Magnesium complexes bearing N,N-bidentate phenanthrene derivatives for the stereoselective ring-opening polymerization of rac-lactides

Abstract

Three unreported magnesium complexes (1: R = C₆H₅, 2: R = 2,6-Et₂C₆H₃, 3: R = 2-^tBuC₆H₄) bearing N,N-bidentate phenanthrene derivatives derived from Schiff bases were synthesized. These complexes were characterized using ¹H and ¹³C NMR spectroscopy and elemental analyses and employed for rac-lactide and L-lactide polymerization. Complex 1 showed the highest activity among these Mg complexes for the ring-opening polymerization (ROP) of L-lactide, and complex 3 showed the highest stereoselectivity for the ROP of rac-lactide, achieving partially heterotactic polylactide (PLA) with a P_r of 0.76. The polymerization kinetics utilizing 2 as a catalyst were researched in detail. The kinetics of the polymerization data revealed that the rate of polymerization was first-order with respect to monomer, and had an order of 0.97 with respect to catalyst. There was a linear relationship between the L-lactide conversion and the number-average molecular weight of PLA.

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Introduction

Biodegradable and biocompatible polymers have been exploited for a wide range of applications such as sutures, disposal containers, bone fracture fixation devices, controlled release drug carriers, scaffolds, textiles and tissue engineering.^1,2 In particular, polylactide (PLA) derived from lactic acid, which is a renewable resource, has proved to be one of the most promising polyesters exhibiting biological degradation.² The presence of two chiral centers in the lactide monomer results in different lactide stereoisomers, namely L-lactide (L-LA), D-lactide (D-LA) and meso-lactide (Fig. 1). The stereochemistry of the polymer chains influences PLA’s physical and chemical properties.^1b Commonly, PLA is synthesized by the ring-opening polymerization (ROP) of lactide initiated by metal complexes, such as some alkoxides of Sn,³ Al,^4,13,14 Zn,⁵ Mg,⁶ Fe,⁷ Ti,⁸ In,⁹ rare-earth metals,¹⁰ organocatalysts,¹¹ and enzymes.¹² Among them, biocompatible zinc and magnesium provoke researchers’ attention because they are essential nutrients and minerals for plants and humans.^1h Moreover, zinc and magnesium catalysts are highly effective initiators employed for the ROP of PLA.⁵ In this context, our group has reported some zinc complexes derived from β-diketone ligands.⁵ⁱ These complexes proved to be highly effective single-site living initiators for the well-controlled ROP of lactides. Furthermore, the more sterically hindered zinc catalysts offered effective control of the polymer microstructures in the ROP of rac-lactide. In the past twenty years, much effort has been expended in the selection of proper ancillary ligands to enhance the performance of initiators in polymerizations. A number of research workers have tried to expound the relation among rac-lactide (rac-LA), metal complexes and the tacticity of polymers (see Fig. 2).^13,14

Fig. 1 Stereoisomers of lactide.

Fig. 2 Initiators for the stereoselective ROP of lactide.

However, to our knowledge, little research on magnesium complexes bearing diamido derivatives derived from phenanthrene Schiff base derivatives (see Scheme 1) has been carried out for the stereoselective ROP of rac-LA. Based on the efficient uses of magnesium complexes,^1h,6b,d,e we believed that magnesium complexes bearing N,N-bidentate diamido ligands might be potential catalysts for the ROP of rac-LA. In this work, the preliminary results of the preparation of magnesium complexes bearing phenanthrene derivatives and their use as initiators to polymerize rac-LA in a controlled manner are reported.

Scheme 1 Synthesis of pro-ligands and Mg complexes.

