Breaking the Paradox between Catalytic Activity and Stereoselectivity

Feb 2, 2018 - Polylactide (PLA), the popular bio-based plastics, has attracted more interest as a benign alternative for oil-based plastics. Lowering ...
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Breaking the Paradox between Catalytic Activity and Stereoselectivity: rac-Lactide Polymerization by Trinuclear Salen−Al Complexes Xuan Pang,† Ranlong Duan,† Xiang Li,† Chenyang Hu,†,‡ Xianhong Wang,† and Xuesi Chen*,† †

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Polylactide (PLA), the popular bio-based plastics, has attracted more interest as a benign alternative for oil-based plastics. Lowering the manufacture cost of PLA and improving the performance of PLA products are the essential challenges in PLA study. PLA stereocomplex is a promising choice to improve the thermal and mechanical properties. Efficient preparation of the PLA stereocomplex by the direct ring-opening polymerization (ROP) of inexpensive racemic lactide (rac-LA) requires the ROP catalysts with high activity and stereoselectivity. For this purpose, the novel trinuclear Salen−aluminum complexes were prepared in this work and exerted extraordinary improvement of catalytic activity and stereoselectivity at the same time. These complexes showed the excellent isoselectivity (typically Pm = 0.98), high activity (typically kp = 15.4 L mol−1 min−1), and low catalyst loading amount (0.01 mol %). The Tm of PLA was 219.9 °C, which is the highest value among the reported PLA derived directly from rac-LA until now. It may foreshadow a new vista in the reduction of overall production cost and performance improvement of PLA to be competitive with oil-based commodity or engineering plastics.

1. INTRODUCTION Synthetic biodegradable polyesters have attracted much interest in recent years as the growing amount of waste created by the petrochemical plastics has generated serious pollution.1−6 Because of its good biodegradability, biocompatibility, and mechanical properties, polylactide (PLA) is regarded as the leading member of the polyester family.7−9 The applications of PLA range from medical to agricultural to industrial fields.10 The future development of PLA industry depends mainly upon the further reduction in manufacture cost and improvement in the product performance. Recently, Sels presented zeolitebased catalyst which could efficiently convert lactic acid directly into lactide.11 The highly productive process was strengthened by facile recovery and practical reactivation of the catalyst. The combination of high efficient catalysis for lactide (LA) and PLA would significantly reduce the manufacture cost of PLA and facilitate its potential applicability. It is well-known that PLA has various stereoisomers, such as 12,13 L-PLA, D-PLA, and DL-PLA, which are prepared by ringopening polymerization (ROP) of L-lactide (L-LA), D-lactide (D-LA), and meso-lactide (meso-LA). The former two have melting points (Tm) of 140−180 °C and glass transition temperatures (Tg) of ∼60 °C, depending on the monomers optical purity. The low Tm and Tg restrained applications of commercial enantiopure PLAs. Fortunately, PLA stereocomplex, first found in 50/50 L-PLA/D-PLA blends in 1987,14 © XXXX American Chemical Society

has many advantages over L-PLA, D-PLA, or DL-PLA. For example, its Tm is as high as 230 °C.15−17 But it has not been widely industrialized because of the low efficiency and high cost of D-LA manufacturing as well as the long process from L-LA/DLA polymerization to stereocomplexation.18 Many efforts have been made to obtain PLA stereocomplex via direct ROP of racLA (a 1:1 mixture of L-LA and D-LA). Several well-defined stereoselective catalysts, in particular complexes of Li,19,20 Zn,21−25 Ga,26−28 Ge,29,30 Sn,31,32 In,33−35 Fe,36,37 and rare earth metals,38−41 have been developed. Among them, aluminum complexes, especially aluminum Salen/Salan complexes, exhibit excellent stereocontrollability.42−59 Multinuclear catalysts showed unique performance in polymer industry.60 In many multinuclear catalysts, two or more metal centers are poised in close proximity to activate both electrophilic and nucleophilic reactants, thereby achieving superior activity and selectivity. These enhanced catalytic properties versus mononuclear catalysts are due to the metal−metal synergistic effect in a single catalytic system. Redshaw,61 Carpentier,62 Yao,63 Jones,64 and Mazzeo65 developed dinuclear or multinuclear complexes for the ROP of LA and ε-caprolactone, in which simple two-coordinated Received: December 15, 2017 Revised: January 23, 2018

