The invention relates to improved methods of grafting polymer extenders onto porous substrates having diffusive pores, such as those used in protein separations, without filing the diffusive pores of the substrate, and restricting diffusion there through. By changing the grafting conditions and/or monomer composition(s) the resulting porous substrates having polymer extenders grafted thereto have increased protein binding capacity and resin selectivity, thereby enhancing the protein separation effectiveness of the substrate. The grafted polymer extenders provide the substrate with significant binding capacity at higher conductivity. The invention also relates to kits, and methods of using and grafting polymer extenders on porous resin substrates having diffusive pores.