Theoretical Studies On Olefins Polymerization Catalyzed By Cationic Rare-Earth Metal Complexes

The development of the polymerization catalysts showing excellent performance has become important impetus for the evolution of new polymer materials. Numerous group 4 and late transition metal complexes as precatalysts, in this context, have been widely synthesized and applied for development of the novel polymer materials. In recent decades, cationic rare earth metal complexes showed good performance in olefin (co)polymerization. However, the real active species is hard to be separated and detected, and some fundamental chemical problems are difficult to be solved just through existing experimental tools. Computational chemistry acts as a powerful tool to explore polymerization mechanism and further help design new polymerization catalysts.In this thesis, density functional theory (DFT) and ONIOM methods have been employed to investigate the mechanisms of various olefin polymerizations catalyzed by cationic rare-earth metal complexes. The origin of high activity and high selectivity have been clarified. The effects of cocatalyst, counterion, and external or internal Lewis-base on catalytic performances have also been investigated. The main results in this thesis are summarized as follows:1.1-hexene polymerization catalyzed by dicationic rare earth metal alkyl species [Ln(iPr-trisox)(CH2SiMe3)]2+(Ln = Sc and Y, trisox = trisoxazoline) has been computationally studied. It has been found that the initiation of 1-hexene polymerization kinetically prefers 1,2-insertion to 2,1-insertion. Such a preference of 1,2-insertion has been also found for chain propagation stage. The isotactic polymerization was computed to be more kinetically preferable in comparison with syndiotactic manner, and the dicationic system resulted in lower insertion free energy barrier and more stable insertion product in comparison with the monocationic system. The stereoselectivity was found to follow chain-end mechanism, and the isospecific polymerization of 1-hexene is mainly controlled by kinetics, In addition, the current computational results, for the first time, indicate that the higher activity of Sc species toward 1-hexene polymerization in comparison with the Y analogue could be ascribed to lower insertion barrier, easier generation of the active species, and its larger chemical hardness.2. Styrene polymerization catalyzed by cationic half-sandwich rare-earth metal complexes [(η5-C5Me5)Ln(CH2SiMe3)(THF)n]+(A, Ln = Sc, Y, Lu, Gd, Sm; n = 0 (ALn),1 (thfALn)), [(η5-C5Me5)Sc(CH2C6H4NMe2-o)]+ (B), and [(η5-C5Me5)Sc(C6H4OMe-o)]+ (C) has been computationally studied. It has been found that THF as external Lewis-base has no effect on the regioselectivity in chain initiation stage, but decreases the activity. THF as external Lewis-base is proposed to leave away from metal Sc center during chain propagation stage. Aminobenzyl as the internal Lewis-base reduces the regioselectivity of 2,1-insertion in chain initiation process but no influence on chain propagation. Strong internal Lewis-base anisyl is prone to produce isotactic chain-end microstructure. In addition, the present computational results_ indicate that the higher activity of Sc species toward styrene polymerization in comparison with its Y, Lu, Sm, and Gd analogues could be ascribed to strong Lewis-acidity, small ionic radius, and much less tendency to generate dormant species.3. The mechanism for regio- and stereoselective polymerization of 1,3-pentadiene, butadiene, isoprene by scandium catalysts bearing different ligands has been computationally investigated. During pentadiene polymerization by half-sandwich Sc catalyst, the insertion of pentadiene into the Sc-R bond in s-cis-1,4 fashion is more kinetically favoured than that in s-trans-1,4 fashion. Furthermore, the isospecific polymerization is more preferable in both kinetics and thermodynamics than syndiotactic process. And the preference of isotactic polymerization could be ascribed to the absence of the repulsion between the ancillary ligand and newly inserting monomer. During butadiene and isoprene polymerization by non-cyclopentadienyl-ligated Sc complex, cis-1,4-selectivity for butadiene polymerization could be ascribed to strong interaction between butadiene moiety and active species, While the bigger steric hinderence between active species and isoprene moieties in 1,4-insertion transition state has explained well the 3,4-selective polymerization of isoprene.4. DFT studies have been performed for trans-1,4-specific polymerization of isoprene catalyzed by a cationic heterobimetallic half-sandwich complex [(C5Me5)La(AlMe4)]+. The possible structures of the active species, viz., [(C5Me5)La(μ2-Me)3AlMe]+ (A), [(C5Me5)La(μ2-Me)2AlMe2]+ (B), and [(C5Me5)La(Me)(μ2-Me)AlMe2]+ (C) have been investigated. On the basis of the chain initiation and the structure transformations among these three species, the C has been proposed to be the true active species smoothly producing trans-1,4-polyisoprene observed experimentally. Both La/Al bimetal-cooperating monomer insertion and La-center based insertion pathways have been calculated and the latter is found to be more favorable, where the AlMe3 moiety serves as a ligand coordinating to the La center via a methyl group. In contrast to this, in the Y analogous system, the AlMe3 ligand is proposed to leave away from the Y center during the chain propagation and the cis-1,4-selectivity is preferred, showing a consistence with experimental results. Such a situation could be ascribed to the smaller ionic radius of Y and thermodynamically favorable dissociation of AlMe3 from Y center in comparison with the La system. These results suggest that such an alkyl aluminum compound plays a crucial role in the regulation of selectivity in the polymerization system investigated.