Study On The Ring-opening Polymerization Of Lactide Catalyzed By Diketiminate Catalysts

Polylactide is a polyester that has attracted great interest in biomedical field due to their good biodegradability and compatibility. The paper study the ring-opening polymerization of lactide catalyzed by β-diketiminate nickel complexes,CH[C(CF3)NAr]2NiBr, Ar=2,6-iPr2C6H3cat.1;Ar=2,6-Me2C6H3:cat.2,and β-diketiminate titanium complex CH[C(CH3)NAr]2TiCl2, Ar=2,6-1Pr2C6H3:cat.3.The effect of the monomer and catalyst molar ratio, the reaction temperature and the reaction time on the yields of polylactide was studied. The structure and the properties of the obtained polylactide was characterized by infrared spectroscopy (IR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and differential scanning calorimetry analysis (DSC).The paper studies the synthesis of racemic lactide using lactic acid as raw material, zinc oxide, zinc acetate and stannous octoate as catalysts. The influence of the monomer and catalyst ratio, the dehydration and the condensation time on the yield of lactide was investigated. The results indicate that the yield is increased first and then decreased with the increasing of the amount of the catalysts, and highest lactide yield was obtained at the amount of the catalyst of1.5%. Additionally, lactide yield had a relationship with the dehydration and the condensation time of lactic acid. With the increase of the condensation time, lactide yield is increased and then decreased; the effect of the dehydration time on lactide yield shows similar trend as the condensation time. Highest lactide yield of54.03%was obtained under the optimized reaction condition of the catalyst content, the dehydration and the condensation time of lactic acid.Melt ring-opening polymerization of lactide was studied using β-diketiminate nickel cat.l, cat.2and the β-diketiminate titanium cat.3complexes as catalysts. The effect of the polymerization condition, such as the monomer and the catalyst molar ratio, the reaction temperature and the reaction time on the polylactide yield was investigated. All three complexes show characteristic of catalyzing polymerization of lactide, the monomer and the catalyst molar ratio, the reaction temperature and the reaction time show similar influence trend on the yield of polylactide. For example, polymer yield increases and then decreases with increases of the monomer and catalyst molar ratio, reaction temperature and reaction time. However, the conditions for obtaining the highest yield polymer are different; the highest polymer yield of88.62%was obtained at140℃for24hours with the molar ratio of monomer and catalyst of500using cat.1, the highest polymer yield of90.47%was obtained at140℃for40hours with the molar ratio of monomer and catalyst of500using cat.2, while the highest polymer yield of89.15%was obtained at150℃for18hours with the molar ratio of monomer and catalyst of1000using cat.3.The sequences of the catalytic activity of the three complexes are: cat.1-cat.2<cat.3. Complexes1and2showed similar activity indicates that the steric effect is not pronounced with β-diketiminate nickel complexes as catalysts for the ring-opening polymerization of lactide. β-Diketiminate titanium complex exhibited highest activity among these three β-diketiminate complexes, indicate that metal center of the β-diketiminate complexes play an important role in the catalytic polymerization of lactide. The molecular weights of the obtained polylactide were characterized by gel permeation chromatography (GPC). With the increases of the reaction temperature, the molecular weights are decreased, this are attributed to the degradation of the polylactide at higher temperature. Additionally, the molecular weights of polylactide increased with increases of the reaction time, however, molecular weights of polylactide dropped at long polymerization time because of degradation of polylactide.The structure of the obtained polylactide was determined by IR, NMR and XRD. The results show that polylactide were successfully prepared using β-diketiminate nickel and titanium complexes1-3. Microstructure analysis of the obtained polylactide proved that heterotactic polylactide was obtained with these catalysts. DSC analysis indicates that the glass transition temperature (Tg) and the decomposition temperature of the obtained polylactide are near the50and300℃, respectively. No melting peak was observed in the GPC curve of polylactide indicate that the obtained polylactide are amorphous racemic polylactide.