Computational Studies On The Copolymerization Of Ethylene And Polar Monomers Catalyzed By Palladium Complexes

Functionalized polyolefin materials have a wide range of applications in the field of life and industry.In this framework,the copolymerization of ethylene and polar monomers catalyzed by late transition metal catalysts have attracted considerable attention.However,there are many challenges in these systems,such as low polymerization activity and low rate of polar monomer insertion.And it is difficult to achieve high activity and high insertion rate simutaneously.Therefore,further study of copolymerization mechanism at molecular and electronic levels is required for developing more efficient copolymerization system.Density functional theory(DFT)calculation is an effective method to study the mechanism of transition metal catalyzed olefin copolymerization.In this thesis,the reaction mechanism of copolymerization of ethylene and polar monomers catalyzed by the late transition metal complexes has been investigated.The main results of this study are as follows:(1)The copolymerization of ethylene and methyl acrylate(MA)catalyzed by α-diimine palladium has been studied.The reasons for the low polar monomer insertion rate and low copolymerization activity of such catalysts have been studied.The results show that the low insertion rate of the polar monomer is not due to the difficulty in the insertion of polar monomer itself,but the less favorable formation of the complex of polar monomer with metal center in comparison with ethylene.The "chain walking" process is investigated also.It is found that,after the polar monomer is inserted,the "chain walking" process is favorable over subsequent ethylene insertion.The rate-determining step of the reaction process is the ethylene insertion process after the formation of the six-membered chelate.The non-covalent interacation analysis shows that the weak interaction between the polar monomer and the catalyst contributes to the stability of the transition state of the rate-determing step.(2)The performance of density functionals has been tested for the insertions of ethylene,methyl acrylate(MA),and vinyl bromide(VB)catalyzed by α-diimine palladium complexes.By screening 67 density functionals and comparing with experimental data,eleven hybrid functionals(M06,BHandH,mPW1PW91,HSEh1PBE,mPW3PBE,LC-α PBE,mPW1PBE,PBE0,M06-HF,M06-2X,M05-2X)showed better performance(energy barrier error of<1.0 kcal/mol)in the ethylene-MA copolymerization in comparison with other functionals tested.Meanwhile,three GGA(PW91PW91,HCTH,HCTH407),two meta-GGA(TPSSTPSS,tHCTH),and ten hybrid functionals(M06,BHandH,TPSSh,B971,B98,B1B95,PBE0,M06-2X,tHCTHhyb,M05-2X)perform well in the ethylene-VB copolymerization.Besides,nine D3 or D3BJ augmented functionals(BPBE-D3,M062X-D3,PBE0-D3,PBEPBE-D3,LC-ω PBE-D3,B3PW91-D3BJ,PBE0-D3BJ,BPBE-D3BJ,PBEPBE-D3BJ)are found to be suitable for the both copolymerization systems.The D2 dispersion correction overestimated insertion energy barriers of these monomers and is unsuitable for such copolymerizations.In addition,the methods of M05-2X/6-31G(d)with SMD model and PBE0/6-31+G(d)with CPCM model for solvation single-point calculations are capable of giving satisfactory results.Considering both accuracy and computational cost,the double-zeta basis set is sufficient for solvation single-point calculations.(3)The copolymerization of ethylene and methyl acrylate(MA)catalyzed by cation bis-phosphine mon-oxide catalyst has been investigated.The reason for the difference in the polymerization activity caused by different ligand and the reason why MA has only 2,1-insertion selectivity catalyzed by the complexes with aryl group have been studied.The results show that the chain initiated by ethylene coordination-insertion at cis-side of the catalyst,duroing which the trans/cis isomerization process could not occur.Compared with 1,2-insertion mode,the 2,1-insertion mode of MA is favorable due to its smaller catalyst deformation energy.The energy barrier of trans/cis isomerization after the insertion of polar monomers is the decisive factor that affects the polymerization activity.It has been also found that both Pd…F non-covalent interaction and the sterics of subsitituents on P atom in the ligand could affect the stability of trans/cis isomerization transition state.