The reactivity ratios of n-butyl acrylate (nBuA) with methyl methacrylate (MMA) and omega-methacryloyl-PMMA macromonomers (MM) in conventional and atom transfer radical copolymerization (ATRP) have been determined. For the copolymerization of nBuA with MMA, good agreement of the ratios is observed between conventional and controlled radical copolymerization, indicating that chemoselectivities in both processes are similar. The relative reactivity of the MM (1/r(nBuA)) in conventional copolymerization is significantly lower than that of MMA. It depends on the concentration of the comonomers but is not significantly influenced by the length of the MM. At high concentrations the relative reactivity decreases due to diffusion control of the MM addition. In ATRP the relative reactivity of the MM is much nearer to the value of MMA. This is explained by the different time scales of monomer addition in both processes: whereas the frequency for monomer addition is in the range of milliseconds for conventional polymerizations, it is in the range of seconds in ATRP; thus, diffusion control is less important here. This gives the opportunity to copolymerize at much higher concentrations than in conventional radical copolymerization. In addition, the graft copolymers obtained by ATRP have lower polydispersities.