We have discovered a synthetic process capable of producing olefin block copolymers (OBCs) with controlled block length polydispersity. Certain compositions of these OBCs self-assemble in the melt to form ordered mesophases. The morphologies and dielectric contrast of the semicrystalline and amorphous blocks produce transparent films exhibiting a partial photonic band gap for frequencies in the visible spectrum. The domain spacings are not only much larger than expected for monodisperse block copolymers of similar molecular weight, but they also exceed the predictions of recent theories for polydisperse block copolymers. An extension to Strong Segregation Theory demonstrates that many molecules have a weak preference for segregation to the interface versus the center of a domain. Minor perturbations can then produce highly swollen but relatively stable domains.