Synthesis and Characterization of N,N,O-Tridentate Aminophenolate Zinc Complexes and Their Catalysis in the Ring-Opening Polymerization of Lactides

A series of aminophenolate ligands with various pendant groups and associated ethyl Zn complexes were synthesized and studied as catalysts for the ring-opening polymerization (ROP) of lactides (LAs). The thiophenylmethyl group (L⁴ZnEt) increased the catalytic activity more than the benzyl group (L¹ZnEt) did, and 2-fluorobenzyl (L³ZnEt) and 2-methoxybenzyl (L²ZnEt) groups had the opposite effect. In addition, the LA polymerization mechanism proved by Nuclear Magnetic Resonance and Density Function Theory was that LA was attracted by H···O bond of an α-hydrogen of the LA molecule and the phenoxyl oxygen of the catalyst. After the dissociation of amino group from the Zn atom, the benzyl alcohol initiated LA without replacing the ethyl group of Zn complex. It is the first case where the ethyl group is regarded as a ligand and cannot be replaced by benzyl alcohol, and this information is very important for the mechanism study of ROP.

Introduction

Because petrochemical polymers such as polystyrene, polypropylene, polyethylene, and poly(vinyl chloride) can be produced easily and cheaply, they are widely used as disposable packaging materials. Because these petrochemical polymers need more than a hundred years to degrade into innocuous soil manure (Rochman et al., 2013), polymer pollution has become a serious problem (Romer, 2010; Romer and Tamminen, 2014; Ladewig et al., 2015; Zhang et al., 2017). The replacement of non-biodegradable polymers with biodegradable materials is therefore a popular field of research (Levis and Barlaz, 2011). Poly(lactide) (PLA) is a biopolymer designed to ameliorate pollution by petrochemical plastics. Owing to its biodegradability, biocompatibility, and permeability, PLA is commonly used for various purposes, such as humidity detection (Sun et al., 2007), MRI contrast agents (Patel et al., 2008), cell/tissue anti-adhesion (Lih et al., 2015), nanocomposites (Raquez et al., 2013), drug delivery (Khemtong et al., 2009), blood circulation (Ma et al., 2008), bone replacement (Simpson et al., 2007), and tissue engineering (Place et al., 2009). Ring-opening polymerization (ROP) by using metal complexes (Bellemin and Dagorne, 2014; Guillaume et al., 2015; Sarazin and Carpentier, 2015; Huang et al., 2016; Fuoco and Pappalardo, 2017; Redshaw, 2017) as catalysts is a common method for the efficient synthesis of PLA. Because no cytotoxic metal residue is required in PLA for the biomaterials, the use of a non-cytotoxic metal such as zinc (Williams et al., 2003; Romain and Williams, 2014; Yang et al., 2015; Binda et al., 2016; Ebrahimi et al., 2016; Thevenon et al., 2016; Wang et al., 2016) in lactide polymerization has been investigated widely. Ligands are crucial for the catalyst design because their catalytic activity can be increased. In a study of Zn complexes (Williams et al., 2003) bearing tridentate aminophenolate ligands, a high catalytic activity was observed during rac-lactide (rac-LA) polymerization, as shown in Figure 1A. As in previous studies, the tetradentate aminophenol ligands reacted with Zn[N(SiMe₃)₂]₂ and four coordinated Zn complexes with a non-coordinated fourth amino group were obtained, as shown in Figures 1B–D. According to the polymerization results of Figures 1B–E, the steric bulky substituents on the phenolate ring, the fourth coordinated amino groups, and chiral N-alkyl groups can increase the stereoselectivity of rac-LA polymerization, and maintain high catalytic activity. A survey of the coordination behavior of Zn complexes (Gao et al., 2013) revealed that five and six are the possible coordination numbers for these complexes, and even seven-coordinated Zn complexes were reported (Vaiana et al., 2007). It would be worthwhile to design the fourth coordinated group of tetradentate aminophenol ligands to interact with the Zn atom, and influence their catalytic activities. In this study, a series of aminophenol ligands and associated Zn complexes were synthesized, and their application in LAs polymerization was investigated.

FIGURE 1

Figure 1. Zn complexes bearing tridentate (A; Williams et al., 2003), (E; Hu et al., 2018), and tetradentate (B–D; Yang et al., 2015) aminophenolate ligands.

Results and Discussion

Syntheses and Characterization

A series of N¹-alkyl- N²,N²-dimethylethylene-1,2-diamines was synthesized by using NaBH₄ to reduce 2-alkylideneamino-N,N-dimethylethylen-1-amines that were synthesized by condensing the aldehyde derivatives with dimethylethylenediamine in ethanol. All ligands L¹-H–L⁴-H were prepared by refluxing a mixture of N¹-alkyl- N²,N²-dimethylethylene-1,2-diamine, para-formaldehyde, and 2,4-bis(α,α-dimethylbenzyl)-phenol (Figure 2). All the ligands reacted with 1.1 equivalents of ZnEt₂ in THF at 0°C to produce a moderate yield (74–83%) of Zn compounds after hexane washing. The Zn complex synthesis can be identified by the ¹H NMR spectrum. The peaks of the methylene groups of HO[(^tBu)₂-Ph]CH₂N and NCH₂Ar from the ¹H NMR spectrum are singlet in ligands and doublet of doublets in Zn complexes, and the proton of PhO-H (10.43–10.15 ppm) disappeared after ZnEt₂ was added. The formulas and structures of the compounds were confirmed on the basis of ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectra.

