| Polylactide,a typical kind of synthetical aliphatic polyesters,has attracted much attention due to its bioenvironmental compatibility,biodegradability and excellent mechanical properties.However,its application is restricted by its high cost in a certain extent.Lactide,can be prepared by lactic acid,is an indispensable material to prepare polylactide.Therefore,preparing high yield lactide is very important to extend application range of polylactide.In the paper,researches to prepare lactide from lactic and to improve the yield of lactide were carried out,and the optimization reaction conditions of lactide's polymerization were probed.Firstly,Solid superacid catalysts SO42-/TiO2-ZnO-La2O3 and SO42-/TiO2-SnO2-La2O3 were prepared by titration precipitation under different conditions and used to catalyze the synthesis of D,L-lactide.The catalytic activity of them were studied.The results show that catalytic activity of SO42-/TiO2-ZnO-La2O3 prepared under conditions of n(Ti4+)∶n(Zn2+)∶n(La3+)=30∶30∶1,calcination temperature 460℃,calcination time 3 h,dipping in 0.7 mol/L H2SO4 for 22 h,and SO42-/TiO2-SnO2-La2O3 prepared under conditions of n(Ti4+)∶n(Sn4+)∶n(La3+)=20∶20∶1,calcination temperature 600℃,calcination time 3 h,dipping in 3.0 mol/L H2SO4 for 12 h,are best to the synthesis of D,L-lactide.Both of the catalysts make the crude yeild of D,L-lactide reach over 71.0%.Effects of catalyst amount,dehydration-polymerization temperature and time,depolymerization temperature,diluents and reaction residue on the crude yield of D,L-lactide were discussed in detail.In the paper,synthesized D,L-lactide was purified by recrystallization with different solvents.The purity of recrystallized D,L-lactide was estimated by measuring its melting point and lactic acid's content.It was found that absolute ethanol is the most suitable solvent for D,L-lactide's purification.After three times recrystallization,the purity of D,L-lactide is very high and the gross percent recovery of D,L-lactide reaches 53.6%.The ring-opening polymerization of L-Lactide initiated by stannous octoate(Sn(Oct)2) was carried out in solution.The effects of different solutions,addition of protic reagent and adding amount on polymerization were studied.The relationship between molecular weight(Mn)and n[L-LA]/n[Sn(Oct)2]ratio,conversion of L-LA was researched.It was found that the rate of polymerization was bigger comparativelly in CH2Cl2,and enhanced rate was observed in the presence of 1.0 equiv of protic reagent, especially of benzyl alcohol.Linear plots of Mn vs conversion and Mn vs n[L-LA]/n[Sn(Oct)2]ratio,were indicative of well-controlled and living polymerizations.Kinetic data were further analyzed by assuming that the propagating active species aggregate,with one inactive,aggregated form and one active, unaggregated form.The kinetic studies revealed a first-order dependence on[L-LA], and one twice-order dependence on[Sn(Oct)2].It was also studied that the polymerization of D,L-lactide catalysed by Sn(Oct)2 was carried out in bulk.The effects on polylactide's molecular weight of catalyst amount,reaction time and reaction temperature were discussed.The results show that the molecular weight of PDLLA,synthesized under conditions of n[D,L-lactide]/n[Sn(Oct)2]=4000,reaction temperature 160℃and reaction time 7h,is about 7.75×104,which is highest.
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