1070-3284/02/2801- $27.00 © 2002 åÄIä “Nauka /Interperiodica” 0025 Russian Journal of Coordination Chemistry, Vol. 28, No. 1, 2002, pp. 25–31. From Koordinatsionnaya Khimiya, Vol. 28, No. 1, 2002, pp. 27–33. Original English Text Copyright © 2002 by Young, Ma, Ting. INTRODUCTION The discovery of the Ziegler–Natta catalyst was of importance to olefin polymerization technology. Such catalysts define name specificity and coordination poly- merization. Most Ziegler–Natta catalysts have been supported on materials such as alumna or silica gel and used as such. The major advantage of coordination polymerization is that the polymer structure can be con- trolled by the geometry of the catalyst around the metal center. In homogeneous polymerization, the ligand of a catalyst largely controls the geometry. Kaminsky [1] discovered the metallocene used in polymerization cat- alyst, indicating the stereocontrol of Ziegler–Natta cat- alysts. The reaction mechanism used most widely for Zie- gler–Natta type catalyst, as well as metallocene, is the Cossee [2] mechanism, as shown in Fig. 1. The Cossee mechanism consists of three major steps. The catalyst activated by a cocatalyst, usually methylalumoxane (MAO) (1a), possesses one positive charge. Then, the π-bond electrons of olefin are attracted by this positive charge in the metal center of the activated catalyst to form a π-complex (1a). The third step is the insertion of olefin into the metal–carbon bond to form a transition state (1c). After the electron transfer process is com- plete, a relatively stable product (1d, 1e) is then formed. The olefin insertion process of metallocene catalysts by various computation methods has received consider- able attention. Kawamura-Kuribayashi et al. [3] stud- ied the model of olefin polymerization by a homoge- neous Ziegler–Natta catalyst. That investigation also studied the mechanism of ethylene and propylene inser- tion into CH 3 TiCl 2 using the ab initio molecular orbital method. The structure and energy of the reactant, inter- mediate, transition state, and product were determined on an RHF/3-21G at the Moller–Plesset perturbation level. Castonguay and Rappe [4] combined ab initio electronic structure and empirical force field molecular mechanics to study the polymerization of isotactic polypropylene. The experimental isotacticity of certain kinds of catalyst was corrected for using computational results. Kawamura-Kuribayashi et al. [5] also studied Activation Energy and Transition State Determination of the Olefin Insertion Process of Metallocene Catalysts Using a Semiempirical Molecular Orbital Calculation 1 Mu-Jen Young*, **, Chen-Chi M. Ma*, and Ching Ting** * Department of Chemical Engineering, National Tsing Hwa University, Hsinchu, Taiwan ** Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan Received January 29, 2001 Abstract—The transition state of the olefin insertion process of metallocene catalysts can be determined by adopting the semiempirical PM3 model. In computational chemistry, the computational methods most employed are the ab initio method and density functional theory, which are very time consuming. The semiem- pirical molecular orbital method requires much less computational resources than the above methods. However, the accuracy and reliability of the semiempirical molecular orbital method remains to be determined. The PM3 model is the most recently developed the semiempirical molecular orbital method and can also be applied to transition metal calculations. This study is intended to investigate the reliability of computational results deter- mined using semiempirical PM3 model on metallocene catalysts through comparison with published results on the density functional theory (DFT). The saddle point finding procedure is adopted to find the transition state of the ethylene insertion process of metallocene catalysts. Results on the geometry and energy trends of the eth- ylene insertion process of metallocene catalysts determined using the PM3 model are in good agreement with the DFT results. In addition, the saddle point of the potential energy surface of ethylene insertion is verified in accordance with the eigenvalue of the vibrational frequency spectrum. Correct eigenvalues indicate that the cor- rect saddle point of the potential energy surface of ethylene insertion has been successfully located. Hence, the eigenvalue of the vibrational frequency spectrum is a valuable reference in terms of saddle point justification. Computational results and vibrational frequency spectrum analysis demonstrate that the PM3 model can be used to locate the correct saddle point of the potential energy surface. The results obtained using the PM3 model confirm that the eigenvalue of the transition state lies nearly on the vibrational frequency spectrum. The eigen- values are also analyzed, providing a valuable reference for further studies of the transition state of olefin inser- tion of metallocene catalysts. The activation energies for the olefin insertion reaction are also studied for eval- uation of the catalyst. 1 This article was submitted by the authors in English.