Significance Molecular sequence and interactions dictate the mesoscale structure of all self-assembling soft materials. Block polymers harness this relationship to access a rich variety of nanostructured materials but typically require energetically unfavorable (high-χ) interactions between blocks. Contrary to this convention, we demonstrate that ABC triblock terpolymers featuring low-χ interactions between end blocks can self-assemble into a unique mixed morphology that subverts the demands of chain connectivity. As a consequence of block–block mixing, the characteristic length scales of these self-assembled structures exhibit an unusual trend: As the total polymer size increases, the domain spacing decreases. These developments expand the vocabulary of block polymer design and open additional avenues for manipulating the self-assembly of synthetic macromolecules.