reach to a high value of Pm = 0.84. The NMR analysis of the reaction mixture of rac-lactide and complex 3 together with kinetic studies suggests the mechanism of ROP in the absence of alcohol is a coordination−insertion mechanism. After addition of BnOH, the ROP rate can increase remarkably due to the cooperation interaction of alcohol and complex 3.

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INTRODUCTION Coordination polymerization reactions have been applied widely in the synthesis of many kinds of polymers due to the good controllability on molecular weights, dispersity, stereoregularity, and other properties of polymers compared to the same polymers prepared by other techniques such as free radical polymerization.1−10 Obviously catalysts play a key role, among which many coordination metal complexes have been widely used in many famous polymerization reactions because their activities and stereoselectivities can be easily adjusted via tuning ligands.11−13 To our interest, alkali metal complexes are rarely utilized to catalyze stereocontrolled polymerization reactions,14−16 which is an important aspect of polymerization reactions because different tactic polymers will own different chemical and physical characters.4,17−21 The most possible reason is that the monomer and polymer chain cannot coordinate strongly on alkali metal ions in the coordination polymerization progress because of the weak Lewis acidity of alkali metal ions; as a result, the important interactions for a high stereoselectivity between monomer and polymer chain or between monomer and ligand are weak. Considering that © XXXX American Chemical Society

sodium and potassium ions are nontoxic for the human body, it is valuable to discover some excellent sodium or potassium catalysts for the synthesis of polymers, especially for medical applications.3,22−27 Recently, we synthesized several sodium and potassium monophenolate complexes with an active center embedded in a confined space constructed by crown ether and bulky group of phenol ligand (Chart 1).28−32 When both monomer and polymer chain approach the active center, the confined space surrounding a metal ion can enforce the interactions between monomer, polymer chain, and ligand; the weaker Lewis acid of alkali metal ions will not be a shortcoming. With this design, we succeeded in the isoselective synthesis of biodegradable and biocompatible polylactide using potassium/sodium phenolate crown ether complexes as catalysts via the ring-opening polymerization (ROP) of raclactide; the isoselectivity even can reach to Pm = 0.94 at −70 °C.28 Received: October 19, 2016 Revised: November 23, 2016

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DOI: 10.1021/acs.macromol.6b02271 Macromolecules XXXX, XXX, XXX−XXX

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polymerization of rac-lactide initiated directly by sulfonamide group will become slow. Importantly, the decreased ROP rate allows us to study and understand this system in details. With this aim in mind, complexes 1−7 with different electronic and bulky effects were synthesized in high yields (Scheme 1).

Chart 1. Alkali Metal Crown Ether Complexes

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RESULTS AND DISCUSSION Structures of Complexes 1−7. As shown in Figure 1, sodium and potassium crown ether ions are coordinated with one nitrogen atom and one oxygen atom of sulfonamide anion (NO chelating model) in complexes of 1 and 4−7. But in complex 2, potassium crown ether cation is coordinated with two oxygen atoms of sulfonamide anion (OO chelating model). The most possible reason is that the four bulky isopropyl groups in complex 2 lead the NO chelating model complex to be unstable. DFT calculations at the B3LYP 6-311+G(d) level showed the OO chelating model isomer of complex 2 is slightly more stable than the NO chelating model isomer, with a decreased energy of 0.56 kcal/mol. The calculation results of sodium complex 1 with a same ligand show that the NO chelating isomer is more stable than OO chelating isomer with a decreased energy of 3.54 kcal/mol because of small hindrance of 15-crown-5 and the slightly higher Lewis acidity of sodium ion. Varied temperature 1H NMR in deuterated toluene was performed in order to find a possibility of exchange between two chelating models, but complexes 1 and 2 do not split into two sets signals even at −60 °C possibly due to very small energy differences between these two type isomers (Figure S1). In spite of this, the small energy differences may hint the exchange between two chelating models is possible especially when the nitrogen atom of sulfonamide anion interacts with other substrates (for example, alcohol via a hydrogen bond). Anyway, compared to sodium/potassium complexes with phenoxy as mono-chelating ligand reported by us (Chart 1), the ligands in these sulfonamidate complexes are bis-chelating and potentially can keep this bis-chelating model when the nitrogen atom of sulfonamide group reacts with alcohol or attacks the carbonyl group of lactide because another oxygen atom of sulfonamide group can replace this nitrogen atom to coordinate to metal ion. ROP of rac-Lactide Catalyzed by Complexes 1−7. All of complexes 1−7 are efficient catalysts for the ROP of raclactide as shown in Table 1. The polymerization of rac-lactide in the presence of alcohol catalyzed by complex 3 was accomplished in 91% conversion within 30 min with a 100:1:1 ratio of [rac-LA]0:[3]0:[BnOH]0 in toluene (Table 1, entry 3, [Cat.]0 = 2.0 mM, room temperature). However, the same reaction consumed 5 h in THF to reach a 31% conversion (Table 1, entry 1), and only 15% monomer can be converted to polymer in CH2Cl2 (Table 1, entry 2). The different reaction rates can be attributed to the competitive coordination of THF and the different solvent polarities which was also found in our previous works.28,30 Therefore, toluene is chosen as an optimized solvent. Compared to previous phenolate sodium and potassium crown ether complexes (Chart 1), the activity of complex 3 is moderate for the ROP of rac-lactide at room temperature. The reason may be the basicity of sulfonamide anion is weaker than phenoxy because the negative charge of nitrogen can distribute to both sulfonyl group and aromatic group. At room temperature, the molecular weight distribution (Đ = 1.32, Table 1, entry 3) of polymer is somewhat broad and the isoselectivity is not very high (Pm = 0.65). When the temperature decreases to 0 °C, a high Pm value of 0.76 and a

Although this type of alkali metal complex seems to be a good catalyst for the ROP of rac-lactide as other excellent isoselective metal complex systems of aluminum,33−50 indium,51−60 zinc,61−75 lanthanum,76−94 and other complexes,95−97 there are some problems which have to be overcome within these phenolate alkali metal complex systems. One important thing is that the phenoxy group as an auxiliary ligand of an alkali metal complex itself is also highly active to initiate an uncontrollable ROP of lactide giving cyclic polylactide via a coordination−insertion mechanism, which is a serious competing side reaction to synthesize desirable linear polymers because this reaction rate is similar to the ROP initiated by alcohol.14,30,32,98 Sometimes this side reaction is not serious upon addition of 1 equiv of alcohol, but in some cases excess equivalents of alcohol toward alkali metal complex is needed to suppress this side reaction which will give rise to a big obstacle to obtain high molecular weight polymers.28,30−32 The molecular weight directly influences the mechanical and sorptive properties of polylactide which sometimes are important for its final use.99,100 For example, high molecular weight polylactide is necessary for application in bone plates or temporary internal fixation of broken or damaged bones.101 To our knowledge, no alkali metal complexes have been reported to synthesize polylactide with molecular weight >100 kg/mol successfully. With an attempt to know the alcohol effect in these alkali metal complex systems and to suppress this side reaction of cyclic polymerization further for improving the highly isoselective alkali metal catalytic system, here we use bichelating sulfonamide to replace monochelating phenol as ligands (Scheme 1). The bichelating effect of sulfonamide ligand can make the dissociation of ligand from sodium/ potassium ion be more difficult and lead to the nucleophilicity of sulfonamide anion to be low;102,103 thus, the rate of cyclic Scheme 1. Synthesis of Complexes 1−7

B


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Figure 1. Molecular structures of 1−2 and 4−7 with probability ellipsoids at 30% (all of the hydrogen atoms are omitted for clarity).

narrow polydispersity Đ = 1.12 can be achieved (Table 1, entry 4). The isoselectively can further be improved up to Pm = 0.84 when the temperature is −30 °C (Table 1, entry 11, Figure 2, and Figure S2). In comparison, sodium complexes 1 and 3 with more bulky substituted groups afford better isoselectivities than the analogous sodium complexes 5−7 (Table 1, entries 4−10). The isoselectivities of potassium complexes 2 and 4 are also lower than that of sodium complexes 1 and 3, respectively, which indicates the isoselectivity of this system is sensitive to the surrounding of the active center constructed by crown ether and substituted groups of sulfonamide ligand. No epimerization was found in the polymerization of L-lactide catalyzed by complex 3 (Figures S3 and S4). To our pleasure, the molecular weights of polymers agree well with calculated values and increase linearly with the ratios of [rac-LA]0 to [BnOH]0 (Table 1, entries 4 and 12−17, and Figure S5), and the molecular weight distributions also are narrow at 0 °C. Secondfeed experiments also confirmed the molecular weights of polymers are under control (Table 1, entry 21). Compared to

sodium complexes 1 and 3, potassium complexes 2 and 4 are slightly less active (Table 1, entries 4−7), which suggests the activation of monomer is important in this system because sodium ion owns stronger Lewis acidity than potassium ion. It is interesting that sodium complexes 5−7 show very different activities for the ROP of rac-lactide; the order of activities is 5 > 6 > 7 (Table 1, entries 8−10). Because these complexes have similar bulky hindrances around the active center but different electronic effect groups at a remote position, the different activities hint that the electron-donating group can accelerate the ROP reaction because the electron-donating group can increase the basicity of nitrogen atom of sulfonamide anion. The two activity tendencies of these complexes with different metal ions and different electronic effect groups demonstrate both activations of monomer and alcohol are important in which the monomer is activated by metal ion and alcohol is activated by sulfonamide anion group. Because complexes 1−7 can be considered as ion-paired complexes, the electronic effect of a substituted group can adjust the basicity of anion ligand C


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Macromolecules Table 1. rac-Lactide Polymerization Catalyzed by 1−7a

entry

Cat.

