Synthesis, characterization, and catalytic activity of sodium ketminiate complexes toward the ring-opening polymerization of L-lactide

Abstract

Studies of the ring-opening polymerization of L-lactide (LA) using Na complexes with Schiff base ligands as catalysts have revealed high catalytic activity but poor controllability of the polymer molecular weight. In this study, Na complexes bearing ketiminate ligands instead of Schiff base ligands were synthesized and their application in LA polymerization was tested. The polymerization results revealed that the catalytic activity of Na complexes bearing ketiminate ligands was higher than that of Na complexes bearing Schiff base ligands, but the poor controllability of polymer molecular weight was still a drawback. However, the poor controllability could be improved by means of using a high concentration of initiators in the polymerization system at 0 °C for 1 min. L^Py–Na revealed the excellent controllability of polymer molecular weight depending on the ratio of [LA]/[initiator] (the molar averages of the number from 4500 to 35 000) with narrow polydispersity indexes, ranging from 1.29 to 1.35.

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Introduction

Environmental pollution caused by abandoned waste composed of petrochemical materials has led people to consider using renewable energy resources and environmentally friendly materials. Polylactide (PLA)¹ is a common replacement for petrochemical plastics because of its short degradation time and its renewable precursor lactide (LA). PLA is also used in protein encapsulation and delivery, hydrogels, medical applications, drug delivery systems, and microsphere development because of its biodegradability, biocompatibility, and permeability.² Many metal catalysts^3a–p and organic catalysts^3r–z have been used in the ring-opening polymerization (ROP) of LA, which is the main method of synthesizing PLA. However, metal residues in polymers must be managed when PLA is used as a biomaterial. Therefore, Na complexes⁴ (Fig. 1) have been extensively researched with regard to LA polymerization. In addition, the precursors of Na complexes are inexpensive, and the synthetic process is simple.

Fig. 1 Na complexes in ROP.

Recently, we reported the ROP of L-LA by using Na complexes (Fig. 1) bearing Schiff base ligands as catalysts, and the results revealed that these Na complexes were highly catalytically active. Compared with Schiff base ligands, ketimine ligands have a similar backbone, but exhibit a weak resonance effect. According to the literature (Fig. 2), Zn complexes bearing ketiminate ligands^5a,b demonstrated higher catalytic activity for LA polymerization than Zn complexes bearing Schiff base ligands did.^5c This led us to consider whether the catalytic activity of Na complexes in LA polymerization can be improved if the ketimine ligands replace Schiff base ligands. In this study, the preparation of a series of ketimine ligands (Scheme 1) with aryl and alkyl amines and associated Na complexes is described to discuss the relationship between the coordination of various amines and the catalytic activity for L-lactide polymerization.

Fig. 2 Comparison of catalytic activity of Zn complexes with ketimine and Schiff base ligands in ROP.

Scheme 1 Synthesis of ketimine ligands and associated Na complexes.

Results and discussion

Synthesis and characterization of Na complexes

All ligands were prepared by refluxing a mixture of 2,4-pentanedione and alkylamine (or aniline) with p-toluenesulfonic acid in EtOH.⁷ The ligands reacted with a stoichiometric quantity of NaN(SiMe₃)₂ in THF to correspondingly produce a moderate yield of Na compounds (Scheme 1). The formula and structure of Na complexes were confirmed through elemental analysis, ¹H and ¹³C NMR spectroscopy, and X-ray crystallographic analysis for L^Py–Na. The X-ray structure of L^Py–Na (Fig. 3) demonstrates that the Na complex is a tetramer and features a five-coordinate Na atom in distorted trigonal bipyramidal geometry with the tau (τ) parameter⁶ of 0.25. According to the literature,⁴ⁱ Na complexes bearing ketiminate ligands revealed a tetramer of cubic form that was similar to that of Na complexes bearing Schiff base ligands.

