Controllable Ring-Opening Polymeriztion Of Racemic Lactide Using Sodium,Potassium And Yttrium Metal Complexes

Poly?lactide??PLA?has been widely utilized in the packaging,textiles,biomedical industry and other fields owing to its good biodegradability and biocompatibility.At present,the ring-opening polymerization?ROP?of lactides by metal catalysts is the most efficient manner to obtain polylactide because of its advantages of a well-controlled molecular weight and low polydispersity?PDI?.So far,a variety of metal complexes including alkali metal,alkaline earth metal,aluminum,zinc,titanium,zirconium,hafnium,indium,tin,and rare earth metal complexes have been developed to initiate/catalyze the ring-opening polymerization of lactides.As you know,the physical and mechanical properties of polymers are highly dependent on their stereoregularity.In the case of polylactide,the thermal stability of isotactic polylactide are much better than that of the heterotactic,syndiotactic,and random polylactide.However,only a few of metal catalysts can afford isotactic enriched PLAs from rac-LA.The reported catalysts mainly include Salen/Salan aluminum,yttrium,zinc,and the crown ether monophenol alkali metal complexes developed by our group.In addition,the alkali metals have certain advantages in the research of synthetic PLA due to their non-toxic and low price.However,the crown ether monophenol alkali system also has many disadvantages,it is necessary to study more deeply the system in order to solve the problems of low molecular weight and obtaining high isotactic polylactide only at low temperature.A lot of efforts have been made in the fieid of synthesis catalyst for the preparation of PLA,but so far,only a metal catalysts?tin octanoate,Sn?Oct?₂?can be industrialized preparation of polylactic acid by the method of ring opening polymerization.The reason is that it can reduce the production cost effectively without the protection of inert gas and the super pure monomers.Meanwhile as a catalyst,Sn?Oct?₂ has many shortcomings,such as serious side effects?ester exchange?,the molecular weight of poly?lactic acid??PLA?can not be controlled,broad molecular weight distribution,etc.,which vastly limits its application in pharmaceutical and other industry.Therefore,it is a significant and challenging focus to develop the air or water stability of the catalyst.Hence,developing the metal base catalysts which are not sensitive to water and air for the synthesis of PLA is a challenging and meaningful work.Based on the above reason,three main research works were conducted,thus this thesis is composed of the following four parts:Part 1:We first provide a brief introduction of the performance of polylactic acid with different stereotacticity and the mechanism of stereoselective ring-opening polymerization of cyclic esters.Then introduce a review of the research progress of stereoselectivity and air stability of the catalysts applied to catalyze ring-opening polymerization of lactide.Part 2:Because the ligand anion can initiate ring-opening polymerization of lactide in the crown ether monophenol sodium and potassium systems,therefore the result cannot obtain high molecular weight polylactide in this way.Based on the above analysis,a series of sodium and potassium metal complexes were synthesized by using sulfonamidate ligands with lower nucleophilicity and multidentate chelating coordination mode to replace monophenol ligand.The experimental results show that the problem of ligand anion initiating ring-opening polymerization of lactide is suppressed in the presence of benzyl alcohol and these complexes catalyze racemic lactide to get high isotactic?P_m=0.84?and high molecular weight?M_n=107 kg/mol?of polylactide under the low temperature.Part 3:Based on the results of the second part of the work,we will further research the crown ether monophenol sodium and potassium systems in more detail.The sulfonamidate ligand was replaced with the amidine ligand,which was to maintain the nucleophilicity of ligand,and further increase the steric hindrance so as to obtain high molecular weight and isotactic polylactide under mild conditions.The results show that the obtained polylactide with molecular weight can reach 117.7kg/mol and isotactic stereoselectivity can reach 0.88 at-70?.In addition,there is an interesting phenomenon of"anti-arrhenius"in the system.Through systemic experimental study,we properly explain the causes of"anti-arrhenius"in the catalytic system.Part 4:Currently,the most of metal catalysts known are poor stability in air or are not active to catalyze lactide in the air,thus the metal catalysts are tremendously restricted to use in industry.Based on the characteristics of rare earth metal yttrium ion,we designed and synthesized stable yttrium complexes to catalyze lactide in air with oxygen-containing multidentate ligands.The results show that the designed catalyst can catalyze with non-purified lactide in air and these catalysts are highly acitive and isoselectiveto catalyze the polymerization of rac-lactidewith the best isoselectivity of P_r value up to 0.74 and a lowest catalyst loading of 10 ppm.Moreover,this part work also studies in detail the mechanism of resistance to water in this system.