[Cat.]0/[rac-LA]0/[BnOH]0

t (h)

convb (%)

Mn,obsdc (g/mol)

Mn,calcdd (g/mol)

Đ

P me

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

3 3 3 3 1 2 4 5 6 7 3 3 3 3 3 3 3 3 3 3 1

1/100/1 1/100/1 1/100/1 1/100/1 1/100/1 1/100/1 1/100/1 1/100/1 1/100/1 1/100/1 1/100/1 1/25/1 1/200/1 1/300/1 1/400/1 1/500/1 1/1000/1 500/1/5 1/25/0 1/100/0 1/50(50)/1

5 5 0.5 1 24 56 6 5 10 56 24 0.5 2 5 9 11 36 3 8 12 1(1)

31 15 91 94 89 41 90 90 91 20 87 97 80 80 97 94 87 90 90 97 92

3500 2800 12800 13800 11300 3100 11600 11200 12300 2400 12700 3500 24100 36300 56400 67700 107000 12200 26600 30400 13100

4600 2300 13200 13600 12900 6000 13100 13100 13200 3000 12600 3600 23100 34700 56000 67800 125400 13100 3200 14000 13400

1.38 1.43 1.32 1.12 1.09 1.06 1.05 1.07 1.08 1.05 1.04 1.07 1.13 1.19 1.26 1.28 1.37 1.07 1.36 1.41 1.06

0.60 0.60 0.65 0.76 0.75 0.69 0.69 0.73 0.71 0.63 0.84 0.78 0.76 0.77 0.78 0.81 0.78 0.76 0.63 0.63 0.75

Conditions: reactions were performed in 5 mL of toluene, 0.01 mmol of catalyst, at 0 °C. bDetermined by 1H NMR spectroscopy. cExperimental Mn and Đ determined by GPC in THF against polystyrene standards and corrected using the factor 0.58.104−106 dCalculated from the molecular weight of rac-LA × [LA]0/[BnOH]0 × conversion + MBnOH. eDetermined by analysis of all of the tetrad signals in the methine region of the homonuclear-decoupled 1H NMR spectrum. fIn 5 mL of THF, at room temperature. gIn 5 mL of CH2Cl2, at room temperature. hIn 5 mL of toluene, at room temperature. kIn 5 mL of toluene, at −30 °C. a

Figure 2. Homonuclear-decoupled 1H NMR spectrum of PLA ([rac-LA]0:[3]0:[BnOH]0 = 100:1:1, Pm = 0.84, Table 1, entry 11).

0.78 at 0 °C (Table 1, entry 17). This phenomenon can change our prejudice of sodium complexes in the synthesis of high molecular weight polylactide because alkali complexes usually suffer from serious side reactions of cyclization, transesterification, and even epimerization.7,107−111 It also indicates this kind of sodium/potassium complex can be comparable to that of known aluminum, rare-earth-metal, and zinc catalysis systems. In addition, this system also can hold high isoselectivity. Kinetic Studies and Proposed Mechanism. In order to get more insight into the reason for the ability of complex 3 in the synthesis of high molecular weight polylactide, we did more experiments to verify the ROP mechanism including NMR and

more remarkably than the ability to adjust the Lewis acidity of central metal ion; the above two activity tendencies are not in conflict. In this system, the key step may be the cooperation of the two activation progresses which can further be confirmed by the following kinetic studies. What is more, complex 3 also can catalyze the immortal ROP of 500 equiv of rac-lactide in the presence of 5 equiv of external BnOH as a co-initiator, affording a controlled molecular weight and a relatively narrow polydispersity (Table 1, entry 18). It is worth to note that complex 3 even can catalyze the ROP of 1000 equiv of rac-lactide in the presence of 1 equiv of BnOH, affording desirable high molecular weight of 107 kg/mol, acceptable polydispersity of Đ = 1.37, and not bad Pm value of D


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Figure 3. 1H NMR spectrum of the mixture of rac-lactide and complex 3 in C6D6, 25 °C with different ratios: (a) 10:1, (b) 10:4, (c) 10:6, (d) 10:8, and (e) 10:10.

Figure 4. MALDI-TOF spectrum (matrix: DCTB; ionization salt: CF3CO2Na; solvent: CHCl3) of poly(rac-LA): (a) [rac-LA]0:[3]0 = 100:1, [3]0 = 2.0 mM, 18% conversion, at room temperature; (b) [rac-LA]0:[3]0 = 100:1, [3]0 = 2.0 mM, 32% conversion, at room temperature; (c) [rac-LA]0: [3]0 = 25:1, [3]0 = 2.0 mM, 90% conversion, at room temperature (Table 1, entry 19).

kinetics experiments. In previous phenolate sodium/potassium complex system, both rates of the ROP progress in the absence and in the presence of alcohol are fast at a similar level.14,28,32 The ROP progress in the absence of alcohol proceeds via a coordination−insertion mechanism and usually gives cyclic polymer, which gives rise to a difficulty of the controllability of the ROP of lactide in the presence of alcohol. While it is gratifying that in this sodium/potassium sulfonamidate complex system the ROP of rac-lactide proceeds slowly in the absence of alcohol with a 100:1 ratio of [rac-LA]0:[3]0 in toluene, the polymerization consumes about 12 h to reach a 97% conversion at room temperature (Table 1, entry 20), but the same reaction in the presence of BnOH just needs 1 h at 0 °C (Table 1, entry

4). The molecular weights are not expected, and the polydispersities are high in the absence of alcohol (Table 1, entries 19 and 20). When rac-lactide and complex 3 with different ratios of 10:1, 10:4, 10:6, 10:8, and 10:10 were mixed in deuterated benzene for about 15 min, the peak of methine of lactide shifts from 3.82 to 4.21 ppm, which may indicate this chemical shift is an equilibrium position between coordinated lactide and free lactide (Figure 3). The VT 1H NMR of the mixture of rac-lactide and complex 3 with a ratio of 1:1 did not succeed to show a split of this peak, but most 1H NMR signals of lactide and ligand shift when temperature decreased from room temperature to −60 °C (Figure S6), which can be attributed to the different dissociation constants of adduct of E


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Figure 5. (a) Semilogarithmic plots of the rac-lactide versus time in toluene at 25 °C with catalyst 3 as an initiator. [3]0 = 1.0, 1.5, 2.0, and 2.5 mM; [rac-LA]0 = 0.1 M. (b) Plots of ln kobs versus ln[3]0 for the polymerization of rac-lactide with catalyst 3 as an initiator in toluene at 25 °C. [rac-LA]0 = 0.1 M.

Scheme 2. Proposed Mechanism for the ROP of rac-Lactide Catalyzed by Complex 3 in the Absence of Alcohol

series peaks of cyclic ester also exist (Figures 4a and 4b). At lower conversions, more linear polymer chains can be found in the isolated products. The end groups of linear polymers are hydroxyl and carboxylic acid, which may result from the hydrolysis of short polymer chain with an amide end. The component of cyclic ester seems to increase when the conversion becomes high (Figures 4a, 4b, and 4c). Thus, the cyclic polymer can happen in the whole ROP progress and does not just result from overrun polymerizations. The sulfonamide group may keep coordinating to sodium ion in the ROP progress, which can lead the cyclic polymerization to happen easily. The linear polymer chains can be looked as precursors of cyclic polymers; thus, the components of cyclic polymers increase at high conversions. Kinetic studies of polymerization of rac-lactide in the absence of alcohol were performed to establish the reaction order with

complex 3 and lactide at different temperatures. Fortunately, a careful analysis of the MALDI-TOF spectrum of a final polymer demonstrated some polymer chains have an end group of sulfonamide (Figure 4c, Table 1, entry 19), which can be verified by a series of weak peaks at 463 + 72n + 23. In the MALDI-TOF spectrum, the main series of peaks of 72n + 23 can be assigned to n(C3H4O2) + Na+ (Figure 4c and Table 1, entry 19), which can prove the polymer mostly is cyclic polylactide. The cyclic structure of a final polymer can further be confirmed by the 1H NMR spectrum in which no obvious polymer chain end can be found (Figure S7). Because overrun polymerization processes can easily lead to significant intramolecular transesterifications to give cyclic polymers, we stopped the ROP reaction with a 100:1 ratio of [LA]0:[3]0 at 18% and 32% conversions, respectively, the MALDI-TOF spectra of isolated polymers demonstrate that F


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Figure 6. 1H NMR spectra in benzene-d6 for (a) [rac-LA]0:[3]0 = 10:1, [3]0 = 1.0 mM, 15 min, at room temperature; (b) [rac-LA]0:[3]0:[BnOH]0 = 10:1:1, [3]0 = 1.0 mM, 15 min, at room temperature; and (c) the mole ratio of (rac-LA:3:BnOH) = 10:1:1, [3]0 = 10 mM, 15 min, at room temperature.

