Theoretical Studies Of Copolymerization Of Isoprene With Ethylene And Lactide Polymerization Catalyzed By Rare-Earth Metal Complexes

The rare-earth metal catalyzed polymerizations have attracted more and more chemists since such metal complexes showed unique performance for polymerization of olefin and cyclic ester,respectively.Although significant advances in this field have been made by experimentalists,the difficulty in isolating and detecting intermediates experimentally hinders a better understanding of polymerization processes.Computational studies have been increasingly used for the characterization of structurally unknown chemical compounds and obtaining mechanistic information.In this thesis,density functional theory(DFT)calculations have been carried out on isoprene/ethylene copolymerization catalyzed by scandium alkyl complex and the mechanism of redox-controlled ring-opening polymerizations of lactide catalyzed by yttrium alkoxide phosfen complex.The main results obtained are as follows:1.In the case of(?5-C5Me5)Sc(CH2SiMe3)2(THF)/[Ph3C][B(C6F5)4] system,theoretical calculations have revealed that the THF-coordinating cation species might work at the chain initiation step,but not be the true active species at chain propagation step.The existence of THF has no effect on the stereoselectivity.Alternative kinetic priority for the insertion of the two monomers could achieve copolymerization according to the kinetic model analysis.In addition,successive 3,4-insertion of isoprene resulted in a noncovalent interaction between the C=C double bond of penultimate unit and the metal center,suppressing the further insertion of monomers due to higher energy barrier and endergonic character.On the other hand,the ethylene pre-inserted species with alkyl active site is more suitable for the subsequent kinetically and thermodynamically favorable isoprene insertion and the copolymerization is therefore realized.2.In the case of(PNPPh)Sc(CH2SiMe3)2 /[Ph3C][B(C6F5)4] system,DFT calculations have revealed that isoprene coordinates to the active species in cis-1,4-si mode followed by 3,4-insertion give ?3-allyl intermediate.The incoming isoprene and the allyl group form exo-endo fashion at the active sites,which ensures the isoselecitivity regularity.And,the unfavorable ethylene insertion into the isoprene pre-inserted species with allyl active site could account for the experimental finding that no isoprene-ethylene copolymer was obtained.3.DFT calculations have been performed to investigate the ring-opening polymerization(ROP)of L-lactide catalyzed by the reduced yttrium alkoxide phosfen complex and its oxidized form.It has been revealed that the hydrogen bond interaction between the redox-active ferrocene backbone and pre-insereted lactide unit plays an important role in the redox-controlled polymerizations.Such kind of hydrogen bond inetaction has been analyzed by noncovalent interaction(NCI)and atom-in-molecule(AIM)methods.These C-H???O hydrogen bonds observed in the oxidized system seem to “link” the ferrocene unit and the ketonic oxygen of pre-inserted L-LA,which make the Y center crowded and thus hampered the polymerization.This could be responsible for the redox-control mechanism.