A theoretical study of the ligand effect of thiobisphenoxytitanium complex catalyst on the catalytic activity for ethylene polymerization

The effect of ortho-substituents (R=methyl, tert-butyl and triisopropylsilyl) of phenoxy groups in thiobisphenoxytitanium complex catalyst on the catalytic activity of ethylene polymerization was analyzed by applying ab initio molecular orbital theory and density functional theory. The calculation results revealed that the activation energy values of ethylene insertion reaction into a methyltitanium bond of thiobisphenoxytitanium complexes with substituents on the ortho position of phenoxy ligands at the RHF level are in good correspondence with the tendency of ethylene polymerization activities. The activation energy values at the B3LYP/DZ, RHF/LANLL2DZ and B3LYP/LANL2DZ//RHF/DZ levels also show similar tendency. While the differences of the molecular weight of polyethylene prepared with these bisphenoxy complexes could not be explained from the activation energy values of β-H elimination reaction and the chain transfer reaction to monomer. The activation energy of ethylene insertion reaction is supposed to control the molecular weight of polyethylene.