Fig. 3 Molecular structure of complex L^Py–Na as 20% ellipsoids (all of the hydrogen atoms were omitted for clarity). CCDC number: 1422660. Selected bond lengths (Å) and bond angles (deg): Na(1)–O(1) 2.255(2), Na(1)–O(4) 2.340(2), Na(1)–O(2) 2.358(2), Na(1)–N(1) 2.367(3), Na(1)–N(2) 2.404(3), O(1)–Na(1)–O(4) 91.70(8), O(1)–Na(1)–O(2) 90.18(8), O(4)–Na(1)–O(2) 90.18(8), O(1)–Na(1)–N(1) 81.74(8), O(4)–Na(1)–N(1) 133.38(9), O(2)–Na(1)–N(1) 135.61(9), O(1)–Na(1)–N(2) 152.09(9), O(4)–Na(1)–N(2) 99.28(9), O(2)–Na(1)–N(2) 115.18(9), and N(1)–Na(1)–N(2) 72.03(9).

Polymerization of L-lactide

The polymerizations of L-LA by using Na complexes with BnOH as an initiator in CH₂Cl₂ were investigated under a nitrogen atmosphere at 25 °C (entries 1–10, Table 1). The catalytic activity of Na complexes with ketiminate ligands was higher (1 min, conversion > 94%) than that of Na complexes bearing Schiff base ligands. However, the controllability of polymer molecular weight was also poor (the values of Mn_Cal., Mn_NMR, and Mn_GPC were not similar) with the broad polydispersity indexs (PDIs: 1.39–2.86), which is due to the transesterification accompanied by high catalytic activity. To improve the controllability of Na complexes, the experiments with more initiators at 0 °C was performed (entries 11–20, Table 1). The results revealed that L^Bn–Na, L^Bu–Na, L^C2O–Na, L^C3O–Na, and L^Py–Na displayed high catalytic activity, and their controllability of polymer molecular weight improved with the acceptable PDIs (1.29–1.48). However, L^C2N–Na, L^PhC2–Na, and L^PhC3–Na exhibited no catalytic activity in this condition. This revealed that aryl groups on N-substituent with bulky substituent and 2-(dimethylamino)ethyl groups on N-substituent of the ketiminate ligands were unfavorable for the catalytic activity of Na complexes for LA polymerization at 0 °C, and the reason may be that these N-substituents increased the repulsion around Al atom and further decreased the L-LA coordination with Al atom. This polymerization character is similar with Al complexes^3p bearing ketiminate ligands. To compare with Na complexes bearing Schiff base ligands, the polymerization comparison between L^Bu–Na and L³Na^4f (Fig. 1) was studied (Table 1, entries 26 and 27). The results revealed that the conversion was up to 80% in 1 min by using L^Bu–Na as a catalyst in the severe condition (0 °C, CH₂Cl₂ (30 mL), [LA] = 0.67 M, [LA] : [BnOH] : [Cat] = 200 : 4 : 0.2); however, L³Na was inactive in this condition. It proves that Na complexes bearing ketiminate ligands have greater catalytic activity than that of Na complexes bearing Schiff base ligands.

Table 1 Polymerizations of L-lactide by using Na complexes with BnOH as an initiator

