Facile Preparation of Stereoblock PLA From Ring-Opening Polymerization of rac-Lactide by a Synergetic Binary Catalytic System Containing Ureas and Alkoxides

Abstract

Ring-opening polymerization (ROP) of cyclic esters/lactones by efficient catalysts is a powerful method for synthesis of biodegradable and biocompatible polyesters with well-defined structures. To develop catalytic systems that are fast, selective and controlled is a persistent effort of chemists. In this contribution, we report a binary urea/alkoxide catalytic system that could catalyze ROP of rac-LA in a fast (over 90% conversion within 1–2 min), stereoselective (P_i up to 0.93) and controlled manner, indicated by narrow MW distributions, linear relationship between the monomer conversions and M_ns, end-group fidelity, and chain extension experiments. Remarkably, the catalytic system described here is simple, easily prepared, and structurally tunable and thus has versatile catalytic performances.

Introduction

Polylactide (PLA) is one of the most popular and commercial polymers in our daily lives (Dechy-Cabaret et al., 2004; Rasal et al., 2010; Raquez et al., 2013). For one thing, it is derived from renewable resources and could completely degrade to metabolites by hydrolysis after several months of exposure to moisture,(Castro-Aguirre et al., 2016; Farah et al., 2016) which could immensely reduce consumption volume of fossil resources and environmental pollution and makes it being a perfect substituent for petroleum-based plastics (Auras et al., 2004; Ray and Bousmina, 2005). On the other hand, it has excellent biocompatibility and wide applications as biomedical and pharmaceutical material (Kim et al., 2003; Armentano et al., 2010; Oh, 2011). The physical and chemical properties of PLA and thus the potential applications are highly dependent on the tacticity of PLA chains (Thomas, 2010). The stereoregular PLA generally has higher degree of crystallinity, melting temperature and mechanical strength as compared to the atactic polymer. For example, atactic PLA is an amorphous material, while heterotactic PLA is a semicrystalline polymer having a low T_m around 120°C. In contrast, isotactic PLLA is a crystalline material with a T_m of 170°C and high mechanical strength, which make it practical useful in the area of degradable packaging, 3D printing and surgical implants (Auras et al., 2004; Guo et al., 2015). Furthermore, the stereocomplex of L-PLA and D-PLA in 1:1 ratio (Ikada et al., 1987; Tashiro et al., 2017) or stereoblock PLA prepared from rac-LA using stereoselective catalysts (Stanford and Dove, 2010; Thomas, 2010; Dijkstra et al., 2011) has an even higher T_m up to 230°C and improved mechanical properties, due to cocrystallization of enantiomers (Ovitt and Coates, 2000; Fujita et al., 2008).

In the past decades, many new catalysts or initiators, such as enzymatic (Duskunkorur et al., 2015), metal-based (O'Keefe et al., 2001; Jensen et al., 2004; Arbaoui et al., 2008; Arbaoui and Redshaw, 2010; Carpentier, 2010; Sauer et al., 2013; Mou et al., 2014; Walton et al., 2014; Liu et al., 2017; Luo et al., 2017; Wang et al., 2017; Xu et al., 2017; Shi et al., 2018a,b) and organic catalysts (Kamber et al., 2007; Song et al., 2017), have been reported for conventional ROP of LA (Dechy-Cabaret et al., 2004). However, there are few able to prepare stereoregular PLA from a feedstock of rac-LA.(Stanford and Dove, 2010; Thomas, 2010; Dijkstra et al., 2011) Salen-Al metal complexes and the derivatives reported by Spassky et al. (1996) and others (Radano et al., 2000; Nomura et al., 2002, 2007; Ovitt and Coates, 2002; Zhong et al., 2002, 2003; Majerska and Duda, 2004; Hormnirun et al., 2006; Du et al., 2007; Luo et al., 2017) have attracted the most attention in the last two decades. Correspondingly, two distinguishing mechanisms, enantiomorphic site control mechanism (Ovitt and Coates, 2002) and chain end control mechanism (Nomura et al., 2002), have been proposed for the production of isotactic polymer (PLLA or PDLA) and stereoblock PLA, respectively. Concerning metal free products or processes desired, ROP of LA using organocatalysts have attracted considerable attention (Kamber et al., 2007; Kiesewetter et al., 2010). Among these, N-heterocyclic carbenes (NHCs) (Dove et al., 2006), phosphorics acids (Makiguchi et al., 2014), phosphazenes (Zhang et al., 2007; Liu et al., 2018), amino acids (Sanchez-Sanchez et al., 2017), and others (Zhu and Chen, 2015) are particularly useful for stereoselective ROP of rac-LA.

Although the degree of stereoselectivity (P_i) can easily exceed 0.9, there are still several problems for either metal-based or organic catalysts: (1) they are suffering from low catalytic rates and need hours or even days to achieve high conversions (Hormnirun et al., 2006); (2) their synthesis are tedious and complicated and thus result in high cost (Sanchez-Sanchez et al., 2017); (3) their structures are hard to modify and tune the catalytic properties (Zhang et al., 2007); (4) conditions of ROP for these systems are usually rigorous (low or high temperature) (Dove et al., 2006; Zhang et al., 2007). Therefore, many scientists have devoted to solving these issues by developing new catalytic systems, of which, however, very few can simultaneously have high activity, selectivity, and easy preparation (Zhang et al., 2016; Lin and Waymouth, 2017).