FIGURE 2

Figure 2. Synthesis of the ligands L¹-H–L⁴-H and associated Zn complexes.

The X-ray structure of L¹ZnEt (CCDC 1574177), Figure 3 revealed a four-coordinated, mononuclear Zn complex with one ethyl group and one aminophenolate ligand. In structural chemistry, the index for four-coordinated complexes (τ₄) is the number that indicates the geometry of the coordination center (Yang et al., 2007). L¹ZnEt showed an intermediate structure varying in geometry between trigonal pyramidal and seesaw (C_2v, θ₆ = 90°), with the corresponding τ₄ value being 0.74. The distances between the Zn(1) atom and C(37) O(1), N(1), and N(2) atoms were 1.983(3), 1.9583(18), 2.159(2), and 2.121(2) Å, respectively.

FIGURE 3

Figure 3. Molecular structure of L¹ZnEt (ellipsoids drawn at the 50% probability, H atoms omitted for clarity). Selected bond lengths (Å) and angles (degrees): Zn(1)-O(1), 1.9583(18); Zn(1)-N(1), 2.159(2); Zn(1)-N(2), 2.121(2); Zn(1)-C(37), 1.983(3); O(1)-Zn(1)-N(1), 95.61(8); O(1)-Zn(1)-N(2), 102.08(9); O(1)-Zn(1)-C(37), 114.38(10); C(37)-Zn(1)-N(1), 134.15(11); C(37)-Zn(1)-N(2), 119.88(11); N(2)-Zn(1)-N(1), 83.55(9).

Ring Opening Polymerization of LAs

The catalytic activity of Zn complexes for L-LA and rac-LA polymerizations with benzyl alcohol (BnOH) as the initiator under a nitrogen atmosphere was investigated, and the results are given in Table 1. As shown by entries 1–4 in Table 1, all Zn complexes were active for L-LA polymerization at 25°C, producing polymers with narrow polydispersity indexes (Ð, 1.08–1.13). L⁴ZnEt had the most controllability of the polymer molar mass with similar values of Mn_cal, Mn_NMR, and Mn_GPC. However, the difference of catalytic activity of these Zn complexes was initially unclear. To determine the difference, the polymerization temperature was set at 100°C and polymerization was terminated after 2 h (entries 5–8 in Table 1). The catalytic activity of L-LA polymerization was in the following order: L⁴ZnEt > L¹ZnEt > L³ZnEt > L²ZnEt. The results revealed that the thiophenylmethyl group of L⁴ZnEt increased the catalytic activity of Zn complexes more than the benzyl group of L¹ZnEt did, and that 2-fluorobenzyl and 2-methoxybenzyl groups decreased the catalytic activity of the complexes. From the polymer data (entries 1–8 in Table 1), the values of Mn_NMR, and Mn_GPC were smaller than that of Mn_cal, and this phenomenon may be attributed to the transesterification. As shown by entries 9–12 in Table 1, the catalytic rates for rac-LA polymerization were the same as those for L-LA polymerization. The rac-PLA polymerized by L⁴ZnEt showed the highest selectivity. According to the literature, electronic donating substituents increased the catalytic activity of Zn catalysts (Chen et al., 2006, 2011, 2015; Huang et al., 2006; Chuang et al., 2011; Fliedel et al., 2014a,b). In our case, the pendant-chelating substituents do not coordinate with the Zn atom in the solid state, but they increase the catalytic activity. Crystal data imply that the fourth coordinated groups, such as thiophenylmethyl, 2-methoxybenzyl, and 2-fluorobenzyl groups, still do not coordinate to Zn atom, just like the behavior of the Zn complexes (1B-1D) shown in Figure 1; however, the low stereoselectivity of rac-LA polymerization was also observed in our case. These phenomena are ascribed, possibly, to the size of the pendant substituents. The reasons will be discussed with DFT results later. In addition, the Ð values of these rac-PLAs were higher than that of PLAs (Table 1), and it may be attributed to more transesterification at a higher temperature in longer polymerization time.

TABLE 1

Table 1. Rinrg opening polymerizations of L-LA and rac-LA by Zn complexes.

The controllability of polymer molar mass by using L⁴ZnEt as a catalyst was investigated, and the results are given in Table 2. The linear relationship between Mn_GPC and ([LA] × conv.)/[BnOH] exhibited in Figure 4 shows that the polymerization of LA by using L⁴ZnEt as a catalyst was highly controllable with acceptable Ð values.

TABLE 2

Table 2. Ring opening polymerization of L-lactide by L⁴ZnEt with BnOH.

FIGURE 4

Figure 4. Linear plot of Mn_GPC vs. [LA] × conv. / [BnOH], with Ðindicated by blue dots (entries 1–7 in Table 2).

