Lactide Research Articles

Selective Copolymerization from Mixed Monomers of Phthalic Anhydride, Propylene Oxide and Lactide Using Nano-Sized Zinc Glutarate.

Selective polymerization with heterogeneous catalysts from mixed monomers remains a challenge in polymer synthesis. Herein, we describe that nano-sized zinc glutarate (ZnGA) can serve as a catalyst for the selective copolymerization of phthalic anhydride (PA), propylene oxide (PO) and lactide (LA). It was found that the ring-opening copolymerization (ROCOP) of PA with PO occurs firstly in the multicomponent polymerization. After the complete consumption of PA, the ring-opening polymerization (ROP) of LA turns into the formation of block polyester. In the process, the formation of zinc-alkoxide bonds on the surface of ZnGA accounts for the selective copolymerization from ROCOP to ROP. These results facilitate the understanding of the heterogeneous catalytic process and offer a new platform for selective polymerization from monomer mixtures.

Monothiolactide, a New Monomer for the Synthesis of Recyclable, Alternating Ester-Thioester Polymers.

Aliphatic polyesters and polythioesters are very interesting alternatives for current fossil-based and degradation-resistant plastics, due to their high (bio)degradability and (chemical) recyclability potential. Two important examples include polylactide (PLD), currently leading the synthetic bioplastics market, and its sulfur analog polythiolactide (PTLD). Both polymers can be made by ring-opening polymerization (ROP) of their corresponding (thio)dilactones, lactide (LD) and thiolactide (TLD) respectively. In this work, the benefits of esters and thioesters were combined in one material by the successful catalytic synthesis and ROP of monothiolactide (MTL), an unprecedented monomer containing half a LD and half a TLD structural unit. MTL can be obtained by a simple direct condensation of biobased lactic acid and thiolactic acid aided by Brønsted acid catalysis. The novel, but simple monomer showed to be easily polymerized with triethylamine to materials containing alternating lactic and thiolactic ester units with a very high molar mass. The lower stability of MTL (vs. TLD) resulted in improved ROP thermodynamics, while also fast and controllable polymerization kinetics were observed. The new polymers feature a good chemical recycling and hydrolytic degradation potential with important improvements compared to PTLD and PLD. Finally, a successful co-polymerization with commercial LD was shown, paving the way towards industrialization.

Open- and Closed-Loop Recycling: Highly Active Zinc Bisguanidine Polymerization Catalyst for the Depolymerization of Polyesters.

In this study, the aliphatic N,N-bisguanidine zinc complex [Zn(DMEG2ch)2](OTf)2 ⋅ THF is introduced as a promising candidate for the chemical recycling of (bio) polyesters. This catalyst is highly active in the ring-opening polymerization (ROP) of lactide (LA) and ϵ-caprolactone (CL). The combination of polymerization and depolymerization activity creates new pathways towards a sustainable circular economy. The catalytic activity of [Zn(DMEG2ch)2](OTf)2 ⋅ THF for the chemical recycling of polylactide (PLA) via alcoholysis was investigated by detailed kinetic and thermodynamic studies. It is shown that various high value-added alkyl lactates can be obtained efficiently under mild reaction conditions. Catalyst recycling was successfully tested using ethanol for the degradation of PLA. In addition, LA can be recovered directly from PLA, enabling either open- or closed-loop recycling. Selective PLA degradation from mixtures with polyethylene terephthalate (PET) and polymer blends are presented. For the first time, a cascade recycling reaction of a PLA/polycaprolactone (PCL) blend is tested with a zinc-based bisguanidine catalyst, whereby PLA is degraded selectively at first and subsequent modification of the reaction conditions leads to efficient degradation of the remaining PCL. The highly active, universally applicable benign zinc catalyst allows the implementation of a circular plastics economy and thus the reduction of plastic pollution in the environment.

