Blends with different compositions of a partly deuterated hydroxy terminated poly(styrene d₈-st-methyl methacrylate) copolymer with average molecular weight (M_n) of 11 200 g mol⁻¹ and a hydroxy terminated poly(styrene-st-methyl methacrylate) copolymer with M_n = 5400 g mol⁻¹ were grafted to a silicon wafer at temperatures ranging from 170 to 250 °C. A partition according to the molecular weight takes place during the “grafting to” reaction in which the lower molecular weight component is selectively incorporated into the polymeric brush. The time and temperature evolution of the layer composition for a specific blend composition as well as the effect of the blend composition on the final brush layer composition were systematically investigated. Hybrid particle-field simulations suggest that the propensity of short chains to preferentially react with the silicon surface comes from a combination of the average number of chains in contact with silica and the substrate surface area occupied by a single chain.