Study On Direct Polycondensation Of Polylactic Acid

Due to the straightforward routing of synthesis polylactic acid and the low costs of products, it has attracted much attention to research the direct polycondensation of PLA.Our main job is synthesis high molecular weight PLA through direct polymerization by solid super-acid and optimizing reaction conditions of PLA's polymerization. Also basic research of synthesis co-polymer PLA-PEG is probed.Firstly, Solid superacid catalysts SO₄^2-/ZrO₂ loaded with rare-earth metal oxide Gd₂O₃,La₂O₃,Y₂O₃,CeO₂ were prepared by titration precipitation under different conditions and used to catalyze the synthesis of Polylactic acid with melt polycondensation. The catalytic activity of them was studied. And the molecular weight of PLA is highest catalyzed by solid super-acid SO₄^2-/ZrO₂-CeO₂. Then SO₄^2-/ZrO₂-CeO₂(SZC) was researched through orthogonal experiment .And three orthogonal experiment L₉(3⁴) were designed and processed with n(Ce)/n(Zr), n% sulphuric acid, calcination time and temperature. The experiment results were analysed by molecular weights of PLA, which were catalyzed by solid super-acid catalysts. The results show that catalytic activity of SO₄^2-/ZrO₂-CeO₂ prepared under conditions of n(Ce)/n(Zr)=0.05, dipping in 0.8 mol/L H₂SO₄, calcination time 3 h,calcination temperature 650℃, are best to the synthesis of PLA. Then we discussed the reaction conditions of PLA.When reaction time is 20h, temperature is abous 170℃, molecular weighe of PLA is highest.Secondly, SO₄^2-/ZrO₂-CeO₂ samples were prepared by impregnation of hydrous zirconium oxides and CeO₂ with different concentration of H2SO4 solutions. The nature and acid strength in SO₄^2-/ZrO₂-CeO₂ samples were measured by FTER, Hammett Indicator, and XRD techniques, NH₃-TPD, FT-IR spectra of pyridine adsorbed. The above catlysts were used in synthesis of poly(lactic acid) (PLA) by solution refluxing polycondensation of lactic acid and the effects of reaction time, catalyst amount and polycondensation methods on polycondensation of lactic acid were studied. The results indicated that the SO₄^2-/ZrO₂-CeO₂ sample, which with m(CeO₂): m(ZrO₂)=1:20 was impregnated in 0.8 mol/L H₂SO₄ solution and calcined at 650℃, was the strongest acid strength (H₀≤14.52). The CeO₂ was high dispersed on catalysts and promoted the acidity of solid superacids. The viscosity average molecular weight of PLA correlated well with the acid strength of those superacid sites. The viscosity-average molecular weight of PLA reached to 1.16×10⁴ when the amount of catalyst was 1% of the mass of lactic acid and reaction time was 30h.Last, The PLA-PEG block polymer catalyzed by solid super acid SO₄^2-/ZrO₂-CeO₂ was prepared with four routes. Then the effects of lactide properties, molecular weight of PEG, reaction time, temperature on copolymer were studied, and the characteristic group of product was measured by FTIR spectra. The results indicated that the best reaction condition are LA and PEG reacting at the same time,PEG 6000,reaction time 9h and temperature 170℃.