A basic limitation in the processing of thermoplastics lies in the high viscosity of their melts. Since high viscosity is largely a consequence of high average molecular weight, this problem would be solved if a polymer could be provided in the form of polymer blocks which are essentially independent at elevated temperatures and which bond together reversibly at ambient temperatures. The theoretical possibility of achieving such a system is considered from the point of view of thermodynamic arguments as applied to an idealized system of monodisperse polymer blocks linearly connected by weak chemical bonds. Average molecular weights are expressed in terms of a molar Gibbs function contribution associated with a weak bond in the polymer. Assuming a requirement for substantial decomposition at 500°K, arguments are presented for an entropy contribution of ca. 200 J °K−1mole−1for such bonds, and on this basis an optimum weak bond energy of 60 ± 20 kJ mole−1is indicated. Possible block terminations giving energies of this magnitude are considered.