Sequential immobilization of ansa-hafnocene complexes for propene polymerization

We report the immobilization of the ultrarigid ansa-hafnocene complexes rac-[Me₂Si(Ind*)₂HfCl₂] (rac-1; Ind* = 7-(3′,5′-Di-tert-butylphenyl)-4-methoxy-2-methylindenyl) on silica as hetereogeneous catalysts for propene polymerization. A sequential three-step synthesis on the siliceous surface led to pre-catalysts of the generalized structure SiO₂–Si(Ind*)₂HfCl₂, which possess the silylene bridge of the substituted bis(indenyl) ligand directly attached to the surface. The immobilized pre-catalysts show very poor performance in the polymerization of propene, independent on the reaction conditions and the employed silica. Based on the results, we suggest that the close proximity of the catalyst to the surface combined with the steric congestion provoked by the ligand prevents a continuous polymerization, most likely due to a blockage of the catalytically active sites with growing polymer.