About Polycondensation, Disproportionation, and Cyclization of Acetylated Poly(L‐Lactide) Esters

AbstractL‐lactide (LA) is polymerized with ethyl L‐lactate or trifluoroethanol as initiators (LA/In = 30/1), and the resulting poly(L‐lactide) esters are acetylated with acetic anhydride. The acetylated PLA esters are mixed with SnOct2, dibutyltin bis(4‐chlorophenoxide), or dibutyltin bis(pentafluorophenoxide) in solution and crystallized at 120 °C. The doped crystals are annealed at 140 °C or at 160 °C for 7, 14, and 28 days, and the chemical modifications that occurred in the solid state are monitored by matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI TOF) mass spectrometry, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC) measurements. All catalysts promoted polycondensation, while no significant formation of cycles is observed. However, in the presence of SnOct2, disproportionation of chains occurred upon annealing at 120 or 140 °C, and crystallites consisting of extended chains with Mn values ≈3500 – 3600 and low dispersities (Ð < 1.5) are formed.

Nanosized “rice ball” like hierarchical beta zeolites with inter-crystalline mesopores for efficient catalytic synthesis of lactide

A series of nanosized “rice ball” like hierarchical beta zeolite aggregates (RBZxAy, x = 6, 9, 12 or 15; y = 2, 4 or 6) with hierarchical structures are successfully synthesized via gel aging and hydrothermal method without mesoporous template. The as-prepared RBZxAy zeolites possess two ranges of inter-crystalline mesoporous that formed by zeolite nanoparticles aggregation compared to the non-aged samples (B12Z4). The effects of Si/Al ratio and crystallization time on the morphology, structure and surface acidic properties of RBZxAy were systematically investigated. Meanwhile, RBZxAy as catalysts were applied in the conversion of lactic acid (LA) to lactide (LT) to investigate their catalytic performance. Benefitting from smaller crystal size, hierarchical structures and higher acid content, the representative catalyst (RBZ12A4) exhibited outstanding catalytic activity that the LA conversion was high to 98 % and the LT yield up to 94 %. In addition, RBZ12A4 presented perfect regeneration ability after 10 times recycles. This work provides an effective and facile route to directly synthesis nanocrystals aluminosilicate zeolite with hierarchal structure, and constructs an approach for efficiently converting LA to LT, it has greatly significance for the development of polylactic acid industry.

From Facile One-Pot Synthesis of Semi-Degradable Amphiphilic Miktoarm Polymers to Unique Degradation Properties.

We report a one-pot synthesis of well-defined A5B and A8B miktoarm star-shaped polymers where N,N-dimethylaminoethyl methacrylate (DMAEMA) and various cyclic esters such as ε-caprolactone (ε-CL), lactide (LA) and glycolide (GA) were used for the synthesis. Miktopolymers were obtained by simultaneously carrying out atom transfer radical polymerization (ATRP) of DMAEMA, ring-opening polymerization (ROP) of cyclic esters, and click reaction between the azide group in gluconamide-based (GLBr5-Az) or lactonamide-based (GLBr8-Az) ATRP initiators and 4-pentyn-1-ol. The relatively low dispersity indices of the obtained miktoarm stars (Đ = 1.2-1.6) indicate that control over the polymerization processes was sustained despite almost complete monomers conversions (83-99%). The presence of salts from phosphate-buffered saline (PBS) in polymer solutions affects the phase transition, increasing cloud point temperatures (TCP) values. The critical aggregation concentration (CAC) values increased with a decreasing number of average molecular weights of the hydrophobic fraction. Hydrolytic degradation studies revealed that the highest reduction of molecular weight was observed for polymers with PCL and PLGCL arm. The influence of the composition on the miktopolymers hydrophilicity was investigated via water contact angle (WCA) measurement. Thermogravimetric analysis (TGA) disclosed that the number of arms and their composition in the miktopolymer affects its weight loss under the influence of temperature.

Stereocomplexed Functional and Statistical Poly(lactide-carbonate)s via a Simple Organocatalytic System.

The stereocomplexation of polylactide (PLA) has been widely relied upon to develop degradable, sustainable materials with increased strength and improved material properties in comparison to stereopure PLA. However, forming functionalized copolymers of PLA while retaining high crystallinity remains elusive. Herein, the controlled ring-opening copolymerization (ROCOP) of lactide (LA) and functionalized cyclic carbonate monomers is undertaken. The produced polymers are shown to remain crystalline up to 25 mol % carbonate content and are efficiently stereocomplexed with homopolymer PLA and copolymers of opposite chirality. Polymers with alkene and alkyne pendent handles are shown to undergo efficient derivatization with thiol-ene click chemistry, which would allow both the covalent conjugation of therapeutic moieties and tuning of material properties.

