Theoretical Studies On The Polymerization Of Polar Monomers Catalyzed By Scandium And Yttrium Metal Complexes

Rare-earth metal complexes showed excellent catalytic activity and selectivity toward polar monomer polymerizations.However,due to the difficulty in isolating and detecting intermediates experimentally,the knowledge of mechanism of rare-earth metal catalysts has remained limited.In this thesis,the effects of ligand structures,Lewis base and the type of substituents on the polymerization of polar monomer catalyzed by scandium and yttrium metal complexes are systematically studied though density functional theory(DFT)and multivariable linear regression analysis,these theoretical informations will contribute to the design and development of high performance rare earth metal polymerization catalysts.The polymerization mechanisms of para-methoxystyrenes(pMOS),meta-methoxystyrenes(mMOS),ortho-methoxystyrenes(oMOS),para-dimethylaminostyrene(pMNS)and para-methylthiostyrene(pMTS)catalyzed by cationic rare-earth metal complexes[(C5Me5)Ln(o-NMe2CH2C6H4)]+(Ln=Sc,Y)and[(Py-CH2-Flu)Y-(CH2SiMe3)(THF)]+has been studied through DFT calculations,respectively.It is found that the small steric hindrance of substituents and the less dispersion of charge distribution on the four-center atoms in the insertion transition state are beneficial to the step-growth polymerization.At the same time,large steric hindrance of the functional group as a main factor prevents monomer from undergoing step-growth polymerization.In addition,the electron-donating pyridine side arm weakened the interaction between monomer and the metal center and meanwhile increased the steric hindrance,which hampering the C-H activation for the step-growth polymerization.The difference in stereospecific polymerization of o-ClSt,m-ClSt,and p-ClSt catalyzed by(C5Me4SiMe3)Sc(CH2SiMe3)(THF)+ and(C5Me4SiMe3-Sc(CH2C6H4NMe2-o)+ complexes have been computationally investigated.It is shown that a coordinating THF participates in regulating the stereoselectivity via C-H…π noncovalent interaction between the THF and phenyl ring of monomer units.Further theoretical investigations of the polymerization of various analogous monomers confirm the role of such C-H…π interactions in stereoselectivity regulation.This is achieved through the electron withdrawing substituent(s)and the substitution position to alter the electron density of phenyl ring of the monomersThrough a combination of DFT calculations and multivariate regression analysis,the origin of the stereoselectivity of 2-methoxyethylamino-bis(phenolate)yttrium catalysed polymerization of 2-vinylpyridine(2VP)has been investigated.It also shows the effectiveness of the combination strategy of this two methods in polymerization selectivity.It has been found that the steric hindrance effect of catalyst ligands is the key factor affecting the isotactic selectivity.And the application of related descriptors and multivariate linear regression models disclosed that the stereoselectivity quantitatively correlates the ligand substituent.The DFT calculations of the cyclopolymerization of 4-phenylthio-1,6-heptadiene(S-2)and 4-benzyloxy-1,6-heptadiene(O-1)catalyzed by a cationic rare-metal complex are preformed.The results show that the heteroatom in monomer coordinates to the metal center to assist si-coordination insertion of the first C=C,the configuration of S-bonded prochiral Cis R in S-2,and the configuration of O-bonded prochiral C is S in O-1.After the first C=C is inserted,the configuration of the chiral carbon connecting with the heteroatom is fixed,it will directly affect the insertion of the second C=C bond via different enantiofaces.The heteroatom-dependent stereoselectivity of the α,ω-diene cyclopolymerization originates form the coordination of different heteroatom in the diene monomers,resulting in different configuration of chiral carbon atom connecting to the heteroatom and therefore diffrernt insertion fashion of the remained C=C bond for cyclization.