Results and discussion

Synthesis of pro-ligands

As shown in Scheme 1, pro-ligands L₁, L₂ and L₃ were prepared in middling yields (35.2–64.7%) by the condensation reaction between phenanthraquinone and modified aniline via catalysis with titanic chloride and dabco in toluene according to the literature.¹⁵ The ¹H NMR spectra of the pro-ligands displayed two sets of signals that could be assigned to two different ortho-substituents at the phenyl rings. For example, the resonances δ 1.07 and 0.74 ppm in the ¹H NMR spectrum of pro-ligand L₂ were assigned to the two sets of primary protons from the ethyls (see Fig. 3) This revealed that these ligands possess Z/E configuration.¹⁶ A similar situation was also reported in Ar′,Ar–BIAN ligands.¹⁷

Fig. 3 Stacked 400 MHz ¹H NMR spectra of pro-ligand L₂ and complex 2, in THF-d₈.

Synthesis of magnesium complexes

As shown in Scheme 1, Mg complexes 1–3 were prepared easily in high yields (84.7–95.6%) by combining magnesium with corresponding pro-ligands in a glovebox and were isolated as red solids from THF. The magnesium was activated by iodine and rinsed with THF three times. All three complexes were sensitive to air and moisture. Complexes 1–3 were characterized using ¹H and ¹³C NMR spectroscopy in THF-d₈ at room temperature and elemental analysis. The ¹H and ¹³C NMR spectra of complexes 1–3 showed one ligand and three THF molecules were coordinated to magnesium in these complexes. For instance, the resonances (δ 3.58 and 1.73 ppm) in the ¹H NMR spectrum of complex 2 were assigned to the secondary protons in three THF molecules coordinating to complex 2 (see Fig. 3). Interestingly, the ¹H NMR spectra of 1–3 revealed that the phenyl groups of the ligands in these complexes were inequivalent. For example, two groups of resonances from ethyls (δ 2.52–2.18, 1.58, 1.05, 0.71 ppm) are observed in the ¹H NMR spectrum (see Fig. 3). It appeared the geometry of 2 in solution was “b” rather than “a” (Fig. S1†), i.e. the Mg atom in 2 exhibits a trigonal bipyramidal geometry and the two N donors occupied axial and equatorial positions. This was consistent with the single crystal structure of 2 in the solid state. Similar configurations have also been seen in salan aluminium complexes.^13e The geometry of complex 2 in the solid state was confirmed via X-ray diffraction analysis. The molecular structure is depicted in Fig. 4. Selected bond distances and angles and other crystallographic data are listed in Table 1 and Table S1,† respectively. X-Ray structural analysis revealed that complex 2 is mononuclear and the central magnesium atom is coordinated by two N atoms and three O atoms. In complex 2 (see Table 1), the Mg1–O1 bond length, 2.161(2) (Å), was the longest among these bond lengths; N1–Mg1–O2, N1–Mg1–N2, N2–Mg1–O3 and O2–Mg1–O3 angles were 116.10(10), 80.60(10), 96.62(10) and 110.67(9)°, respectively. The quantity of the distortion can be calculated by structural index parameter τ.¹⁸ The τ values range from 0 (perfectly square pyramidal) to 1 (perfectly trigonal bipyramidal). For complex 2, the τ value was 0.59, which suggests the magnesium is in a distorted trigonal bipyramidal environment. The C–N bond average distance of 1.397 Å for 2 (see Table 1) approximated that of 1.390 Å in BIAN magnesium complex 1 (ref. 19) reported previously. It showed that the ligand bonded in a dianionic diamido manner. In other words, the pro-ligand was a Schiff base, but upon reaction with Mg, oxidative addition occurred in which Mg(0) was oxidized to Mg(2⁺) and the Schiff base was reduced to a diamido ligand. Hence, the Mg complex should be referred to as a diamido magnesium complex.

Fig. 4 Perspective view of 2 with thermal ellipsoids drawn at the 30% probability level. Hydrogens are omitted for clarity.

