Application Of Salen-Manganese Complexes In The Synthesis Of Degradable Polyester

Realizing the industrialization transformation of scientific research results has become one of the most constructive goals in the field of polymer science.To achieve industrialization,a new type of polymer material must meet the requirements of excellent performance,high production efficiency,easy processing,and good controllability,which is a challenge for both the polymer itself and the catalytic system.For decades,degradable aliphatic polyesters and polycarbonate plastics have been extensively studied,including polylactic acid(PLA),polycaprolactone(PCL),and copolymers of epoxides with carbon dioxide or anhydride.Catalysts that combine high efficiency and biocompatibility with the ability to control the block sequence of polymers are among our best goals in the development of highly efficient polymerization catalysts,particularly metal complexes that catalyze cyclic ester ring opening polymerization(ROP).Based on the previous research of Schiff base metal complexes in our group,in the first part of this paper,we synthesized a group of Salen manganese(III)complexes,and investigated their catalytic activity in the ROP of lactide(LA)and ε-caprolactone(ε-CL).Under the activation of propylene oxide(PO),the manganese complexes can efficiently catalyze the ROP of LA and ε-CL,and the molecular weight distribution can be well controlled.We also fully discussed the effect of different bridging parts of ligands on the polymerization.Surprisingly,the chlorine substituents had a negative effect on the catalytic activity.Theoretical simulation results showed that the center metal of 7a becomes more electronegative.By changing the ligand amine bridging parts and substituents,the configuration of PLA chains shifted from a slight isotactic to a slight syndiotactic.In the second part of this paper,gas controlled switchable polymerization of lactide or lactones by commercial Salen manganese(Mn)complexes was firstly developed through the insertion and deinsertion of CO2 into Mn alkoxide active species.The handy implementation of switchable procedure benefited from the inherent sensitivity of Mn complexes to the reversible CO2 absorption,which essentially was the transition from metal alkoxide(Mn-OR)to metal carbonate(Mn-OCOOR).Given the extremely low chain-growth activity of Mn carbonate and the toggling mechanism of two active species as chemoselective agent for different polymerizations,the single switchable catalyst was further applied to the chemoselective ROCOP of epoxide,anhydride and lactide.Unlike CO2,anhydride not only acts as metal chain end group regulator but also as monomer participating in the polymerization.Different proportions of poly(anhydride-epoxide)segments were introduced into the PLA chain in batches to obtain various novel multi-block ABAB polymer chains.Theoretical experiments were conducted to achieve robust explanation for the whole metal chain end control selective mechanism.This work was a significant complement to the field of controlled polymerization with switchable catalysis.