We describe the morphological implications of broad molecular weight dispersity on the bulk and thin film self-assembly behavior of seven model poly(styrene-block-methyl methacrylate) (SM) diblock copolymers. Derived from sequential nitroxide-mediated polymerizations, these unimodal diblock copolymers are comprised of narrow dispersity S blocks (Crossed D sign ≤ 1.14) and broad dispersity M blocks (Crossed D sign ∼ 1.7) with total molecular weights Mn,total = 29.2-42.9 kg/mol and M volume fractions f M = 0.35-0.63. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) analyses demonstrate that these diblock copolymers microphase separate into lamellar and cylindrical morphologies with substantially larger microdomain spacings at lower overall molecular weights as compared to their narrow dispersity analogues. The observed microphase-separated melt stabilization is also accompanied by a substantial shift in the lamellar phase composition window to higher values of fM. In thin films, these polydisperse copolymers form perpendicularly oriented morphologies with modest degrees of lateral order on substrates functionalized with P(S-ran-MMA) neutral polymer brush layers.