Entry	Catalyst	Time (min)	Conv.^a (%)	MnCal^b	MnNMR^a	MnGPC^c	PDI^c
a Obtained from ^1H NMR analysis.b Calculated from the molecular weight of LA × [LA]0/[BnOH]0 × conversion yield + Mw(BnOH).c Obtained from GPC analysis and calibrated using the polystyrene standard. Values in parentheses are the values obtained from GPC × 0.58.d Reaction condition: room temperature, CH2Cl2 (10 mL), [LA] = 1.00 M, [LA] : [BnOH] : [Cat] = 100 : 1 : 0.125.e Reaction condition: 0 °C, CH2Cl2 (20 mL), [LA] = 1.00 M, [LA] : [BnOH] : [Cat] = 200 : 5 : 0.375.f Reaction condition: 0 °C, CH2Cl2 (15 mL), [LA] = 0.16 M, [LA] : [BnOH] : [Cat] = 25 : 1 : 1.g Reaction condition: 0 °C, CH2Cl2 (15 mL), [LA] = 0.66 M, [LA] : [BnOH] : [Cat] = 100 : 1 : 1.h Reaction condition: 0 °C, CH2Cl2 (15 mL), [LA] = 1.32 M, [LA] :[ BnOH] : [Cat] = 200 : 1 : 1.i Reaction condition: 0 °C, CH2Cl2 (15 mL), [LA] = 1.98 M, [LA] : [BnOH] : [Cat] = 300 : 1 : 1.j Reaction condition: 0 °C, CH2Cl2 (15 mL), [LA] = 2.64 M, [LA] : [BnOH] : [Cat] = 400 : 1 : 1.k Reaction condition: 0 °C, CH2Cl2 (30 mL), [LA] = 0.67 M, [LA] : [BnOH] : [Cat] = 200 : 4 : 0.2.
1^d	L^Bn–Na	1	99	14 500	18 200	10 600	2.57
2^d	L^Bu–Na	1	99	14 500	28 200	11 100	2.86
3^d	L^C2N–Na	1	99	14 500	6300	13 100	2.31
4^d	L^C2O–Na	1	99	14 500	17 300	20 100	1.95
5^d	L^C3O–Na	1	94	14 500	14 600	30 300	2.04
6^d	L^Py–Na	1	97	14 100	11 200	11 700	2.32
7^d	L^THF–Na	1	99	14 500	12 000	13 300	1.93
8^d	L^PhC2–Na	1	99	14 500	6600	17 500	2.24
9^d	L^PhC3–Na	1	99	14 500	22 600	20 100	1.98
10^d	L^PhiPr–Na	1	99	14 500	17 300	16 000	2.19
11^e	L^Bn–Na	1	99	5900	6400	11 000	1.65
12^e	L^Bu–Na	1	99	5900	5200	7600	1.46
13^e	L^C2N–Na	1	0	—	—	—	—
14^e	L^C2O–Na	1	99	5900	6200	4600	1.36
15^e	L^C3O–Na	1	99	5900	8000	6900	1.29
16^e	L^Py–Na	1	99	5900	5000	4000	1.08
17^e	L^THF–Na	1	40	2400	2400	1900	1.04
18^e	L^PhC2–Na	1	0	—	—	—	—
19^e	L^PhC3–Na	1	0	—	—	—	—
20^e	L^PhiPr–Na	1	99	5900	9800	65 300	1.13
21^f	L^Bu–Na	5	99	3700	7100	9000	1.19
22^g	L^Bu–Na	5	99	14 500	13 200	24 500	1.25
23^h	L^Bu–Na	5	99	28 900	22 200	35 400	1.26
24^i	L^Bu–Na	5	99	43 300	32 000	43 400	1.18
25^j	L^Bu–Na	5	99	57 700	46 100	54 200	1.10
26^k	L^Bu–Na	1	80	5900	9600	9200	1.17
27^k	L^3Na	1	0	—	—	—	—

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LA polymerization was satisfactorily controlled using L^Bu–Na as a catalyst, as demonstrated in the linear relationship between Mn_GPC and [LA]₀ × conv./[BnOH]₀ (entries 21–25, Table 1, Fig. 4), and polymers with low PDIs, ranging from 1.10–1.26 were obtained.

Fig. 4 Linear plot of Mn_GPC vs. [LA]₀/[BnOH], with polydispersity indexes indicated by closed circles (Table 1, entries 21–25).

The ¹H NMR spectrum of PLA prepared using L^Py–Na (entry 19, Table 1) showed that one benzyl group and hydroxy chain ends with an integral ratio of 5 : 2 for H_a and H_e, suggesting that initiation occurred through the insertion of the BnOH into L-LA (Fig. 5).

Fig. 5 ¹H NMR spectrum of PLLA (polymerization by L^Py–Na, entry 19, Table 1).