Scheme 3. Proposed Mechanism for the ROP of rac-Lactide Catalyzed by Complex 3 in the Presence of Alcohol

values of kobs are 0.0020, 0.0028, 0.0042, and 0.0054 min−1, respectively. The overall rate equation is shown in eq 1, and the reaction constant of k1 is 2.03 M−1 min−1 at 25 °C.

respect to lactide and complex 3. The results indicated that the reaction rate has first-order dependence on both the concentration of rac-lactide and complex 3 (Figure 5). At various concentrations of complex 3 ([3]0 = 1.0, 1.5, 2.0, and 2.5 mM) and the concentration of [rac-LA]0 fixed at 0.1 M, the

v1 = −d[LA]/dt = k1[LA]1 [3]1 G

(1) DOI: 10.1021/acs.macromol.6b02271 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Thus, a coordination−insertion mechanism is reasonable for the ROP of lactide in the absence of alcohol. As proposed in Scheme 2, monomer first coordinates to sodium ion as evidenced by the 1H NMR spectrum (Figure 3 and Figure S6). The sulfonamide end-group found in the MALDI-TOF spectrum suggests sulfonamide anion itself can attack the carbonyl group of lactide to initiate the ROP progress directly; consequently, the lactide inserts into the sodium nitrogen bond in complex 3; after that, a new polymer alkoxy was formed which can propagate to extend the polylactide chain. The sulfonamide anion can be regenerated by the backbiting reaction of alkoxy group of polymer chain to give a cyclic polylactide, and complex 3 is also regenerated. The peaks with a difference of molar mass of ∼72 Da in MALDI-TOF MS indicate an intramolecular attack by the metal−alkoxy group to any ester positions of the propagation chain also can happen. In the whole progress, the sulfonamide group may keep coordinating on sodium ion which leads the cyclic polymerization to become easy. As shown in Figure 6 and Figure S8, when rac-lactide, BnOH, and complex 3 were mixed in deuterated benzene with a ratio of 10:1:1 for about 15 min (Figure 6b), 93% monomer was converted to polymer, which confirmed further the polymerization of rac-lactide in the presence of BnOH is faster than that in the absence of alcohol (Figure 6a). In Figure 6c, the concentration of complex 3 was increased from 1.0 to 10 mM, and 10 equiv of rac-LA was added into the NMR tube to keep the mole ratio of rac-LA:3:BnOH at 10:1:1. In this situation, rac-LA cannot completely dissolve at this high concentration in deuterated benzene; the presence of large amounts of unconsumed lactide in the system allows us more easily to detect details of this reaction. The new complicated multiple signals from 4.04 to 4.30 ppm can be ascribed to the methine proton close to the hydroxyl group of alcohol, which is similar to the chemical shift of the methene protons of BnOH (4.29 ppm) and the methine proton of ethyl lactate (4.04 ppm) (Figures S9 and S10). Because partial alcohol molecules may coordinate to complex 3 or be hydrogen-bonded to complex 3, and some may be free in solution, the proton signals of different alcohol seem to be so complicated. The reaction of complex 3 and BnOH was conducted in a 1H NMR tube with deuterated benzene as a solvent (Figure S9); the methene proton of BnOH downshifts from 4.29 to 4.38 ppm. This shift value proves that there are some interactions between BnOH and complex 3. We think the interactions include a hydrogen bond interaction between alcohol and sulfonamide group and the coordination of an oxygen atom of BnOH to a sodium ion. We believe the hydrogen bond interaction is important in the key step of catalytic cycle (Scheme 3); otherwise, the ROP rate cannot increase so dramatically relative to the ROP in the absence of alcohol, and both metal effect and the electronic effect of substituted group on ligand for the ROP of lactide cannot be so remarkable (Table 1, entries 5 vs 6, 4 vs 7, and 8 vs 10). The 1H NMR spectrum of final polymer obtained with complex 3 as a catalyst in the presence of alcohol proved that the polymer is the type of HO-[PLA]-OBn, capped with a benzyl ester group on one end and a hydroxyl group on the other with an integral ratio close to 5:1 between Hf (aromatic proton of benzyl end group) and Hc (neighbor methine of hydroxyl end group), which indicates the polymers are linear and BnOH acts as a real initiator (Figure S11). The MALDITOF spectrum (Figure 7 and Table 1, entry 12) also confirmed this further by a series of peaks at 144m + 108 + 19 with a

Figure 7. MALDI-TOF spectrum of poly(rac-LA) prepared by the ROP of rac-LA (Table 1, entry 12).

charge of +1, which can be assigned to m(C6H8O4) + BnOH + H3O+. A series of weak peaks with a difference in molecular mass of ∼72 Da suggest some transesterification reaction happens during this polymerization process. But we believe the transesterification reaction is not serious because the molecular weights of polymers are desirable and polydispersities are narrow. Kinetic studies of polymerization of rac-lactide in the presence of alcohol were performed to establish the reaction order with respect to lactide, complex 3, and BnOH. The results indicated that the reaction rate exhibits first-order dependence on all components of rac-lactide, complex 3, and BnOH (Figure 8). At various concentrations of complex 3 ([3]0 = 1.0, 1.5, 2.0, and 2.5 mM) and both concentrations of [rac-LA]0 and [BnOH]0 fixed at 0.1 M and 1.0 mM, the values of kobs1 are 0.0206, 0.0334, 0.0413, and 0.0518 min −1 respectively. The overall rate equation is shown in eq 2, and the reaction constant of k2 is 2.11 × 104 M−2 min−1 at 25 °C. v2 = −d[LA]/dt = k 2[LA]1 [3]1 [BnOH]1

(2)

Based on all above information, the mechanism as shown in Scheme 3 seems to be suitable for the ROP of lactide in the presence of BnOH. Lactide can be activated after coordinating to K+ or Na+; at the same time BnOH can be activated by the sulfonamide anion via a hydrogen bond, and then the activated BnOH can attack the carbonyl group of lactide to initiate the ROP reaction. The analysis of Lewis acidity of metal ions and electronic effect of substituted group for the ROP progress informs us both activations of monomer and BnOH are important in the key step of catalytic progress, which also can be proved by the kinetic equation. The cooperation of the two activation reactions leads the ROP reaction to proceed smoothly, and the hydrogen bond interaction between alcohol and sulfonamide anion should be an important factor for increasing the ROP reaction rate because it can activate alcohol (A, Scheme 3) and even can help to break the carbon−oxygen bond of lactide (C, Scheme 3). Now we can find the ROP progress to give linear polylactide in the presence of alcohol should be faster than the cyclic polymerization progress when the concentration of BnOH is not very small. For example, when the concentration of BnOH as applied in all entries of Table 1 is 2 mM, v2 is about 20 times faster than v1. In addition, upon addition of BnOH in this H


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Figure 8. (a) Semilogarithmic plots of the rac-lactide versus time in toluene at 25 °C with catalyst 3 and BnOH as a co-initiator. [3]0 = 1.0, 1.5, 2.0, and 2.5 mM, [rac-LA]0 = 0.1 M, [BnOH]0 = 1.0 mM. (b) Plots of ln kobs1 versus ln[3]0 for the polymerization of rac-lactide with catalyst 3 and BnOH as a co-initiator in toluene at 25 °C. [rac-LA]0 = 0.1 M, [BnOH]0 = 1.0 mM. (c) Semilogarithmic plots of the rac-lactide versus time in toluene at 25 °C with catalyst 3 and BnOH as a co- initiator. [BnOH]0 = 1.0, 1.5, 2.0, and 2.5 mM, [rac-LA]0 = 0.1 M, [3]0 = 1.0 mM. (d) Plots of ln kobs2 versus ln[BnOH]0 for the polymerization of rac-lactide with catalyst 3 and BnOH as an initiator in toluene at 25 °C. [rac-LA]0 = 0.1 M, [3]0 = 1.0 mM.

complex 3 confirmed the hydrogen interaction between alcohol and complex 3 can accelerate the ROP rate. Kinetic studies also confirmed these hypotheses

system, the hydrogen bond interaction between of alcohol and complex 3 possibly can further reduce the ability of sulfonamide anion to initiate the cyclic polymerization directly. What is more, because the cyclic polymerization initiated by sulfonamide anion directly is slow, the little amount of very small cyclic polymer also can be consumed to be polymerized into a linear polymer with alcohol as an initiator before enlarging the size of cyclic polymer. Therefore, the alcohol plays a very important role in the whole polymerization progress for the successful synthesis of high molecular weight linear polylactide with a desirable molecular weight.

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations with air- and moisture-sensitive materials were carried out under a dry argon atmosphere using standard Schlenk techniques or in a glovebox. Toluene, THF, and n-hexane were distilled from sodium benzophenone ketyl before use. CH2Cl2 was dried by refluxing from P2O5. Chloroform-d, benzene-d6, DMSO-d6, toluene-d8, and other reagents were carefully dried and stored in a glovebox. The monomer raclactide (rac-LA; Daigang BIO Engineer Ltd. of China) was recrystallized from dry toluene and then sublimed twice prior to use. BnOH was dried over CaH2 under argon prior to distillation. All other chemicals were commercially available and used after appropriate purification. Methods. NMR spectra were recorded on Varian Mercury Plus 300 MHz and JNM-ECS 400 MHz spectrometer. The 1H and 13C NMR spectra were referenced internally using the residual solvent resonances and reported relative to TMS. The elemental analysis data 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 determined by gel permeation chromatography (GPC) using THF as the eluent (flow rate: 1 mL min−1 at 25 °C) and narrow polystyrene standards as reference samples. The Mn values of PLAs were corrected with a Mark−Houwink factor of 0.58 to account

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CONCLUSION In summary, we developed a new sodium/potassium crown ether complex system to improve the catalytic performance of sodium/potassium complexes in the ROP of rac-lactide. In this system, bichelating sulfonamide can effectively suppress the cyclic polymerization progress via a coordination−insertion mechanism which leads the synthesis of high molecular weight linear polylactide to become a reality for the sodium/potassium complex system. Because the active centers of these sodium/ potassium sulfonamidate complexes have especial sandwich structure constructed by the plane of the crown and the plane of substituted groups of the sulfonamide anion, the isoselectivity also can reach to a high value of Pm = 0.84. The NMR analysis of the reaction mixture of rac-lactide, BnOH, and I