Synthesis and characterization of amino-functional polyesters composed of lactic acids and serine amino acids

Functional polyesters with good biocompatibility are always important biomaterials. In this study, poly (latide-co-N-trityl serine ester)s (P (LA-co-TSL)s) were synthesized through the copolymerization of lactide (LA) and N-trityl serine lactone (TSL) in toluene under 90 °C with the catalysis of diethylzinc. The molecular weight (Mn) of P (LA-co-TSL) with different TSL molar ratios (4.5%, 8.8%, 25.2%, 41.7% and 100 mol%) ranged from 3.7 kDa to 7.2 kDa with the polydipersity index (PDI) from 1.40 to 1.57. The chemical structure of poly (LA-co-TSL)s was characterized by FTIR, 1H NMR and 13C NMR spectroscopy. As the proportion of TSL in P (LA-co-TSL) increased, the glass transition temperature (Tg) of P (LA-co-TSL) increased gradually, from 56 °C of PLA to 161 °C of Poly (TSL). The initial weitht loss temperature of P (LA-co-TSL) increased as the ratio of TSL increased, from 172 °C of PLA to 230 °C of Poly (TSL). Upon removal of the N-trityl protecting group, amino functional poly (latide-co-serine ester) (P (LA-co-SL)) was realized and confirmd by FTIR and 1H NMR spectra. GPC displayed approximately 19.5% polyester backbone degradation during deprotection with trifluoroacetic acid. DSC analysis illustrated better crystallization properties of the copolyester after amino group deprotection. Using mPEG as an initiator, a new type of block copolymer, mPEG-b-poly (latide-co-N-trityl serine ester)s (mPEG-b-P (LA-co-TSL)s), was obtained. After N-trityl removal, mPEG-b-P (LA-co-TSL) displayed excellent cytocompatibility in L929 cell culture experiments.

Metalloporphyrin catalyzed self-switchable terpolymerization of propene oxide, phthalic anhydride and lactide for one-pot prepare block copolyesters

The copolymerization of polylactic acid with other monomers or polymers can enhance its performance and expand its range of applications. In this work, a range of metalloporphyrin catalysts featuring diverse substituents and metal coordination centers were synthesized and employed in conjunction with co-catalyst bis(triphenylphosphoranylidene) ammonium chloride (PPNCl) for the terpolymerization of phthalic anhydride (PA), propylene oxide (PO), and lactide (LA) ester without the use of initiators. The catalytic activity of the porphyrin ligand is significantly influenced by its substituents and different metal centers. Under optimal conditions, catalyst TOMePP-CoCl exhibits the most outstanding reactivity among all catalysts. In particular, this terpolymerization is a one-pot self-switching polymerization catalyzed by the metalloporphyrin/PPNCl system. PA selectively undergoes ring-opening copolymerization with PO first, while LA remains inert until PA totally consumed. Through continuous feeding experiments, a multiblock copolymer with fine structure controllability was successfully synthesized, which has been verified by DOSY NMR analysis.

Investigation of the correlation between the substituent effect of ligands and the catalytic activity/magnetic properties in fine-tuned rare-earth complexes

