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The second generation hyperbranched polyesters(HBP) were prepared from trimethylol-propane, 2,2-dimethylol acid and p-toluenesulphonic acid.Polypropylene/hyperbranched polyester blends were prepared.Isothermal crystallization kinetics of the blends was studied by DSC.The results showed that Avrami equation was satisfying in describing the crystallization kinetics of polypropylene/hyperbranched polyester blends.HBP mainly acted as a nucleating agent for PP.Addition of the second generation hyperbranched polyesters can decrease the half crystallization time (t1/2) and increase the crystallization rate of PP greatly.The Avrami exponents of PP and polypropylene/hyperbranched polyester blends are all close to 2.5 , which indicates that the addition of nucleating agents doesnt change the crystallization growth mode of PP under isothermal conditions.Meanwhile Hoffman theory is adopted to calculate the interfacial free energy per unit area of PP and polypropylene/hyperbranched polyester blends.The result shows that the addition of the second generation hyperbranched polyesters can decrease theδe value of PP so that the nucleating rate of PP is increased.