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above for HL2, 4-nitrobenzenesulfonyl chloride (2.22 g, 10 mmol) was converted to give HL5 as yellow powder (0.38 g, 10% yield). 1H NMR (400 MHz, chloroform-d, 25 °C): δ 8.31 (d, J = 8.0 Hz, Ar−H, 2H), 7.98 (d, J = 8.0 Hz, Ar−H, 2H), 7.28−7.33 (m, Ar−H, 2H), 7.20 (d, J = 8.0 Hz, Ar−H, 2H), 7.13 (d, J = 8.0 Hz, Ar−H, 2H), 7.00−7.04 (m, Ar−H, 2H), 5.88 (d, J = 8.0 Hz, CH, 1H), 5.03 (d, J = 8.0 Hz, NH, 1H). 13C NMR (100 MHz, DMSO-d6, 25 °C): δ 151.03, 149.81, 129.33, 127.82, 124.52, 123.41, 120.61, 116.35, 48.40. Anal. Calcd (%) for C19H14N2O5S: C 59.68, H 3.69, N 7.33. Found: C 59.53, H 3.56, N 7.21. Synthesis of Complex 1. To a solution of N-(2,6-diisopropylphenyl)-2,4,6-triisopropylbenzenesulfonamide (HL1) (0.443 g, 1.0 mmol) and 15-crown-5 ether (0.220 g, 1.0 mmol) in toluene (20 mL) was slowly added NaN(SiMe3)2 (0.5 mL of 2.00 M solution in THF, 1.0 mmol) at 0 °C under a nitrogen atmosphere. During this progress, the colorless solution changed to faint yellow. After stirring 6 h at room temperature, the white precipitate formed was separated by filtration. The solid residue was washed with 20 mL of hexane and dried in vacuo to give complex 1 as a white powder (0.523 g, 76%). Colorless crystals of 1 suitable for X-ray diffraction studies were obtained from a mixture of toluene and n-hexane at room temperature. 1 H NMR (400 MHz, benzene-d6, 25 °C): δ 7.27 (s, Ar−H, 1H), 7.26 (s, Ar−H, 1H), 7.21 (br, Ar−H, 2H), 7.11 (t, J = 8.0 Hz, Ar−H, 1H), 4.70 (m, CH, 2H), 4.30 (m, CH, 2H), 3.20 (s, crown ether−H, 20H), 2.77 (m, CH, 1H), 1.40 (d, J = 8.0 Hz, CH3, 12H), 1.28 (d, J = 8.0 Hz, CH3, 12H), 1.20 (d, J = 8.0 Hz, CH3, 6H). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 148.74, 148.29, 145.97, 143.32, 142.99, 123.43, 122.84, 121.94, 69.07, 34.52, 30.61, 28.12, 25.51, 24.97, 24.16. Anal. Calcd (%) for C37H60NNaO7S: C 64.79, H 8.82, N 2.04. Found: C 64.58, H 8.69, N 1.98. Synthesis of Complex 2. To a solution of N-(2,6-diisopropylphenyl)-2,4,6-triisopropylbenzenesulfonamide (HL1) (0.443 g, 1.0 mmol) and 18-crown-6 ether (0.264 g, 1.0 mmol) in toluene (20 mL) was slowly added KN(SiMe3)2 (1.0 mL of 1.00 M solution in THF, 1.0 mmol) at 0 °C under a nitrogen atmosphere. During this progress, the colorless solution changed to faint yellow. After stirring 6 h at room temperature, the white precipitate formed was separated by filtration. The solid residue was washed with 20 mL of hexane and dried in vacuo to give complex 2 as a white powder (0.642 g, 86%). Colorless crystals of 2 suitable for X-ray diffraction studies were obtained from a mixture of toluene and n-hexane at room temperature. 1H NMR (400 MHz, benzene-d6, 25 °C): δ 7.34 (s, Ar−H, 1H), 7.32 (s, Ar−H, 1H), 7.23 (br, Ar−H, 2H), 7.16 (t, J = 8.0 Hz, Ar−H, 1H), 4.99 (m, CH, 2H), 4.47 (m, CH, 2H), 3.14 (s, crown ether−H, 24H), 2.76 (m, CH, 1H), 1.47 (d, J = 8.0 Hz, CH3, 12H), 1.36 (d, J = 8.0 Hz, CH3, 12H), 1.19 (d, J = 8.0 Hz, CH3, 6H). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 148.60, 147.70, 145.69, 144.58, 143.93, 123.15, 122.53, 121.20, 70.04, 34.52, 30.33, 28.20, 25.57, 24.84, 24.27. Anal. Calcd (%) for C39H64KNO8S: C 62.78, H 8.65, N 1.88. Found: C 62.70, H 8.52, N 1.75. Synthesis of Complex 3. According to the procedure described above for 1, HL2 (0.463 g, 1.0 mmol) was converted to give complex 3 as white powder (0.603 g, 85% yield). 1H NMR (400 MHz, benzened6, 25 °C): δ 7.93 (d, J = 8.0 Hz, Ar−H, 2H), 7.40 (s, Ar−H, 2H), 7.05 (d, J = 8.0 Hz, Ar−H, 2H), 6.91−6.98 (m, Ar−H, 4H), 5.96 (s, CH, 1H), 5.24 (m, CH, 2H), 3.07 (s, crown ether−H, 20H), 2.88 (m, CH, 1H), 1.49 (d, J = 8.0 Hz, CH3, 12H), 1.30 (d, J = 8.0 Hz, CH3, 6H). 13 C NMR (100 MHz, benzene-d6, 25 °C): δ 152.12, 149.61, 148.82, 142.87, 132.51, 129.73, 126.86, 123.17, 122.89, 115.43, 68.83, 48.73, 34.57, 30.26, 26.04, 24.29. Anal. Calcd (%) for C38H52NNaO8S: C 64.66, H 7.43, N 1.98. Found: C 64.53, H 7.32, N 1.88. Synthesis of Complex 4. According to the procedure described above for 2, HL2 (0.463 g, 1.0 mmol) was converted to give complex 4 as white powder (0.618 g, 81% yield). Colorless crystals of 4 suitable for X-ray diffraction studies were obtained from a toluene solution at room temperature. 1H NMR (400 MHz, benzene-d6, 25 °C): δ 8.04 (d, J = 8.0 Hz, Ar−H, 2H), 7.41 (s, Ar−H, 2H), 7.06 (d, J = 8.0 Hz, Ar−H, 2H), 6.92−6.99 (m, Ar−H, 4H), 6.00 (s, CH, 1H), 5.38 (m, CH, 2H), 3.09 (s, crown ether−H, 24H), 2.88 (m, CH, 1H), 1.53 (d, J = 8.0 Hz, CH3, 12H), 1.30 (d, J = 8.0 Hz, CH3, 6H). 13C NMR (100