Catalysis employing H-bonding for initiator or monomer activation is an effective and versatile strategy in ROP (Thomas and Bibal, 2014), but stereoselective ROP of a chiral monomer using this methodology is still in the infancy. For example, β-isocupreidine reported by Chen and benzyl bispidine/thiourea reported by Dove, containing chiral centers, are efficient bifunctional catalysts for stereocontrolled ROP of rac-LA through the kinetic resolution, and thus produce isotactic-enriched PLLA (Miyake and Chen, 2011; Todd et al., 2013). So far, the examples of bifunctional catalysis employing H-bonding strategy that could prepare stereoblock PLA from rac-LA are rare and limited. Recently, Waymouth reported a binary catalytic system containing alkali metal alkoxides and organic ureas (Lin and Waymouth, 2017), which was hyperactive for ROP of different classes of cyclic monomers at room temperature, but no research on stereocontrol performances was reported. Herein, we reported that this binary urea/alkoxide system could stereoselectively catalyze ROP of rac-LA to produce stereoblock PLA for the first time with a superfast rate and excellent controlled manner (Scheme 1) under varying conditions.

Scheme 1

Results and discussion

At a first test, solo KOMe was used for ROP of rac-LA at room temperature (Table 1, run 1). The polymerization was slow and a conversion of 67% was obtained within 30 min. As expected, the produced PLA was atactic polymer (P_i = 0.52, Figure 1, blue) with a broad molecular weight distribution (Ð = M_w/M_n = 1.46), which was consistent with previous report for KOMe catalyzed ROP (Zhang et al., 2016).

Figure 1

When 1 equiv. urea 1 was mixed with KOMe (Table 1, run 2), the resulted mixture could rapidly polymerize rac-LA (68% in 1 min) and produce isotactic-enriched products (P_i = 0.62). Surprisingly, when the ratio of urea to KOMe was increased to 3, superfast polymerization rate and remarkable isotacticitity were observed (90% conversion in 1 min; P_i = 0.87, Table 1 run 3, Figure 1, green). However, further increasing the quantity of urea would lead to a lower activity with similar stereoselectivity (Table 1 run 4). Therefore, next investigations were performed with a 3:1 ratio for urea/KOMe unless specified. These results are significant since the ROP of rac-LA with fast rate and high stereoselectivity has been achieved using a simple catalytic system under mild conditions (room temperature). In contrast, with comparative conditions (room temperature), the degrees of stereoregularity (P_i) of PLA prepared by NHC and dimeric phosphazene were 0.59 and 0.72, respectively (Dove et al., 2006; Zhang et al., 2007). As compared to metal based catalysts, this urea/KOMe binary system showed comparable stereoselectivity but higher activity (Chamberlain et al., 2001; Nomura et al., 2002; Ovitt and Coates, 2002; Hormnirun et al., 2004, 2006; Bakewell et al., 2012; Pilone et al., 2014; Xiong et al., 2015).

It was reported that decreasing reaction temperature could enhance the isotacticity but at the cost of low activity (Dove et al., 2006; Zhang et al., 2007). Dimeric phosphazene with 1-pyrenebutanol as initiator produced PLAs with a P_i of 0.72 at 20°C and a P_i of 0.95 at −75°C, but the activity tremendously decreased to 1/50 (Zhang et al., 2007). Recently, we developed a new type of organic phosphazene base (CTPB) (Li et al., 2017; Zhao et al., 2017) and catalyzed ROP of rac-LA at −75°C with high stereoselectivity (P_i = 0.93) (Liu et al., 2018). In our current case, the increased isotacticity up to 0.93 was observed along with decreasing temperature (Table 1, runs 3 and 5–7). However, the polymerization rates remained fast (Figures S4–S9). At −20°C, a conversion of 91% was achieved in 1 min and the resulted PLA had a high P_i of 0.90 (Table 1 run 5). Even at −60°C, both fast polymerization rate (93% conversion in 2 min) and high P_i (0.93, Figure 1, red) were obtained (Table 1, run 7). The high isoselectivity was also confirmed by characterization of methane carbon (Figure S1, ¹³C NMR spectrum). Moreover, the T_m of the resulted PLA was 178°C (Figures S2, S10, S11) (Nomura et al., 2009), that is higher as compared to PLLA having similar MW and confirmed the formation of stereoblock PLA (Scheme 1C) (Zhang et al., 2007).

