Preparation Of High Molecular Weight Polylactic Acid By Lactide Ring-opening Polymerization

Polylactic acid not only has excellent biodegradability,but also has excellent mechanical,physical properties and biocompatibility,suitable for various processing methods.The broad application prospect and high research value of polylactic acid make polylactic acid occupy a place in the future society with sustainable development and ecological protection as the main melody,and become the research hotspot of scientists all over the world.There are roughly two ways to synthesize polylactic acid.One is direct polycondensation method,polylactic acid is made by dehydration polymerization of lactic acid.This method has low cost and simple process,and the molecular weight of the synthesized polylactic acid is generally not high.The other is the intermediate lactide ring-opening polymerization,this method can get higher molecular weight and relatively pure polylactic acid.In this paper,the indirect synthesis of polylactic acid（PLA）was explored,that is,polylactic acid was prepared by ring-opening polymerization of lactide.Indirect method is divided into bulk method and solution method,the difference between the two is there is no solvent.The synthesis experiment was carried out with stannous octoate（Sn（Oct）2）as catalyst and butaerythritol as initiator.The structure of polylactic acid was determined by nuclear magnetic resonance and infrared spectroscopy,the molecular weight of polylactic acid was determined by gel permeation chromatography（GPC）,and the rotation of polylactic acid was determined by visual gyroscope.The lactide of Henan Jindan was selected from some commercial and experimental grades,and the relationship among polymerization temperature,polymerization time,catalyst dosage and polylactic acid molecular weight was investigated.The experimental results show that polylactic acid with relatively high molecular weight can be obtained by regulating the polymerization conditions.The optimum process conditions of bulk method are that the polymerization temperature is gradually increased to 175℃,the polymerization time is 6 h,the molar ratio of catalyst to monomer is 1:6000,and the highest weight average molecular weight of the synthesized polymer is more than 60,000.The optimum conditions of solution method were polymerization temperature of 120℃,polymerization time of 24 h,molar ratio of catalyst to monomer of 1:400,and the highest average molecular weight of the synthesized polymer was about 60,000.A 5 L polymerization reactor was designed and prepared on the basis of the technological conditions explored in the laboratory experiment.Considering the technological requirements of early lactide preparation and late lactide ring-opening polymerization,the reactor can allow two kinds of reactions to be carried out separately,and can realize the function of multiple use of one reactor,which greatly improves the application range and application value of this reactor in polylactic acid amplification experiment.Lactic acid,as a raw material,is highly corrosive,which will corrode the inner wall of the metal reactor and affect the color of the polymerization product.The samples made of several metal materials were put into the monomer lactic acid,and the corrosion experiment was carried out by heating reflux at 96 ℃.In the end,2205 stainless steel was selected as the material for the contact part of the 5L polymerization kettle.According to the process conditions and parameters in the glass reaction bottle,a series of scale-up experiments of bulk ring-opening polymerization were carried out in a 5 L polymerization kettle to optimize the experimental process parameters.Using Henan Jindan lactide as monomer,polylactic acid with molecular weight of more than 100,000 was obtained,and the process repeatability was good.Therefore,the optimum process conditions can be obtained as segmenting polymerization,that is,0.5 h prepolymerization at 130℃,2 h polymerization at 175℃,and then about 0.5 h discharging at 185℃.In view of the defects and deficiencies of stannous caprylate catalysts,a new type of composite catalyst was developed.Through lactide ring-opening experiment,MnO2 and Co2O3 were selected as two components of the composite catalyst.The MnO2-Co2O3 composite catalyst was fired by tube furnace according to the program.The mixing ratio of the two components was explored and the composite catalyst was characterized by various means.The ring-opening polymerization of lactide catalyzed by MnO₂-Co₂O₃ composite catalyst was studied.