Hybrid materials were synthesized by grafting polymer chains from the surface of ordered mesoporous silica (OMS) particles via surface-initiated atom transfer radical polymerization (SI−ATRP) of methyl methacrylate or styrene. Various types of OMS particles were used as substrates: micrometric particles with ill-defined shape and varying mesopore diameters (9−14 nm), submicrometric polydisperse spherical OMS particles, and monodisperse core−shell particles composed of a dense silica core and an OMS shell, the latter two materials exhibiting ordered mesopores (diameter 2.5 nm) with radial orientation. This work proposes a systematic investigation of the molar mass, molar mass distribution and chain-end structure of both the grafted chains grown from the silica surface and the free chains produced in solution from an additional free initiator. The polymerizations of methyl methacrylate and styrene were perfectly controlled in the homogeneous medium via the ATRP mechanism whereas the study of the grafted chains highlighted the formation of a large fraction of dead species together with the expected population of living chains. The influence of the polymerization conditions and of the OMS particle structure on the occurrence of the extensive termination reactions was studied and discussed.