Kinetic Studies of Polymerization of L-LA by Using L³ZnEt

Kinetic studies of the polymerization of L-LA catalyzed by L³ZnEt in the presence of BnOH were performed to establish the reaction order with respect to [L-LA], [L³ZnEt], and [BnOH]. The experiments were performed at [L-LA]₀/[L³ZnEt]/[BnOH] ratios of 50:1:1, 50:1:2, 50:1:3, 50:1:4, 50:2:1, 50:3:1, and 50:4:1 ([L-LA] = 0.5 M in 10 mL toluene at 25°C), respectively. The preliminary results indicated that the reaction rate was a first-order dependent on [L-LA] for all seven ratios (Figures 5, 7) according to Equation (1), where k_obs = k_prop[L³ZnEt]^x[BnOH]^y and k_prop is the propagation rate constant. To determine the order (x) of [L³ZnEt], different [L³ZnEt] (10, 20, 30, and 40 mM) with the same [L-LA] (0.5 M) and [BnOH] (10 mM) were used (Figure 6). In addition, [BnOH] is regarded as a constant and incorporated into k₁ (k₁ = k_prop[BnOH]^y). The variable k_obs is first assumed to be as described by Equation (2). When k_obs is plotted against [L³ZnEt], a k₁ values of 15.034 (M⁻¹h⁻¹) for x = 1 is obtained (Figure 6). The variable k_obs is then assumed to be as described by Equation (3), where [L³ZnEt] is regarded as a constant and incorporated into k₂. Furthermore, various concentrations of [BnOH] (10, 20, 30, and 40 mM) with the same [L-LA] (0.5 M) and [L³ZnEt] (10 mM) (Figure 7) are used. When k_obs is plotted against [BnOH], a k₂ values of 13.227 (M⁻¹h⁻¹) for y = 1 is obtained (Figure 8). Next, k_prop is calculated to be 1,413 by averaging k₁/[BnOH] and k₂/[L³ZnEt]. L-LA polymerization using [L³ZnEt] and [BnOH] followed by an overall kinetic law given by Equation (4).

$\begin{array}{cc}-d[\text{L}-\text{LA}]/\text{d}t=k_{\text{obs}}[L-\text{LA}]& (1)\end{array}$

$\begin{array}{cc}{k}_{\text{obs}}=k_1[{\text{L}}^3\text{ZnEt}{]}^{\text{x}}& (2)\end{array}$

$\begin{array}{cc}{k}_{\text{obs}}=k_2[\text{BnOH}{]}^{\text{y}}& (3)\end{array}$

$\begin{array}{cc}-\text{d}[\text{L}-\text{LA}]/\text{dt}=1413[L-\text{LA}][L^{3}ZnEt][\text{BnOH}]& (4)\end{array}$

FIGURE 5

Figure 5. First-order kinetic plots for L-LA polymerizations vs. time in toluene (10 mL) at 25°C with various concentrations of complex [L³ZnEt].

FIGURE 6

Figure 6. Linear plot of k_obs vs. [L³ZnEt] for the polymerization of L-LA with [L-LA] = 0.5 M in toluene (10 mL) at 25°C.

FIGURE 7

Figure 7. First-order kinetic plots for L-LA polymerizations vs. time in toluene (10 mL) at 25°C with various concentrations of [BnOH].

FIGURE 8

Figure 8. Linear plot of k_obs vs. [BnOH] for the polymerization of L-LA with [L-LA] = 0.5 M in toluene (10 mL) at 25°C.

Proposed Mechanism

In order to realize the mechanism of LA polymerization by using Zn catalysts, the ¹H NMR study of the reaction of L⁴ZnEt with one equivalent BnOH was investigated as shown in Figure S19. In Figure S19D, the ¹H NMR spectrum revealed that L⁴ZnEt did not react with BnOH. This phenomenon was very surprising because most alkyl Zn complexes could react with alcohol to form Zn alkoxide complexes (Chuang et al., 2011; Fliedel et al., 2014a,b; Chen et al., 2015). To prove that the ethyl group of L⁴ZnEt could not be replaced by BnOH in polymerization process, the LA polymerization ([LA]:[Zn]:[BnOH] = 4:1:1, [LA] = 0.02 M in d⁸-toluene (0.5 mL) at 25°C) was monitored by ¹H NMR as shown in Figure 9, Figure S20, and the ethyl group was always at 0.25 ppm from the beginning to the end of the polymerization (Figure S20). When these ¹H NMR spectra revealed that the BnOH did not replace the ethyl group of the Zn complex in the LA polymerization process, we were curious about how BnOH initiated the monomer. To understand the polymerization mechanism, the interactions between the catalysts, the initiator, LA, and relative free energies of the intermediates of the catalytic reaction were studied using the DFT calculation.

FIGURE 9

Figure 9. ¹H NMR spectra of the LA polymerization ([LA]:[Zn]:[BnOH]=4:1:1, [LA] = 0.02 M in d⁸-toluene (0.5 mL) at 25°C).

DFT Calculations: Mechanistic Study of LA Polymerization by Using Zn Complexes Bearing Aminophenolate Ligands as Catalysts

In our DFT calculations, the cumyl group at the 4 position of the phenol ring L²ZnEt was simplified to a hydrogen atom, since it is far from the zinc catalytic center. The initiator was replaced by a methanol molecule because different alcohol molecules often show similar activity (Chang et al., 2015). The mechanism is shown in Figure 10.