A survey of recent studies on LA polymerization by using Na complexes as catalysts is provides in Fig. 6. L^Bu–Na revealed the highest catalytic activity of L-LA polymerization with the poor controllability of polymer molecular weight and the broad PDI at room temperature; however, the controllability could be improved in the condition with more initiators at 0 °C. Only Na complexes^4k bearing 6,6′-(oxobis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenolate) ligand was active for L-LA polymerization at 0 °C in CH₂Cl₂ (5 min, conversion > 99%), compared with other Na complexes at room temperature. In this study, the catalytic activity of Na complexes bearing ketiminate ligands for LA polymerization (1 min, conversion > 99%) was higher than that of Na complexes^4k bearing 6,6′-(oxobis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenolate) ligand.

Fig. 6 Survey of recent studies on LA polymerization, in which Na complexes were used as catalysts.

Conclusions

A series of ketimine ligands and associated Na complexes were synthesized. These complexes had a cubic tetramer which was similar to that of Na complexes bearing Schiff base ligands. All Na complexes demonstrated high catalytic activity for L-LA polymerization but the poor controllability of polymer molecular weight with the broad PDIs at room temperature. The controllability of polymer molecular weight was improved using a high concentration of BnOH in the polymerization system at 0 °C. However, the Na complexes bearing the ketiminate ligands with 2-(dimethylamino)ethyl and aryl groups on the N-substituent of were inactive at 0 °C. Compared with other Na complexes mentioned in the literature,⁴ Na complexes bearing ketiminate ligands as catalysts had the highest catalytic activity for the L-LA polymerization at 0 °C.

Experimental section

General

Standard Schlenk techniques and a N₂-filled glovebox were used throughout the isolation and handling of all the compounds. Solvents, ε-caprolactone, and deuterated solvents were purified prior to use. 2,4-Pentanedione, p-toluenesulfonic acid, 2,4,6-trimethylaniline, 3,5-dimethylaniline, 2,6-diisopropylaniline, pyridin-2-ylmethanamine, (tetrahydrofuran-2-yl)methanamine, benzylamine, N,N-dimethylethane-1,2-diamine, 2-methoxyethan-1-amine, 3-methoxypropan-1-amine, tert-butylamine, and deuterated chloroform were purchased from Acros. Benzyl alcohol was purchased from Alfa. ¹H and ¹³C NMR spectra were recorded on a Varian Gemini2000-200 (200 MHz for ¹H and 50 MHz for ¹³C) spectrometer with chemical shifts given in ppm from the internal TMS or center line of CDCl₃ and C₆D₆. Microanalyses were performed using a Heraeus CHN-O-RAPID instrument. GPC measurements were performed on a Jasco PU-2080 PLUS HPLC pump system equipped with a differential Jasco RI-2031 PLUS refractive index detector using THF (HPLC grade) as an eluent (flow rate 1.0 mL min⁻¹, at 40 °C). The chromatographic column was JORDI Gel DVB 103 A, and the calibration curve was made by primary polystyrene standards to calculate Mn(GPC). L^Bn–H,⁷ L^C2N–H,⁷ L^Py–H,⁷ L^PhC2–H,⁷ L^THF–H,⁷ L^PhC3–H,⁷ L^Bu–H,^8a L^PhiPr–H,⁷ L^C2O–H,^8b and L^C3O–H,^8a,b were prepared following literature procedures.

Synthesis of L^Bn–Na

A mixture of L^Bn–H (3.79 g, 20 mmol) and sodium hydride (0.6 g, 20 mmol) in THF (30 mL), was stirred for 24 h. Volatile materials were removed under vacuum to give yellow powder, and then the residue was washed with hexane (50 mL) and a light yellow powder was obtained after filtration. Yield: 2.75 g (65%). ¹H NMR (C₆D₆, 200 MHz): δ 7.19–7.05 (4H, m, ArH), 4.71 (1H, s, β-H), 4.28 (2H, s, NCH₂Ph), 1.79 (3H, s, CH₃CN), 1.64 (3H, s, CH₃C=O). ¹³C NMR (C₆D₆, 50 MHz): δ 178.19 (C=O), 170.15 (C=C–N), 142.41, 128.89, 127.51, 126.50 (Ar), 97.02 (β-C) 54.31 (NCH₂Ph), 29.26 (CH₃CO), 20.70 (NCCH₃)₂. Anal. calcd (found) for C₁₁H₁₅N₂ONa: C, 68.23 (68.19); H, 6.68 (7.02); N, 6.63 (6.76)%. Mp: 125 °C.