Table 1 Selected bond lengths (Å) and angles (°) for complex 2

Mg1–N1	2.051(3)	N1–Mg1–O2	116.10(10)
Mg1–N2	2.071(3)	N1–Mg1–N2	80.60(10)
Mg1–O1	2.161(2)	N2–Mg1–O3	96.62(10)
Mg1–O2	2.078(2)	O2–Mg1–O3	110.67(9)
Mg1–O3	2.085(2)
C1–N1	1.384(4)
C14–N2	1.409(4)

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Lactide polymerization

All magnesium complexes were studied as catalysts for the ROP of L-LA or rac-LA. The polymerization reactions were performed in tetrahydrofuran or toluene, and representational polymerization data are recorded in Table 2. These magnesium complexes showed moderate to high activities (87.8–97.7% monomer conversion) with the co-catalysis of isopropanol at 70 °C. ¹H NMR and GPC were used to confirm the M_n values of the PLA. The number-averaged molecular weights (measured via ¹H NMR spectroscopy²⁰) were close to theoretical values (calculated from the monomer/magnesium complexes molar ratio). The activities of these complexes declined with the increase of substituent bulk on the phenyl rings. Among the three complexes, the monomer conversion for complex 1 was the highest under the same conditions (see Fig. 5, Table 2, Entries 1–6).

Table 2 Polymerization data of LA with complexes 1–3^a

Entry	Catalyst	Monomer	Temp. (°C)	t (h)	[LA]/[cat.]	Conv.^b (%)	Mn(calcd)^c × 10^−4	MnGPC^d × 10^−4	PDI^d	kapp (h^−1) × 10^−3	Pr
a The polymerization reactions were carried out in toluene solution except that several actions processed in THF at 25 and 40 °C, [LA]0 = 0.5 mol L^−1, [isopropanol]/[cat.] = 1.0.b Measured using ^1H NMR spectroscopy.c Computed from the 144 × [LA]0/[cat.]0 × conversion + Mw^isopropanol.d From GPC analysis and calibration against a polystyrene standard. The veritable value of Mn(calcd) could be computed according to the formula Mn = 0.58MnGPC.^20
1	1	L-LA	70	6	100	97.7	1.41	2.52	1.30	n.a.	0
2	2	L-LA	70	6	100	93.6	1.35	2.40	1.17	n.a.	0
3	3	L-LA	70	6	100	89.4	1.29	2.26	1.21	n.a.	0
4	1	L-LA	40	12	100	71.3	1.03	1.72	1.15	103.2	0
5	2	L-LA	40	12	100	64.0	0.92	1.54	1.10	82.5	0
6	3	L-LA	40	12	100	58.3	0.84	1.45	1.13	72.4	0
7	2	L-LA	40	12	75	70.5	1.05	1.71	1.11	107.6	0
8	2	L-LA	40	12	125	55.9	1.01	1.74	1.12	65.8	0
9	2	L-LA	40	12	150	51.7	1.12	1.94	1.10	53.7	0
10	1	rac-LA	70	6	100	95.6	1.38	2.31	1.35	n.a.	0.51
11	2	rac-LA	70	6	100	92.2	1.33	2.17	1.22	n.a.	0.55
12	3	rac-LA	70	6	100	87.8	1.26	2.21	1.28	n.a.	0.58
13	1	rac-LA	40	12	100	69.2	1.00	1.53	1.21	n.a.	0.56
14	2	rac-LA	40	12	100	61.9	0.89	1.57	1.13	n.a.	0.65
15	3	rac-LA	40	12	100	56.9	0.82	1.44	1.14	n.a.	0.71
16	3	rac-LA	25	20	100	52.4	0.75	1.29	1.13	n.a.	0.76
17	1	L-LA	90	16	400	98.0	5.64	9.85	1.40	n.a.	0
18	2	L-LA	90	18	400	92.3	5.32	9.77	1.33	n.a.	0
19	3	L-LA	90	18	400	87.5	5.04	8.82	1.38	n.a.	0

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Fig. 5 Kinetics of the ROP of L-LA by 2 with isopropanol at 40 °C in THF with ([LA]₀ = 0.5 mol L⁻¹; k_p = k_app/[cat.]₀). ■: L-LA, [cat.]₀ = 0.005 mol L⁻¹, k_app = 82.5 × 10⁻³ h⁻¹; □: L-LA, [cat.]₀ = 0.0067 mol L⁻¹, k_app = 107.6 × 10⁻³ h⁻¹; ●: L-LA, [cat.]₀ = 0.004 mol L⁻¹, k_app = 65.8 × 10⁻³ h⁻¹; ▲: L-LA, [cat.]₀ = 0.0033mol L⁻¹, k_app = 53.7 × 10⁻³ h⁻¹.