for the difference in hydrodynamic volume of polystyrene and polylactide. The MALDI-TOF mass spectroscopic data were obtained using DCTB as the matrix in a Bruker Daltonics Inc. BIFLEX III MALDI-TOF mass spectrometer. Synthesis of N-(2,6-Diisopropylphenyl)-2,4,6-triisopropylbenzenesulfonamide (HL 1 ). A solution containing 2,4,6triisopropylbenzenesulfonyl chloride (3.02 g, 10 mmol) in pyridine (20 mL) was added dropwise to a pyridine solution (20 mL) of 2,6diisopropylaniline (1.77 g, 10 mmol) at 118 °C. Then the reaction mixture was stirred under reflux for 2 h and cooled to room temperature. Pyridine was then removed under reduced pressure. The residue was diluted with water and extracted with ethyl acetate (3 × 30 mL). The combined organic extracts were washed with brine, dried over MgSO4, and concentrated under reduced pressure. The residue was recrystallized from ethyl acetate and petroleum ether to give HL1 as white solid (1.56 g, 35% yield). 1H NMR (400 MHz, chloroform-d, 25 °C): δ 7.23 (t, J = 8.0 Hz, Ar−H, 1H), 7.11 (s, Ar−H, 2H), 7.10 (d, J = 8.0 Hz, Ar−H, 2H), 6.09 (s, NH, 1H), 3.71 (m, CH, 2H), 3.28 (m, CH, 2H), 2.87 (m, CH, 1H), 1.22 (d, J = 8.0 Hz, CH3, 6H), 1.11 (d, J = 8.0 Hz, CH3, 12H), 0.97 (d, J = 8.0 Hz, CH3, 12H). 13C NMR (100 MHz, chloroform-d, 25 °C): δ 152.87, 150.26, 148.43, 134.28, 129.32, 128.89, 124.23, 123.92, 34.34, 30.95, 28.49, 24.79, 23.82, 23.75. Anal. Calcd (%) for C27H41NO2S: C 73.09, H 9.31, N 3.16. Found: C 72.96, H 9.28, N 3.09. Synthesis of 2,4,6-Triisopropyl-N-(9H-xanthen-9-yl)benzenesulfonamide (HL 2 ). A solution containing 2,4,6triisopropylbenzenesulfonyl chloride (3.02 g, 10 mmol) in pyridine (20 mL) was added dropwise to a pyridine solution (20 mL) of 9Hxanthen-9-amine112 (1.97 g, 10 mmol) at 118 °C. Then the reaction mixture was stirred under reflux for 2 h and cooled to room temperature. Pyridine was then removed under reduced pressure. The residue was diluted with water and extracted with ethyl acetate (3 × 30 mL). The combined organic extracts were washed with brine, dried over MgSO4, and concentrated under reduced pressure. The residue was recrystallized from ethyl acetate and petroleum ether to give HL2 as white solid (1.23 g, 27% yield). 1H NMR (300 MHz, chloroform-d, 25 °C): δ 7.22−7.28 (m, Ar−H, 2H), 7.17 (s, Ar−H, 2H), 7.03−7.10 (m, Ar−H, 4H), 6.90−6.95 (m, Ar−H, 2H), 5.86 (d, J = 9.0 Hz, CH, 1H), 4.86 (d, J = 9.0 Hz, NH, 1H), 4.12 (m, CH, 2H), 2.95 (m, CH, 1H), 1.30 (d, J = 6.0 Hz, CH3, 6H), 1.17 (d, J = 6.0 Hz, CH3, 12H). 13 C NMR (100 MHz, chloroform-d, 25 °C): δ 153.00, 151.44, 149.73, 134.98, 129.98, 129.48, 123.97, 123.63, 120.64, 116.76, 49.24, 34.36, 29.92, 24.80, 23.89. Anal. Calcd (%) for C28H33NO3S: C 72.54, H 7.17, N 3.02. Found: C 72.46, H 7.04, N 3.09. Synthesis of 4-Methoxy-N-(9H-xanthen-9-yl)benzenesulfonamide (HL3). According to the procedure described above for HL2, 4-methoxybenzenesulfonyl chloride (2.06 g, 10 mmol) was converted to give HL3 as white powder (1.10 g, 30% yield). 1H NMR (400 MHz, chloroform-d, 25 °C): δ 7.86 (d, J = 8.0 Hz, Ar−H, 2H), 7.29 (d, J = 8.0 Hz, Ar−H, 2H), 7.19 (d, J = 8.0 Hz, Ar−H, 2H), 7.09 (d, J = 8.0 Hz, Ar−H, 2H), 7.00−7.03 (m, Ar−H, 4H), 5.77 (d, J = 8.0 Hz, CH, 1H), 4.87 (d, J = 8.0 Hz, NH, 1H), 3.91 (s, OCH3, 3H). 13 C NMR (100 MHz, chloroform-d, 25 °C): δ 163.13, 151.39, 133.33, 129.42, 123.75, 120.56, 116.87, 114.47, 55.83, 49.24. Anal. Calcd (%) for C20H17NO4S: C 65.38, H 4.66, N 3.81. Found: C 65.19, H 4.54, N 3.67. Synthesis of 4-Methyl-N-(9H-xanthen-9-yl)benzenesulfonamide (HL4). According to the procedure described above for HL2, 4-methylbenzenesulfonyl chloride (1.90 g, 10 mmol) was converted to give HL4 as white powder (1.08 g, 31% yield). 1H NMR (400 MHz, chloroform-d, 25 °C): δ 7.79 (d, J = 8.0 Hz, Ar−H, 2H), 7.33 (d, J = 8.0 Hz, Ar−H, 2H), 7.24−7.28 (m, Ar−H, 2H), 7.14 (d, J = 8.0 Hz, Ar−H, 2H), 7.07 (d, J = 8.0 Hz, Ar−H, 2H), 6.96−7.00 (m, Ar−H, 2H), 5.76 (d, J = 8.0 Hz, CH, 1H), 4.90 (d, J = 8.0 Hz, NH, 1H), 2.47 (s, CH3, 3H). 13C NMR (100 MHz, chloroform-d, 25 °C): δ 151.37, 143.73, 138.71, 129.95, 129.59, 127.26, 123.71, 120.51, 116.85, 49.28, 21.73. Anal. Calcd for C20H17NO3S: C 68.36, H 4.88, N 3.99. Found: C 68.26, H 4.69, N 3.87. Synthesis of 4-Nitro-N-(9H-xanthen-9-yl)benzenesulfonamide (HL5). According to the procedure described J


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MHz, benzene-d6, 25 °C): δ 152.45, 149.38, 148.41, 143.64, 132.25, 130.54, 126.66, 122.96, 122.82, 115.48, 69.98, 49.40, 34.60, 30.08, 26.10, 24.33. Anal. Calcd (%) for C40H56KNO9S: C 62.72, H 7.37, N 1.83. Found: C 62.64, H 7.19, N 1.76. Synthesis of Complex 5. According to the procedure described above for 1, HL3 (0.367 g, 1.0 mmol) was converted to give complex 5 as white powder (0.512 g, 84% yield). Colorless crystals of 5 suitable for X-ray diffraction studies were obtained from a deuterated benzene solution at room temperature. 1H NMR (400 MHz, benzene-d6, 25 °C): δ 8.30 (d, J = 8.0 Hz, Ar−H, 2H), 7.85 (d, J = 8.0 Hz, Ar−H, 2H), 6.95−6.99 (m, Ar−H, 2H), 6.85−6.91 (m, Ar−H, 4H), 5.90 (s, CH, 1H), 3.32 (s, OCH3, 3H), 3.11 (s, crown ether−H, 20H). 13C NMR (100 MHz, benzene-d6, 50 °C): δ 160.59, 151.84, 132.26, 129.28, 129.04, 127.04, 115.48, 113.53, 69.28, 54.92, 49.46. Anal. Calcd (%) for C30H36NNaO9S: C 59.10, H 5.95, N 2.30. Found: C 59.06, H 5.83, N 2.24. Synthesis of Complex 6. According to the procedure described above for 1, HL4 (0.351 g, 1.0 mmol) was converted to give complex 6 as white powder (0.470 g, 79% yield). Colorless crystals of 6 suitable for X-ray diffraction studies were obtained from a deuterated benzene solution at room temperature. 1H NMR (400 MHz, benzene-d6, 25 °C): δ 8.33 (d, J = 8.0 Hz, Ar−H, 2H), 7.91 (d, J = 8.0 Hz, Ar−H, 2H), 7.10 (d, J = 8.0 Hz, Ar−H, 2H), 7.04 (d, J = 8.0 Hz, Ar−H, 2H), 6.97 (t, J = 8.0 Hz, Ar−H, 2H), 6.90 (t, J = 8.0 Hz, Ar−H, 2H), 5.96 (s, CH, 1H), 3.05 (s, crown ether−H, 20H), 2.13 (s, CH3, 3H). 13C NMR (100 MHz, benzene-d6, 50 °C): δ 151.58,148.27, 138.15, 132.15, 129.41, 128.85, 127.57, 126.91, 122.66, 115.26, 68.86, 49.17, 21.31. Anal. Calcd (%) for C30H36NNaO8S: C 60.69, H 6.11, N 2.36. Found: C 60.33, H 6.18, N 2.56. Synthesis of Complex 7. According to the procedure described above for 1, HL5 (0.382 g, 1.0 mmol) was converted to give complex 7 as white powder (0.521 g, 83% yield). Yellow crystals of 7 suitable for X-ray diffraction studies were obtained from a toulene solution at room temperature. 1H NMR (400 MHz, benzene-d6, 25 °C): δ 8.24 (d, J = 8.0 Hz, Ar−H, 2H), 8.05 (d, J = 8.0 Hz, Ar−H, 2H), 7.65 (d, J = 8.0 Hz, Ar−H, 2H), 7.03 (d, J = 8.0 Hz, Ar−H, 2H), 6.95 (t, J = 8.0 Hz, Ar−H, 2H), 6.81 (t, J = 8.0 Hz, Ar−H, 2H), 5.84 (s, CH, 1H), 2.96 (s, crown ether−H, 20H). 13C NMR (100 MHz, benzene-d6, 50 °C): δ 151.58,148.05, 131.84, 128.46, 127.27, 123.60, 122.80, 115.52, 68.77, 49.43. Anal. Calcd (%) for C29H33N2NaO10S: C 55.76, H 5.33, N 4.48. Found: C 55.62, H 5.19, N 4.39. General Polymerization Procedure. A typical polymerization procedure is illustrated by the synthesis of PLA-([rac-LA]0/[cat.]0/ [BnOH]0 = 100:1:1; Table 1, entry 4). A solution of complex 3 (0.007 g, 0.010 mmol) and BnOH (100 μL, 0.10 M in toluene) in toluene (5 mL) were cooled to 0 °C, and then rac-lactide (0.144 g, 1.0 mmol) was added. The mixture was then rapidly stirred at 0 °C for 1 h and quenched by a few drops of water. The polymer was precipitated by adding hexane (20 mL). A white crystalline solid was obtained by recrystallization from a CH2Cl2/hexane mixed solvent. The collected polymer sample was further dried in a vacuum oven at 50 °C for 8 h to a constant weight for GPC and 1H NMR analyses. General Procedure for Kinetic Studies. The polymerizations were carried out 25 °C in a glovebox. Complex 3 (0.007 g, 0.010 mmol) and BnOH (100 μL, 0.10 M in toluene) were added a solution of rac-lactide (0.144 g, 1.0 mmol) in toluene (10 mL). At appropriate time intervals, 0.5 mL aliquots were removed and quenched with benzoic acid (1.0 μL, 0.10 M in toluene). The solvent was removed in vacuo, and the percent conversion was determined by 1H NMR in CDCl3. Crystal Structure Determination. X-ray diffraction measurements were performed on a SuperNova (Dual) X-ray diffraction diffractometer with a graphite-monochromated Cu/Mo Kα radiation source (λ = 1.54184/0.71073 Å). The structures were solved by direct methods using the Siemens SHELXTL PLUS program.113 Nonhydrogen 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 parameters and results of the structure refinements are summarized in Table S1.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02271. 1 H NMR and 13C NMR spectra of compounds HL1− HL5 and complexs 1−7, the polymerization studies, computational details (PDF) Crystallographic data for complex 1 (CIF) Crystallographic data for complex 2 (CIF) Crystallographic data for complex 4 (CIF) Crystallographic data for complex 5 (CIF) Crystallographic data for complex 6 (CIF) Crystallographic data for complex 7 (CIF)

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AUTHOR INFORMATION

Corresponding Author

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

Jincai Wu: 0000-0002-8233-2863 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, 21401161, and 21671087), 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. We also appreciated the help of Bingbing Wu at the School of Chemistry and Materials Science, University of Chinese science and technology for the experiments.

