A series of symmetric poly(4-tert-butylstyrene-block-methyl methacrylate) (PtBS-b-PMMA) diblock copolymers with varying molar masses and narrow molar mass distributions were prepared using sequential anionic polymerization. Order-to-disorder transition (ODT) temperatures were determined as a function of the overall degree of polymerization, N, using a combination of low-frequency dynamic mechanical spectroscopy (DMS) and variable temperature small-angle X-ray scattering (SAXS), leading to a mean-field expression for the segment-segment interaction parameter, χ = (41.2 ± 0.9)/T - (0.044 ± 0.002). This material is characterized by a larger value of χ, and much greater temperature sensitivity, than polystyrene-b-PMMA, providing access to tunable lamellar periods (pitch) down to 14 nm at technologically relevant temperatures. Nucleation and growth of lamellae, following cooling from the disordered state, was characterized by SAXS well above the glass transition temperature (T g ≈ 130 °C) and extrapolated to lower processing temperatures based on polymer chain relaxation times extracted from time-temperature superposed DMS data. These results qualify PtBS-b-PMMA as an attractive candidate for development as a new lithographic material.