Controlled Synthesis Of Environmentally-friendly Poly(lactic Acid) With Biogenic 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 biogenic organic guanidine compound creatinine was successfully used as catalysts for synthesis of PLA with high bio-safety and environmentally friendship via controlled polycondensation;a total green process for lactide synthesis was developed for the first time;controlled ring-opening polymerization of L-lactide was successfully conducted with a non-toxic binary catalyst system of triazabicyclodecenylguanidinium acetate-alcohol,designed and synthesized by the authors.(1)Isotactic polycondensation of L-lactic acid(LLa)and D-lactic acid(DLa)was successfully realized under the conditions of 150-198? and sequentially reduced pressure(30-10 torr)with biogenic creatinine(CR),a guanidine derivative existing in human body,for the first time.The products poly(L-lactic acid)(PLLA)and poly(D-lactic acid)(PDLA)possess high isotacticities(97.8%)and optical purities(e.e.96.1-96.4%),as well as narrow molecular weight distribution(PDI 1.78-1.88).13C NMR follow-up monitor of the polymerization demonstrated that the isotacticities of PLLA and PDLA formed in the polymerization kept constant high values(isotacticities 97.8-99.5%)throughout the polymerization.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%)and PDLA(Mw 1.0 × 105,isotacticity 98.6%)were successfully synthesized via melt/solid polycondensation(MP/SSP)using catalyst CR.The follow-up monitor of the polycondensation products with 13C NMR technique revealed that the polymerization of MP/SSP catalyzed by CR proceeded in a stereochemical controlled way throughout the whole process as evidenced by the constant high values of isotacticity(96.0-99.0%)of the product polymers.Thermogravimetric(TG)analysis demonstrated that the decomposition temperatures(Td,init 324.3 0C,Td,5%347.0 ?,Td,max 400.2 0C)of PLLA synthesized with catalyst CR are over 100 ? above those of PLLA synthesized with catalyst SnCl2 ·2H2O.(3)Enantiomerically pure lactides(100%e.e.)of L-lactide(LLA)and D-lactide(DLA)were synthesized with high yield(68.5-69.5%)via the depolymerization of preformed oligo-PLLA(O-PLLA)and oligo-PDLA(O-PDLA)catalyzed by CR.The fine molecular structures of synthesized LLA and DLA were characterized with XRD analysis of the prepared single crystals.Gas chromatograph(with a chiral stationary phase capillary column)analysis demonstrated that the synthesized LLA and DLA possessed enantiomeric excess of 100%.The catalytic efficiency of the catalyst creatinine reached to a high level as proved by the high turnover frequency of 338.0-347.7 h-1 which exceeded the values reported till now.The residues of the depolymerization were utilized for resynthesis of the enantiomerically pure lactides without the decrease in the yields of enantiomerically pure LLA/DLA.This total green process for lactide synthesis featured a closed-cycle without any waste release.A mechanism of CR-catalyzed LLA/DLA synthesis was proposed based on the experimental investigation.(4)A binary catalyst system of triazabicyclodecenylguanidinium acetate(TBDA)—alcohol(R'OH)i.e.TBDA—R'OH was successfully developed and used to realize the controlled ROP of LLA.Experimental results demonstrated that the catalytic system is of high catalytic activity.1H NMR structural characterization of the living polymeric species in progress of the ROP revealed that the active bond of the growing polymer was the guanidinium alkoxide(G+-OR')formed in situ.The mechanism of the polymerization was postulated to follow the coordination-insertion mechanism on the basis of the end-group characterization of the living polymeric species.The PLLA produced has higher isotacticity of 82.0%as evidenced by the 13C NMR characterization.