Synthesis of L-Lactide from Lactic Acid and Production of PLA Pellets: Full-Cycle Laboratory-Scale Technology

Lactide is one of the most popular and promising monomers for the synthesis of biocompatible and biodegradable polylactide and its copolymers. The goal of this work was to carry out a full cycle of polylactide production from lactic acid. Process conditions and ratios of reagents were optimized, and the key properties of the synthesized polymers were investigated. The influence of synthesis conditions and the molecular weight of lactic acid oligomers on the yield of lactide was studied. Lactide polymerization was first carried out in a 500 mL flask and then scaled up and carried out in a 2000 mL laboratory reactor setup with a combined extruder. Initially, the lactic acid solution was concentrated to remove free water; then, the oligomerization and synthesis of lactide were carried out in one flask in the presence of various concentrations of tin octoate catalyst at temperatures from 150 to 210 °C. The yield of lactide was 67-69%. The resulting raw lactide was purified by recrystallization in solvents. The yield of lactide after recrystallization in butyl acetate (selected as the optimal solvent for laboratory purification) was 41.4%. Further, the polymerization of lactide was carried out in a reactor unit at a tin octoate catalyst concentration of 500 ppm. Conversion was 95%; Mw = 228 kDa; and PDI = 1.94. The resulting products were studied by differential scanning calorimetry, NMR spectroscopy and gel permeation chromatography. The resulting polylactide in the form of pellets was obtained using an extruder and a pelletizer.

Syntheses of polylactides by means of tin catalysts

Reaction mechanisms and synthetic methods used for the preparation of homo- and copolylactides based on tin(ii) and tin(iv) catalysts are reviewed.

High T m linear poly(l-lactide)s prepared via alcohol-initiated ROPs of l-lactide

Alcohol-initiated ROPs of l-lactide were performed in bulk at 160 °C for 72 h with variation of the catalyst or with variation of the initiator (aliphatic alcohols). Spontaneous crystallization was only observed when cyclic Sn(ii) compounds were used as a catalyst. Regardless of initiator, high melting crystallites with melting temperatures (T m) of 189-193 °C were obtained in almost all experiments with Sn(ii) 2,2'-dioxybiphenyl (SnBiph) as catalyst, even when the time was shortened to 24 h. These HTm poly(lactide)s represent the thermodynamically most stable form of poly(l-lactide). Regardless of the reaction conditions, such high melting crystallites were never obtained when Sn(ii) 2-ethylhexanoate (SnOct2) was used as catalyst. SAXS measurements evidenced that formation of HTm poly(l-lactide) involves growth of the crystallite thickness, but chemical modification of the crystallite surface (smoothing) seems to be of greater importance. A hypothesis, why the "surface smoothing" is more effective for crystallites of linear chains than for crystallites composed of cycles is discussed.

About the transformation of low T m into high T m poly(l-lactide)s by annealing under the influence of transesterification catalysts

Cyclic polylactides were prepared in bulk at 170 °C, crystallized at 120 °C and then annealed at temperatures between 130 and 170 °C with variation of catalyst, catalyst concentration and annealing time. The transformation of the initially formed low melting (LT m) crystallites, having melting temperatures (T m) < 180 °C into high melting (HT m) crystallites having T m values > 189 °C was monitored by means of DSC measurements and characterized in selected cases by SAXS measurements. It was confirmed that the formation of HT m crystallites involves a significant growth of the thickness of the lamellar crystallites along with smoothing of their surface. Annealing at 170 °C for 1 d or longer causes thermal degradation with lowering of the molecular weights, a gradual transition of cyclic into linear chains and a moderate decrease of lamellar thickness. An unexpected result revealed by MALDI TOF mass spectrometry is a partial reorganization of the molecular weight distribution driven by a gain of crystallization enthalpy.

Cyclic polylactide synthesis initiated by a lithium anthraquinoid: understanding the selectivity through DFT and diffusion NMR

We present herein the application of a lithium anthraquinoid in the catalytic synthesis of cyclic PLA, showing that the aggregation plays a critical role in cyclic vs. linear selectivity.

Investigation of the influence of impurities on the ring-opening polymerisation of L-Lactide from biogenous feedstock

In order to use a L-lactide monomer that is derived from fermentation processes it is necessary to understand, how the polymerisation process is influenced by impurities which derive from the production process. We have selected a group of likely contaminants and added them at various concentrations to the polymerisation of L-lactide using tin (II)-2-ethylhexanoate as catalyst and 2-methoxyethanol as initiator. The effect of impurities onto the global properties of the polymers such as glass transition temperature, melting point and molecular mass distribution were investigated and NMR and MALDI mass spectrometry were used to identify structural changes within the polymers. Thus, it could be shown that in reference experiments cyclic polymers and linear polymers with different starting groups are formed. Addition of ethanol and sodium carbonate showed the strongest influence on molecular masses as well as polymer structures, which could be elucidated by interpretation of the MALDI mass spectra and NMR data.

The Ring-Opening Polymerization-Polycondensation (ROPPOC) Approach to Cyclic Polymers

A new concept called ring-opening polymerization-polycondensation (ROPPOC) is presented and discussed. This synthetic strategy is based on the intermediate formation of chains having two end groups that can react with each other. The ROPPOC syntheses are subdivided into three groups according to the nature of the chain ends: two ionic end groups, one ionic and one covalent chain end, and a combination of two reactive covalent end groups may be involved, depending on the catalyst. The usefulness for the preparation of cyclic polymers is discussed with a review of numerous previously published examples. These examples concern the following classes of cyclic polymers: polypeptides, polyamides, and polyesters, including polycarbonates and cyclic polysiloxanes. It is demonstrated that the results of certain ROPPOC syntheses are in contradiction to the Jacobson-Stockmayer theory. Finally, the usefulness of ROPPOCs for the detection of polydisperse catenanes is discussed.