The synthesis of a vinyl-terminated, hyperbranched polysilane and the additional hydroboration with bis(pentafluorophenyl) borohydride are reported. Quantitative transformation was proved by H-1 NMR and FT-IR analysis. The polymeric borane was tested as cocatalyst in the metallocene-catalyzed polymerization of propylene. The corresponding catalyst system comprising rac-[1-(9-eta(5)-fluorenyl)-2-(5,6cyclopenta-2-methyl-1-eta(5)-indenyl)e thane]zirconocene dichloride and the polymeric borane was activated in situ in the presence of triisobutylaluminum (TIBA). Polymerizations were performed at temperatures of 30 and 50 degreesC with a propylene concentration of 3 mol/L in toluene solution using various cocatalyst: catalyst ratios (10:1, 50:1, and 100:1). This polymer-supported catalyst system produces polypropylenes with excellent activities and good molecular weights. These results were compared with an additional set of polymerization experiments using molecular dispersed B(C6F5)(3) as cocatalyst, resulting in significantly reduced polymerization activities. This indicates a hindered catalyst mobility inside the hyperbranched cocatalyst matrix, leading to a higher concentration of active species within the hyperbranched nanostructure.