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

摘要： 为研究工业化生产中左旋(L-)丙交酯和内消旋(meso-)丙交酯的聚合反应及产物性能,通过对L-丙交酯进行高温热处理制备更加符合工业生产的meso-丙交酯,并以L-丙交酯和meso-丙交酯为聚合单体,探究丙交酯性质对开环聚合反应产物的分子量及其性能的影响。借助凝胶渗透色谱仪、核磁共振波谱仪、傅里叶红外光谱仪、热重分析仪、差示扫描量热仪和二维X射线衍射仪分析了聚乳酸共聚产物的分子量、化学结构、热性能和结晶性能。结果表明:含有0~5.0% meso-丙交酯的L-丙交酯可以成功进行开环聚合得到聚乳酸共聚物,并且meso-丙交酯的存在不会显著影响聚合物分子量的分布均一性。随着meso-丙交酯含量的增加,聚乳酸的热分解温度、玻璃化转变温度、熔点和熔融焓等热性能参数均明显降低。与聚左旋乳酸(PLLA)相比,共聚物的结晶速度与结晶能力降低,二者的最佳结晶温度均在100 ℃左右。meso-丙交酯的加入使共聚物大分子链的有序化程度降低,但不会改变聚乳酸的晶体结构。

关键词: L-丙交酯, meso-丙交酯, 结晶, 共聚反应