FIGURE 10

Figure 10. Mechanism of L-lactide polymerization with N,N,O-tridentate aminophenolate Zn complexes as catalysts and benzyl alcohol as an initiator.

When the catalyst reacted with LA, the LA molecule was initially stably bonded in an open pocket formed between the R¹ group on the 2 position of the phenol ring and the pendant R² group to form intermediate I as shown in Figure 11. The major interaction to stabilize this structure was found to be a unique hydrogen bond between an α-hydrogen of the LA molecule and the phenoxyl oxygen of the catalyst with a H···O distance of 2.204 Å. I then went through two possible pathways to form key catalytic intermediate IV. One of possible pathways is that the NMe₂ group dissociated from the Zn center to produce an empty space to be coordinated by a carbonyl oxygen atom with a Zn-O bond (2.383 Å) to form intermediate II. After that, the initiator came into the complex to form intermediate IV. Another possible pathway is that the initiator (ROH) bonded to the Zn complex with a coordination bond between its oxygen atom and the Zn center (2.508 Å) and a hydrogen bond between the hydroxyl hydrogen atom of ROH and phenoxyl oxygen of catalyst to form intermediate III. The NMe₂ group subsequently left the Zn atom to make a rearrangement to generate IV. In intermediate IV, the O atom of the initiator was close to the carbonyl group of LA with a distance of 2.855 Å to facilitate the ring opening reaction. Moreover, the hydrogen bond between the initiator and the phenoxyl O atom of Zn catalyst stabilized this transient intermediate structure (IV) and also activated the initiator. After the LA ring opening, a new hydroxyl group formed at one end of the polymer (or oligomer) and interacted with phenoxyl oxygen through a hydrogen bond with a coordination between the zinc center and the carbonyl group on the other end to form intermediate V. The NMe₂ group then came back to re-coordinate on the zinc center to assist the leaving of the carbonyl group of the polymer to form intermediate VI and re-activated the catalyst. Finally, another LA monomer came in to reform intermediate III to continue the polymerization reaction or to expel the polymer from the catalyst to regenerate intermediate I to finish one catalytic cycle.

FIGURE 11

Figure 11. The stick (A) and spacefill (B) model of intermediate I.

From the thermochemical data of our DFT studies as shown in Figure 12 and Table 3, it can be found that to generate IV from I, the pathway through II was a little more favored, since the relative free energy of II + MeOH was slightly lower than that of III (2.276 kcal mol⁻¹ lower). This is because the molecular freedom (entropy) of II + methanol was much higher than that of III. After forming IV, the ring opening reaction to form V makes its relative free energy increase, due to the fact that the bulky polymer chain was restricted on the catalyst. However, after the carbonyl group dissociated from the zinc atom, the relative free energy of VI decreased dramatically. This large free energy decrease is a strong driving force to make the catalyst open the lactide ring.

FIGURE 12

Figure 12. The relative free energy level diagram of all intermediates in the mechanism. M stands for methanol.

TABLE 3

Table 3. Thermochemical data of intermediates derived from DFT calculations^a.

From our DFT studies, some important experimental phenomena can be well-explained. First, the stability of the basic Et group on the Zn atom of the catalyst can be explained by noting that the acidic proton of the initiator (RO-H) was retained on the phenoxyl oxygen of catalyst by hydrogen bond and the crowded structures of the catalysts (III, IV, and VI). These protected the Et group on the Zn atom from the acid proton. Second, the relative activities of the four catalysts can also be rationalized, possibly by the size of the pocket for accepting LA on the catalysts found in intermediate I, besides the electronic effects on the zinc center. The size of the pocket decides the easiness of LA entering the catalyst center, and the smaller pocket size also makes monomer coordination slightly different between D-LA and L-LA to reveal the higher stereselectivity in rac-LA polymerization. The pocket sizes of the four catalysts according to the sizes of the pendant groups and the percent buried volume (%V_bur) of the ligands with a sphere radius of 7 Å measured from the optimized L'ZnEt structures (Supporting Information) are in the following order: L^4′ZnEt (32.6%) > L^1′ZnEt (32.8%) > L^3′ZnEt (33.4%) > L^2′ZnEt (33.6%) (Falivene et al., 2016). This order is coincident with that of their catalytic activities and opposed to stereselectivity.

Conclusion

In this study, a series of aminophenolate ligands with various pendant groups and associated ethyl Zn complexes were synthesized to investigate the effect of pendant groups on the catalytic activity of the complexes during LA polymerization. The thiophenylmethyl group (L⁴ZnEt) increased the catalytic activity more than the benzyl group (L¹ZnEt) did, and 2-fluorobenzyl (L³ZnEt) and 2-methoxybenzyl (L²ZnEt) groups showed the opposite effect. The new LA polymerization mechanism was proven by NMR study and DFT calculation, which showed that LA was attracted by the H···O bond of an α-hydrogen of the LA molecule and the phenoxyl oxygen of the catalyst. After the dissociation of the amino group from the Zn atom, the benzyl alcohol initiated LA without replacing the ethyl group of the Zn complex. It is the first case that the ethyl group is regarded as a ligand and cannot be replaced by benzyl alcohol. This information provides researchers with another possible mechanisms of ROP by using alkyl zinc complexes as catalysts.