Stereoselective polymerization

We have also studied poly(rac-LA) (Table 2, Entry 16) from the homonuclear decoupled ¹H NMR spectrum of the methane fragment²¹ (see Fig. 6). The P_r (ref. 22) value, 0.76, certified that these polymer chains were heterotactic enriched. The results indicated that the P_r selectivities increased from 0.51 to 0.58 with the increase of the bulk of the substituents on the pro-ligands (from hydrogen atoms to tert-butyls on the phenyl rings) at 70 °C (see Table 2, Entries 10 and 12). The stereoregularity of the poly(rac-LA) was also affected by the reaction temperature. For complex 3, on reducing the temperature from 70 to 25 °C, the P_r value increased from 0.58 to 0.76 (Table 2, Entries 12 and 16).

Fig. 6 Homonuclear decoupled ¹H NMR spectrum of the methine part of poly(rac-LA) prepared using complex 3 at 25 °C, P_r = 0.76, (Table 2, Entry 16, 400 MHz, CDCl₃).

Kinetics of lactide polymerization

The variation of the kinetics of the ROP of L-LA with some conditions, such as monomer/initiator ratio, were researched in THF employing 2. The molecular weight (M_n) of the polymers increased linearly with increasing monomer conversion. The PDI values of these polymers remained low (1.05–1.10). This illustrated the living characteristic of the catalytic systems (Fig. 7). In order to calculate the order in initiator, the k_app value was plotted versus the concentration of complex 2 (see Fig. 8). For each condition, first-order kinetics in monomer was seen (see Fig. 8). So the ROP of LA by 2 speculatively agreed with the formula:

−d[LA]/dt = k_app[LA] (1)

where k_app = k_p[cat.]^x, and k_p is the propagation speed constant. In order to calculate the order in catalyst, ln k_app was plotted versus ln[cat.]₀ (Fig. 9). The order in catalyst was calculated as 0.97–1. So the ROP of LA by 2 followed the entire kinetic equation:

−d[LA]/dt = k_p[cat.]^0.97[LA] (2)

Fig. 7 Plots of PLA M_n and PDI in the light of L-LA conversion employing complex 2/isopropanol, [LA]₀/[cat.]₀ = 100, at 40 °C in THF.

Fig. 8 Kinetics of the ROP of L-LA by 1–3 with isopropanol at 40 °C in toluene with [LA]₀ = 0.5 mol L⁻¹, [cat.]₀ = 5 × 10⁻³ mol L⁻¹, [LA]₀/[cat.]₀ = 100, ▲: 1, k_app = 103.2 × 10⁻³ h⁻¹; ■: 2, k_app = 82.5 × 10⁻³ h⁻¹; ●: 3, k_app = 72.4 h⁻¹ × 10⁻³ h⁻¹.

Fig. 9 ln k_app versus the ln[cat.]₀ of 2/isopropanol initiator for the L-LA polymerization at 40 °C in THF.

In addition, complexes 1–3 were also examined as catalysts in high [LA]₀/[cat.]₀ molar ratio ([LA]₀/[cat.]₀ = 400) for the ROP of L-LA in toluene solution at 90 °C, where high degrees of monomer conversion (87.5–98.0%) were achieved (Table 2, Entries 17–19).

Mechanism of lactide polymerization

Chain end analysis of the oligomer of L-LA, which was prepared by the polymerization of L-LA at low monomer/initiator ratio ([LA]_t : [2]_t = 41 : 1) (Fig. 10) was measured using ¹H NMR spectroscopy. The integral ratio of the two peaks at δ 1.24 ppm (assigned to the methyl protons from the isopropoxycarbonyl end-group, “a” in Fig. 10) and δ 4.34 ppm (assigned to the methine proton neighboring the hydroxyl end-group, “d” in Fig. 10) approximated 6 : 1. This meant the aggregating chain was end-capped by an isopropyl ester with a hydroxyl.^14a,d In other words, the magnesium complex had transformed isopropoxy magnesium reactive species at the initiation of the aggregation, so the genuine initiator might be the isopropoxy magnesium species. This ROP of lactide using magnesium complexes adopted a coordination insertion mechanism.^6a

Fig. 10 ¹H NMR spectrum of oligomer of L-LA at low monomer/initiator ratio ([LA]_t : [2]_t = 41 : 1) in CDCl₃.