The molecular weight distributions (Ð) are narrow as shown in Figure 2A (Ð = 1.06–1.11), and the M_ns of resulted PLA linearly increase with enhanced conversions of monomer as shown in Figure 2B. These results indicate that the ROP of rac-LA by urea 1/KOMe at low temperature is highly controlled. The structural analysis of a low-M_n PLA was carried out with MALDI-TOF MS (Figure 3A). In Figure 3A, the ion peaks are separated by 144 mass units and consistent with linear PLA having MeO/H chain ends [M_n = 144.13 n + 32.0 (MeOH) + 23.0 (Na⁺) (g·mol⁻¹)]. The transesterification reactions are negligible due to the absence of ions separated by 72 mass units, consistent with the highly controlled polymerization by urea/KOMe catalytic system. ¹H NMR spectrum of resulted polymer (Figure S3) shows a singlet peak at 3.74 ppm, which confirms the group of OMe as chain end. In contrast, the MALDI-TOF MS of PLA prepared by KOMe alone exhibited three series of ions separated by 72 mass units (Figure 3B), corresponding to linear PLA having MeO/H chain end [square-series, M_n = 72.06 n + 32.0 (MeOH) + 23.0 (Na⁺) (g·mol⁻¹); triangle-series, M_n = 72.06 n + 32.0 (MeOH) + 39.1 (K⁺) (g·mol⁻¹)] and cyclic PLA without chain end [cycle-series, M_n = 72.06 n + 39.1 (K⁺) (g·mol⁻¹)], respectively. This result indicated competitive transesterification reactions and back-biting reactions during ROP by KOMe alone.

Figure 2

Figure 3

Moreover, a chain extension experiment was carried out, in which the second 100 equiv. monomers were added into a pre-formed PLA before quenching. GPC (Figure 2C) and homonuclear decoupled ¹H NMR analysis (Figure 2D) showed an excellent control on the M_ns, distributions and stereoregulatities before (M_n = 13.9 kDa, Ð = 1.09, P_i = 0.93) and after (M_n = 27.6 kDa, Ð = 1.06, P_i = 0.92, Table 1 run 11) chain extension. This result further confirmed the highly controlled behavior of urea/KOMe system.

Besides the hyperactive, stereoselective and controlled properties, the other advantage of urea/alkoxide system is the tunable catalytic performances benefitted from the diversities of alkoxides and ureas. Replacement of the KOMe with KO^tBu led to a faster polymerization rate (90% conversion in 1 min), while the high control on molecular weight and stereoselectivity remained (Table 1 run 13). In contrast, using sodium alkoxide (NaOMe) led to a lower polymerization rate (85% conversion in 2 min) and a lower isoselectivity (P_i = 0.85, Table 1 run 12). On the other hand, a series of commercially available ureas (Scheme 1A) were studied in detail for the ROP of rac-LA. Generally, the ureas containing at least one phenyl group are more active than aliphatic urea under identical conditions (ureas 1–4 vs. 5, Table 1 runs 7 and 14–17). It was found that homogeneous clear solutions were obtained by mixing ureas 1–4 and KOMe in THF, while the combination of urea 5 and KOMe led to less soluble compound, which probably resulted in low activity. According to previous report, the phenyl ring of urea could coordinate with alkali metal during the ROP, which probably increased the solubility of urea/alkoxide system (Zhang et al., 2016). The substituents on the phenyls with bulky steric hindrance and electron-withdrawing effect decreased the catalytic efficiency (ureas 2, 3 vs. 1), similar as previous report (Lin and Waymouth, 2017). Urea 6, reported by Waymouth as the most active one combining with KOMe for ROP, was also investigated for comparison. Under similar conditions, it was proved to be efficient for stereocontrol ROP of rac-LA (P_i = 0.91; Table 1 run 18).

Since the urea/alkoxide system had no chirality, the high stereoselectivity of ROP of rac-LA should be achieved through the chain end control mechanism (Table S1), similar as the previously reported one for phosphazene catalyst (Zhang et al., 2007; Liu et al., 2018). In the case of phosphazene catalyzed ROP of rac-LA, the bulky dimeric phosphazene and CTPB molecules could enhance the steric hindrances at the propagating chain end, and thus increased the stereocontrol, particularly when the mobility of the molecule was limited at low temperature. Inspired by Waymouth's recent work (Zhang et al., 2016), a synergetic bifunctional mechanism was proposed for the formation of stereoblock polymer using a feedstock of racemic monomers by achiral urea/KOMe system (Scheme 2). Reaction of the neutral urea with KOMe formed a complex of urea anion associated with activated MeOH by H-bonds (Scheme 2, A) that can initiate the ROP of rac-LA. In the first turnover, D-LA and L-LA monomers have an equal chance to open the ring, considering the achiral nature of urea/KOMe system. The urea anion could interact with the terminal alkoxide by H-bonds (Scheme 2B). Next, the coming monomer would interact with the N-H and insert in the polymer chain with stereocontrol because of chiral induction derived from the last inserted monomer (Schemes 2C,D). With this mechanism, R or S blocks of PLA could be constructed in the chain propagation. Occasionally, a monomer with opposite chirality was enchained and then built a new block having different configurations (Schemes 2E–G). This process (Scheme 2) was similar as that in chain end control mechanism by achiral metal based catalysts (Nomura et al., 2002). However, the existence of H-bonds (between urea anion and chain end, urea anion and the coming monomer) provided a unique bifunctional mechanism for ROP of rac-LA with high stereoselectivity. Note that the proposed hydrogen bonding association between urea anion and polymer chain end may also prevent back-biting forming cyclic product and transesterification reactions, which were absence in ROP by urea/KOMe system but occurred in the scenario of KOMe alone (Figure 3).