Experiment Section

General

Standard Schlenk techniques and a N₂-filled glove box were used throughout the isolation and handling of all the compounds. Solvents, L-LA, and deuterated solvents were purified prior to use. N,N-dimethylethylenediamine, 2,4-bis(α,α-dimethylbenzyl)phenol, sodium borohydride, benzaldehyde, 2-methoxybenzaldehyde, 2-fluorobenzaldehyde, 2-thiophenecarboxaldehyde, and diethyl zinc were purchased from Aldrich. ¹H and ¹³C NMR spectra were recorded on a Varian Unity Inova-600 (600 MHz for ¹H and 150 MHz for ¹³C) or a Varian Mercury-400 (400 MHz for ¹H and 100 MHz for ¹³C) spectrometer with chemical shifts given in ppm from the internal tetramethylsilane or the central line of CDCl₃. Gel permeation chromatography (GPC) measurements were performed on a Jasco PU-2080 plus system equipped with a RI-2031 detector using THF (high-performance liquid chromatography grade) as an eluent (flow rate 1.0 mL/min, at 40°C). The chromatographic column was Phenomenex Phenogel 5 μm 103 Å, and the calibration curve used to calculate Mn(GPC) was produced from polystyrene standards. The GPC results were calculated using the Scientific Information Service Corporation (SISC) chromatography data solution 3.1 edition.

Ligands and Associated Zn Complexes Synthesis

Synthesis of L¹-H

N,N-Dimethylethylenediamine (0.88 g, 10 mmole) and benzaldehyde (1.06 g, 10 mmole) were refluxed in ethanol (20 mL) for 6 h. When the solution was cooled to 25°C, sodium borohydride (0.42 g, 11 mmole) was added and stirred overnight to form N¹-benzyl-N²,N²-dimethylethane-1,2-diamine. After removing the precipitate from the solution, 2,4-bis(α,α-dimethylbenzyl)phenol (3.3 g, 10 mmole) and paraformaldehyde (0.45 g, 15 mmole) were transferred into the solution and refluxed for 12 h. Volatile materials were removed under vacuum to produce yellow mud. The mud was dissolved in CH₂Cl₂ (20 mL) and the solution was washed with water (3 × 40 mL), and the solvent removed at reduced pressure to produce the white powders. Yield: 3.08 g (59%). ¹H NMR (CDCl₃, 400 MHz): δ10.42 (1H, s, ArOH), 7.27 (5H, m, PhH), 7.23 (4H, m, PhH), 7.17 (5H, m, PhH), 6.91 (1H, m, PhH), 6.89 (1H, m, ArH), 6.75 (1H, d, ArH), 3.60 (2H, s, NCH₂Ar), 3.37 (2H, s, NCH₂Ph), 2.39 (2H, t, J = 4 Hz), NCH₂CH₂N), 2.27 (2H, t, J = 4 Hz, NCH₂CH₂N), 1.96 (6H, s, NCH₃), 1.68 (12H, s, C(CH₃)₂Ph). ¹³C NMR (CDCl₃, 100 MHz): δ153.46 (ArC-OH), 151.65, 151.46, 139.50, 137.52, 135.31, 129.69, 128.14, 127.80, 127.55, 127.07, 126.70, 126.37, 125.66, 125.30, 124.62, 124.54, 122.19 (ArC and PhC), 57.85 (NCPh), 57.37 (CN(CH₃)₂), 55.82 (ArCN), 49.84 (NCCN), 45.07 (CN(CH₃)₂), 42.40, 41.99 (ArC(CH₃)₂Ph), 31.10, 29.44 (ArC(CH₃)₂Ph). Anal. Calc. (found) for C₃₆H₄₄N₂O: C 82.76% (83.03%), H 8.35% (8.52%), N 5.36% (5.38%).

Synthesis of L²-H

We used a method similar to that for L¹-H, except 2-methoxybenzaldehyde was used to replace benzaldehyde. Yield: 2.86 g (52%). ¹H NMR (CDCl₃, 400 MHz): δ10.22 (1H, s, ArOH), 7.24 (5H, m, PhH), 7.20 (4H, m, PhH), 7.16 (3H, m, PhH), 6.88 (1H, m, ArH), 6.78 (2H, m, PhH), 6.71 (1H, d, ArH), 3.64 (3H, s, PhOCH₃), 3.59 (2H, s, NCH₂Ar), 3.54 (2H, s, NCH₂Ph), 2.41 (2H, t, J = 4 Hz, NCH₂CH₂N), 2.18 (2H, t, J = 4 Hz, NCH₂CH₂N), 2.01 (6H, s, NCH₃), 1.67 (12H, s, C(CH₃)₂Ph). ¹³C NMR (CDCl₃, 100 MHz): δ157.92 (PhCOCH₃), 153.62 (ArC-OH), 151.64, 151.45, 139.42, 135.06, 131.63, 128.68, 127.76, 127.45, 126.66, 125.84, 125.65, 125.26, 125.23, 124.47, 124.36, 122.14, 120.26, 110.15 (ArC and PhC), 58.09 (NCPh), 56.08 [CN(CH₃)₂], 54.96 (ArCN), 53.05 (PhOCH₃), 50.18 (NCCN), 45.23 [CN(CH₃)₂], 42.34, 41.86 [ArC(CH₃)₂Ph], 31.04, 29.35 [ArC(CH₃)₂Ph]. Anal. Calc. (found) for C₃₇H₄₆N₂O₂: C 79.79% (80.69%), H 8.30% (8.42%), N 5.04% (5.09%).