Experimental section

General

All experimental operations were performed using Schlenk techniques.^23,24 Elemental analysis was accomplished using a Varian EL microanalyzer; ¹H NMR and ¹³C NMR spectra were measured on Bruker AV 400M or 300M instruments at RT in THF-d₈ or CDCl₃ with TMS for compounds and polymers. Gel permeation chromatography (GPC) measurements were conducted with a Waters 515 GPC with CHCl₃ as the eluant (flow rate: 1 mL min⁻¹, at 35 °C). The molecular weight was adjusted through a PS standard. Crystallographic data were gathered and analyzed according to the experimental sections of ref. 14b–d, 25. Magnesium, titanic chloride, isopropanol, 9,10-phenanthrenequinone, 1,4-diazabicyclo[2.2.2]octane (dabco), aniline, 2,6-diethylaniline and 2-tert-butylaniline were purchased from J&K Scientific Ltd.

Synthesis of pro-ligands (general procedure)

To a stirred solution of aniline (50 mmol) and dabco (150 mmol) in toluene (130 mL), was added TiCl₄ (50 mmol) in toluene (25 mL) dropwise at 90 °C, then 9,10-phenanthrenequinone (25 mmol) was added. The reaction was maintained at 140–145 °C for 20–24 h and was then cooled to ca. 80 °C. Insoluble substance was removed by hot filtration and then the solvent was removed under vacuum. The product was isolated as a deep red solid by silica-gel column chromatography (V_petroleum : V_{ethyl acetate} = 6 : 1).

L₁. Yield 64.7%. ¹H NMR (400 MHz, THF-d₈, δ, ppm): 8.42 (d, J = 7.2 Hz, 1H, ArH), 8.28 (d, J = 7.9 Hz, 2H, ArH), 8.20 (t, J = 7.6 Hz, 1H, ArH), 7.56 (t, J = 7.4 Hz, 1H, ArH), 7.46 (t, J = 7.5 Hz, 1H, ArH), 7.21–6.84 (m, 11H, ArH), 6.69 (d, J = 7.8 Hz, 1H, ArH). ¹³C NMR (100 MHz, THF-d₈, δ, ppm): 162.2, 158.5, 149.1, 147.8, 136.0, 132.4, 133.6, 131.9, 131.8, 129.4, 128.6, 128.0, 127.4, 127.0, 126.8, 125.5, 124.6, 124.3, 123.6, 123.3, 122.7. Anal. calcd for C₂₆H₁₈N₂ (%): C, 87.12; H, 5.06; N, 7.82. Found: C, 87.15; H, 5.10; N, 7.86. HRMS (m/z): calcd for C₂₆H₁₈N₂: 358.15; found: 358.20 [M + H]⁺.

L₂. Yield 45.3%. ¹H NMR (400 MHz, THF-d₈, δ, ppm): 8.32 (d, J = 7.4 Hz, 1H, ArH), 7.93 (m, 2H, ArH), 7.49 (t, J = 7.4 Hz, 1H, ArH), 7.36 (t, J = 7.6 Hz, 1H, ArH), 7.09–6.94 (m, 3H, ArH), 6.92–6.84 (m, 5H, ArH), 6.68 (d, J = 7.9 Hz, 1H, ArH), 2.36 (ddt, J = 36.5, 15.0, 7.6 Hz, 4H, CH₂CH₃), 1.72–1.50 (m, 4H, CH₂CH₃), 1.07 (t, J = 6.0 Hz, 6H, CH₂CH₃), 0.74 (t, J = 6.0 Hz, 6H, CH₂CH₃). ¹³C NMR (100 MHz, THF-d₈, δ, ppm): 159.04, 134.86, 134.15, 133.62, 131.91, 131.40, 130.77, 130.43, 129.08, 128.78, 128.12, 127.41, 127.27, 125.42, 124.66, 123.61, 123.55, 122.80, 24.82, 23.39, 14.89, 13.36. Anal. calcd for C₃₄H₃₄N₂ (%): C, 86.77; H, 7.28; N, 5.95. Found: C, 86.66; H, 7.27; N, 5.90. HRMS (m/z): calcd for C₃₄H₃₄N₂: 470.27; found: 470.30 [M + H]⁺.