Scheme 2

Conclusion

The synergetic binary urea/MOR catalytic system was investigated for stereoselective ROP of rac-LA for the first time. At room temperature, the ROP of rac-LA with fast rate (90% conversion within 1 min) and high stereoselectivity (P_i = 0.87) has been achieved. At low temperature, the increased P_i up to 0.93 was obtained and the polymerization rates remained fast (over 90% conversion within 2 min). Narrow MW distributions, linear increment MW with enhanced monomer conversion and chain extension experiments suggest highly controlled ROP of rac-LA by urea/KOMe binary catalytic system. The microstructures and properties of the produced PLA were carefully analyzed and characterized by GPC, NMR, DSC, and MALDI-TOF. A synergetic bifunctional mechanism was proposed based on these results to elucidate the formation of stereoblock polymer using a feedstock of racemic monomers (rac-LA) by the binary urea/MOR catalytic system. These high-performances on catalysis together with the easy preparation and tunable structures make this binary system of great potential in practical application. Our research expands the use to H-bonding catalysis to prepare advanced well-defined polymers.

Experimental section

General considerations

All moisture/oxygen sensitive reactions/compounds were performed using standard Schlenk techniques or glovebox techniques in an atmosphere of high-purity nitrogen. THF, DCM, toluene was purified by first purging with dry nitrogen, followed by passing through columns of activated alumina. rac-LA was purchased from TCI and used without further purification. CDCl₃ was dried over CaH₂ and stored over activated 4 Å molecular sieves. All other chemicals were purchased from commercial suppliers and used without further purification unless otherwise noted. Reaction temperatures were controlled using an IKA temperature modulator.

NMR spectra were recorded on Bruker AV500 FT-NMR spectrometer. Matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) measurements were carried out on Bruker BIFLEX III equipped with a 337 nm nitrogen laser. α-Cyano-4-hydroxycinnamic acid was used as the matrix and sodium chloride as the cationizing agent. The GPC measurements of PLAs were collected on Agilent 1260 Infinity using THF as the eluent (flow rate: 1 mL min⁻¹, at 40°C) and polystyrenes as standard. Differential scanning calorimetry (DSC) was performed using a TA differential scanning calorimeter DSC25 that was calibrated using high purity indium at a heating rate of 5°C/min. Melting temperature were determined from the second scan at a heating rate of 5°C/min following a slow cooling rate of 3°C/min to remove the influence of thermal history. N-(4-Chlorophenyl)-N′-phenylurea (1), N-(4-chlorophenyl)-N′-(2,6-dimethylphenyl)urea (2), N-(4-trifloromethylphenyl)-N′-phenylurea (3), N-(4-chlorophenyl)-N′-cyclohexylurea (4) and 1,3-diisopropylurea (5) were purchased from Aldrich and used as received.

General procedure for the ROP of rac-LA

Polymerization of rac-LA using urea 1/KOMe = 3/1 at −60°C (Table 1, run 7) was carried out as following processes. In a N₂-filled glovebox, 288 mg of rac-LA (2.0 mmol) was dissolved in 8 mL of THF in a 25 mL vial containing a micro stir bar. A stock solution containing 14.3 mg of KOMe (0.2 mmol) and 148 mg of Urea 1 (0.6 mmol) in 20 mL of THF was prepared in a separate vial. The solutions were taken out of the glovebox and immersed in the cooling bath at −60°C. After equilibration at this temperature for 10 min, the polymerization was initiated by rapid addition of 2 mL of the stock solution containing the initiator and the catalyst into the rac-LA solution. At 2 min, the reaction was quenched by a couple drops of acetic acid. A small part of solution was taken and dried for ¹H NMR characterization to determine the conversion. The other solution was poured into methanol (100 mL) to precipitate polymer.

Chain extension experiment

Chain extension experiment (Table 1, run 7) was carried out as following processes. In a N₂-filled glovebox, 288 mg of rac-LA (2.0 mmol) was dissolved in 8 mL of THF in a 25 mL vial containing a micro stir bar. A stock solution containing 14.3 mg of KOMe (0.2 mmol) and 148 mg of Urea 1 (0.6 mmol) in 20 mL of THF was prepared in a separate vial. The solutions were taken out of the glovebox and immersed in the cooling bath at −60°C. After equilibration at this temperature for 10 min, the polymerization was initiated by rapid addition of 2 mL of the stock solution containing the initiator and the catalyst into the rac-LA solution. At 2 min, a small part of solution was taken and dried for ¹H NMR characterization to determine the conversion of the first step. Meanwhile, a pre-arranged rac-LA solution (288 mg, 2.0 mmol in 2 mL of THF) was added into the rest of polymerization solution at −60°C. After another 3 min, a small part of solution was taken and dried for ¹H NMR characterization to determine the conversion of the second step. The other solution was poured into methanol (100 mL) to precipitate polymer.