Synthesis of L^Bu–Na

Using a method similar to that for L^Bn–Na except L^Bu–H was used in place of L^Bn–H. The white powder was obtained. Yield: 2.37 g (67%). ¹H NMR (C₆D₆, 200 MHz): δ 4.70 (1H, s, β-H), 2.11 (3H, s, CH₃CN), 1.88 (3H, s, CH₃C=O), 1.34 (9H, s, C(CH₃)₃. ¹³C NMR (C₆D₆, 50 MHz): δ 194.07 (C=O), 168.17 (C=C–N), 96.16 (β-C), 54.03 (NC(CH₃)₃), 32.01 (CH₃CO), 25.70 (NCCH₃)₂), 20.14 (NC(CH₃)₃). Anal. calcd (found) for C₉H₁₆NONa: C, 61.00 (61.19); H, 9.10 (8.79); N, 7.90 (7.76)%. Mp: 106 °C.

Synthesis of L^C2N–Na

Using a method similar to that for L^Bn–Na except L^C2N–H was used in place of L^Bn–H. The brown powder was obtained. Yield: 3.30 g (86%). ¹H NMR (C₆D₆, 200 MHz): δ 4.82 (1H, s, β-H), 3.10 (2H, br, NCH₂CH₂), 2.35 (2H, br, NCH₂CH₂), 2.01 (6H, s, N(CH₃)₂), 1.95 (3H, s, CH₃CN), 1.73 (3H, s, CH₃C=O). ¹³C NMR (C₆D₆, 50 MHz): δ 193.95 (C=O), 161.61 (C=C–N), 95.37 (β-C) 61.17 (NCH₂), 59.07 (NCH₂CH₂), 45.32 (N(CH₃)₂), 28.98 (CH₃CO), 18.59 (NC(CH₃)₂). Anal. calcd (found) for C₉H₁₇N₂ONa: C, 56.23 (55.99); H, 8.91 (9.02); N, 14.57 (14.76)%. Mp: 129 °C.

Synthesis of L^Py–Na

Using a method similar to that for L^Bn–Na except L^Py–H was used in place of L^Bn–H. The deep brown powder was obtained. Yield: 2.21 g (52%). ¹H NMR (C₆D₆, 200 MHz): δ 8.58 (1H, d, J = 6.00 Hz, Py-H), 7.03–6.68 (3H, m, Py-H), 4.81 (1H, s, β-H), 4.41 (2H, s, NCH₂Py), 1.94 (3H, s, CH₃CN), 1.68 (3H, s, CH₃C=O). ¹³C NMR (C₆D₆, 50 MHz): δ 190.62 (C=O), 162.09 (C=C–N), 150.00, 136.21, 128.48, 127.99, 127.50 (Py), 96.89 (β-C) 56.14 (NCH₂Py), 29.28 (CH₃CO), 20.88 (NC(CH₃)₂). Anal. calcd (found) for C₁₁H₁₃N₂ONa: C, 62.25 (61.99); H, 6.17 (6.09); N, 13.20 (13.56)%. Mp: 148 °C.

Synthesis of L^C2O–Na

Using a method similar to that for L^Bn–Na except L^C2O–H was used in place of L^Bn–H. The yellow powder was obtained. Yield: 2.58 g (72%). ¹H NMR (C₆D₆, 200 MHz): δ 4.85 (1H, s, β-H), 3.23–3.10 (7H, br, NCH₂CH₂OCH₃), 2.09 (3H, s, CH₃CN), 1.66 (3H, s, CH₃C=O). ¹³C NMR (C₆D₆, 50 MHz): δ 177.65 (C=O), 168.10 (C=C–N), 96.56 (β-C), 74.00 (CH₂OCH₃), 58.51 (CH₂OCH₃), 50.21 (NCH₂), 29.46 (CH₃C=O), 20.73 (NC(CH₃)₂). Anal. calcd (found) for C₈H₁₄NO₂Na: C, 53.62 (53.92); H, 7.88 (8.05); N, 7.82 (7.51)%. Mp: 116 °C.

