A polymeric alloy (SP-A) containing syndiotactic polystyrene (sPS), atactic polypropylene (aPP), and about 66 wt % sPS-b-aPP diblock copolymer, was prepared by the sequential feed of monomers in the presence of the half-titanocene Cp*Ti(OBz)(3) (where Cp* is C5Me5 and Bz is PhCH2), modified methylaluminoxane, and external tri-isobutylaluminum. The effects of the SP-A alloy as a compatibilizer for sPS and isotactic polypropylene (iPP) blends were evaluated. The blending of sPS and iPP, with and without SP-A, was performed in a single-screw miniextruder with a side channel that allowed the continuous recycling of materials. The influence of SP-A on the mechanical and thermal properties of the immiscible sPS/iPP blends was investigated over a range of composition. The presence of the SP-A alloy resulted in a significant improvement of the impact strength of the blends compared with that of pure sPS and their pure blends. This improvement was particularly obvious in the sPS/iPP (90/10 wt %) blend containing 5 wt % SP-A. Morphological analysis of the impact-fractured surface of the ternary blends indicated that the sPS-b-aPP diblock copolymer contained in the SP-A alloy acted as an efficient compatibilizer by decreasing the dispersed-phase iPP particle size, improving the interfacial adhesion, and generating a stable microphase-separated state. (C) 2003 Wiley Periodicals, Inc.