A theoretical reasoning on why coordination catalysts supported on mesoporous supports produce HDPE crystalline nanofibers but not iPP crystalline nanofibers

Since 1999, when Takuzo Aida proposed the preparation of high density polyethylene (HDPE) crystalline nanofibers through polymerization of ethylene by a Cp2 TiCl2 /MCM-41 catalyst, many researchers have published various papers on different aspects of the idea. The published researches show that the endeavors to polymerize other types of alpha olefins, especially propylene, to obtain polyolefins with crystalline nanofiberous morphology have not been successful and no one has reported such morphology for them. In the present article, a possible reason behind these observations is proposed. Comparing the direction of the growth of HDPE chains and isotactic polypropylene (iPP) helices to become appropriate for making polymer crystals shows that HDPE chains are able to form crystal unit cells without concerning upward or downward chain growth direction, while the growth direction of adjacent iPP chains might be responsible for unsuccessful synthesis of crystalline iPP nanofibers in confined channels of the mesoporous catalysts. iPP helices can crystallize beside each other only in the form of anticline isochiral helices.