Synthesis Of Poly(L-Lactic Acid) With High Molecular Weight And Isotacticity Via Melt/Solid Polycondensation Of L-Lactic Acid Catalyzed By Organic Guanidine

In recent years, with the continued increase of the awareness on the environmental protection which mankind live on and the increasingly aggravation of white pollution caused by the blind production and abuse of the disposable petroleum-based plastics, the development and utilization of environmentally-friendly and biodegradable polymers based on renewable resources has aroused growing concern. Among the family of environmentally-friendly polymers, poly(lactic acid) (PLA) is the most valuable synthetic polyester which shows great potential to be an alternative to the disposable petroleum-based plastics. Synthesis of biodegradable PLA with metal-free biogenic compounds is a new challenge in the field of biodegradable materials. In this thesis, the organic guanidine compound creatinine (CR) was successfully used as catalysts for synthesis of PLA with high bio-safety and environmentally friendship via controlled polycondensation.(1) Isotactic polycondensation of L-lactic acid (L-La) was successfully realized under the conditions of 150-175℃ and sequentially reduced pressure (20-10 torr) with biogenic creatinine, for the first time. The optimal reaction conditions:the catalyst is CR, the amount of it was 0.1wt%, the reaction temperature is 175±2℃。The products poly(L-lactic acid) (PLLA) possess high isotacticity (97.8%) and optical purity (e.e. 96.1-96.4%), as well as narrow molecular weight distribution (PDI 1.78-1.88). The 1H NMR structural characterization of the growing polymeric species in progress of the polycondensation revealed that the active catalytic species is a guanidinium formed in situ. The mechanism of the creatinine-catalyzed polycondensation was proposed based on the experimental investigation.(2) High stereoregularity and molecular weight polymers of PLLA (Mw 1.2×105, isotacticity 96.0%) was successfully synthesized via melt/solid polycondensation (MP/SSP) using catalyst Organic Guanidine. The optimal reaction conditions:the catalyst is CR, the amount of it was 0.005wt%, the reaction temperature is gradient(150℃-153oC-154℃-157℃).Thermogravimetric (TG) analysis demonstrat-ed that the decomposition temperatures (Td, init 324.3℃, Td,5% 347.0℃, Td, max 400.2℃) of PLLA synthesized with catalyst CR are over 100℃ above those of PLLA synthesized with catalyst SnCl2·2H2O.