References

• 1

  ArbaouiA.RedshawC. (2010). Metal catalysts for epsilon-caprolactone polymerisation. Polym. Chem.1, 801–826. 10.1039/b9py00334g

  • CrossRef
  • Google Scholar

• 2

  ArbaouiA.RedshawC.HughesD. L. (2008). Multinuclear alkylaluminium macrocyclic Schiff base complexes: influence of procatalyst structure on the ring opening polymerisation of epsilon-caprolactone. Chem. Commun.21, 4717–4719. 10.1039/b810417d

  • CrossRef
  • Google Scholar

• 3

  ArmentanoI.DottoriM.FortunatiE.MattioliS.KennyJ. M. (2010). Biodegradable polymer matrix nanocomposites for tissue engineering: a review. Polym. Degrad. Stab.95, 2126–2146. 10.1016/j.polymdegradstab.2010.06.007

  • CrossRef
  • Google Scholar

• 4

  AurasR.HarteB.SelkeS. (2004). An overview of polylactides as packaging materials. Macromol. Biosci.4, 835–864. 10.1002/mabi.200400043

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 5

  BakewellC.CaoT.-P.-A.LongN.Le GoffX. F.AuffrantA.WilliamsC. K. (2012). Yttrium phosphasalen initiators for rac-lactide polymerization: excellent rates and high iso-selectivities. J. Am. Chem. Soc.134, 20577–20580. 10.1021/ja310003v

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 6

  CarpentierJ. F. (2010). Discrete metal catalysts for stereoselective ring-opening polymerization of chiral racemic β-lactones. Macromol. Rapid Commun.31, 1696–1705. 10.1002/marc.201000114

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 7

  Castro-AguirreE.Iniguez-FrancoF.SamsudinH.FangX.AurasR. (2016). Poly(lactic acid)-mass production, processing, industrial applications, and end of life. Adv. Drug Delivery Rev.107, 333–366. 10.1016/j.addr.2016.03.010

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 8

  ChamberlainB. M.ChengM.MooreD. R.OvittT. M.LobkovskyE. B.CoatesG. W. (2001). Polymerization of lactide with zinc and magnesium β-diiminate complexes: stereocontrol and mechanism. J. Am. Chem. Soc.123, 3229–3238. 10.1021/ja003851f

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 9

  Dechy-CabaretO.Martin-VacaB.BourissouD. (2004). Controlled ring-opening polymerization of lactide and glycolide. Chem. Rev.104, 6147–6176. 10.1021/cr040002s

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 10

  DijkstraP. J.DuH.FeijenJ. (2011). Single site catalysts for stereoselective ring-opening polymerization of lactides. Polym. Chem.2, 520–527. 10.1039/C0PY00204F

  • CrossRef
  • Google Scholar

• 11

  DoveA. P.LiH.PrattR. C.LohmeijerB. G. G.CulkinD. A.WaymouthR. M.et al. (2006). Stereoselective polymerization of rac- and meso-lactide catalyzed by sterically encumbered N-heterocyclic carbenes. Chem. Commun.2006, 2881–2883. 10.1039/b601393g

  • CrossRef
  • Google Scholar

• 12

  DuH.PangX.YuH.ZhuangX.ChenX.CuiD.et al. (2007). Polymerization of rac-lactide using Schiff base aluminum catalysts: structure, activity, and stereoselectivity. Macromolecules40, 1904–1913. 10.1021/ma062194u

  • CrossRef
  • Google Scholar

• 13

  DuskunkorurH. O.BegueA.PolletE.PhalipV.GuvenilirY.AverousL. (2015). Enzymatic ring-opening (co)polymerization of lactide stereoisomers catalyzed by lipases. Toward the in situ synthesis of organic/inorganic nanohybrids. J. Mol. Catal. B Enzym.115, 20–28. 10.1016/j.molcatb.2015.01.011

  • CrossRef
  • Google Scholar

• 14

  FarahS.AndersonD. G.LangerR. (2016). Physical and mechanical properties of PLA, and their functions in widespread applications-A comprehensive review. Adv. Drug Delivery Rev.107, 367–392. 10.1016/j.addr.2016.06.012

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 15

  FujitaM.SawayanagiT.AbeH.TanakaT.IwataT.ItoK.et al. (2008). Stereocomplex formation through reorganization of Poly(l-lactic acid) and Poly(d-lactic acid) crystals. Macromolecules41, 2852–2858. 10.1021/ma7024489

  • CrossRef
  • Google Scholar

• 16

  GuoS.-Z.YangX.HeuzeyM.-C.TherriaultD. (2015). 3D printing of a multifunctional nanocomposite helical liquid sensor. Nanoscale7, 6451–6456. 10.1039/C5NR00278H

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 17

  HormnirunP.MarshallE. L.GibsonV. C.PughR. I.WhiteA. J. P. (2006). Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators. Proc. Natl. Acad. Sci. U.S.A.103, 15343–15348. 10.1073/pnas.0602765103