Synthesis of L³-H

We used a method similar to that for L¹-H, except 2-florobenzaldehyde was used to replace benzaldehyde. Yield: 3.39 g (63%). ¹H NMR (CDCl₃, 400 MHz): δ10.32 (1H, s, ArOH), 7.25 (5H, m, PhH), 7.22 (4H, m, PhH), 7.15 (3H, m, PhH), 6.93 (1H, m, PhH), 6.91 (1H, m, ArH), 6.76 (1H, m, PhH), 6.73 (1H, d, ArH), 3.61 (2H, s, NCH₂Ar), 3.47 (2H, s, NCH₂Ph), 2.42 (2H, t, J = 4 Hz, NCH₂CH₂N), 2.28 (2H, t, J = 4 Hz, NCH₂CH₂N), 1.98 (6H, s, NCH₃), 1.68 [12H, s, C(CH₃)₂Ph]. ¹³C NMR (CDCl₃, 100 MHz): δ162.51 (PhCF), 160.07 (ArC-OH), 153.48, 151.64, 151.42, 139.56, 135.30, 132.36, 132.32, 128.85, 128.77, 127.80, 127.52, 126.69, 126.34, 125.66, 125.31, 124.67, 124.54, 124.15, 124.11, 124.03, 122.03 (ArC and PhC), 57.31 (NCPh), 55.84 [CN(CH₃)₂], 49.89 (ArCN), 43.59 (NCCN), 45.06 [CN(CH₃)₂], 42.39, 41.99 [ArC(CH₃)₂Ph], 31.07, 29.42 [ArC(CH₃)₂Ph]. Anal. Calc. (found) for C₃₆H₄₃N₂OF: C 80.12% (80.26%), H 8.07% (8.04%), N 5.40% (5.20%).

Synthesis of L⁴-H

We used a method similar to that for L¹-H, except 2-thiophenecarboxaldehyde was used to replace benzaldehyde.Yield: 3.16 g (60%). ¹H NMR (CDCl₃, 400 MHz): δ10.15 (1H, s, ArOH), 7.27 (4H, d, PhH), 7.22 (4H, d, PhH), 7.15 (2H, m, PhH), 7.10 (1H, m, PhH), 6.87 (1H, t, CHCHCHS), 6.72 (1H, d, CHCHCHS), 6.63 (1H, d, CHCHCHS), 3.69 (2H, s, NCH₂Ar), 3.59 (2H, s, NCH₂Ph), 2.51 (2H, t, J = 4 Hz, NCH₂CH₂N), 2.32 (2H, t, J = 4 Hz, NCH₂CH₂N), 2.05 (6H, s, NCH₃), 1.68 [12H, s, C(CH₃)₂Ph]. ¹³C NMR (CDCl₃, 100 MHz): δ153.60 (ArC-OH), 151.53, 151.43, 139.68, 138.53, 135.23, 127.81, 127.56, 126.70, 126.64, 126.48, 125.64, 125.31, 125.14, 124.69, 124.61, 121.76 (ArC, PhC, and thioC), 56.70 (NCPh), 56.27 [CN(CH₃)₂], 50.88 (ArCN), 49.71 (NCCN), 45.19 [CN(CH₃)₂], 42.38, 42.05 [ArC(CH₃)₂Ph], 31.03, 29.48 [ArC(CH₃)₂Ph]. Anal. Calc. (found) for C₃₄H₄₂N₂OS: C 77.08% (77.52%), H 7.69% (8.04%), N 5.11% (5.32%).