Amine-bis(benzothiazole phenol) ligands, C1NNBiBTHP-H2 and CLNNBiBTHP-H2 ligands, were designed for yttrium complexes with analogous structures. A series of nitrophenolate (NP)-type of ligands possessing R substituents with variable electronic properties (R = NO2, Cl, H, CH3) on ortho and/or para position attached to the phenolate rings have been selected as secondary ligands for complexes syntheses. Ten novel yttrium complexes have been synthesized, [Y(C1NNBiBTHP)(NP)(MeOH)] (1–5) and [Y(CLNNBiBTHP)(NP)(MeOH)] (6–10) where the secondary ligand NP = 2,4-dinitrophenol(1 and 6), 2,5-dinitrophenol (2 and 7), 2-nitrophenol (3 and 8), 4-chloro-2-nitrophenol (4 and 9) and 4-methyl-2-nitrophenol (5 and 10), as well as the structures and catalytic properties for lactide (LA) polymerization were both studied. All the core structures and coordination geometries for complexes 1–10 were isostructural. All complexes were demonstrated to be active catalysts for LA polymerization, and the catalytic performance for these complexes was compared. The dysprosium analogues (11, 12 with C1NNBiBTHP-H2 and 13, 14 with CLNNBiBTHP-H2) also been synthesized and the magnetic properties of these compounds are fully studied. Comparing a series of nitrophenolate co-ligands in two different ligand systems, we presented the first example to study the correlation between electronic effect of the ligands and catalytic properties for mononuclear rare-earth complexes.

High-Tg PLA copolymers via base-catalyzed transesterification of PLA with 2,5,7-trioxabicyclo[2.2.2]octan-6-one

Ring-opening copolymerization of a new bicyclic lactone, 2,5,7-trioxabicyclo[2.2.2]octan-6-one (TOB), with lactide (LA) provided a series of degradable copolyesters with tunable high glass transition temperatures ranging from 50 to 100 °C.

MICROWAVE ASSISTED LACTIDE-GLYCOLIDE HOMO AND COPOLYMERIZATION INITIATED BY MAGNESIUM LACTATE

In this work, microwave-assisted ring opening polymerization (ROP) of lactide (LA) and glycolide (GA) initiated by magnesium lactate was investigated. The influence of microwave power, temperature, and reaction time on homopolymers and copolymer’s structure and thermal properties is discussed. Polymerization of L-lactide (LLA) initiated by the magnesium compound resulted in yields up to 78% and poly(L-lactic acid) (PLLA) with average molar mass of up to 30,600 g mol-1. Homopolymerization of glycolide generate poly(glycolic acid) (PGA) in yields up to 90% and in L-lactide/glycolide copolymerizations yields between 34-82%. PLLA prepared with the initiator presented microstructure with significant amount of stereo errors as the result of a racemization process during polymerization as well as low glass transition temperature (Tg) and melting temperature (Tm), while the PGAs presented high Tg, due to high crystallinity, and Tm in accordance with the literature. LLA and GA copolymerization result in poly(lactic acid-co-glycolic acid) (PLGA) with composition close to polymerization feed composition.

Selective Transesterification to Control Copolymer Microstructure in the Ring-Opening Copolymerization of Lactide and ε-Caprolactone by Lanthanum Complexes.

A series of novel lanthanum amido complexes, supported by ligands designed around the salan framework (salan = N,N'-bis(o-hydroxy, m-di-tert-butylbenzyl)-1,2-diaminoethane) were synthesized and fully characterized in the solid and solution states. The ligands incorporate benzyl or 2-pyridyl substituents at each tertiary amine center. The complexes were investigated as catalysts in the ring-opening homopolymerization of lactide (LA) and ε-caprolactone (ε-CL) and copolymerization of equimolar amounts of LA and ε-CL at ambient temperature. Solvent (THF or toluene) and the number of 2-pyridyl groups in the complex were found to influence the reactivity of the catalysts in copolymerization reactions. In all cases, complete conversion of LA to PLA was observed. The use of THF, a coordinating solvent, suppressed ε-CL polymerization, while the presence of one or more 2-pyridyl groups promoted ε-CL polymerization. Each copolymer gave a monomodal trace in gel permeation chromatography-size-exclusion chromatography (GPC-SEC) experiments, indicative of copolymer formation over homopolymerization. Copolymer microstructure was found to be dependent on catalyst structure and reaction solvent, ranging from blocky to close to alternating. Experiments revealed rapid conversion of LA in the initial stages of the reaction, followed by incorporation of ε-CL into the copolymer by either transesterification or propagation reactions. Significantly, the mode of transesterification (TI or TII) that occurs is determined by the structure of the metal complex and the reaction solvent, leading to the possibility of controlling copolymer microstructure through catalyst design.