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DOI: 10.1021/acs.macromol.7b02662 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Pathway of Ligands and Complexes

Kα radiation (λ = 0.710 73 Å) at 187 K, and calculations were performed using the SHELXTL-2014 crystallographic software package. In a typical polymerization experiment, the aluminum complex (30 mmol) and the required amount of rac-lactide or εcaprolactone in toluene (60 mL) were loaded in a flame-dried vessel containing a magnetic bar. The vessel was placed in an oil bath. After a certain reaction time, the polymer was isolated by precipitation with cold methanol. The precipitate was collected and dried under vacuum at RT for 36 h. For detailed information on synthesis and characterization of ligands L1a−L2d and complexes 1a−2d, LA, and CL polymerizations please see the Supporting Information.

moieties was linked together in various ways. In all reports the close metal centers boosted the activity under cooperative effects and exhibited modest stereoselectivity. Inspired by the success of multinuclear Salen catalysts in polymerization of olefin and lactone and copolymerization of propylene oxide and CO2,66−69 highly active and selective multinuclear aluminum− Salen complexes are recently given more interest in developing PLA study and marketing.62 Herein, a series of the trinuclear aluminum complexes (Scheme 1, 2a−2d) are reported with a central benzene ring shared by three Salen−Al subunits. Owing to the crowding of the three Salen−Al subunits, high activity and stereoselectivity are expected. For comparison, the mononuclear Salen−Al complexes 1a and 1c were also prepared.

3. RESULTS AND DISCUSSION 3.1. Characterization of Complexes. Four trinuclear Salen−Al complexes 2a−2d and two mononuclear Salen−Al complexes 1a and 1c were prepared (Scheme 1, Figures S1 and S2).70,71 Among them, 1a and 2a carried 1,2-cyclohexylenediimine as the bridge between the two imines, and the other four had 1,1-dimethylethylene-diimine as linking groups. Different substituents were introduced to the 2- and 4-positions of the salicylaldehyde to examine their influence on the catalytic activity and stereoselectivity. Interestingly, 1H NMR peaks of the OH, NCH, and methylene protons of the trinuclear ligands L2a−L2d clearly displayed spectral splitting. For example, the NCH protons showed chemical shifts at δ 8.40 and 8.30 for L2c (Figure 1a). The spectral splitting of L2c is attributed to the asymmetric environment of aforementioned protons. Upon treatment with 3 equiv of AlEt3, L2c was converted into 2c, as evidenced by disappearance of OH peaks and shift of major peaks. 2c also maintained the above inequivalence, in which methyl protons and methylene protons of the ethylaluminum exhibited four sets of peaks at 0.80, 0.62, −0.33, and −0.45 ppm, respectively, confirming the inequivalence of the chemical environment of AlCH2CH3 group (Figure

2. EXPERIMENTAL SECTION All experiments were carried out under argon using Schlenk techniques or in an argon-filled glovebox. Starting materials for the synthesis of ligands were purchased from Aldrich Inc. Toluene was distilled from Na−benzophenone. Ethyl acetate and 2-propanol were distilled from CaH2 under the protection of argon. NMR spectra were recorded on Bruker AV 400M in CDCl3 at 25 °C. Chemical shifts were given in parts per million from tetramethylsilane. Gel permeation chromatography (GPC) measurements were conducted with a Waters 515 GPC with CHCl3 as eluent. The molecular weights were calibrated against polystyrene (PS) standards. Differential scanning calorimetry (DSC) was carried out with a TA Q100 thermal analyzer. UV−vis spectra were recorded by Shimadzu UV-2401PC at room temperature with a sealed quartz cuvette (path length: 10 mm). Ligand L1c was dissolved in CH3CN, while ligand L2c as well as complexes 1c and 2c were dissolved in CH2Cl2. The concentration of ligands L1c and L2c were 1.923 × 10−5 M. Complexes 1c and 2c were 1.667 × 10−5 M. Crystal of 2c suitable for an X-ray structure determination was grown from toluene solution. Crystallographic data were collected using a Bruker APEX CCD diffractometer with graphite monochromated Mo B

DOI: 10.1021/acs.macromol.7b02662 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 3. Ellipsoid representation (30% probability) of molecular structure of 2c. The disorder molecule and H atoms are omitted for clearity: A: 1 − y, x − y, z, B: 1 − x − y, 1 − x, z).

