Switchable Polymerization And Sequence Controlled Polyester

As one of the most cost-effective degradable polymer,polylactic(PLA)is widely used in daily life,agriculture,industry,medicine and other fields.Through the unremitting efforts of researchers,the properties of polylactic acid have been thoroughly studied and improved.However,the inherent disadvantages of PLA such as poor flexibility and inability to further chemical modification cannot be solved by changing the polymerization method.These defects can be improved by the copolymerization of lactdie(LA)and other monomers.On this basis,the switching copolymerization of polyester can realize the precise control of the polymerization process and generate highly regular block structure,which is a feasible scheme to obtain regular and functional polyesters.In this paper,we have studied different switchable copolymerization systems,synthesized a variety of polyblock copolymers of LA and other monomers,and finally realized some functionalization.1.Switchable polymerization is a new method of polyester ring-opening polymerization in recent years.It can control the "on" and "off" of catalyst activity through the addition of external stimuli.Inspired by the emerging field of switchable polymerization,a new monomer controlled self-switchable polymerization route was developed.Chemoselective ring opening copolymerization of O-carboxyanhydrides(OCAs)and LA was explored without cocatalyst.The sequential conversion of OCAs and LA into the polymer chain could form multi-block polyesters.Based on this strategy,a series of multi-block polyesters with different pendant groups were synthesized.Furthermore,by modifying the propargyl-containing copolymers with quaternary ammo-nium groups,we have realized antibacterial functionalization of PLA.These results imply the potential application of this strategy for the fabrication of functional polymers for biomedical applications.2.Due to the unique optical properties and heavy metal adsorption properties of sulfur-containing polymers,the preparation of sulfur-containing polymers has become a research hotspot.In order to introduce the special sulfur element into the polylactide system,the Sulfur-containing Carboxyanhydride(5-Methyl Dithiolane-2,4-dione,MDTD)was synthesized and polymerized by Schiff base-manganese(Salen-Mn).The formation of small-molecule by-products were inhibited and the active homopolymerization of MDTD was realized under the condition of low feed ratio(50:1)using 4-dimethylaminopyridine(DMAP)as cocatalyst,at lower temperature(60℃).Based on the reversible insertion of carbon oxysulfide(COS)by Salen-Mn,a new monomer controlled self-switchable polymerization route was developed by sequentially addition of the MDTD and the removal of COS.Chemoselective ring opening MDTD and LA was explored with DMAP as cocatalyst.The monomer controlled switchable copolymerization reaction process and the formation of polyblock polyester/polythioester(p(LA-b-MDTD-b-LA))were confirmed by nuclear magnetic resonance(NMR)and gel permeation chromatography(GPC).Eventually,a two-step switchable copolymerization was realized by sequential addition of MDTD and degassing,a five-block polyester/polythioester copolymer was obtained.This emerging polymerization strategy could synthesize the corresponding multi-block polyester/polythioester by changing the structure of the Sulfur-containing Carboxyanhydride monomer,which lays a foundation for improving physical and chemcial properties of the thiopolyesters.3.Copolymers of lactide and OCA or MDTD were successfully prepared by self-switching copolymerization route of step feeding.However,due to the inevitable complexity of polymerization system caused by step feeding,the polymerization stage became more and more uncontrollable.In order to improve the effect of step feeding on the switching polymerization system,an electrochemical REDOX process was used to control the polymerization direction.based on the chemical selectivity of heterometal-lic Salen-Co-Mn complexes of different valences,an electrochemically switchable strategy was developed for the copolymerization of LA,CO2 and epoxides.The switchable redox reactions endowed this system with the capability to easily synthesize a multi-block copolymer of PLA and polycarbonate(PC).Moreover,the multi-block copolymer could be further modified by introducing various monomers with different microstructures and functional groups.