关键词: L-丙交酯, meso-丙交酯, 结晶, 共聚反应

Abstract: Polylactic acid (PLA), as a biodegradable material, can replace the current non degradable plastics and greatly reduce the environmental pollution caused by plastics. In addition, PLA has good thermal processability, biocompatibility and degradability, and will not pollute the environment after use. It can replace many traditional petroleum-based plastics, and has a wide range of applications. Generally, high-quality PLA is mainly prepared through stereoisomeric and specific ring opening polymerization of high-purity L-lactide or D-lactide. In the production process, L/D-lactide will be affected by factors such as catalyst, high temperature and reaction time, which will easily lead to a large amount of meso-lactide in the final product. The molecular weight, crystallinity, mechanical properties and thermal stability of PLA will be affected when meso-lactide enters the ring opening polymerization stage of L/D-lactide. Due to the similar properties of the three isomers and their characteristics of heat sensitivity, high freezing point and high boiling point, it is very difficult to purify L/D-lactide. This is an important factor affecting the quality and yield of lactide at present, and it is also a key and difficult point in the preparation process of lactide. It is of great significance to study the ring opening polymerization of meso-lactide and L-lactide obtained in actual industrial production and the product properties to guide the preparation process and application of PLA.
At present, research on the polymerization of L-lactide and meso-lactide focuses on the polymerization with L-lactide using high-purity meso-lactide as the monomer, and the properties are far from those of meso-lactide produced in the actual industrial production of L-lactide. Therefore, in this paper, the meso-lactide produced in the production process that is more in line with the industry is prepared by high-temperature heat treatment of L-lactide, and it is used as the polymerization monomer to explore the ring opening polymerization of L-lactide and meso-lactide, and to analyze the molecular weight, chemical structure, thermal performance and crystallization performance of the copolymer.
It is found that L-lactide could successfully undergo ring opening polymerization to obtain PLA copolymer with 0-5.0% meso-lactide, and the presence of meso-lactide does not significantly affect the molecular weight distribution uniformity of the polymer. With the increase of the content of meso-lactide, the molecular chain regularity of the copolymer decreases, and the crystallization region has a significant repulsion effect on the stereoisomers, resulting in the decrease of the crystallization rate and crystallization ability of the copolymer. Besides, the thermal performance parameters such as the thermal decomposition temperature, glass transition temperature, melting point and melting enthalpy of the copolymer are significantly reduced. Compared with PLA, the copolymer shows slower crystallization kinetics and the optimum crystallization temperature is about 100 ℃, which is consistent with PLA. The addition of meso-lactide will reduce the degree of ordering of copolymer macromolecular chain, but will not change the crystal structure of PLA.

Key words: L-lactide, meso-lactide, crystallization, copolymerization