Figure 1. 1H NMR spectrum of (a) ligand L2c and (b) complex 2c.

S3). Although the three AlCH2CH3 moieties exhibit identical layout in solid-state structure (Figure 3), it is deduced that the structure is not completely maintained in solution. The flexibility of the ethyl group in AlCH2CH3 differentiates its angle or even orientation to metal species and thus produces the splitting in NMR spectrum. Electronic absorption spectra of the mono- and trinuclear ligands L1c and L2c and the mono- and trinuclear complexes 1c and 2c were recorded to further investigate the conjugate structures. The overall intensity of L2c at 280−400 nm was approximately 10 times higher than that of L1c (Figure 2a). A

central benzene ring so that the whole complex was not flat but looked like a “bowl” (Figure S4). Three ethyl groups were located inside the bowl. This specific structure may endow 2c potential synergistic effect in the catalytic process, leading to high catalytic efficiency. 3.2. Kinetic Studies. All complexes were tested as ROP catalysts of rac-LA in toluene solution with identical LA concentration (Table 1). Mononuclear 1a was impressive in terms of stereocontrollability, but it suffered from low activity and unacceptably high catalyst loading. Its propagation rate constant kp value was only 0.009 L mol−1 min−1.42,43 To our surprise, the trinuclear 2a, which had identical Y, R1, and R2 groups to those of 1a, gave a kp value (1.2 L mol−1 min−1, Table S3) of 133 times higher than that of 1a with similarly high stereocontrollability. The great improvement in activity from 1a to 2a encouraged us to move forward. It is believed that different diimine linker (the bridging part between two nitrogen atoms) could influence catalytic activity of Salen−Al complexes.45,52 Therefore, complexes 2b−2d were prepared by introducing 1,1-dimethylethylene instead of 1,2-cyclohexyl as the new diimine linker. A linear relationship was obtained in the plot of ln([LA]0/[LA]) against time (Figure 4 and Figure S5), indicating a first-order kinetics of the polymerization reaction. 2b−2d showed superior kp values: 15.4 L mol−1 min−1 for 2b, 9.0 for 2c, and 1.6 for 2d, respectively (Table S3). As for monomer conversion, taking 2c (kp was in the middle of the three) as an example, at 110 °C and [LA]:[2c] = 3000, a conversion of 83% was reached in 17 h. At [LA]:[2c] = 10 000, a conversion of 77% was still achieved in 48 h. Even at lower temperature of 25 °C, the conversion reached 68% after 42 h under [LA]:[2c] = 200 (Table S4, entries 1, 9−11). In comparison, the corresponding mononuclear 1c, which had the same diimine linker and R1/R2 substituents to those of 2c, only gave a kp of 0.008 L mol−1 min−1.48 The tremendous activity increase from 1c to 2c indicated that the influence of the trinuclear structure was overwhelming compared with diimine linker, attributed to an intramolecular cooperation among the three Salen−Al subunits (evidenced by similar numberaveraged molecular weights to calculated ones and narrow PDIs (Table 1 and Table S4); the trinuclear centers in 2a−2d were all involved in the polymerization).

Figure 2. Electronic spectra of (a) ligands L1c and L2c and (b) complexes 1c and 2c.

similar difference was observed for 1c and 2c (Figure 2b). Clearly, a simple 3-fold superposition from mono- to trinuclear complex cannot be counted on the large discrepancy. It was possible that there existed an electronic interaction among the three Salen−Al subunits via the phloroglucinol moieties. It was this electronic interaction that resulted in greatly enhanced electronic absorbance of 2c compared to 1c (Table S1).71 Crystals of 2c suitable for X-ray crystallography were obtained by evaporation from toluene (Figure 3 and Table S2). The three salicyl rings were located twistedly from the C

