Narrow molecular weight distribution polypropylene and methods for their preparation

PROBLEM TO BE SOLVED: To provide a narrow molecular weight distribution propylene polymer prepared directly by polymerization using a Ziegler-Natta catalyst.SOLUTION: A Ziegler-Natta catalyst contains (1) a titanium-containing solid catalyst made by reacting an alkoxy magnesium compound, a titanium compound and an internal electron donor compound represented by the formula (IV), (2) an alkyl aluminum compound, and (3) an external electron donor compound preferably consisting of an organic silicon compound. There is provided a polypropylene having a molecular weight distribution index Mw/Mn of 2.5 to 5.5, a polydispersion index of a high molecular weight tail in a molecular weight distribution width PIof 1.9 or more, isotactic pentad [mmmm] alignment of 85% or more, a crystallization temperature of 113°C or more and xylene-soluble content of less than 4.5 wt.%. The polymer has advantages on cost, energy saving and environmental protection in comparison with a narrow molecular weight distribution polypropylene by a conventional decomposition process. It is suitable especially for use accompanied by relatively high flowability.