Partly deuterated hydroxy-terminated poly(styrene d(8)-r-methyl methacrylate) statistical copolymers with different molecular weights were grafted to a silicon substrate. Then, fully hydrogenated hydroxy-terminated poly(styrene-r-methyl methacrylate) statistical copolymers were spun over the pregrafted substrate and let to react at 250 degrees C following the time evolution of the brush thickness and composition. Over time, the grafting density of the deuterated polymers decreases, whereas the grafting density of the hydrogenated polymers increases thus indicating the simultaneous occurrence of grafting and degrafting reactions. Moreover, keeping constant the grafting time, the total grafting density decreases as the molecular weight of the hydrogenated polymers increases, until a limiting value is reached when the molecular weight of the incoming polymer is equal to the one of the preformed brush. The overall picture of the experimental data, further supported by hybrid-particle field coarse-grained simulations, suggests that the reactivity of the system is related to the degree of stretching of the brush layer induced by the entering of additional polymer chains. This new vision of the grafting to mechanism implies that the self-limiting nature of this process derives from a mechanochemical control of the reaction rather than from diffusion/penetration effects.