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REFERENCES

(1) Tschan, M. J. L.; Brule, E.; Haquette, P.; Thomas, C. M. Synthesis of Biodegradable Polymers from Renewable Resources. Polym. Chem. 2012, 3, 836−851. (2) Zhao, Y.; Wang, L.; Xiao, A.; Yu, H. The Synthesis of Modified Polyethylene via Coordination Polymerization Followed by ATRP, RAFT, NMRP or ROP. Prog. Polym. Sci. 2010, 35, 1195−1216. (3) Nakamura, A.; Ito, S.; Nozaki, K. Coordination-Insertion Copolymerization of Fundamental Polar Monomers. Chem. Rev. 2009, 109, 5215−5244. (4) Chen, E. Y. X. Coordination Polymerization of Polar Vinyl Monomers by Single-Site Metal Catalysts. Chem. Rev. 2009, 109, 5157−5214. (5) Yamago, S. Precision Polymer Synthesis by Degenerative Transfer Controlled/Living Radical Polymerization Using Organotellurium, Organostibine, and Organobismuthine Chain-Transfer Agents. Chem. Rev. 2009, 109, 5051−5068. (6) Rosen, B. M.; Percec, V. Single-Electron Transfer and SingleElectron Transfer Degenerative Chain Transfer Living Radical Polymerization. Chem. Rev. 2009, 109, 5069−5119. (7) Ouchi, M.; Terashima, T.; Sawamoto, M. Transition MetalCatalyzed Living Radical Polymerization: Toward Perfection in Catalysis and Precision Polymer Synthesis. Chem. Rev. 2009, 109, 4963−5050. (8) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Recent Developments in Main Group Metal Complexes Catalyzed/Initiated Polymerization of Lactides and Related Cyclic Esters. Coord. Chem. Rev. 2006, 250, 602−626.

K


Article

Macromolecules

for Highly Isoselective Polymerization of rac-Lactide. ACS Macro Lett. 2015, 4, 556−560. (32) Zhang, J.; Xiong, J.; Sun, Y.; Tang, N.; Wu, J. Highly IsoSelective and Active Catalysts of Sodium and Potassium Monophenoxides Capped by a Crown Ether for the Ring-Opening Polymerization of rac-Lactide. Macromolecules 2014, 47, 7789−7796. (33) Kan, C.; Ge, J.; Ma, H. Aluminum Methyl, Alkoxide and αalkoxy Ester Complexes Supported by 6,6′-dimethylbiphenyl-bridged Salen Ligands: Synthesis, Characterization and Catalysis for racLactide Polymerization. Dalton Trans. 2016, 45, 6682−6695. (34) Press, K.; Goldberg, I.; Kol, M. Mechanistic Insight into the Stereochemical Control of Lactide Polymerization by Salan-Aluminum Catalysts. Angew. Chem., Int. Ed. 2015, 54, 14858−14861. (35) Pilone, A.; Press, K.; Goldberg, I.; Kol, M.; Mazzeo, M.; Lamberti, M. Gradient Isotactic Multiblock Polylactides from Aluminum Complexes of Chiral Salalen Ligands. J. Am. Chem. Soc. 2014, 136, 2940−2943. (36) Pang, X.; Duan, R.; Li, X.; Chen, X. Bimetallic Salen-Aluminum Complexes: Synthesis, Characterization and Their Reactivity with racLactide and ε-Caprolactone. Polym. Chem. 2014, 5, 3894−3900. (37) Meduri, A.; Fuoco, T.; Lamberti, M.; Pellecchia, C.; Pappalardo, D. Versatile Copolymerization of Glycolide and rac-Lactide by Dimethyl(salicylaldiminato)aluminum Compounds. Macromolecules 2014, 47, 534−543. (38) Maudoux, N.; Roisnel, T.; Dorcet, V.; Carpentier, J. F.; Sarazin, Y. Chiral (1,2)-Diphenylethylene-Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide. Chem. - Eur. J. 2014, 20, 6131−6147. (39) Chen, H.-L.; Dutta, S.; Huang, P.-Y.; Lin, C.-C. Preparation and Characterization of Aluminum Alkoxides Coordinated on Salen-Type Ligands: Highly Stereoselective Ring-Opening Polymerization of racLactide. Organometallics 2012, 31, 2016−2025. (40) Whitelaw, E. L.; Loraine, G.; Mahon, M. F.; Jones, M. D. Salalen Aluminium Complexes and Their Exploitation for the Ring Opening Polymerisation of rac-Lactide. Dalton Trans. 2011, 40, 11469−11473. (41) Darensbourg, D. J.; Karroonnirun, O. Stereoselective RingOpening Polymerization of rac-Lactides Catalyzed by Chiral and Achiral Aluminum Half-Salen Complexes. Organometallics 2010, 29, 5627−5634. (42) Du, H.; Velders, A. H.; Dijkstra, P. J.; Sun, J.; Zhong, Z.; Chen, X.; Feijen, J. Chiral Salan Aluminium Ethyl Complexes and Their Application in Lactide Polymerization. Chem. - Eur. J. 2009, 15, 9836− 9845. (43) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Aluminum and Yttrium Complexes of an Unsymmetrical Mixed Fluorous Alkoxy/Phenoxy-Diimino Ligand: Synthesis, Structure, and Ring-Opening Polymerization Catalysis. Organometallics 2009, 28, 1469−1475. (44) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; Pugh, R. I.; White, A. J. Study of Ligand Substituent Effects on the Rate and Stereoselectivity of Lactide Polymerization Using Aluminum SalenType Initiators. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15343−15348. (45) Majerska, K.; Duda, A. Stereocontrolled Polymerization of Racemic Lactide with Chiral Initiator: Combining Stereoelection and Chiral Ligand-Exchange Mechanism. J. Am. Chem. Soc. 2004, 126, 1026−1027. (46) 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. (47) 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. (48) 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.

(9) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Controlled Ring-Opening Polymerization of Lactide and Glycolide. Chem. Rev. 2004, 104, 6147−6176. (10) Kuran, W. Coordination Polymerization of Heterocyclic and Heterounsaturated Monomers. Prog. Polym. Sci. 1998, 23, 919−992. (11) Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds. In Comprehensive Asymmetric Catalysis I-III; Springer-Verlag: Berlin, 1999. (12) Baugh, L. S., Canich, J. A. M., Eds. In Stereoselective Polymerization with Single-Site Catalysts; CRC Press/Taylor & Francis Group: Boca Raton, FL, 2007. (13) Ojima, I., Ed. In Catalytic Asymmetric Synthesis, 3rd ed.; John Wiley & Sons: New York, 2010. (14) García-Valle, F. M.; Estivill, R.; Gallegos, C.; Cuenca, T.; Mosquera, M. E. G.; Tabernero, V.; Cano, J. Metal and LigandSubstituent Effects in the Immortal Polymerization of rac-Lactide with Li, Na, and K Phenoxo-imine Complexes. Organometallics 2015, 34, 477−487. (15) Calvo, B.; Davidson, M. G.; García-Vivó, D. PolyamineStabilized Sodium Aryloxides: Simple Initiators for the Ring-Opening Polymerization of rac-Lactide. Inorg. Chem. 2011, 50, 3589−3595. (16) Khougaz, K.; Zhong, X. F.; Eisenberg, A. Aggregation and Critical Micelle Concentrations of Polystyrene-b-poly(sodium acrylate) and Polystyrene-b-poly(acrylic acid) Micelles in Organic Media. Macromolecules 1996, 29, 3937−3949. (17) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Stereoselective Epoxide Polymerization and Copolymerization. Chem. Rev. 2014, 114, 8129−8152. (18) Tsuji, H. Poly(lactide) Stereocomplexes: Formation, Structure, Properties, Degradation, and Applications. Macromol. Biosci. 2005, 5, 569−597. (19) Slager, J.; Domb, A. J. Biopolymer Stereocomplexes. Adv. Drug Delivery Rev. 2003, 55, 549−583. (20) Södergård, A.; Stolt, M. Properties of Lactic Acid Based Polymers and Their Correlation with Composition. Prog. Polym. Sci. 2002, 27, 1123−1163. (21) Okamoto, Y.; Nakano, T. Asymmetric Polymerization. Chem. Rev. 1994, 94, 349−372. (22) Ugartemendia, J. M.; Larrañaga, A.; Amestoy, H.; Sarasua, J. R. Supramolecular Evolution over an Initial Period of Biodegradation of Lactide and Caprolactone Based Medical (co)polyesters. Polym. Degrad. Stab. 2014, 108, 87−96. (23) Slomkowski, S.; Penczek, S.; Duda, A. PolylactidesAn Overview. Polym. Adv. Technol. 2014, 25, 436−447. (24) Davachi, S. M.; Kaffashi, B.; Roushandeh, J. M. Synthesis and Characterization of a Novel Terpolymer Based on L-Lactide, Glycolide, and Trimethylene Carbonate for Specific Medical Applications. Polym. Adv. Technol. 2012, 23, 565−573. (25) Thomas, C. M.; Lutz, J. F. Precision Synthesis of Biodegradable Polymers. Angew. Chem., Int. Ed. 2011, 50, 9244−9246. (26) Lo, C. T.; Van Tassel, P. R.; Saltzman, W. M. Simultaneous Release of Multiple Molecules From Poly(lactide-co-glycolide) Nanoparticles Assembled onto Medical Devices. Biomaterials 2009, 30, 4889−4897. (27) Dove, A. P. Controlled Ring-Opening Polymerisation of Cyclic Esters: Polymer Blocks in Self-Assembled Nanostructures. Chem. Commun. 2008, 6446−6470. (28) Sun, Y.; Xiong, J.; Dai, Z.; Pan, X.; Tang, N.; Wu, J. Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(raclactide) Synthesis. Inorg. Chem. 2016, 55, 136−143. (29) Dai, Z.; Sun, Y.; Xiong, J.; Pan, X.; Tang, N.; Wu, J. Simple Sodium and Potassium Phenolates As Catalysts for Highly Isoselective Polymerization of rac-Lactide. Catal. Sci. Technol. 2016, 6, 515−520. (30) Xiong, J.; Zhang, J.; Sun, Y.; Dai, Z.; Pan, X.; Wu, J. Iso-Selective Ring-Opening Polymerization of rac-Lactide Catalyzed by Crown Ether Complexes of Sodium and Potassium Naphthalenolates. Inorg. Chem. 2015, 54, 1737−1743. (31) Dai, Z.; Sun, Y.; Xiong, J.; Pan, X.; Wu, J. Alkali-Metal Monophenolates with a Sandwich-Type Catalytic Center as Catalysts L


