Synthesis Of Biodegradable Polyester Materials Catalyzed By Lewis Acid-Base Pairs

As the most widely used biodegradable polyesters,polylactide（PLA）and poly（ε-caprolactone）（PCL）are mainly synthesed by ring-opening polymerization（ROP）of cyclic esters.Compared with traditional catalysts,Lewis Acid-Base Pairs cooperate to activate the monomers,affording high molecular weight cyclic polyesters.There is no transesterification in ROP and the molecular weight distribution is narrow.In this paper,firstly common organic bases DMAP,DBU and MTBD were selected as Lewis base to cooperate with Zn（C₆F₅）₂.The interaction mode between Lewis Pairs was characterized by XRD and ¹⁹F NMR.Less steric bulky DMAP could coordinate with zinc atom and form Classical Lewis Acid-Base adducts（CLA）,whereas the sterically demanding MTBD or DBU interacted with Zn（C₆F₅）₂ and formed Frustrated Lewis pairs（FLP）.In situ ¹H NMR spectroscopy of Lewis Pairs at different temperatures confirmed that the interaction between acid and base was decreased with the increase of temperature,Lewis activity was not completely quenched and the polymerization can be initiated.In addition,in situ ¹H NMR and MALDI-TOF MS showed that the active species is zwitterionic species.Based on the experimental results,a mechanism involving bifunctional activation was proposed.This mechanism was verified by the kinetic experiments under different conditions.Furthermore,the type of Lewis acid was changed to select a Lewis acid with simple structure and high activity,which could cooperate with DMAP to catalyze the homopolymerization of L-LA andε-CL.The reactivity of Zn（C₆H₅）₂ or ZnEt₂ is higher than that of Zn（C₆F₅）₂ through the kinetic experiments.Finally,the copolymerization between L-LA andε-CL were initiated by Lewis Pairs.The block copolymer was polymerized by sequential addition.The ideal diblock copolymer can only be obtained via“PCL block first”route and CL was not initiated by the living PLA^*.The ideal random copolymer was not obtained by one-pot addition,but the gradient copolymer was synthesized.