Silanes are versatile coupling agents. They represent an easy way of chemically bonding polymers to silica. Two processes have been considered: polymerization from a coupling agent grafted on the surface; and reaction of a prefunctionalized polymer by condensation of its alkoxy head with silanols of the surface. Each of them has advantages and limitations. We focus here on the ＇onto＇ grafting mechanism. The main advantages are the control of polymerization degree in the first step of polymer synthesis which occurs in a homogeneous medium and the easy polymer characterization of the soluble precursor. The semi-batch procedure avoids molecular weight shift and broadening with conversion. Grafting efficiency is measured in function of polymer chain length, which indicates a steric limitation by the silica pore size. Trialkoxy silanes are more reactive than monoalkoxy for silica coverage and exhibit a better chemical resistance.