A 4th generation commercial hyperbranched polyester (HBP) based on bis(hydroxymethyl) propionic acid, bis(MPA), was used as an original crosslinker for polyurethane networks. However a partial modification of the numerous hydroxyl groups was necessary to obtain complete miscibility between the three polyurethane precursors, i.e. macrodiol, diisocyanate and hyperbranched, HB, crosslinking agent. A monofunctional aromatic isocyanate, para-tolylisocyanate, pTI, was used for that purpose. The pristine hyperbranched polyester was at first quite fully characterized, especially concerning the initial distribution of hydroxyl groups between linear, L, and terminal, T, bis(MPA) units. H-1 and C-13 NMR (both for urethane formation and hydroxyl consumption) and chemical titration were then used and compared for the determination of OH conversion. It appeared that a small part of the modifier was systematically lost in a side reaction with moisture, even though the reactive species had been thoroughly dried. All the remaining pTI molecules produced exclusively urethane end groups. NMR also demonstrated that the hydroxyls born by linear units had a lower reactivity as compared to T-type OH groups. Therefore their relative proportion increases with the modification degree, and more and more modified crosslinkers become less and less reactive. Calorimetric studies performed on pristine and partially modified HB polyesters revealed that hydrogen bonds played an important role as far as thermal behavior was concerned. With increasing modification, T-g increases but also ordered domains resulting from strong interactions between hydroxyl groups are progressively replaced by new structures created by self-assembling aromatic urethanes. The latter are responsible for a high-temperature pseudo-melting for highly modified samples. However most of these interactions seem to remain intramolecular since the newtonian behavior of pristine HBP is maintained up to about 110-120 degrees C for pTI-modified oligomers; but annealing at higher temperature can result in even more ordering and in a final loss of the newtonian behavior. (c) 2006 Elsevier B.V. All rights reserved.