Investigating Effects of Using Mixtures of Two External Electron Donors on Microstructure and Properties of Polypropylene/Poly(ethylene- co-propylene) in-Reactor Blends Based on Ziegler–Natta Catalyst Leila Moballegh, Shokoufeh Hakim,* Jalil Morshedian, Mehdi Nekoomanesh Novel results on using mixtures of two external electron donors ‘‘phenyltriethoxysilane (donor A)’’ and ‘‘cyclohexylmethyldimethoxysilane (donor C)’’ in the synthesis of polypropylene/poly (ethylene-co-propylene) in-reactor blends were presented. Thermal gradient elution fractionation (TGEF) was carried out to separate crystallizable copolymer fractions from the blends. FTIR, DSC, DMTA, and SEM were used to analyze the microstructure of the blends and separated fractions. Although using donor A as pure decreased the catalyst activity and molecular weight, using ‘‘donor A’’ in combination with donor C had a synergistic effect on the molecular weight of polypropylene homopolymers and the blends. The glass transition temperature (T g) of the blend rubbery phase was the lowest in the case of the blend based on pure donor A because of formation of higher sequences of polyethylene and the T g increased with increasing donor C concentration. PP–PE copoly- mer concentration and impact strength were found to be higher in the case of the blend based on donor ratio of A/C: 75/25 than other blends. 1. Introduction Polypropylene (PP) is one of the most widely used plastics because of having excellent properties like low cost, low density, high mechanical strength, and good processability. However, its brittleness especially at low temperature has limited its use in many applications. Various methods have been applied to improve PP toughness, including mechan- ical blending of PP with an elastomer like propylene/a- olefin copolymer or other thermoplastics like polyethy- lene. [1–4] But, one of the most important techniques for improving impact properties is in-reactor blending of PP with poly(propylene/a-olefin) especially a copolymer of propylene and ethylene. The technique provides the possibility of controlling the chemical structure to improve product properties. [4,5] L. Moballegh, Dr. S. Hakim, Prof. J. Morshedian, Prof. M. Nekoomanesh Iran polymer and Petrochemical Institute, Pajoohesh Blvd, km15 Tehran-Karaj Hwy, Tehran 115/14965, Iran E-mail: s.hakim@ippi.ac.ir Full Paper 350 ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mren.201500002 Macromol. React. Eng. 2015, 9, 350–359 wileyonlinelibrary.com