"Block-random" copolymers-where one or more blocks are themselves random copolymers-offer a flexible modification to the usual block copolymer architecture. For example, in a poly(A)-poly(A-ran-B) diblock consisting of monomer units A and B, the interblock segregation strength can be continuously tuned through the B content of the random block, allowing the design of block copolymers with accessible order-disorder transitions at arbitrarily high molecular weights. Moreover, the development of controlled radical polymerizations has greatly expanded the palette of accessible monomer units A and B, including units with strongly interacting functional groups. We synthesize a range of copolymers consisting of styrene (S) and acetoxystyrene (AS) units, including copolymers where one block is P(S-ran-AS), through nitroxide-mediated radical polymerization. At sufficiently high molecular weights, near-symmetric PS-PAS diblocks show well-ordered lamellar morphologies, while dilution of the repulsive S-AS interactions in PS-P(S-ran-AS) diblocks yields a phase-mixed morphology. Cleavage of a sufficient fraction of the AS units in a phase-mixed PS-P(S-ran-AS) diblock to hydrogen-bonding hydroxystyrene (HS) units yields, in turn, a microphase-separated melt. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2106-2113, 2009.