Bimetallic Manganese Catalysts: A Route to Controlled and Switchable Polymerization of Lactones.

The sustainable solution to the environmental problem of polymeric materials calls for efficient and well-controlled ring-opening polymerization catalytic systems. Inspired by the highly reactive and stereospecific bimetallic catalysts, three kinds of bimetallic Salen-Mn catalysts supported by biaryl linking moieties are synthesized and applied to polymerization catalysis of lactide (LA) and ϵ-caprolactone (ϵ-CL) in this work. The polymerization is initiated in situ by the ring-opening of epoxide compounds, in which the ionic cocatalyst could accelerate the reaction process. The Mn-Mn coordination effect contributes to the higher activity and iso-selectivity towards LA compared to the mononuclear Salen-Mn catalyst. The reactivity and stereoselectivity are determined by the conformation of catalysts, specifically the Mn-Mn separation and dihedral angle. Finally, the CO2 -controlled switchable polymerizations are carried out with LA and ϵ-CL. The reversibility of the on-off switching operation is influenced by the combination between CO2 molecules and active species. The success in binuclear Salen-Mn catalysts not only expands the range of bimetallic catalyst analogues but also claims the promising potential of Mn-based catalysts in practical and theoretical research.

One‐Pot/Simultaneous Synthesis of PHPMA‐G‐PLA Copolymers via Metal‐Free Rop/Raft Polymerization and their Self‐Assembly from Micelles to Thermoresponsive Vesicles

AbstractThe present article reports a simple and straightforward approach to access thermoresponsive graft copolymers based on lactide (LA) and a methacrylic monomer, 2‐hydroxypropyl methacrylate (HPMA), using a synthesized carboxy‐functionalized trithiocarbonate‐based chain transfer agent. One protocol involves a metal‐free simultaneous synthesis through a combination of reversible addition‐fragmentation chain transfer polymerization and organic acid‐catalyzed ring‐opening polymerization, which follows first‐order kinetics. The resulting copolymers with a controlled structure exhibit remarkably narrow molecular weight distributions (Ð < 1.10). Within this framework, the self‐assembly of PHPMA‐g‐PLA graft copolymers (GCs) into nanoparticles (NPs) is demonstrated at concentrations of 0.2 and 0.5 wt.%, respectively. The displacement method, based on the rapid injection of the organic solvent (acetone) into an aqueous medium under vigorous stirring, produces spherical NPs such as micelles, vesicles, or non‐spherical “lumpy rods”. The presence of a pseudo‐thermoresponsive segment (PHPMA) in GCs facilitates stimulus‐responsive self‐assembly behavior. Well‐defined spherical NPs—primarily vesicles of substantial size—develop upon heating above the glass transition temperature (Tg ≈35–36 °C) of the GCs in an acetone–water (80/20 wt.%) mixture. Last, specific interactions between the obtained PHPMA‐g‐PLA nano‐objects and blood proteins in human plasma are studied using isothermal calorimetry.

Accelerated Polymerization of Lactide Driven by a DBU‐based Salt and a 15‐Crown‐5 Ether Sacrificial Kinetic Booster

The recent advances in polymerization of lactide (LA) from organic salts enriched the toolbox to prepare well-defined materials. Herein we report the use of crown ether (CE) to catalyze the polymerization of L-LA initiated by 12-hydroxydodecanoic acid (HDA) activated by one equivalent of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). While both size and structure of the ether catalyst play an important role on the overall rate of reaction, the CE decomplexation is crucial to outpace transfer reactions and also permit the process to be moderately conducted under air atmosphere by an efficient water trapping process.

Degradation Behavior of Poly(Lactide-Co-Glycolide) Monolayers Investigated by Langmuir Technique: Accelerating Effect.

