The blending of various types of polyurethanes with rubbers has extensively been attempted to obtain new materials with an improved property spectrum. However, the opted solution-and melt-blending methodologies are highly time- and energy-intensive, which tends to forfeit the commercial realization of the reported blends offering limited performance benefits. This study realizes the in-situ synthesis of structurally different polyurethane-ureas during reactive blending with polar (CR, NBR) and nonpolar (NR, EPDM, sSBR) rubbers [1-2]. The critical processing parameters of the proposed reactive blending method are determined from the chemo-rheological investigations of the polyurethane-urea (PUU) precursors. The compositional, structural and morphological aspects influence the static- and dynamic-mechanical response of the prepared highly reinforced blends. Importantly, the scanning electron microscopy of the cryofractured specimens reveals strong interfacial adhesion for an effective stress transfer between phases of heterogeneous blends. The blends are always compatibilized irrespective of the kind of polyurethane-urea and rubber polymers. References: Tahir, M.; Stöckelhuber, K.W; Mahmood, N., Komber, H.; Heinrich G. Macromolecular Materials & Engineering 300 (2015) 242-250. 2 Tahir, M; Stöckelhuber, K.W.; Mahmood, N.; Komber, H.;Formanek, P.; Wießner, S.; Heinrich, G. European Polymer Journal 73 (2015) 75-87