Synthesis of L¹ZnEt

ZnEt₂ (0.617 g, 5.0 mmol) was added slowly to an ice cold (0°C) solution of L¹-H (2.6 g, 5.0 mmol) in THF (15 mL), and the solution was stirred for 3 h. Volatile materials were removed under vacuum to yield a yellow mud. The mud was washed with hexane (20 mL) and a white powder was obtained after filtration. Yield: 2.54 g (83%). ¹H NMR (CDCl₃, 400 MHz): δ7.48 (2H, d, PhH), 7.41 (3H, t, PhH), 7.27 (7H, m, PhH), 7.18 (1H, m, PhH), 7.10 (1H, t, PhH), 4.01, 3.93 (2H, d, J = 6 Hz, NCH₂Ph), 3.92, 3.23 (2H, d, J = 6 Hz, NCH₂Ph), 2.67 (1H, m, NCH₂CH₂N), 2.45 (1H, m, NCH₂CH₂N), 2.33 (1H, m, NCH₂CH₂N), 2.16 (1H, m, NCH₂CH₂N), 2.11 (3H, s, NCH₃), 1.98 [3H, s, C(CH₃)₂Ph], 1.71 [3H, s, C(CH₃)₂Ph], 1.68 [6H, s, C(CH₃)₂Ph], 1.33 (3H, t, ZnCH₂CH₃), 1.28 (3H, s, NCH₃), 0.12 (2H, m, ZnCH₂CH₃). ¹³C NMR (CDCl₃, 100 MHz): δ164.87 (ArC-OH), 152.52, 151.02, 136.60, 133.63, 132.30, 131.75, 131.59, 128.41, 128.41, 128.36, 128.15, 127.82, 127.69, 127.56, 127.28, 127.13, 126.98, 126.72, 125.88, 125.71, 124.98, 124.18, 124.84 (ArC and PhC), 59.96 (NCPh), 58.44 [CN(CH₃)₂], 57.25 (ArCN), 46.85 (NCCN), 45.15 [CN(CH₃)₂], 44.01, 42.14 [ArC(CH₃)₂Ph], 31.02, 26.78 [ArC(CH₃)₂Ph], 13.21 (ZnCH₂CH₃),−3.17 (ZnCH₂CH₃). Anal. Calc. (found) for C₃₈H₄₈N₂OZn: C 74.66% (74.31%), H 7.97% (7.88%), N 4.86% (4.56%).

Synthesis of L²ZnEt

We used a method similar to that for L²ZnEt, except L²-H was used to replace L¹-H. L²ZnEt followed the procedure of L²ZnEt. Yield: 2.51g (78%). ¹H NMR (CDCl₃, 400 MHz): δ7.45 (2H, d, PhH), 7.33 (2H, m, PhH), 7.19 (7H, m, PhH), 7.07 (2H, m, PhH), 6.93 (2H, m, PhH), 6.51 (1H, s, PhH), 4.14, 4.06 (2H, d, J = 6 Hz, NCH₂Ph), 3.91, 3.12 (2H, d, J = 6 Hz, NCH₂Ph), 3.76 (3H, s, PhOCH₃), 2.50 (2H, m, NCH₂CH₂N), 2.34 (2H, m, NCH₂CH₂N), 2.01 (3H, s, NCH₃), 1.95 [3H, s, C(CH₃)₂Ph], 1.64 [3H, s, C(CH₃)₂Ph], 1.61 [6H, s, C(CH₃)₂Ph], 1.32 (3H, t, ZnCH₂CH₃), 1.06 (3H, s, NCH₃), 0.09 (2H, m, ZnCH₂CH₃). ¹³C NMR (CDCl₃, 100 MHz): δ165.03 (PhCOCH₃), 164.14 (ArC-OH), 158.42, 152.59, 152.10, 150.76, 150.31, 133.85, 129.91, 128.23, 127.61, 127.48, 127.44, 126.89, 126.63, 125.69, 124.89, 124.12, 120.28, 111.00, 110.78 (ArC and PhC), 67.91 (PhOCH₃), 58.91 (NCPh), 57.52 [CN(CH₃)₂], 55.29 (ArCN), 52.47 (NCCN), 45.34 [CN(CH₃)₂], 42.08, 31.10 [ArC(CH₃)₂Ph], 29.39, 26.21 [ArC(CH₃)₂Ph], (ZnCH₂CH₃) 13.21,−3.63 (ZnCH₂CH₃). Anal. Calc. (found) for C₃₉H₅₀N₂O₂Zn: C 72.27% (72.71%), H 7.28% (7.82%), N 4.52% (4.35%).

Synthesis of L³ZnEt

We used a method similar to that for L³ZnEt, except L³-H was used to replace L¹-H.Yield: 2.58 g (82%). ¹H NMR (CDCl₃, 400 MHz): δ7.42 (2H, d, PhH), 7.33 (2H, m, PhH), 7.18 (7H, m, PhH), 7.11 (2H, m, PhH), 7.06 (2H, m, PhH), 6.48 (1H, s, PhH), 4.08, 3.94 (2H, d, J = 6 Hz NCH₂Ph), 3.91, 3.14 (2H, d, J = 6 Hz, NCH₂Ph), 2.47-2.44 (2H, m, NCH₂CH₂N), 2.08 (3H, s, NCH₃), 2.00-1.94 (2H, m, NCH₂CH₂N), 1.92 [3H, s, C(CH₃)₂Ph], 1.63 [3H, s, C(CH₃)₂Ph], 1.61 [6H, s, C(CH₃)₂Ph], 1.28 (3H, t, ZnCH₂CH₃), 1.10 (3H, s, NCH₃), 0.05 (2H, m, ZnCH₂CH₃). ¹³C NMR (CDCl₃, 100 MHz): δ164.91 (PhCF), 163.01 (ArC-OH), 160.57, 152.55, 150.87, 136.62, 134.07, 133.66, 130.53, 128.23, 127.56, 127.39, 126.96, 126.70, 125.86, 124.98, 124.19, 121.63, 119.53, 119.37, 115.87, 115.64 (ArC and PhC), 58.36 (NCPh), 57.24 [CN(CH₃)₂], 51.92 (ArCN), 46.11 (NCCN), 45.15 [CN(CH₃)₂], 42.15, 41.96 [ArC(CH₃)₂Ph], 31.04, 26.62 [ArC(CH₃)₂Ph], (ZnCH₂CH₃) 13.18,−3.68 (ZnCH₂CH₃). Anal. Calc. (found) for C₃₈H₄₇N₂OFZn: C 71.96% (72.20%), H 7.49% (7.49%), N 4.52% (4.43%).

