AB-INITIO THEORETICAL-STUDY ON ETHYLENE POLYMERIZATION WITH HOMOGENEOUS SILYLENE-BRIDGED GROUP-4 METALLOCENE CATALYSTS - ETHYLENE INSERTION AND BETA-ELIMINATION

The mechanism of homogeneous polymerization of ethylene with silylene-bridged group 4 cationic metallocene model catalysts (H(2)SiCp(2)M-Me(+); M = Ti, Zr, Hf) has been studied by the ab initio MO method, with focus on two important reactions: the repetitive insertion of ethylene into the metal-alkyl bond along Cossee's direct insertion mechanism and the beta-elimination, considered to be a termination or chain-transfer step. All stationary point structures are optimized at the RHF level (some for Zr at the RMP2 level as well), and the energetics is calculated at the RQCISD level. The insertion of ethylene into the M-Me bond proceeds through a four-centered transition state with a small activation energy of 7-10 kcal/mol, leading to the kinetic product having the gamma-agostic propyl ligand with an exothermicity of about 30 kcal/mol. There is a clear difference in energy of reaction between Ti and the other metals, Ti being the least exothermic, consistent with the experimental relative order of insertion activity: Zr greater than or equal to Hf > Ti. The peculiarity of Ti has been attributed to the size effect; the small radius of Ti makes its Ti transition states and intermediates more compact and crowded than those. of the other metals. Isomerization of the gamma-agostic product to the more stable beta-agostic propyl complex requires a high activation energy, especially for Hf a factor related to the ability to produce polymer with higher molecular weight. beta-Elimination from the beta-agostic propyl complex is endothermic by about 50 kcal/mol and takes place with difficulty. In addition, comparison has been made with recent theoretical calculations which gave no barrier.