Synthesis of L^C3O–Na

Using a method similar to that for L^Bn–Na except L^C3O–H was used in place of L^Bn–H. The yellow powder was obtained. Yield: 2.66 g (69%). ¹H NMR (C₆D₆, 200 MHz): δ 4.76 (1H, s, β-H), 3.28 (4H, br, NCH₂CH₂CH₂OCH₃), 3.06 (3H, s, OCH₃), 2.11 (3H, s, CH₃CN), 1.91 (2H, br, NCH₂CH₂CH₂OCH₃), 1.75 (3H, s, CH₃C=O). ¹³C NMR (C₆D₆, 50 MHz): δ 178.19 (C=O), 169.09 (C=C–N), 96.19 (β-C), 74.13 (CH₂OCH₃), 58.46 (CH₂OCH₃), 50.45 (NCH₂), 32.12 (NCH₂CH₂CH₂O), 29.13 (CH₃C=O), 20.73 (NC(CH₃)₂). Anal. calcd (found) for C₉H₁₆NO₂Na: C, 55.95 (55.92); H, 8.35 (8.21); N, 7.25 (7.69)%. Mp: 123 °C.

Synthesis of L^THF–Na

Using a method similar to that for L^Bn–Na except L^THF–H was used in place of L^Bn–H. The yellow powder was obtained. Yield: 1.84 g (45%). ¹H NMR (C₆D₆, 200 MHz): δ 4.89 (1H, s, β-H), 3.85–3.71 (3H, br, NCH₂CHOCH₂), 3.19–2.94 (2H, m, NCH₂), 2.12 (3H, s, CH₃CN), 1.74 (3H, s, CH₃C=O), 1.51–1.15 (4H, br, CH₂CH₂). ¹³C NMR (C₆D₆, 50 MHz): δ 176.78 (C=O), 166.96 (C=C–N), 96.47 (β-C), 80.58, 67.89 (CH₂OCH₂), 56.49 (NCH₂), 29.35 (CH₃C=O), 25.97, 25.92 (CH₂CH₂), 20.89 (NC(CH₃)₂). Anal. calcd (found) for C₁₀H₁₆NO₂Na: C, 58.52 (59.01); H, 7.86 (7.99); N, 6.82 (6.25)%. Mp: 132 °C.

Synthesis of L^PhC2–Na

Using a method similar to that for L^Bn–Na except L^PhC2–H was used in place of L^Bn–H. The white powder was obtained. Yield: 2.52 g (56%). ¹H NMR (C₆D₆, 200 MHz): δ 6.51 (1H, s, Ar-H), 6.16 (2H, s, Ar-H) 4.91 (1H, s, β-H), 2.14 (9H, br, CH₃CN, Ph(CH₃)₂), 1.68 (3H, s, CH₃C=O). ¹³C NMR (C₆D₆, 50 MHz): δ 191.76 (C=O), 180.69 (C=C–N), 154.07, 137.86, 120.16, 117.67 (Ar), 96.77 (β-C), 25.67 (CH₃C=O), 22.54 (NCCH₃), 21.51 (Ph(CH₃)₂). Anal. calcd (found) for C₁₃H₁₆NONa: C, 69.31 (69.03); H, 7.16 (6.95); N, 6.22 (6.53)%. Mp: 168 °C.

