Theoretical Studies On The Factors Governing Conjugated Olefin Polymerization Catalyzed By Cationic Rare-earth Metal Complexes

Cationic rare-earth metal complexes have unique catalytic activity and selectivity toward olefin polymerizations.However,the factors govering the observed activity and slectivity are unclear.In this thesis,DFT calculations are utilized to investigate the mechanism of olefin polymerization catalyzed by cationic rare-earth metal complexes.Based on the clarified mechanism,the effects of ancillary ligand and rare-earth metal on the activity and selectivity are further investigated at the level of molecular and electronic levels.The effects of counterion and Lewis bases on the catalytic performance and their interactions with metal center are also computationally studied.The main results are summarized as follows:(1)The mechanism of styrene polymerization catalyzed by five analogous cationic rare-earth metal complexes[(R-CH2-Py)Y(CH2SiMe3)]+(R = C5Me4(Cp'),?+;R = C9H6(Ind),?+;R = C13H8(Flu),?+),[(Flu-Py)Y(CH2SiMe3)]+(?+),and[(Flu-CH2CH2-NHC)Y(CH2SiMe3)]+(?+)has been studied through DFT calculations.Having achieved an agreement between theory and experiment in the activity discrepancy and selectivity,it is found that styrene polymerization kinetically prefers 2,1-insertion to 1,2-insertion.The free energy profiles for the insertion of the second monomer molecule have been computed for both migratory and stationary insertion manners,and the former resulting in a syndiotactic enchainment indicates obvious kinetic preference.The results suggest that the coordination of styrene to the active metal center could play an important role in the observed activity difference.Interestingly,the charge on central metal of the cationic species accounts for the activities of ?+,?+ and ?+:the higher the charge on the central metal is,the higher the activity is.The coordination of THF molecule to the central metal and more difficult generation of the active species could be responsible for the low activity of ?+.For species ?+,the resulting product of the first styrene insertion is quite stabilized,and the ancillary ligand and styryl group hampered the insertion of the incoming styrene molecule.This could be responsible for the absolute inertness of ?+ toward styrene polymerization.The calculated results also suggest that longer alkyl chain of side-arm of the ancillary ligand could be against monomer coordination and thus decrease the polymerization activity.The present calculation revealed whenever the orbitals of the pyridyl-methylene fluorenyl ligand overlapped with those of the rare-earth metals,the LUMO energy of the active species was lowered and thus the catalytic activity was high.The LUMO energy of the active species could be adopted as a potential criterion to estimate the activity of a catalytic system for styrene polymerizations.(2)The mechanism difference in regio-and stereospecific polymerization of butadiene and isoprene by[(?5-Flu-CH2-Py)Ln(CH2SiMe3)]+(Ln = Sc or Y)complexes has been computationally investigated.According to the DFT calculations,the unprecedented regularity combination is mainly attributed to the backbiting interaction of the penultimate unit of the polymer chain to the active central metal ion.The geometrical features indicate that there is more space in[(?5-Flu-CH2Py)Y(CH2SiMe2)]+ for monomer coordination and insertion in comparison with the Sc analogue.The[(?5-Flu-CH2Py)Y(CH2SiMe3)]+ is more electron-deficient and is a stronger Lewis acid with bigger chemical hardness compared to Sc analogue.The optimized transition state structures indicate that the steric hindrance of the methyl group of isoprene and the growing chain could attribute to the lower reactivity of isoprene polymerization.The mechanism of chain transfer polymerizations(CTP)of isoprene have been computationally investigated.(3)DFT calculations have been carried out for the highly selective cis-1,4-polymerization of butadiene catalyzed by a cationic rare-earth metal complex bearing an ancillary PNP ligand.It has been found that the chain initiation and propagation of butadiene polymerization occurs via the favorable cis-1,4-insertion route.The trans-1,4 and 1,2-insertion are unfavorable both kinetically and thermodynamically.The chain growth follows the ?-allyl-insertion mechanism.The analyses of energy decomposition of transition states indicate that the likelihood of rival insertion pathways is predominantly controlled by the interaction energy of butadiene with metal center and the deformation energy of butadiene moiety.The electronic factor of the central metal has a decisive influence on the cis-vs.trans-insertion and the regioselectivity(cis-1,4-vs.cis-1,2-insertion)is mainly determined by steric hindrance.The trans-monomer insertion was more kinetically impressed in the case of THF coordination to the metal center compared with the THF-uncoordinated species.During the chain propagation,cis-insertion of monomer facilitates THF de-coordination and the THF molecule could therefore dissociate from the central metal.(4)A full account of theoretical analyses at the DFT level,focusing on the formation and reactivity of a family of catalysts[(R-(CH2)n-Py)Sc(CH2SiMe3)]+,including the effects of anion and solvation,has been reported.Two sets of model systems have been considered:(a)structures having identical bridging group but with varying cyclopentadienyl groups(R = Cp')(A),R= Ind(B),and R = Flu(C)),and(b)systems with identical cyclopentadienyl group(Flu)but with varying bridging groups(n= 1(C),n=0(D),and n=2(E)).For complex C,metals Sc,Y,La,Pr,Nd,Gd,Tb,Dy,Ho,Er,Tm,and Lu were considered to investigate the effect of central metals on the contact ion pairs.The calculated energy data indicates that solvation decrease the exothermicity of the contact ion pair formation reaction and the endothermicity of the contact ion pair separation.The solvation hardly affects the energy barrier of the styrene insertion into[(Flu-CH2-Py)Sc(C17H19)][B(C6F5)4).