The role of prepolymer on the interfacial strength of polyurethane-based assemblies is examined for two hydroxyl-terminated (telechelic) polybutadienes, R45M and R45HT. Although it is known that both prepolymers lead to very different adhesive behavior, the reasons that explain this are not understood. In this study, we show that two main effects have to be considered: the length of the free chains, which can diffuse across the interface during the formation of the joint and the relative reactivity of the various hydroxyl groups of the chains and chain extenders with the isocyanate. The R45M chains are more efficient connectors than the R45HT chains. Migration of chain extenders and isocyanate over large distances creates a thick interphase where physical crosslinks appear. This interphase is thicker for R4BM-based polyurethane showing the role of the chain reactivity. (C) 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 665-671, 2000.