Study On The Synthesis Of Polylactic Acid And Simulation Of The Synthesis Process Of Lactide

It is researched on the synthesis of lactide, lactide purification techniques and simulating the synthesis of lactide by Materials Studio. The main conclusions are as follows:1. Through the single factor experiments, the optimal conditions for synthesis of lactide which we determined is:The catalyst is the mass fraction of 1% of the Sn(Oct)2, the temperature of removal of free water is 80℃, the pressure is reduced slowly through the whole process, dehydrates 1h; Prepolymerization temperature is 140℃,3h reaction; Then, the system of reaction is heated to 230℃quickly, the broken chain esterification is carried on. During the distillation process, the prolong is wrapped with asbestos, to prevent the crystallization of lactide in the pipe. The maximum production of lactide is 55.6%.2. Through experiments, lactide recrystallized technological conditions is optimized: Absolute ethyl alcohol is selected as a solvent for recrystallization, the temperature of recrystallization is 0℃, recrystallized three times, each time for 1h, The best solvent ratio is 0.4mL/g.In this way, lactide receiving rate enhanced to 73.9%.3. Lactide ring-opening polymerization technological conditions was optimized by response surface methodology (RSM) to obtain the optimum conditions. The conclusions are as follows:The amount of catalyst is 0.29ml, the polymerization temperature is 160℃, the polymerization lasts 10h, relative molecular mass of polylactic acid is 12.4×104. At the same time, the equation of catalyst(A),Polymerization temperature(B) and Polymerization time(C) is obtained: Y=12.40+1.37X,+1.48X2+0.68X3-3.81 X12-1.91 X22-2.66 X32+1.38 X, X2+0.42 X1 X3-0.18X2X34. Equilibrium geometric configuration and dipole moment of LA,LAoligomers and lactide is optimized by MS.Through analysing frontier molecular orbital, it is indicated that:The HOMO of the LA oligomers is mainly collected to hydroxyl terminal and ester linked, however, the LUMO is mainly collected to carboxyl terminal and ester linked. These sites is the active site of depolymerization reaction. the enthalpy change of each reaction is positive value,the suitable temperature elevation is advantageous in the reaction toward positive. Entropy change of each of depolymerization reaction is negative, but absolute value is minor, so all of depolymerization reactions are reversible reactions, product removal is advantageous in the reaction toward positive.By the QST/LST transition state search, which is the module in Dmol3, the various transition states are received and determined. And all reaction intermediates are obtained from the MEP curve, and thus mechanism of the synthesis of lactide is proposed theoretically.