Article

Macromolecules (49) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. Stereoselective RingOpening Polymerization of Racemic Lactide Using Aluminum-Achiral Ligand Complexes: Exploration of a Chain-End Control Mechanism. J. Am. Chem. Soc. 2002, 124, 5938−5939. (50) Ovitt, T. M.; Coates, G. W. Stereoselective Ring-Opening Polymerization of meso-Lactide: Synthesis of Syndiotactic Poly(lactic acid). J. Am. Chem. Soc. 1999, 121, 4072−4073. (51) Aluthge, D. C.; Ahn, J. M.; Mehrkhodavandi, P. Overcoming Aggregation in Indium Salen Catalysts for Isoselective Lactide Polymerization. Chem. Sci. 2015, 6, 5284−5292. (52) Yu, I.; Acosta-Ramírez, A.; Mehrkhodavandi, P. Mechanism of Living Lactide Polymerization by Dinuclear Indium Catalysts and Its Impact on Isoselectivity. J. Am. Chem. Soc. 2012, 134, 12758−12773. (53) Normand, M.; Kirillov, E.; Roisnel, T.; Carpentier, J.-F. Indium Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands for Ring-Opening Polymerization of rac-Lactide: How Does Indium Compare to Aluminum? Organometallics 2012, 31, 1448−1457. (54) Bompart, M.; Vergnaud, J.; Strub, H.; Carpentier, J.-F. Indium(III) Halides as Exceptionally Active, Water-Tolerant Catalysts for Cationic Polymerization of Styrenics. Polym. Chem. 2011, 2, 1638− 1640. (55) Blake, M. P.; Schwarz, A. D.; Mountford, P. Sulfonamide, Phenolate, and Directing Ligand-Free Indium Initiators for the RingOpening Polymerization of rac-Lactide. Organometallics 2011, 30, 1202−1214. (56) Pietrangelo, A.; Knight, S. C.; Gupta, A. K.; Yao, L. J.; Hillmyer, M. A.; Tolman, W. B. Mechanistic Study of the Stereoselective Polymerization of d,l-Lactide Using Indium(III) Halides. J. Am. Chem. Soc. 2010, 132, 11649−11657. (57) Buffet, J.-C.; Okuda, J.; Arnold, P. L. Chiral Indium Alkoxide Complexes as Initiators for the Stereoselective Ring-Opening Polymerization of rac-Lactide. Inorg. Chem. 2010, 49, 419−426. (58) Pietrangelo, A.; Hillmyer, M. A.; Tolman, W. B. Stereoselective and Controlled Polymerization of d, l-Lactide Using Indium(III) Trichloride. Chem. Commun. 2009, 2736−2737. (59) Peckermann, I.; Kapelski, A.; Spaniol, T. P.; Okuda, J. Indium Complexes Supported by 1,ω-Dithiaalkanediyl-Bridged Bis(phenolato) Ligands: Synthesis, Structure, and Controlled RingOpening Polymerization of L-Lactide. Inorg. Chem. 2009, 48, 5526− 5534. (60) Douglas, A. F.; Patrick, B. O.; Mehrkhodavandi, P. A Highly Active Chiral Indium Catalyst for Living Lactide Polymerization. Angew. Chem., Int. Ed. 2008, 47, 2290−2293. (61) Yang, Y.; Wang, H.; Ma, H. Stereoselective Polymerization of rac-Lactide Catalyzed by Zinc Complexes with Tetradentate Aminophenolate Ligands in Different Coordination Patterns: Kinetics and Mechanism. Inorg. Chem. 2015, 54, 5839−5854. (62) Bakewell, C.; Fateh-Iravani, G.; Beh, D. W.; Myers, D.; Tabthong, S.; Hormnirun, P.; White, A. J. P.; Long, N.; Williams, C. K. Comparing a Series of 8-Quinolinolato Complexes of Aluminium, Titanium and Zinc as Initiators for the Ring-Opening Polymerization of rac-Lactide. Dalton Trans. 2015, 44, 12326−12337. (63) Wang, H.; Yang, Y.; Ma, H. Stereoselectivity Switch between Zinc and Magnesium Initiators in the Polymerization of rac-Lactide: Different Coordination Chemistry, Different Stereocontrol Mechanisms. Macromolecules 2014, 47, 7750−7764. (64) Honrado, M.; Otero, A.; Fernández-Baeza, J.; Sánchez-Barba, L. F.; Garcés, A.; Lara-Sánchez, A. n.; Rodríguez, A. M. Stereoselective ROP of rac-Lactide Mediated by Enantiopure NNO-Scorpionate Zinc Initiators. Organometallics 2014, 33, 1859−1866. (65) Abbina, S.; Du, G. Zinc-Catalyzed Highly Isoselective Ring Opening Polymerization of rac-Lactide. ACS Macro Lett. 2014, 3, 689−692. (66) Poirier, V.; Roisnel, T.; Carpentier, J.-F.; Sarazin, Y. Zinc and Magnesium Complexes Supported by Bulky Multidentate AminoEther Phenolate Ligands: Potent Pre-Catalysts for the ImmortalringOpening Polymerisation of Cyclic Esters. Dalton Trans. 2011, 40, 523−534.

(67) Drouin, F.; Oguadinma, P. O.; Whitehorne, T. J. J.; Prud’homme, R. E.; Schaper, F. Lactide Polymerization with Chiral β-Diketiminate Zinc Complexes. Organometallics 2010, 29, 2139− 2147. (68) Di Iulio, C.; Jones, M. D.; Mahon, M. F.; Apperley, D. C. Zinc(II) Silsesquioxane Complexes and Their Application for the Ring-Opening Polymerization of rac-Lactide. Inorg. Chem. 2010, 49, 10232−10234. (69) Darensbourg, D. J.; Karroonnirun, O. Ring-Opening Polymerization of Lactides Catalyzed by Natural Amino-Acid Based Zinc Catalysts. Inorg. Chem. 2010, 49, 2360−2371. (70) Labourdette, G.; Lee, D. J.; Patrick, B. O.; Ezhova, M. B.; Mehrkhodavandi, P. Unusually Stable Chiral Ethyl Zinc Complexes: Reactivity and Polymerization of Lactide. Organometallics 2009, 28, 1309−1319. (71) Knight, P. D.; White, A. J. P.; Williams, C. K. Dinuclear Zinc Complexes Using Pentadentate Phenolate Ligands. Inorg. Chem. 2008, 47, 11711−11719. (72) Chen, H.-Y.; Tang, H.-Y.; Lin, C.-C. Ring-Opening Polymerization of Lactides Initiated by Zinc Alkoxides Derived from NNOTridentate Ligands. Macromolecules 2006, 39, 3745−3752. (73) Rieth, L. R.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. Single-Site β-Diiminate Zinc Catalysts for the Ring-Opening Polymerization of β-Butyrolactone and β-Valerolactone to Poly(3-hydroxyalkanoates). J. Am. Chem. Soc. 2002, 124, 15239−15248. (74) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. Polymerization of Lactide with Zinc and Magnesium β-Diiminate Complexes: Stereocontrol and Mechanism. J. Am. Chem. Soc. 2001, 123, 3229−3238. (75) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. Single-Site Catalysts for Ring-Opening Polymerization: Synthesis of Heterotactic Poly(lactic acid) from rac-Lactide. J. Am. Chem. Soc. 1999, 121, 11583−11584. (76) Hermans, C.; Rong, W.; Spaniol, T. P.; Okuda, J. Lanthanum Complexes Containing a Bis(phenolate) Ligand with a Ferrocene-1,1′diyldithio Backbone: Synthesis, Characterization, and Ring-Opening Polymerization of rac-lactide. Dalton Trans. 2016, 45, 8127−8133. (77) Gu, W.; Xu, P.; Wang, Y.; Yao, Y.; Yuan, D.; Shen, Q. Synthesis and Characterization of Yttrium and Ytterbium Complexes Supported by Salen Ligands and Their Catalytic Properties for rac-Lactide Polymerization. Organometallics 2015, 34, 2907−2916. (78) Mou, Z.; Liu, B.; Liu, X.; Xie, H.; Rong, W.; Li, L.; Li, S.; Cui, D. Efficient and Heteroselective Heteroscorpionate Rare-Earth-Metal Zwitterionic Initiators for ROP of rac-Lactide: Role of σ-Ligand. Macromolecules 2014, 47, 2233−2241. (79) Bakewell, C.; White, A. J. P.; Long, N. J.; Williams, C. K. MetalSize Influence in Iso-Selective Lactide Polymerization. Angew. Chem., Int. Ed. 2014, 53, 9226−9230. (80) Kratsch, J.; Kuzdrowska, M.; Schmid, M.; Kazeminejad, N.; Kaub, C.; Oña-Burgos, P.; Guillaume, S. M.; Roesky, P. W. Chiral Rare Earth Borohydride Complexes Supported by Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of rac-Lactide. Organometallics 2013, 32, 1230−1238. (81) Bakewell, C.; Cao, T.-P.-A.; Le Goff, X. F.; Long, N. J.; Auffrant, A.; Williams, C. K. Yttrium Phosphasalen Initiators for rac-Lactide Polymerization. Organometallics 2013, 32, 1475−1483. (82) Zhao, W.; Wang, Y.; Liu, X.; Chen, X.; Cui, D. Synthesis of Isotactic-Heterotactic Stereoblock (Hard-Soft) Poly(lactide) with Tacticity Control Through Immortal Coordination Polymerization. Chem. - Asian J. 2012, 7, 2403−2410. (83) Nie, K.; Fang, L.; Yao, Y.; Zhang, Y.; Shen, Q.; Wang, Y. Synthesis and Characterization of Amine-Bridged Bis(phenolate)lanthanide Alkoxides and Their Application in the Controlled Polymerization of rac-Lactide and rac-β-Butyrolactone. Inorg. Chem. 2012, 51, 11133−11143. (84) Bakewell, C.; Cao, T.-P.-A.; Long, N.; Le Goff, X. F.; Auffrant, A.; Williams, C. K. Yttrium Phosphasalen Initiators for rac-Lactide Polymerization: Excellent Rates and High Iso-Selectivities. J. Am. Chem. Soc. 2012, 134, 20577−20580. M


