Polymerization-induced microphase separation (PIMS) is a rapid and efficient process to produce nanostructured polymer materials in large-scale. However, so far, it remains a challenge to produce well defined morphologies with long-range ordering and controlled dimension. Herein, we demonstrate that nitroxide-mediated PIMS can generate ordered asymmetric lamellar morphologies of variable size over a long-range, by simple thermal dissociation of the first block. For that purpose, a low fraction (<7.5 wt %) of star poly(n-butyl acrylate-co-styrene) macroalkoxyamine (s-P(BA-co-S)-SG1), end functionalized with the SG1 nitroxide, was initially solubilized in methyl methacrylate monomer (MMA). The s-P(BA-co-S)-SG1 macroinitiator is thermally activated to be chain extended by bulk MMA nitroxide-mediated polymerization, inducing simultaneous self-assembly of the in situ synthesized s-P(nBA-co-S)-b-PMMA block copolymer. This strategy produces asymmetric lamellar morphologies with a large lamella period, up to 350 nm. The addition of varying fractions of a presynthesized PMMA-b-PnBA-b-PMMA triblock copolymer (from 0 to 16 wt %) in the initial formulation is a key point to fine-tune the dimensions of the lamella periods from 350 nm down to 100 nm via a cosurfactant effect.