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 18

  HormnirunP.MarshallE. L.GibsonV. C.WhiteA. J. P.WilliamsD. J. (2004). Remarkable stereocontrol in the polymerization of racemic lactide using aluminum initiators supported by tetradentate aminophenoxide ligands. J. Am. Chem. Soc.126, 2688–2689. 10.1021/ja038757o

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 19

  IkadaY.JamshidiK.TsujiH.HyonS. H. (1987). Stereocomplex formation between enantiomeric poly(lactides). Macromolecules20, 904–906. 10.1021/ma00170a034

  • CrossRef
  • Google Scholar

• 20

  JensenT. R.BreyfogleL. E.HillmyerM. A.TolmanW. B. (2004). Stereoselective polymerization of D,L-lactide using N-heterocyclic carbene based compounds. Chem. Commun.2004, 2504–2505. 10.1039/B405362A

  • CrossRef
  • Google Scholar

• 21

  KamberN. E.JeongW.WaymouthR. M.PrattR. C.LohmeijerB. G. G.HedrickJ. L. (2007). Organocatalytic ring-opening polymerization. Chem. Rev.107, 5813–5840. 10.1021/cr068415b

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 22

  KiesewetterM. K.ShinE. J.HedrickJ. L.WaymouthR. M. (2010). Organocatalysis: opportunities and challenges for polymer synthesis. Macromolecules43, 2093–2107. 10.1021/ma9025948

  • CrossRef
  • Google Scholar

• 23

  KimK.YuM.ZongX.ChiuJ.FangD.SeoY.-S.et al. (2003). Control of degradation rate and hydrophilicity in electrospun non-woven poly(d,l-lactide) nanofiber scaffolds for biomedical applications. Biomaterials24, 4977–4985. 10.1016/S0142-9612(03)00407-1

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 24

  LiH.ZhaoN.RenC.LiuS.LiZ. (2017). Synthesis of linear and star poly(?-caprolactone) with controlled and high molecular weights via cyclic trimeric phosphazene base catalyzed ring-opening polymerization. Polym. Chem.8, 7369–7374. 10.1039/C7PY01673E

  • CrossRef
  • Google Scholar

• 25

  LinB.WaymouthR. M. (2017). Urea anions: simple, fast, and selective catalysts for ring-opening polymerizations. J. Am. Chem. Soc.139, 1645–1652. 10.1021/jacs.6b11864

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 26

  LiuS.LiH.ZhaoN.LiZ. (2018). Stereoselective ring-opening polymerization of rac-lactide using organocatalytic cyclic trimeric phosphazene base. ACS Macro Lett.7, 624–628. 10.1021/acsmacrolett.8b00353

  • CrossRef
  • Google Scholar

• 27

  LiuS.ZhangJ.ZuoW.ZhangW.SunW.-H.YeH.et al. (2017). Synthesis of aluminum complexes bearing 8-anilide-5,6,7-trihydroquinoline ligands: highly active catalyst precursors for ring-opening polymerization of cyclic esters. Polymers9:83. 10.3390/polym9030083

  • CrossRef
  • Google Scholar

• 28

  LuoW.ShiT.LiuS.ZuoW.LiZ. (2017). Well-designed unsymmetrical salphen-al complexes: synthesis, characterization, and ring-opening polymerization catalysis. Organometallics36, 1736–1742. 10.1021/acs.organomet.7b00106

  • CrossRef
  • Google Scholar

• 29

  MajerskaK.DudaA. (2004). Stereocontrolled polymerization of racemic lactide with chiral initiator: combining stereoselection and chiral ligand-exchange mechanism. J. Am. Chem. Soc.126, 1026–1027. 10.1021/ja0388966

  • CrossRef
  • Google Scholar

• 30

  MakiguchiK.YamanakaT.KakuchiT.TeradaM.SatohT. (2014). Binaphthol-derived phosphoric acids as efficient chiral organocatalysts for the enantiomer-selective polymerization of rac-lactide. Chem. Commun.50, 2883–2885. 10.1039/C4CC00215F

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 31

  MiyakeG. M.ChenE. Y. X. (2011). Cinchona alkaloids as stereoselective organocatalysts for the partial kinetic resolution polymerization of rac-lactide. Macromolecules44, 4116–4124. 10.1021/ma2007199

  • CrossRef
  • Google Scholar

• 32

  MouZ.LiuB.LiuX.XieH.RongW.LiL.et al. (2014). Efficient and heteroselective heteroscorpionate rare-earth-metal zwitterionic initiators for ROP of rac-lactide: role of σ-ligand. Macromolecules47, 2233–2241. 10.1021/ma500209t

  • CrossRef
  • Google Scholar

• 33

  NomuraN.HasegawaJ.IshiiR. (2009). A direct function relationship between isotacticity and melting temperature of multiblock stereocopolymer poly(rac-lactide). Macromolecules42, 4907–4909. 10.1021/ma900760z

