Propylene polymerization by C₁-symmetric {ONNO′}-type salan zirconium complexes

The activities of C₁-symmetric dibenzyl zirconium complexes of Salan ligands that bear a halo-substituted phenolate ring and an alkyl-substituted phenolate ring in propylene polymerization with methylaluminoxane as cocatalyst were studied. These {ONNO′}ZrBn ₂-type catalysts exhibited moderate-to-high activities and yielded polypropylene of low molecular weight. The degree of tacticity was found to depend on the steric bulk of the substituents on both phenolate rings and ranged from practically atactic to substantially isotactic (74-78% [mmmm] for polymerizations at room temperature by Lig⁵ZrBn₂). Hemi-isotactic polypropylene was not obtained, despite the diastereotopicity of the two positions. The pattern of stereo errors was consistent with the enantiomorphic site control of propylene insertion typically observed for C ₂-symmetric catalysts and implied a facile site-averaging mechanism. A regular 1,2-insertion and a β-H transfer to an incoming monomer correspond to the main propagation and termination processes, respectively.