| 초록 |
We have investigated the insertion mechanism of propylene monomer in MgCl2-supported heterogeneous Ziegler-Natta catalysts in order to elucidate the origin of the stereoselectivity of Ziegler-Natta-based olefin polymerization by means of a non-local density functional theory (DFT). Two different cluster models of Mg8Cl16 for (110) surface and Mg9Cl18 for (100) surface are used for the description of MgCl2 support. Active sites formed by adsorption of TiClx (x = 4 or 3) or Ti2Cl6 on two different surfaces of MgCl2 (110 and 100 surface) are examined as model catalytic systems. A propylene monomer is inserted into the model catalytic systems according to the Cossee-Arlman mechanism revised by Brookhart and Green: (i) The catalytic system is activated by exchange of one chlorine ligand at the titanium center for an alkyl group of the alkyl aluminum as a growing polymer chain. (ii) Then the π-complex is formed by coordination of the monomer with the activated site, which is a barrierless, exothermic process. (iii) From the π-complex, the monomer insertion proceeds through a transition state assisted by agostic interaction. (iv) The final structure, the product state, is obtained by the monomer insertion. Calculations of monomer insertion are performed by a stepwise decrease in the reaction coordinate, followed by geometry optimization with respect to all other degrees of freedom at each of discrete reaction coordinates. It is found that propylene is polymerized isotactically at active sites formed on (100) surface of MgCl2 and atactically at active sites formed on (110) surface of MgCl2. At active sites formed on (100) surface, the sterically repulsive interaction between the growing chain and Cl ligand of MgCl2 surface responsible for the orientation of growing chain results in α-agostic assisted transition state. This intermediate structure leads to the discrimination between the prochiral faces of the monomer because it is energetically more favorable when the second carbon atom of the growing chain and the methyl group of propylene are located on the opposite side of the plane defined by the double bond of inserting monomer and the metal atom than when they are on the same side of the plane. It is also found that low oxidation state of Ti enhances the stability of π-complex due to π back-donation. However, the energy barrier of low oxidation state is higher than that of high oxidation state because of additional energy costed for breaking of π back-donation in low oxidation state. |
