The Study Of Stereoselective Rop Of Cyclic Esters Catalyzed By Metal Complexes

Poly(α-hydroxy acids)(PAHA)have received great attetion over the last decades as a replacement for conventional petroleum-based synthetic materials due to its biodegradeablity,biocompatiablity and accessibility from natural resouse.Among them,polylactide(PLA)is an important one which has been applied in a wide range of fields such as packaging,fiber technology,and some important biomedical and pharmaceutical applications.A promising method for preparing PLA is the ring-opening polymerization(ROP)of lactide for its prominent features of a controllable molecular weight and low polydispersity(?).By far,a number of metal based catalysts,which consist of an appropriate organic ligand and a central electropositive metal atom such as alkali metals,Mg,Zn,Al,In,Sn,rare earth metals and other metals,were reported to initiate/catalyze the polymerization of lactide.Stereoselective polymerization of lactide is a valuable research subject because different tactic polymers will own different chemical and physical characters.Obviously,catalysts play a key role in this area,among which a metal catalyst with a suitable ligand is very attractive for the high efficiency,high selectivity,and easy tuning of the ligand.The lack of side chian functionality of conventional PLA also appears as an important limitation,which make it diffcult to tune the physicochemical properties of PAHA-derived materials.Increasing efforts have been devoted to development a methodology to preparation PAHAs via ROP of functional LA derivatives,but this strategy can be complicated by the moderate accessibility and poor reactivity in ROP of the required 1,4-dioxane-2,5-diones.During the last years,O-carboxyanhydride(OCA)have emerged as suitable alternatives to 1,4-dioxane-2,5-diones for the efficient preparation of functionalized PAHA under mild conditions.The stereoselective polymerization of O-carboxyanhydrides are still big challenges until now due to the absence of suitable catalysts.In this thesis,we studied the above two issues,and four chapters are presented:,1.A brief review of stereoslective ring-opening polymerization of cyclic esters and the modification of PAHA via rop of derivate of LA and OCAs.2.Eight highly active sodium and potassium phenolates as highly isoselective catalysts for the ROP of rac-lactide were sythesied,the best isoselectivity value of Pm = 0.94 was achieved.The isoselective mechanism is the chain-end control through the analysis of the stereoerrors in the microstructure of a final polymer,thus isotactic multi-block structures polymers were obtained and the highest melt point can reach to 192.5 °C.The donating group in phenolate can clearly accelerate the ROP reaction,potassium complexes are more active than the analogous sodium complexes,and the big spacial hindrance of ligand can decrease the activies.The high isoselectivities of these complexes mostly result from their sandwich structure constructed by the plane of crown and the plane of anthryl group.3.Two binuclear magnesium and zinc alkoxides supported by a bis-salalen type dinucleating heptadentate Schiff base ligand were synthesized and fully characterized.The two complexes are efficient initiators for the ring-opening polymerization(ROP)of L-lactide,affording polymers with narrow polydispersities and desirable molecular weights.Interestingly,the mechanisms for the ROP of lactide are different at different temperatures.At a high temperature of 130 °C,a coordination-insertion mechanism is reasonable for the bulk polymerization of lactide.At a low temperature,the alkoxide cannot initiate the ROP reaction;however,upon addition of external benzyl alcohol into the system,the ROP of lactide can smoothly proceed via an “activated monomer” mechanism.In addition,these complexes display slight stereo-selectivity for the ring-opening polymerization of rac-lactide,affording partially isotactic polylactide in toluene with a Pm value of 0.59.4.Four Zr/Hf amine triphenolate complexes were synthesized.These complexes can highly syndioselective catalyze ROP of OCAs form high sydiotactic poly(α-hydroxy acids)with a highest Pr value up to 0.97.Furthermore,these catalysts were successfully applied in the precisely alternating sequence controlled copolymerization of Phe OCA and Tyr(Bn)OCA.