Asymmetric miktoarm star polymers produce unique material properties, yet existing synthetic strategies are beleaguered by complicated reaction schemes restricted in both the monomer scope and yield. Here, we introduce a new synthetic approach coined "mu STAR", miktoarm synthesis by termination after ring-opening metathesis polymerization, that circumvents these traditional synthetic limitations by constructing the block-block junction in a scalable one-pot process involving (1) grafting-through polymerization of a macromonomer followed by (2) in situ enyne-mediated termination to install a single mikto-arm with exceptional efficiency. This modular mu STAR platform cleanly generates AB(n) and A(BA')(n) miktoarm star polymers with unprecedented versatility in the selection of A and B chemistries as demonstrated using many common polymer building blocks. The average number of B or BA' arms (n) is easily controlled by the equivalents of Grubbs catalyst. While these materials are characterized by dispersity in n that arises from the statistics of polymerization, they self-assemble into mesophases that are identical to those predicted for precise miktoarm stars. In summary, the mu STAR technique provides a significant boost in design flexibility and synthetic simplicity while retaining the salient phase behavior of precise miktoarm star materials.