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In this work, high melt flow rate (MFR) polypropylene (HF-PP) and polypropylene/poly(ethylene-co-propylene) inreactor alloys (HF-PP/EPR) with MFR % 30 g/10 min were synthesized by spherical MgCl 2 -supported Ziegler-Natta catalyst with cyclohexylmethyldimethoxysilane (CHMDMS) or dicyclopentyldimethoxysilane (DCPDMS) as external donor (De). The effects of De on polymerization activity, chain structure, mechanical properties, and phase morphology of HF-PP and HF-PP/EPR were studied. Adding CHMDMS caused more sensitive change of the polymers MFR with H 2 than DCPDMS, and produced PP/EPR alloys containing more random ethylene-propylene copolymer (r-EP) and segmented ethylene-propylene copolymer (s-EP). CHMDMS also caused formation of s-EP with higher level of blockiness than DCPDMS. HF-PP/EPR alloy prepared in the presence of DCPDMS exhibited higher flexural properties but lower impact strength than that prepared with CHMDMS. Toughening efficiency of the rubber phase was nearly the same in the alloys prepared using CHMDMS or DCPDMS as De, but stiffness of the alloy can be improved by using DCPDMS.

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