DOI: 10.1021/acs.macromol.7b02662 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Polymerization Data of rac-LA with Complexes 1a, 1c, and 2a−2da entry 1 2 3 4 5 6 7

complex 43

1a 1c48 2a 2b 2c 2d 2d

temp (°C)

t (h)

[LA]0/[cat.]/[iPrOH]

convb (%)

Mn(calcd) × 10−4 c

Mn(GPC) × 10−4 d

PDId

P me

70 70 70 70 70 25 70

288 72 10 3 5 72 10

62:1:1 56:1:1 150:1:3 600:1:3 300:1:3 100:1:3 300:1:3

85.3 28.3 93 90 97 83 83

0.76 0.24 0.67 2.59 1.40 0.40 1.19

0.77 0.28 0.70 2.71 1.44 0.41 1.15

1.06 1.08 1.08 1.35 1.04 1.13 1.05

0.93 0.72 0.90 0.64 0.90 0.98 0.94

All polymerizations were carried out in toluene solution, [LA]0 = 0.5 mol L−1. bMeasured by 1H NMR. cCalculated from the molecular weight of LA × [LA/3]0/[cat.] × conversion + Mw(iPrOH). dObtained from GPC analysis with polystyrene standard and multiplied by a correction factor of 0.58.72 ePm.73

a

by lactide monomer and, as a result, slow down the polymerization rate (Figure 4). The linear increase of number-average molecular weights of PLA with the LA conversion as well as narrow molecular weight distributions by complex 2c indicated well-controlled polymerizations (Figure S7). All the resulting polymers had similar numberaveraged molecular weights to calculated ones (Table 1 and Table S4). This suggested that the trinuclear centers in 2a−2d were involved in the polymerizations. Following work focused on the density functional theory (DFT) calculation of this intramolecular cooperation mechanism is ongoing. End-group analysis of PLA oligomer was investigated by 1H NMR and MALDI-TOF mass spectra (Figures S10 and S11). These studies indicated that the PLA was capped with one isopropyl ester group and one hydroxyl group, and the ring-opening event occurred through a coordination−insertion mechanism.43,74 Based on the above-mentioned result, a plausible coordination−insertion mechanism has been proposed to elucidate the cooperation between Al centers in trinuclear complexes. The alcohol reacted with metal alkyls to form the Al alkoxide groups as active species. Then, one or two Al centers worked as a

Figure 4. Kinetic plots of the rac-lactide conversion vs the reaction time. All polymerizations were carried out under N2 in toluene solution, [LA]0 = 0.5 mol L−1, T = 70 °C. (a) Complex 2b, [M]0/ [cat.] = 600; (b) complex 2c, [M]0/[cat.] = 600; (c) complex 2a, [M]0/[cat.] = 150; (d) complex 2d, [M]0/[cat.] = 300.

The activity was also remarkably influenced by the substituent group on the salicyl rings. The more bulky substituents may keep active species from being approached

