The preparation of block copolymers with acrylonitrile (AN) and n-butyl acrylate (n-BA) was examined using two controlled radical polymerization (CRP) processes: atom transfer radical polymerization (ATRP) and nitroxide mediated polymerization (NMP). When crossing from poly(n-butyl acrylate) (PBA) to polymerization of AN, the use of halogen exchange in an ATRP process improved control of polymerization. However, when switching from polyacrylonitrile (PAN) to n-BA, the cross-propagation was well controlled without halogen exchange. These differences in blocking efficiency can be explained by differences in the bond dissociation energy of the terminal carbon-halogen bond. In NMP, an efficient transition from a preformed PBA block to AN polymerization required the presence of excess of nitroxide. However, chain-extension from PAN to PBA, even under homogeneous conditions and with an excess nitroxide, was less efficient, and GPC traces showed bimodality.