Well-defined block copolymers poly(n-butyl acrylate)-b-poly(methyl methacrylate-co-styrene) and polystyrene-b-poly(methyl methacrylate-co-styrene) were synthesized using activators regenerated by electron transfer (ARGET) and initiators for continuous activator regeneration (ICAR) for atom transfer radical polymerization (ATRP). In order to overcome the poor initiation efficiency of polystyrene and poly(n-butyl acrylate) macroinitiators during chain extension with methacrylates, a small amount of styrene was used as a comonomer in the second block. To determine the percentage of styrene necessary for efficient chain extension, studies with low molecular weight alkyl halide initiators that model the poly(n-butyl acrylate) and polystyrene macroinitiators, respectively, were performed. Extension from the macroinitiators was then conducted using the appropriate methyl methacrylate/styrene comonomer mixture to obtain well-defined block copolymers, for example, starting from poly(n-butyl acrylate) macroinitiator (M-n = 18 000, M-w/M-n = 1.12), a poly(n-butyl acrylate)-b-poly(methyl methacrylate-co-styrene) (M-n = 39 100, M-w/M-n = 1.23) was obtained. Without styrene, a block copolymer with a bimodal molecular weight distribution was formed: poly(n-butyl acrylate)-b-poly(methyl methacrylate): M-n = 36 400, M-w/M-n = 1.70). Computational simulations also indicated that the improved initiation efficiency was due to higher concentration of Cu-I species caused by lower K-ATRP of polystyrene chain ends.