Figure 5. Methine region of the homonuclear-decoupled 1H NMR spectra of PLAs. D

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Macromolecules Lewis acid bound the incoming LA and activate it for the following attack by the proximal Al alkoxide groups. The polymer chain growth could be achieved via nucleophilic attack of the polymer chain at the coordinated LA by shuttling between the Al centers. A similar cooperative effect was also reported in dinuclear systems61−65 in which the distance between metal centers is appropriate (5.8−6.6 Å62). The Al−Al distance for 2c in this work is 6.384 Å and is close enough for Al−Al interactions. Considering the existence of electronic interaction among the three Salen−Al subunits as shown in Figure 2, it was believed that this special electron communication through central benzene ring is also responsible for the boost of activity for the trinuclear complexes. 3.3. Stereoselectivity Analysis. Stereoselectivity analysis of PLA was carried out by determining the homonuclear decoupled 1H NMR spectra (Figure 5) and calculating the isotactic sequence forming probability Pm.73 Pm values of 2b, 2c, and 2d were 0.64, 0.90, and 0.94, respectively, while 1c showed a Pm value of only 0.72 (Table 1, entries 2, 4, 5, and 7). Bulky substitution on salicyl rings had a remarkable effect on the stereoselectivity. Considering that more bulky substituents on the salicyl rings led to lower kp in kinetics, it can be concluded that bulky substitution has a positive effect on stereoselectivity but a negative effect on polymerization rate. However, it is worth noting that the trinuclear complexes are more active than reported mononuclear aluminum catalysts. The stereoselectivity of 2b−2d decreased with increasing temperature. For example, the Pm values of 2c reduced from 0.90 at 70 °C to 0.80 at 110 °C (Figure S12, Table 1, entry 5, and Table S4, entry 8). This P m reduction was corresponding to the loss of microenvironmental inequivalency upon temperature rise as revealed by the variable-temperature NMR (Figure S3). XRD profiles also confirmed the formation of stereocomplex (Figure S13). 3.4. Thermal Properties. It is believed that the PLA obtained via ROP of rac-LA by stereoselective catalysts is a stereoblock copolymer that consists of alternative D- and L-PLA blocks. Because of the stereocomplexation between different blocks, the copolymer exhibited melting temperature (Tm) between 180 and 230 °C, depending on block lengths and thus on Pm of the catalyst used.19,75 Du reported a family of chiral zinc complexes, in which Tm of the resultant PLA was 214 °C.21 Of note, Tm of the PLA prepared in this work by 2d was determined to be 219.9 °C (Figure 6). As far as we know, it is the highest Tm reported up to date for PLA derived directly from rac-LA. 3.5. ε-Caprolactone Polymerization. The great performances of the trinuclear complexes encouraged us to further use them in the ROP of ε-caprolactone (ε-CL). Similar to the case of rac-LA polymerization, the trinuclear complexes 2a−2d exhibited much higher catalytic activity than mononuclear one; the trinuclear complexes with Y = 1,1-dimethylethylene were more active than that with Y = 1,2-cyclohexyl, and bulky substitution on the salicyl ring led to relatively poor activity (Figures S14 and S15, Table S5). For example, at 25 °C and [CL]:[2b] = 1000, a conversion of 97% was reached within 18 h; even at [CL]:[2b] = 2000, it still reached 70% conversion within 18 h. In contrast, no detectable PCL polymer was obtained for mononuclear complexes 1a and 1c at 25 °C (Table S5, entries 1 and 3). First-order kinetics in monomer, i.e., linear plot of ln([CL]0/[CL]) versus time, was observed by using 2a−2d (Figure S14). The number-average molecular weights of PCL increased linearly with the CL conversion by

Figure 6. DSC thermogram of second heating runs of PLA samples.

complex 2c. Combining with the narrow molecular weight distributions, the well-controlled polymerizations were thus interpreted (Figure S15). All the resulting polymers had similar number-averaged molecular weights to calculated ones. This suggested that the trinuclear centers in 2a−2d were involved equally in the polymerizations.

4. CONCLUSIONS Trinuclear Salen−Al complexes were prepared and characterized in this work. These complexes exhibited satisfactory combination of catalytic activity and stereoselectivity. Their exceptionally high activities made it possible to proceed under very low catalyst/monomer ratio, and their good stereoselectivity resulted in highly isotactic PLA stereoblock copolymers. Consequently, the Tm of stereoblock PLA prepared was as high as 219.9 °C, which is the highest Tm reported for PLA derived directly from rac-LA. It was temporarily attribute to the topology of the trinuclear catalysts with the central benzene ring shared by the three Salen−Al subunits. Specific spatial arrangement of the three Salen−Al subunits, electronic interactions among the three Salen−Al subunits via the central benzene ring, and asymmetry of each Salen−Al subunit were responsible for the combination of catalytic activity and stereoselectivity. Further work in our group aims at understanding the intrinsic topology of the trinuclear system, further improving the system and extending its application scope.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02662. Figures S1−S15 and Tables S1−S5 (PDF) checkCIF/PLATON report (PDF) X-ray crystallographic data of complex 2c (CIF)

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Corresponding Author

*E-mail: [email protected] (X.C.). E

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Xuan Pang: 0000-0003-3293-832X Xianhong Wang: 0000-0002-4228-705X Xuesi Chen: 0000-0003-3542-9256 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Nos. 21574124, 51503203, and 51233004).

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DOI: 10.1021/acs.macromol.7b02662 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b02662 Macromolecules XXXX, XXX, XXX−XXX