Article

Macromolecules

(102) Schwarz, A. D.; Chu, Z.; Mountford, P. SulfonamideSupported Aluminum Catalysts for the Ring-Opening Polymerization of rac-Lactide. Organometallics 2010, 29, 1246−1260. (103) Forcato, M.; Lake, F.; Blazquez, M. M.; Renner, P.; Crisma, M.; Gade, L. H.; Licini, G.; Moberg, C. Chiral, Enantiopure Aluminum(III) and Titanium(IV) Azatranes. Eur. J. Inorg. Chem. 2006, 2006, 1032−1040. (104) Biela, T.; Duda, A.; Penczek, S. Control of Mn, Mw/Mn, EndGroups, and Kinetics in Living Polymerization of Cyclic Esters. Macromol. Symp. 2002, 183, 1−10. (105) Save, M.; Schappacher, M.; Soum, A. Controlled Ring-Opening Polymerization of Lactones and Lactides Initiated by Lanthanum Isopropoxide, 1. General Aspects and Kinetics. Macromol. Chem. Phys. 2002, 203, 889−899. (106) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S. Intermolecular Chain Transfer to Polymer with Chain Scission: General Treatment and Determination of kp/ktr in L,L-Lactide Polymerization. Macromol. Rapid Commun. 1997, 18, 325−333. (107) Sarazin, Y.; Carpentier, J.-F. Discrete Cationic Complexes for Ring-Opening Polymerization Catalysis of Cyclic Esters and Epoxides. Chem. Rev. 2015, 115, 3564−3614. (108) Brocas, A.-L.; Mantzaridis, C.; Tunc, D.; Carlotti, S. Polyether Synthesis: From Activated or Metal-Free Anionic Ring-Opening Polymerization of Epoxides to Functionalization. Prog. Polym. Sci. 2013, 38, 845−873. (109) Krief, A.; Kremer, A. Synthesis of Alkali Metal Carboxylates and Carboxylic Acids Using “Wet” and “Anhydrous” Alkali Metal Hydroxides. Chem. Rev. 2010, 110, 4772−4819. (110) Labet, M.; Thielemans, W. Synthesis of Polycaprolactone: A Review. Chem. Soc. Rev. 2009, 38, 3484−3504. (111) Otera, J. Transesterification. Chem. Rev. 1993, 93, 1449−1470. (112) Filippatos, E.; Papadaki-Valiraki, A.; Roussakis, C.; Verbist, J. F. Synthesis and Antitumor Activity of Some New 2-Chloroethylnitrosoureas. Arch. Pharm. (Weinheim, Ger.) 1993, 326, 451−456. (113) Sheldrick, G. M. SHELXL-97; University of Göttingen: Göttingen, Germany, 1996.

(85) Sinenkov, M.; Kirillov, E.; Roisnel, T.; Fukin, G.; Trifonov, A.; Carpentier, J.-F. Rare-Earth Complexes with Multidentate Tethered Phenoxy-Amidinate Ligands: Synthesis, Structure, and Activity in Ring-Opening Polymerization of Lactide. Organometallics 2011, 30, 5509−5523. (86) Zhao, W.; Cui, D.; Liu, X.; Chen, X. Facile Synthesis of Hydroxyl-Ended, Highly Stereoregular, Star-Shaped Poly(lactide) from Immortal ROP of rac-Lactide and Kinetics Study. Macromolecules 2010, 43, 6678−6684. (87) Dyer, H. E.; Huijser, S.; Susperregui, N.; Bonnet, F.; Schwarz, A. D.; Duchateau, R.; Maron, L.; Mountford, P. Ring-Opening Polymerization of rac-Lactide by Bis(phenolate)amine-Supported Samarium Borohydride Complexes: An Experimental and DFT Study. Organometallics 2010, 29, 3602−3621. (88) Clark, L.; Cushion, M. G.; Dyer, H. E.; Schwarz, A. D.; Duchateau, R.; Mountford, P. Dicationic and Zwitterionic Catalysts for the Amine-Initiated, Immortal Ring-Opening Polymerisation of racLactide: Facile Synthesis of Amine-Terminated, Highly Heterotactic PLA. Chem. Commun. 2010, 46, 273−275. (89) Buchard, A.; Platel, R. H.; Auffrant, A.; Le Goff, X. F.; Le Floch, P.; Williams, C. K. Iminophosphorane Neodymium(III) Complexes As Efficient Initiators for Lactide Polymerization. Organometallics 2010, 29, 2892−2900. (90) Kramer, J. W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.; Thomas, C. M.; Coates, G. W. Polymerization of Enantiopure Monomers Using Syndiospecific Catalysts: A New Approach To Sequence Control in Polymer Synthesis. J. Am. Chem. Soc. 2009, 131, 16042−16044. (91) Ma, H.; Spaniol, T. P.; Okuda, J. Rare-Earth Metal Complexes Supported by 1,ω-Dithiaalkanediyl-Bridged Bis(phenolato) Ligands: Synthesis, Structure, and Heteroselective Ring-Opening Polymerization of rac-Lactide. Inorg. Chem. 2008, 47, 3328−3339. (92) Arnold, P. L.; Buffet, J. C.; Blaudeck, R. P.; Sujecki, S.; Blake, A. J.; Wilson, C. C3-Symmetric Lanthanide Tris(alkoxide) Complexes Formed by Preferential Complexation and Their Stereoselective Polymerization of rac-Lactide. Angew. Chem., Int. Ed. 2008, 47, 6033−6036. (93) Ma, H.; Spaniol, T. P.; Okuda, J. Highly Heteroselective RingOpening Polymerization of rac-Lactide Initiated by Bis(phenolato)scandium Complexes. Angew. Chem., Int. Ed. 2006, 45, 7818−7821. (94) Amgoune, A.; Thomas, C. M.; Ilinca, S.; Roisnel, T.; Carpentier, J. F. Highly Active, Productive, and Syndiospecific Yttrium Initiators for the Polymerization of Racemic β-Butyrolactone. Angew. Chem., Int. Ed. 2006, 45, 2782−2784. (95) Hild, F.; Neehaul, N.; Bier, F.; Wirsum, M.; Gourlaouen, C.; Dagorne, S. Synthesis and Structural Characterization of Various N,O,N-Chelated Aluminum and Gallium Complexes for the Efficient ROP of Cyclic Esters and Carbonates: How Do Aluminum and Gallium Derivatives Compare ? Organometallics 2013, 32, 587−598. (96) Bakewell, C.; White, A. J. P.; Long, N. J.; Williams, C. K. 8Quinolinolato Gallium Complexes: Iso-selective Initiators for racLactide Polymerization. Inorg. Chem. 2013, 52, 12561−12567. (97) Chmura, A. J.; Davidson, M. G.; Frankis, C. J.; Jones, M. D.; Lunn, M. D. Highly Active and Stereoselective Zirconium and Hafnium Alkoxide Initiators for Solvent-Free Ring-Opening Polymerization of rac-Lactide. Chem. Commun. 2008, 1293−1295. (98) Alhashmialameer, D.; Ikpo, N.; Collins, J.; Dawe, L. N.; Hattenhauer, K.; Kerton, F. M. Ring-Opening Polymerization of racLactide Mediated by Tetrametallic Lithium and Sodium DiaminoBis(phenolate) Complexes. Dalton Trans. 2015, 44, 20216−20231. (99) Park, T. G. Degradation of Poly(d,l-lactic acid) Microspheres: Effect of Molecular Weight. J. Controlled Release 1994, 30, 161−173. (100) Nakamura, T.; Hitomi, S.; Watanabe, S.; Shimizu, Y.; Jamshidi, K.; Hyon, S. H.; Ikada, Y. Bioabsorption of Polylactides with Different Molecular Properties. J. Biomed. Mater. Res. 1989, 23, 1115−1130. (101) Proikakis, C. S.; Tarantili, P. A.; Andreopoulos, A. G. Synthesis and Characterization of Low Molecular Weight Polylactic Acid. J. Elastomers Plast. 2002, 34, 49−63. N


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