Well-defined diblock copolymers of pentafluorostyrene and methyl methacrylate with narrow molecular weight distributions (PDI < 1.15) were successfully synthesized by atom transfer radical polymerization (ATRP). Both polypentafluorostyrene and poly( methyl methacrylate) can serve as effective macroinitiators for the polymerization of the subsequent block. Because of thermodynamic immiscibility between the two blocks, these diblock copolymers undergo microphase separation upon annealing to form ordered nanostructures. The order-disorder transition temperatures for a series of symmetric, low molecular weight samples were determined to ascertain the temperature dependence of the Flory-Huggins interaction parameter (chi). Poly(pentafluorostyrenebmethyl methacrylate) is only about twice as segregated as its nonfluorinated counterpart, poly(styrene-b-methyl methacrylate), or the quintessential poly( styrene-b-isoprene), at 440 K. Despite perfluorination of the ring, the symmetric placement of the C-F bonds on the benzene ring causes the polarities at the ortho and meta positions to cancel in polypentafluorostyrene. Replacing polystyrene with polypentafluorostyrene thus only leads to a small net polarity increase, which in turn results in a reduced polarity difference between polypentafluorostyrene and poly( methyl methacrylate), as compared to that predicted on the basis of the increase in fluorine content alone.