Among biodegradable polymers, polylactides (PLAs) have attracted considerable interest because the monomer can be produced from renewable resources. Since their initial degradability strongly affects commercial application fields, it is necessary to manage the degradation properties of PLAs to make them more commercially attractive. To control their degradability, poly(lactide-co-glycolide) (PLGA) copolymers of glycolide and isomer lactides (LAs) were synthesized, and their enzymatic and alkaline degradation rates of PLGA monolayers as functions of glycolide acid (GA) composition were systematically investigated by the Langmuir technique. The results showed that the alkaline and enzymatic degradations of PLGA monolayers were faster than those of l-polylactide (l-PLA), even though proteinase K is selectively effective in the l-lactide (l-LA) unit. Alkaline hydrolysis was strongly affected by their hydrophilicity, while the surface pressure of monolayers for enzymatic degradations was a major factor.

A wide range of poly(glycolic acid)‐based di‐ and tri‐block polyesters by a single organocatalyst: Synthesis and characterization

AbstractThe ring‐opening polymerization (ROP) of glycolide (GA), the copolymerization of GA with ε‐caprolactone (ε‐CL), and the terpolymerization of GA with ε‐CL and lactide (LA) were studied over the cheap non‐toxic metal‐free organocatalyst tetrabutylammonium 2‐(carbamyl)benzoate (TBACB). The catalyst with comparable catalytic activity to the comonomers afforded a series of well‐defined PGA‐based block polymers with ε‐CL, LA monomers at high yields by varying feeding methods under bulk and solution conditions. The poly(ɛ‐caprolactone‐b‐glycolic acid)s (PCGAs) and poly(ε‐caprolactone‐b‐lactide‐b‐glycolide) (PCLGAs) with controllable chain segment composition were obtained by “one‐pot” solution copolymerization in mixtures, or semi‐batch copolymerization. Due to the high thermal stability of TBACB, the PCGA and PCLGA with high molecular weight (Mn = 6.9 and 10.0 kg mol−1) could also be obtained by bulk polymerization at 120 °C by sequential addition of ε‐CL, (LA), and GA in a short time (<2.5 h). Compared to the polymers with similar compositions by different methods, the bulk polymerization showed more effective and higher molecular weight. The microstructure characterized by NMR showed highly ordered block polymers with defined chain segments. Two melting transitions (Tm) of PCGA suggested exclusively associated with PCL and PGA segmental chain in the product.

Synthesis and Structure‐Driven Acid‐Catalyzed Degradation of Benzoic Cyclic Acetal‐Labeled PEG Precursors toward Shell‐Sheddable PLA Block Copolymer Synthesis

AbstractThe synthesis of acid‐degradable poly(ethylene glycol) (PEG) precursors labeled with benzoic cyclic acetal (BzCA) is reported. To get an insight into their acid‐catalyzed hydrolysis, three precursors are designed with spacers exhibiting different inductive effects between PEG and BzCA linkage. The 1H‐NMR analysis confirms the order of increasing acid‐catalyzed hydrolysis to be ether > ester oxygen > ester carbonyl, which can be attributed to their ability to stabilize benzylic carbocation intermediates formed during hydrolysis. The formed precursors tend to be stable under a tin‐catalyzed condition for ring opening polymerization of lactide (LA), thus enabling the synthesis of well‐controlled acid‐degradable PEG‐based PLA block copolymers labeled with BzCA linkage at the block junction. When being exposed to acidic pH, the copolymers degrade through the cleavage of the junction BzCA linkages. These results guide the design principle of acid‐degradable shell‐sheddable BzCA‐bearing block copolymers for control over their acid‐catalyzed degradation and potentially drug release kinetics.

2nd generation PLA; Lactide formation directly from aqueous lactic acid

In order to develop a 2nd generation polylactic acid (PLA) process, lactide (LD) was synthesized directly from a commercially available 90% aqueous lactic acid (LA) solution. Experiments were conducted in an ordinary continuous flow fixed bed reactor. The reaction was carried out over various zeolites and γ-Al2O3 catalysts. Furthermore, the influence of pressure, temperature and water content in the inlet was studied. The highest LD selectivity of 98.9% was obtained using 90% LA and γ-Al2O3 as catalyst. In addition, it is concluded that LD yield increases when using highly concentrated LA oligomer with a minor amount of water. The catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray powder diffraction (XRD), nitrogen adsorption isotherms (BET), temperature programmed desorption of ammonia (NH3-TPD) and pyridine Fourier transformed infrared spectroscopy (Py-FTIR).