The silane compounds including dicyclohexyldimethoxysilane (Donor-H) and dipiperidinedimethoxysilane (Donor-Py) were first synthesized.Then they and the industrial external donor cyclohexylmethyldimethoxysilane (Donor-C) were employed for propylene polymerization with MgCl2-supported Ziegler-Natta catalyst

respectively.The effects of the changeable substitutes of the three external donors on the catalytic activity

hydrogen response

molecular weight distribution

crystalline ability

isotactic sequence length and isotactic sequence length distribution were studied by differential scanning calorimetry (DSC)

gel permeation chromatography (GPC)

self-nucleation and annealing (SSA) and 13C-NMR.The results indicated that the electron-donating ability of the external electron donors was enhanced with the increase of the bulkiness of the changeable substituted groups in the external donors.Compared with the external donor Donor-C

the synthesized external donors Donor-H and Donor-Py had the excellent performance in the catalytic activity

crystallization ability and isospecificity of polypropylene for propylene polymerization with the increase of the bulkiness of the substituted groups in the external donors Donor-H and Donor-Py.Especially

when the Donor-Py was used as external donor

the catalyst showed the highest catalytic activity

and the prepared polypropylene exhibited the highest isotacticity

the strongest crystallization ability and the widest melt index.The GPC data also showed the external donor Donor-Py was benefit for obtaining broad molecular weight distribution polypropylene than the other two external donors Donor-H and Donor-C.The SSA results showed that the polypropylene prepared by the external donor Donor-Py had the largest relative content of higher isotactic components and the longest isotactic sequence length of polypropylene

and the narrowest isotactic sequence length distribution.The 13C-NMR results also showed that the increase of the bulkiness of the substituted groups in the external donors was benefit for improving the average meso isotactic sequence length and the molecular chain stereoregularity of polypropylene.