L₃. Yield 35.2%. ¹H NMR (400 MHz, THF-d₈, δ, ppm): 8.26 (d, J = 7.4 Hz 1H, ArH), 7.88 (dd, J = 13.2, 7.9 Hz, 2H, ArH), 7.59 (m, 1H, ArH), 7.51 (t, J = 7.5 Hz, 1H, ArH), 7.30 (m, 1H, ArH), 7.09–7.01 (m, 4H, ArH), 6.96–6.81 (m, 5H, ArH), 6.63 (d, J = 7.9 Hz, 1H, ArH), 1.25 (s, 9H, C(CH₃)₃), 1.13 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, THF-d₈, δ, ppm): 159.82, 146.57, 144.91, 135.56, 135.40, 135.11, 134.76, 124.49, 131.78, 131.05, 129.42, 129.10, 128.77, 127.40, 127.04, 124.07, 123.47, 123.41, 123.37, 122.50, 58.23, 54.59, 28.30, 27.25. Anal. calcd for C₃₄H₃₄N₂ (%): C, 86.77; H, 7.28; N, 5.95. Found: C, 86.69; H, 7.23; N, 5.89. HRMS (m/z): calcd for C₃₄H₃₄N₂: 470.27; found: 470.20 [M + H]⁺.

Synthesis of Mg complexes

A mixture of pro-ligand (10 mmol) and magnesium powder (50 mmol) activated with iodine in tetrahydrofuran (50 mL) was stirred for ca. 1 h in a glovebox at RT. The deep red solid complex was isolated through filtering and removing the solvent under vacuum.

1. Yield 94.5%. ¹H NMR (400 MHz, THF-d₈, δ, ppm): 8.38 (d, J = 7.4 Hz, 1H, ArH), 8.25 (t, J = 7.6 Hz, 2H, ArH), 7.62 (t, J = 7.5 Hz, 1H, ArH), 7.58–7.45 (m, 2H, ArH), 7.39 (t, J = 7.9 Hz, 1H, ArH), 7.12–6.95 (m, 9H, ArH), 6.91–6.82 (m, 2H, ArH), 3.61 (bs, 12H, α-CH₂(THF)), 1.75 (bs, 12H, β-CH₂(THF)). ¹³C NMR (100 MHz, THF-d₈, δ, ppm): 162.00, 135.21, 134.05, 132.77, 130.56, 129.85, 129.43, 129.22, 128.87, 128.33, 127.92, 127.80, 127.64, 126.39, 125.91, 125.72, 124.98, 124.40, 124.03, 123.88, 123.44, 69.02 (α-CH₂ (THF)), 26.43 (β-CH₂(THF)). Anal. calcd for C₃₈H₄₂MgN₂O₃ (%): C, 76.19; H, 7.07; N, 4.68. Found: C, 76.24; H, 7.11; N, 4.75. M.p. 189 °C.