Synthesis of L⁴ZnEt

We used a method similar to that for L⁴ZnEt, except L⁴-H was used to replace L¹-H. Yield: 2.29 g (74%). ¹H NMR (CDCl₃, 400 MHz): δ 7.40 (2H, d, J = 4 Hz, PhH), 7.29 (1H, d, J = 2 Hz, thio-H), 7.21-7.10 (7H, m, PhH), 7.05-6.99 (1H, m, PhH), 6.88 (1H, s, PhH), 6.56 (1H, s, PhH), 4.18, 3.98 (2H, d, J = 6 Hz, NCH₂Ph), 3.73, 3.31 (1H, d, J = 6 Hz, NCH₂Ph), 2.66, 2.46 (2H, m, NCH₂CH₂N), 2.30, 2.23 (2H, m, NCH₂CH₂N), 2.11 (3H, s, NCH₃), 1.89 [3H, s, C(CH₃)₂Ph], 1.65 [3H, s, C(CH₃)₂Ph], 1.62 [6H, s, C(CH₃)₂Ph], 1.35 (3H, s, NCH₃), 1.22 (3H, t, ZnCH₂CH₃), 0.01 (2H, m, ZnCH₂CH₃). ¹³C NMR (CDCl₃, 100 MHz): δ164.78 (ArC-OH), 152.49, 151.11, 136.79, 133.75, 133.65, 130.30, 128.10, 127.58, 127.21, 127.11,126.98, 126.73, 126.58, 125.97, 124.98, 124.17, 121.85 (ArC, PhC and thioC), 57.26 (NCPh), 57.10 [CN(CH₃)₂], 53.07 (ArCN), 47.49 (NCCN), 46.84 [CN(CH₃)₂], 44.28, 42.06 [ArC(CH₃)₂Ph], 30.99, 26.98 [ArC(CH₃)₂Ph], (ZnCH₂CH₃) 13.18,−3.84 (ZnCH₂CH₃). Anal. Calc. (found) for C₃₆H₄₆N₂OSZn: C 69.65% (69.74%), H 7.43% (7.48%), N 4.68% (4.52%).

General Procedures for the Polymerization of L-LA and rac-LA

A typical polymerization procedure was exemplified by the synthesis of entry 1 of Table 1. The mixture of L-lactide (0.72 g, 5 mmol) and BnOH (5 mmol) in 5 mL toluene was added in the solution of zinc complex in toluene (5 mL) at 25°C. After the solution was stirred for 6.5 h, the reaction was then quenched by adding to a drop of ethanol, and the polymer was precipitated pouring into n-hexane (20.0 mL) to give white solids. The white solid was dissolved in CH₂Cl₂ (2.0 mL) and then n-hexane (20.0 mL) was added to give white crystalline solid. For rac-PLA, the Pr values for the selectivity of the heterotactic PLA were identified through ¹H NMR spectra (5.0−5.2 ppm) of rac-PLA after decoupling at 1.57 ppm (methine group of rac-PLA) as shown in Figure S21.

X-Ray Crystallographic Study

X-ray diffraction data for a suitable crystal of L¹ZnEt was collected on an Oxford diffraction limited Gemini S with graphite-monochromated Mo-Ka (λ = 0.71073 Å) radiation. All data were collected at 150 K with the ω-scan techniques. The structure was solved by direct methods and refined using Fourier techniques. An absorption correction based on SADABS was applied. All non-hydrogen atoms were refined by full-matrix least squares on F² using the SHELXTL program package. Hydrogen atoms were located and refined by the geometry method. The cell refinement, data collection and reduction were done through the use of CrysAlisPro, Agilent Technologies, Version 1.171.37.31. The structure solution and refinement were performed by using SHELXS-97 and SHELXL-97, respectively. Molecular structure was generated by using the SHELXTL program.

DFT Calculation

The DFT calculation was carried out using the Gaussian09 program. The structures were built according to the X-ray structure of L¹ZnEt. Their geometry optimizations were carried out using B3LPY (Becke, 1993). The LanL2DZ basis set (Hay and Wadt, 1985a,b) was used for Zn atom, and the 6-31G(d) basis set (Ditchfield et al., 1971) was used for other atoms. The minimum energy stationary point was confirmed by frequency analysis with the same calculation level.

Author Contributions

Theoretical calculation was performed by K-HW and the rest of work was performed by W-YL under the supervision of C-CL and H-YC.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This study was supported by the Ministry of Science and Technology of Taiwan (Grant MOST 107-2113-M-037-001 and 104-2113-M-005-015-MY3) and Kaohsiung Medical University NSYSU-KMU JOINT RESEARCH PROJECT (NSYSUKMU 107-P010). We thank the Center for Research Resources and Development at Kaohsiung Medical University for instrumentation and equipment support.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2019.00189/full#supplementary-material

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