"Block-random" copolymers-wherein one or more blocks is itself a random copolymer-present a useful and convenient variation on the typical block copolymer architecture, as the interblock interactions and physical properties can be tuned continuously through the random block's composition. However, typical living or controlled polymerizations produce compositional gradients along the "random" block, which can in turn influence the phase behavior. Organolithium initiation in a cyclohexane/triethylamine mixture is shown herein to yield narrow-distribution copolymers of styrene and isoprene of any desired composition, with no measurable down-chain gradient. These random copolymers (SrI) are also successfully incorporated into well-defined symmetric block copolymers (I-SrI diblocks). Isoprene-selective hydrogenation yields thermally stable hI-SrhI diblocks, which self-assemble into well-defined lamellar morphologies with sharply defined order-disorder transitions, whose temperatures T-ODT scale predictably with diblock molecular weight. The use of SrhI in lieu of a styrene homopolymer block allows the diblock molecular weight and domain period to be substantially increased for a given value of T-ODT. The measured interaction energy density between hI and SrhI is consistent with the mean-field "copolymer equation", providing a first step toward the design of styrene-isoprene block-random copolymers of desired molecular weight and T-ODT.