Efficient Synthesis of Lactide with Low Racemization Catalyzed by Sodium Bicarbonate and Zinc Lactate

Abstract

Lactide is an important monomer in the industrial production of PLA, which is one of the materials with the greatest potential for replacing petroleum-based polymers. In view of the low yields of lactide when synthesized by metal catalysts, as well as the low purity and racemization seen in recently reported catalytic systems, we have developed a new catalytic methoda complex system of Zn­(La)2 and NaHCO3 that offers both high yield and high purity. The yield of lactide reached above 95.63% with a purity up to 97.86% in only 3.25 h. Furthermore, the effects of the catalyst loading and the addition time of the catalyst were studied to optimize the yield of lactide and deduce a reasonable reaction mechanism in conjunction with NMR analysis. The reaction of NaHCO3 and the oligomer is thought to generate carboxylate anions, which would dramatically inhibit further polymerization. Thus, the bond between ester oxygen and methyne carbon was broken due to the attack of the carboxylate anion on the asymmetrical carbon atom, and the ester bond was broken due to the attack of the hydroxyl group on the carbonyl carbon, which results in the decrease of the molecular weight of the residual oligomers. As a result, the rate of lactide production increased rapidly as the molecular weight of the polymer decreased.

Controlled Synthesis of l-Lactide Using Sn-Beta Zeolite Catalysts in a One-Step Route

“…With the occurrence of environmental pollution owing to the accumulation of petroleum-based plastics, there is an urgent need to develop renewable and biodegradable materials as substitutes. − Polylactic acid (PLA), as a biomass-derived − and environmentally friendly polymer, has been widely used in biomedical and chemical products. − High-quality PLA is predominately produced by ring-opening polymerization of lactide, a cyclic dimer of lactic acid (La). ,− Industrially, lactide is produced through a two-step method that is costly and energy-intensive. , In this process, La is polymerized to form oligo-PLA ( M w < 3000 Da) by self-catalysis and then depolymerized into lactide via a back-biting mechanism in the presence of a metal catalyst, which can be a salt or an oxide of tin or zinc. − Among the catalysts developed, tin­(II)-based compounds (SnCl 2 , SnOct 2 , and others) are considered to be the most efficient to synthesize lactide. − The lactide must be separated rapidly from the reaction system to avoid the thermodynamically favored reverse reaction. Since the viscosity of the substrate gradually increases with the progress of the reaction, it becomes difficult to remove water and lactide from the system.…”

“…Since the viscosity of the substrate gradually increases with the progress of the reaction, it becomes difficult to remove water and lactide from the system. Therefore, both steps of lactide synthesis are carried out at high temperature under vacuum for a long time for the removal of water and lactide. ,, The yield of lactide is only 60–70% because of the formation of meso -lactide by racemization and polymeric waste residue by the polymerization of oligo-PLA. − Moreover, the catalyst present in the waste residue cannot be reused as it is complicated to separate it from the residue. , …”

Stereogradient Poly(Lactic Acid) from meso‐Lactide/L‐Lactide Mixtures

“…[13,14] In recent years, vast efforts have been invested in developing of alternative methods for production of L-lactide wherein the meso-lactide contamination would be minimized, including "one-step" processes. [8,[15][16][17] To our knowledge, though, these processes are yet to be implemented on an industrial scale. [18] Herein we describe a new approach for attaining highly crystalline PLLA which relies on accommodating rather than removing lactide stereoisomers.…”

Development of recoverable and recyclable Fe3O4‐supported organocatalysts for ring‐opening polymerization

“…Indeed, those ROP catalysts are mainly single use. 15,16 Although many of the commercial ROP catalysts are quite active such that a single use is acceptable, catalyst residues could be an unnegligible issue for some specialized polymer products, such as, polymers for biomedical and microelectronic applications. [17][18][19] Recoverable and recyclable catalysts have been widely studied in organic synthesis via covalent tethering homogeneous organocatalysts to heterogeneous supports, including porous inorganic oxides, 20 organic polymers, [21][22][23][24] and magnetic nanoparticles (MNPs).…”

“…However, it is very difficult to remove or recover these catalysts, which leads to the product contamination and subsequently increasing processing cost. Indeed, those ROP catalysts are mainly single use 15,16 . Although many of the commercial ROP catalysts are quite active such that a single use is acceptable, catalyst residues could be an unnegligible issue for some specialized polymer products, such as, polymers for biomedical and microelectronic applications 17–19 …”