Zero shear viscosities, eta0, were determined by means of a magnetoviscometer for melts of poly(n-butyl methacrylate) (M = 8.7 to 450 kg/mol, T = 53.5 to 200-degrees-C) and for concentrated solutions of the highest molecular weight sample in isopropanol (T = 34.8 to 131.5-degrees-C). Master curves can be constructed in both cases if the reference temperature is set proportional to the gelation temperature of the particular fluid. Special intersegmental interactions (eventually leading to thermoreversible gelation) can above all be felt in eta0 (T) and in M(c), the critical molecular weight determined in plots of log eta0 vs. log M. As the temperature is lowered, the behavior changes from WLF to Arrhenius, and M(c) declines considerably. The former observation is explained by analogy to the transition from fragile to hard glasses resulting from increased crosslinking. The latter effect can also be rationalized in terms of a physical network reducing the molecular weight that is required for the formation of entanglements.