PTT was successfully toughened by incorporation of core-shell impact modifier (CSIM) materials in the aid of polycarbonate (PC) as an effective dispersing agent. Adding small amounts of PC (∼ 5 wt%) greatly improves both the dispersion of the rubber particles and the mechanical properties of ternary blends. Melt viscosity of the PTT matrix significantly affects blend performance. Blends prepared from high melt viscosity showed superior low temperature toughness. However, different types of PC (ie, melt viscosity or molecular weight) plays less important role to improve blend toughness than the presence of the PC itself in ternary blend. Melt viscosity of the PTT can be increased by a means of chain extension induced by reactions between triphenyl phosphite (TPP) and the PTT end groups during melt processing process. Addition of small amounts of TPP (ie, 1%) leads to an increase in melt viscosity of the polyester matrix resulting in greater shear force that facilitate better dispersion of the CSIM particles. PTT/CSIM (80/20) blends showed improved toughness performance when 2% of TPP are added. The desirable chain extension effects appeared to be subdued in ternary PTT/CSIM/PC blends. The presence of PC may result in more complicated reaction paths during melt processing process. The use of GMA-grafted core-shell impact modifiers has been shown to be effective in toughening of PTT. The reactions between the GMA units and the PTT end groups lead to promoted interfacial adhesion, resulting in improved particle dispersion and accordingly good blend performances. Small amounts of GMA (ie,> 0.17 wt%) grafted on the shell is required in order to adequately improved dispersion of the particles to be achieved. Larger level of grafting shows no advantages in terms of improved morphology and blend performance. Reactive ethylene-acrylate ester rubbers containing GMA functionality have been demonstrated to successfully toughen PTT. PTT/non-reactive blends exhibit general features of incompatible polymer blends such as coarse morphology and inferior mechanical properties while PTT/GMA-contained rubber blends display a fine but complex morphology. Improved toughness performance can be significantly achieved when sufficient GMA content (ie,> 3%) is presented in the rubber phase.