Synthesis of L^PhC3–Na

Using a method similar to that for L^Bn–Na except L^PhC3–H was used in place of L^Bn–H. The white powder was obtained. Yield: 2.68 g (56%). ¹H NMR (C₆D₆, 200 MHz): δ 6.79 (2H, s, Ar-H), 4.86 (1H, s, β-H), 2.19 (3H, s, CH₃CN), 2.02 (9H, br, Ph(CH₃)₃), 1.50 (3H, s, CH₃C=O). ¹³C NMR (C₆D₆, 50 MHz): δ 194.07 (C=O), 176.23 (C=C–N), 168.17, 127.92, 127.52, 127.44 (Ar), 98.16 (β-C), 32.52 (Ph(CH₃)₂), 31.05 (Ph(CH₃)) 25.70 (CH₃C=O), 20.14 (NCCH₃). Anal. calcd (found) for C₁₄H₁₈NONa: C, 70.27 (69.87); H, 7.58 (7.59); N, 5.85 (5.55)%. Mp: 175 °C.

Synthesis of L^PhiPr–Na

Using a method similar to that for L^Bn–Na except L^PhiPr–H was used in place of L^Bn–H. The brown powder was obtained. Yield: 2.11 g (75%). ¹H NMR (C₆D₆, 200 MHz): δ 7.31–7.12 (3H, m, Ar-H), 5.20 (1H, s, β-H), 3.03 (2H, Sep, J = 8 Hz, CH(CH₃)₂), 2.12 (3H, s, CH₃CN), 1.63 (3H, s, CH₃C=O), 1.22, 1.15 (12H, d, J = 8 Hz, CH(CH₃)₂). ¹³C NMR (C₆D₆, 50 MHz): δ 195.81 (C=O), 170.00 (C=C–N), 162.81, 146.26, 128.21, 123.44 (Ar), 95.46 (β-C), 29.52 (CH₃CO), 28.39 (CH(CH₃)₂), 24.50, 22.56 (CH(CH₃)₂), 19.00 (CH₃CN). Anal. calcd (found) for C₁₇H₂₄NONa: C, 71.92 (72.57); H, 9.05 (8.60); N, 4.98 (4.72)%. Mp: 196 °C.

X-ray crystallographic studies

The suitable crystals of complexes L^Py–Li were sealed in a thin-walled glass capillary under a dry nitrogen atmosphere and mounted on a Bruker AXS SMART 1000 diffractometer. Intensity data were collected in 1350 frames with increasing (width of 0.3° per frame). The absorption correction was based on the symmetry-equivalent reflections using the SADABS program. The space group determination was based on a check of the Laue symmetry and systematic absences and confirmed by using the structure solution. The structure was solved by direct method using an SHELXTL package. All non-H atoms were located from successive Fourier maps, and hydrogen atoms were refined using a riding model. Anisotropic thermal parameters were used for all non-H atoms, and fixed isotropic parameters were used for H atoms.

General procedures for the polymerization of L-lactides

A typical polymerization procedure was exemplified by the synthesis of entry 1 (Table 1) using complex L^Bn–Na as a catalyst. The polymerization conversion was analyzed by ¹H NMR spectroscopic studies. CH₂Cl₂ (10.0 mL) was added to a mixture of complex L^Bn–Na (0.01 g, 0.0125 mmol), BnOH (0.01 g, 0.1 mmol), and L-lactide (1.44 g, 10 mmol) at 25 °C. After the solution was stirred for 6.5 min, the reaction was then quenched by adding to a drop of ethanol, and the polymer was precipitated pouring into n-hexane (30.0 mL) to give white solids. The white solid was dissolved in CH₂Cl₂ (5.0 mL) and then n-hexane (70.0 mL) was added to give white crystalline solid. Yield: 1.24 g (86%).

Acknowledgements

This study is supported by Kaohsiung Medical University “Aim for the top 500 universities grant" under Grant No. KMU-DT103007, NSYSU-KMU JOINT RESEARCH PROJECT (NSYSU KMU 103-I004), NSYSU-KMU JOINT RESEARCH PROJECT, (#NSYSUKMU 104-P006), and the Ministry of Science and Technology (Grant MOST 104-2113-M-037-010). We thank Center for Research Resources and Development at Kaohsiung Medical University for the instrumentation and equipment support.

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Footnote

† Electronic supplementary information (ESI) available: Polymer characterization data, and details of the kinetic study. CCDC 1422660. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra00373g

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