Green Synthesis Of Glycolide Via A Closed Repetitive Process Using Organic Guanidine As Catalyst

With the increasingly environmental problem caused by the nondegradable petroleum-based plastics,the use of biodegradable and environmental-friendly materials is an urgent task.Synthetic biodegradable polyesters are ultimately degraded into carbon dioxide and water in nature.Poly（glycolic acid）（PGA）,Poly（lactic acid-glycolic acid）（PLGA）,two commercialized biodegradable polymers have found many useful applications in the field of packaging materials,biomedical and tissue engineering.High performance PGA and PLGA were generally prepared by ring opening polymerization（ROP）of high-purity GA.Therefore,the synthesis of high-purity glycolide is important for the synthesis of poly（glycolic acid）and its copolymers.Commercially GA was synthesized with heavy metal catalysts Sn（Oct）₂,SnCl₂,Sb₂O₃.The main disadvantage of the process is that the residue polymer（r-PGA）containing the deactivated toxic catalyst can not be reused and is deserted as a solid waste.To solve the problem,a green process for glycolide（GA）synthesis was explored for the first time in this thesis.The new process features repetitive utilization of the residue polymer（r-PGA）as a raw material for GA synthesis,and a metal free,non-toxic organic catalyst.The r-PGA was a byproduct formed in decomposing the preformed o-PGA.1.Three guanidine-based catalysts i.e.CR,TBD,bicyclic guanidine chloride（TBDC）were used as catalyst for the synthesis of GA.Under the optimum conditions（TBDC,280℃,4 torr,2 h）,the yield of glycolide reached up to 84.6%.2.Two different processes were employed for repetitively utilizing the r-PGA,the utilization efficiency of the Ga reached up to 100%.（1）r-PGA was firstly degraded to o-PGA with the raw material Ga（70%）.The o-PGA was then repeatedly utilized for GA synthesis.After 6 repetitive utilization of r-PGA,the yield of GA reached up to 97.1%which is the highest value reported up to date.（2）With r-PGA as the raw material,high molecular weight PGA（Mw 9.9×104 Da）was synthesized by solid-state polycondensation under the temperature of 190-195℃ for 30 h.3.Two methods for GA purification were investigated:（1）By the method of ethyl acetate/acetone（6:1 v/v）recrystallization GA of 99.9%purity was obtained,which meets the standard for biomedical polymer synthesis.（2）By the method of deionized water washing for three times GA of 99.3%purity was obtained with yield of 85%,which meets the requirment for environmental-friendly polymer synthesis.4.The structure and purity of GA were characterized by ¹H NMR,IR,GC and DSC.The synthesis and degradation of r-PGA were monitored by GPC.