| Functional polymers are wildely used in the field of industry and biomedicine.For example,the application of ethylene-vinylacetate copolymer(E/VAC)in medicine packing,medical device,and drugs have attracted considerable attention.In the field of functional polyolefin obtained from copolymerization of ethylene and polar monomers,late-transition metal complexes have attracted researchers’ interests maily due to their advantage in low oxyphilicity of the central metal.Coordination insertion polymerization of ethylene and polar monomers has made great progress.It has been experimentally found that ligand skeleton,ligand substituents and metal center have an impact on the reactivity,monomer insertion rate,molecular weight and branching degree of obtained polymers.However,only through current experimental approaches,it is difficult to elucidate the reaction mechanism and the role of aforementioned factors at molecular and electronic levels.This situation hamppers further development of this field.Therefore,it is of importance to elucidate the copolymerization mechanism and further clarify the molecular and electronic factors on catalytic activity,incoopration of polar monomer and banching ratio through density functional theory calculations.In this thesis,the copolymerization mechanisms of ethylene and methacrylate,methyl methacrylate,and ethyl-2-cyanoacrylate,respectively,have been elucidated by DFT calculations,with the purpose of addressing the regioselectivity and catalytic activity.The main contents and conclusions of this study are as follows:(1)The cationic five-membered ring diphosphazane-monoxide(PNPO)palladium and neutral six-membered ring phosphine-sulfonate palladium catalyst systems have been comparably studied.The results show that the rigidity and cationic feature of PNPO catalyst are benefical for the catalytic copolymerization.In addition,in the PNPO system,MMA monomer was inserted in 2,1-fashion because the geometric deformation of the insertion transition state was less compared with 1,2-insertion case.In addition,the 2,1-insertion transition state shows a MeO???H noncovalent interaction between the methyl group of MMA and the auxiliary ligand and therefore was stabilized.In contrast,the barriers of 1,2-and 2,1-inserion fashions in phosphon-sulfonate palladium catalyzed MMA insertion are similar,which accounts for the low regionselectivity of this catalyst.(2)It was previously reported that [P,O] nickel catalysts showed different activity in copolymerization of ethylene and various polar monomers.The SHOP-type [P,O] catalysts have poor activity in ethylene/MA copolymerization,but could copolymerize methyl 4-pentenoate(4-MP)with two methylene spaces between vinyl and functional group.However,the modified [P,O] phosphine phenol catalyst can copolymerize ethylene with these two polar monomers well because the MeO group of ancillary ligand tends to coordinate to the metal center and the catalyst deformation was thus reduced.The copolymerization activity of ethylene with methacrylate could be improved when the substituent at P atom of ancillary ligand was replaced by fluorobenzene.(3)It was previously reported that the neutral phosphine-sulfonate palladium catalyst could copolymerize ethylene with 1,1-disubstituted difunctional monomers(ethyl-2-cyanoacrylate).The related chain initiation,chain growth,and chain termination have been calculated.The results suggest that the nitrile N atom is thermodynamically easier to coordinate to metal center in comparison with carbonyl O atom,rendering higher stability to the N-coordinating structure.In addition,monomers are inserted mainly in 2,1-form during chain growth.This is mainly because,in the 2,1-insertion transition state,the geometrical deformation energy of catalyst moiety is smaller in comparison with 1,2-insertion case,which stabilized corresponding transition state and lowered the energy barrier. |