  • CrossRef
  • Google Scholar

• 34

  NomuraN.IshiiR.AkakuraM.AoiK. (2002). Stereoselective ring-opening polymerization of racemic lactide using aluminum-achiral ligand complexes: exploration of a chain-end control mechanism. J. Am. Chem. Soc.124, 5938–5939. 10.1021/ja0175789

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 35

  NomuraN.IshiiR.YamamotoY.KondoT. (2007). Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes. Chem. Eur. J.13, 4433–4451. 10.1002/chem.200601308

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 36

  OhJ. K. (2011). Polylactide (PLA)-based amphiphilic block copolymers: synthesis, self-assembly, and biomedical applications. Soft Matter7, 5096–5108. 10.1039/c0sm01539c

  • CrossRef
  • Google Scholar

• 37

  O'KeefeB. J.HillmyerM. A.TolmanW. B. (2001). Polymerization of lactide and related cyclic esters by discrete metal complexes. J. Chem. Soc. Dalton Trans.15, 2215–2224. 10.1039/b104197p

  • CrossRef
  • Google Scholar

• 38

  OvittT. M.CoatesG. W. (2000). Stereoselective ring-opening polymerization of rac-lactide with a single-site, racemic aluminum alkoxide catalyst: synthesis of stereoblock poly(lactic acid). J. Polym. Sci. A Polym. Chem.38, 4686–4692. 10.1002/1099-0518(200012)38:1+<4686::AID-POLA80>3.0.CO;2-0

  • CrossRef
  • Google Scholar

• 39

  OvittT. M.CoatesG. W. (2002). Stereochemistry of lactide polymerization with chiral catalysts: New opportunities for stereocontrol using polymer exchange mechanisms. J. Am. Chem. Soc.124, 1316–1326. 10.1021/ja012052

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 40

  PiloneA.PressK.GoldbergI.KolM.MazzeoM.LambertiM. (2014). Gradient isotactic multiblock polylactides from aluminum complexes of Chiral Salalen ligands. J. Am. Chem. Soc.136, 2940–2943. 10.1021/ja412798x

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 41

  RadanoC. P.BakerG. L.SmithM. R. (2000). Stereoselective polymerization of a racemic monomer with a racemic catalyst: direct preparation of the polylactic acid stereocomplex from racemic lactide. J. Am. Chem. Soc.122, 1552–1553. 10.1021/ja9930519

  • CrossRef
  • Google Scholar

• 42

  RaquezJ.-M.HabibiY.MurariuM.DuboisP. (2013). Polylactide (PLA)-based nanocomposites. Prog. Polym. Sci.38, 1504–1542. 10.1016/j.progpolymsci.2013.05.014

  • CrossRef
  • Google Scholar

• 43

  RasalR. M.JanorkarA. V.HirtD. E. (2010). Poly(lactic acid) modifications. Prog. Polym. Sci.35, 338–356. 10.1016/j.progpolymsci.2009.12.003

  • CrossRef
  • Google Scholar

• 44

  RayS. S.BousminaM. (2005). Biodegradable polymers and their layered silicate nanocomposites: In greening the 21st century materials world. Prog. Mater. Sci.50, 962–1079. 10.1016/j.pmatsci.2005.05.002

  • CrossRef
  • Google Scholar

• 45

  Sanchez-SanchezA.RivillaI.AgirreM.BasterretxeaA.EtxeberriaA.VelosoA.et al. (2017). Enantioselective ring-opening polymerization of rac-lactide dictated by densely substituted amino acids. J. Am. Chem. Soc.139, 4805–4814. 10.1021/jacs.6b13080

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 46

  SauerA.KapelskiA.FliedelC.DagorneS.KolM.OkudaJ. (2013). Structurally well-defined group 4 metal complexes as initiators for the ring-opening polymerization of lactide monomers. Dalton Trans.42, 9007–9023. 10.1039/c3dt00010a

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 47

  SaveM.SchappacherM.SoumA. (2002). Controlled ring-opening polymerization of lactones and lactides initiated by lanthanum isopropoxide, 1. General aspects and kinetics. Macromol. Chem. Phy.203, 889–899. 10.1002/1521-3935(20020401)203:5/6<889::AID-MACP889>3.0.CO;2-O

  • CrossRef
  • Google Scholar

• 48

  ShiT.LuoW.LiuS.LiZ. (2018a). Controlled random copolymerization of rac-lactide and ε-caprolactone by well-designed phenoxyimine Al complexes. J. Polym. Sci. Part A Polym. Chem.56, 611–617. 10.1002/pola.28932

  • CrossRef
  • Google Scholar

• 49

  ShiT.ZhengQ.ZuoW.LiuS.LiZ. (2018b). Bimetallic aluminum complexes supported by bis(salicylaldimine) ligand: synthesis, characterization and ring-opening polymerization of lactide. Chinese J. Polym. Sci.36, 149–156. 10.1007/s10118-018-2039-5

  • CrossRef
  • Google Scholar

• 50

  SongQ.-L.HuS.-Y.ZhaoJ.-P.ZhangG.-Z. (2017). Organocatalytic copolymerization of mixed type monomers. Chinese J. Polym. Sci.35, 581–601. 10.1007/s10118-017-1925-6