2. Yield 91.0%. ¹H NMR (400 MHz, THF-d₈, δ, ppm): 8.26 (d, J = 7.5 Hz, 1H, ArH), 8.01 (t, J = 8.1 Hz, 2H, ArH), 7.59 (t, J = 7.4 Hz, 1H, ArH), 7.53–7.42 (m, 2H, ArH), 7.36 (t, J = 7.5 Hz, 1H, ArH), 6.99 (d, J = 7.4 Hz, 2H, ArH), 6.94–6.64 (m, 3H, ArH), 3.58 (bs, 12H, α-CH₂ (THF)), 2.52–2.18 (m, 4H, CH₂CH₃), 1.73 (bs, 12H, β-CH₂(THF)), 1.58 (dt, J = 15.3, 7.7 Hz, 4H, CH₂CH₃), 1.05 (t, J = 7.4 Hz, 6H, CH₂CH₃), 0.71 (t, J = 7.4 Hz, 6H, CH₂CH₃). ¹³C NMR (100 MHz, THF-d₈, δ, ppm): 159.65, 132.49, 132.11, 131.12, 129.92, 129.44, 128.89, 128.55, 128.27, 128.07, 127.83, 127.7, 127.55, 126.10, 125.81, 125.42, 124.43, 124.15, 123.91, 123.26, 122.95, 68.34 (α-CH₂(THF)), 28.73, 26.05 (β-CH₂(THF)), 23.96, 13.39, 13.32. Anal. calcd for C₄₆H₅₈MgN₂O₃ (%): C, 77.68; H, 8.22; N, 3.94. Found: C, 77.65; H, 88.19; N, 3.89. M.p. 213 °C. A crystal of 2 suitable for X-ray structural determination was grown in tetrahydrofuran solution.†

3. Yield 87.3%. ¹H NMR (400 MHz, THF-d₈, δ, ppm): 8.24 (d, J = 7.8 Hz, 1H, ArH), 7.94 (t, J = 7.9 Hz, 2H, ArH), 7.57 (m, 1H, ArH), 7.50–7.38 (m, 2H, ArH), 7.36 (t, J = 7.8 Hz, 1H, ArH), 6.96–6.60 (m, 5H, ArH), 3.55 (bs, 12H, α-CH₂(THF)), 1.70 (bs, 12H, β-CH₂(THF)), 1.23 (s, 9H, C(CH₃)₃), 1.10 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, THF-d₈, δ, ppm): 159.45, 133.00, 132.54, 131.03, 129.83, 129.35, 128.71, 128.48, 128.12, 128.00, 127.56, 127.66, 127.48, 126.01, 125.59, 125.33, 124.37, 124.26, 123.75, 123.11, 122.77, 66.87 (α-CH₂(THF)), 29.02, 28.95, 26.05 (β-CH₂(THF)). Anal. calcd for C₄₄H₅₄MgN₂O₃ (%): C, 77.35; H, 7.97; N, 4.10. Found: C, 77.41; H, 8.00; N, 4.14. M.p. 227 °C.

General procedure for lactide polymerization

In a typical polymerization reaction: magnesium complex (0.4 mmol) and isopropanol (0.4 mmol) in toluene (80 mL) were added in a flame-dried ampule including a magnetic stir bar. The ampule was placed in an oil bath at 70 °C. The solution was stirred for ca. 10 min, when the catalyst was activated by isopropanol and whereafter the selected quantity of lactide was added. The polymer was isolated by precipitating with cold methanol or refrigerated centrifuge, after the required length of reaction time. The solid was attained and dried in vacuo at 30 °C for 40 h.

Conclusions

In conclusion, three magnesium complexes 1–3 were synthesized in moderate yields from the reactions of magnesium with Schiff bases containing phenanthrene derivatives. These complexes were employed as catalysts for the polymerization of L-LA and rac-LA. Bulky substituents on the ligand affect the tacticities of the polymers. Microstructural analysis of the polymers catalyzed by these complexes revealed that the Schiff base ligands have a certain ability to affect the tacticity of the polymer, and this ability varies according to ligand bulk. In addition, kinetic analysis indicated that polymerizations of lactide by complex 2 are first-order with respect to monomer, and have an order of 0.97 with respect to catalyst.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (nos 21204082, 51173183, 51021003, 51321062); the Ministry of Science and Technology of China (no. 2011AA02A202); Jilin Science & Technology Department, Science and Technology Development Project (no. 20140204017GX); Science and Technology Bureau of Changchun City (no. 2013060).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic data and refinement for complex 2 in CIF format. CCDC 1029408. For ESI and crystallographic data in CIF format see DOI: 10.1039/c4ra12548g

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