Theoretical Studies On Redox-Switchable Polymerization Of Cyclic Lactone Or Epoxide Catalyzed By Transition Metal Complexes

Redox-switchable catalysis,being capable of serving as an effective polymerization methodology,has been applied to cyclic ester and epoxide polymerization because it can modulate polymerization process and polymer microstructure.Transition metal complexes have attracted considerable attention for their excellent performance in this context.In the switchable polymerization catalytic system,the active species are less stable.Especially,in-situ oxidation and reduction polymerization,it is difficult to isolate and characterize the true catalytically active species and clarify the related basic issues through currently existing experimental tools.Therefore,it is very necessary to carry out theoretical calculations to solve basic scientific issues at the molecular and electronic levels,such as the structure of active species,the mechanism of switchable polymerization,and the monomer dependence,which will provide theoretical guidance for designing novel redox switchable catalysts and developing new polymerization systems.In this thesis,density functional theory(DFT)has been employed to investigate the mechanisms of redox-switchable polymerization of cyclic lactone and epoxide catalyzed by transition metal complexes.The main results in this thesis are summarized as follows:1.The opposite redox-dependent activity of bis(imino)pyridine iron bis(alkoxide)complexes(Aox/Ared)toward the ring-opening polymerization of lactide(LA)and cyclohexene oxide(CHO)was comparatively investigated by DFT calculations.Compared with the reduced form(Ared),the lower activity of the oxidized form(Aox)for LA polymerization originated from the larger geometrical distortion of the metal complex to achieve corresponding transition state.In contrast,in the case of CHO polymerization,the higher activity of Aox compared with Ared was mainly due to stronger interaction between CHO and the metal center.Such a discrepancy originated from the higher nucleophilic ability of CHO compared with LA monomer,suggesting that CHO was more sensitive to the strong Lewis acidic metal center.Following this trend,analogous iron complexes with stronger Lewis acidity were computationally modeled through regulating ligand.It is found that the electron-withdrawing substituents are capable of reducing the energy barrier for CHO polymerization.Notably,the LUMO energies of these iron complexes with various substituents on the backbone of bis(imino)pyridine ligand or on the initial p-position of phenoxyl groups positively correlate with the energy barrier of rate-determining step of CHO enchainment.These findings are expected to provide some useful information for developing redox-switchable polymerization systems.2.The redox-switchable polymerization mechanism of trimethylene carbonate(TMC)catalyzed by indium alkoxide complexes[(phosfen)In(OPh)](B)and[(salfen)In(OtBu)](C)bearing ferrocene-based ligand was comparatively elucidated by DFT calculations.In the case of the fonner,although the oxidation occurred at the ferrocenyl unit,the electronic effect induced longer In…N(ligand)coordination bond distance and thus increased the coordination ability of the indium center toward monomer.This made the monomer unit strongly bind to the indium center and ultimately stabilized the oxidized species.This could account for the higher activity of oxidized indium complex(Box)compared with its reduced state(Bred).However,the lower activity of Cox compared to Cred was due to an increased stability of the intermediate resulting in a high energy barrier.Higher activity of Cred than Bred stemed from the less crowded catalytic metal,which facilitated the coordination and insertion of monomers.3.The redox-switchable ring-opening polymerization mechanism of ?-caprolactone catalyzed by four pairs of group 4 metal complexes i.e.,[(salfan)Zr(OtBu)2](D),[(thiolfan*)Ti(OiPr)2](E),[(thiolfan*)Zr(OtBu)2](F),and[(thiolfan)Zr(OtBu)2](G)was investigated by DFT calculations.Having achieved an agreement between calculation and experiment,it has been found that the redox switchable mechanism Dox/Dred system was similar to Box/Bred,where the oxidization of ferrocenyl unit could induce the dissociation of partial ligation atoms and make the interaction between the catalyst and monomer stronger.Therefore,the activity of Dox was much higher than Dred.Whereas,the higher activity of the oxidized forms Eox and Gox than their corresponding reduced forms originated from the higher Lewis acidity of the catalytic metal center in the oxidized species.In contrast,the lower activity of the oxidized species Fox than that of Fred was due to the increased stability of the intermediate and resulted in a higher energy barrier.This mechanism of switchable polymerization was the same as the system of[(salfen)In(OtBu)](C).Based on this,our computational modelling indicates that a Hf analogue may possess better redox-switchable property for the ROP of CL than its corresponding Zr complex.Furthermore,the redox-switchable activity of the Zr complexes with different bridging-heteroatoms in their ancillary ligands follows the order of O<S<Se.These findings are expected to provide useful information for developing new redox-switchable polymerization catalysts for the synthesis of biodegradable polymers from biomass-derived monomers.