  • CrossRef
  • Google Scholar

• 51

  SpasskyN.WisniewskiM.PlutaC.Le BorgneA. (1996). Highly stereoelective polymerization of rac-(D,L)-lactide with a chiral Schiff's base/aluminum alkoxide initiator. Macromol. Chem. Phys.197, 2627–2637. 10.1002/macp.1996.021970902

  • CrossRef
  • Google Scholar

• 52

  StanfordM. J.DoveA. P. (2010). Stereocontrolled ring-opening polymerisation of polylactide. Chem. Soc. Rev.39, 486–494. 10.1039/B815104K

  • CrossRef
  • Google Scholar

• 53

  TashiroK.WangH.KounoN.KoshobuJ.WatanabeK. (2017). Confirmation of the X-ray-analyzed heterogeneous distribution of the PDLA and PLLA chain stems in the crystal lattice of poly(lactic acid) stereocomplex on the basis of the vibrational circular dichroism IR spectral measurement. Macromolecules50, 8066–8071. 10.1021/acs.macromol.7b01573

  • CrossRef
  • Google Scholar

• 54

  ThomasC.BibalB. (2014). Hydrogen-bonding organocatalysts for ring-opening polymerization. Green Chem.16, 1687–1699. 10.1039/C3GC41806E

  • CrossRef
  • Google Scholar

• 55

  ThomasC. M. (2010). Stereocontrolled ring-opening polymerization of cyclic esters: synthesis of new polyester microstructures. Chem. Soc. Rev.39, 165–173. 10.1039/B810065A

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 56

  ToddR.RubioG.HallD. J.TempelaarS.DoveA. P. (2013). Benzyl bispidine as an efficient replacement for (-)-sparteine in ring opening polymerization. Chem. Sci.4, 1092–1097. 10.1039/c2sc22053a

  • CrossRef
  • Google Scholar

• 57

  WaltonM. J.LancasterS. J.RedshawC. (2014). Highly selective and immortal magnesium calixarene complexes for the ring-opening polymerization of rac-lactide. Chemcatchem6, 1892–1898. 10.1002/cctc.201402041

  • CrossRef
  • Google Scholar

• 58

  WangX.ZhaoK. Q.Al-KhafajiY.MoS.PriorT. J.ElsegoodM. R. J.et al. (2017). Organoaluminium complexes derived from anilines or schiff bases for the ring-opening polymerization of epsilon-caprolactone, delta-valerolactone and rac-lactide. Eur. J. Inorg. Chem.2017, 1951–1965. 10.1002/ejic.201601415

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 59

  XiongJ.ZhangJ.SunY.DaiZ.PanX.WuJ. (2015). Iso-selective ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium naphthalenolates. Inorg. Chem.54, 1737–1743. 10.1021/ic502685f

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 60

  XuT.-Q.YangG.-W.LiuC.LuX.-B. (2017). Highly Robust yttrium bis(phenolate) ether catalysts for excellent isoselective ring-opening polymerization of racemic lactide. Macromolecules50, 515–522. 10.1021/acs.macromol.6b02439

  • CrossRef
  • Google Scholar

• 61

  ZhangL.NederbergF.MessmanJ. M.PrattR. C.HedrickJ. L.WadeC. G. (2007). Organocatalytic stereoselective ring-opening polymerization of lactide with dimeric phosphazene bases. J. Am. Chem. Soc.129, 12610–12611. 10.1021/ja074131c

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 62

  ZhangX.JonesG. O.HedrickJ. L.WaymouthR. M. (2016). Fast and selective ring-opening polymerizations by alkoxides and thioureas. Nat. Chem.8, 1047–1053. 10.1038/nchem.2574

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 63

  ZhaoN.RenC.LiH.LiY.LiuS.LiZ. (2017). Selective ring-opening polymerization of none-strained γ-butyrolactone catalyzed by cyclic trimeric phosphazene base. Angew. Chem. Int. Ed.56, 12987–12990. 10.1002/anie.201707122

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 64

  ZhongZ.DijkstraP. J.FeijenJ. (2002). [(salen)Al]-mediated, controlled and stereoselective ring-opening polymerization of lactide in solution and without solvent: synthesis of highly isotactic polylactide stereocopolymers from racemic D,L-lactide. Angew. Chem.41, 4510–4513. 10.1002/1521-3773(20021202)41:23<4510::AID-ANIE4510>3.0.CO;2-L

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 65

  ZhongZ.DijkstraP. J.FeijenJ. (2003). Controlled and stereoselective polymerization of lactide: kinetics, selectivity, and microstructures. J. Am. Chem. Soc.125, 11291–11298. 10.1021/ja0347585

  • Pubmed Abstract
  • CrossRef
  • Google Scholar

• 66

  ZhuJ.-B.ChenE. Y. X. (2015). From meso-lactide to isotactic polylactide: epimerization by b/N Lewis Pairs and kinetic resolution by organic catalysts. J. Am. Chem. Soc.137, 12506–12509. 10.1021/jacs.5b08658

  • Pubmed Abstract
  • CrossRef
  • Google Scholar