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The titanium catalyst (C5(CH3)4Si(CH3)2NR)TiCl2 has been tethered to the interior pore surfaces of p-aminophenylsilyl-functionalized mesoporous silica materials in a single-step process to form supported constrained-geometry catalysts (CGCs), and the polymerization of ethylene and copolymerization of ethylene and 1-octene by the resulting heterogenized CGCs have been studied. Confirmation that the intended supported CGC is correctly assembled and is the only catalytically active species is provided by spectroscopic characterization (²⁹Si CP MAS NMR) and by ethylene/1-octene copolymerization. The use of tailored mesoporous silica supports with different uniform pore sizes (25−70 Å) permits investigation of the influence of the structure of the support matrix (i.e., the “second sphere of influence”, where the ligand environment provides the primary influence) on catalyst activity and on product properties. Homopolymerization of ethylene by the supported CGCs shows a clear trend in the crystallinity of the high-density polyethylene (HDPE) produced, with a higher degree of crystallinity observed for HDPE formed by the heterogenized CGC within the smaller-pore substrates. The observation that a “second sphere of influence” is indeed significant introduces an additional tunable parameter for the tailored synthesis of polyolefins using supported catalysts.