PURPOSE: To improve the impact resistance by melt compounding a polypropylene having a specified weight-average molecular weight and a specified content of room-temperature p-xylene-solubles in the presence of an organic peroxide.

CONSTITUTION: Propylene is prepolymerized in a solvent in the presence of a catalyst consisting of a Ti compound and an organoaluminum compound and then fully polymerized at about 80°C or lower in the presence of a catalyst consisting of a Ti compound, an organoaluminum compound, an organoalkoxyaluminum compound and an electron donor compound, such as alcohol, at a prescribed molar ratio to produce a polypropylene powder having a weight-average molecular weight of at least 1,000,000, especially 1,500,000 to 3,000,000, a room-temperature p-xylene-solubles content of 5-30wt.% and a bulk specific gravity of about 0.30-0.50 g/cc. 100 pts.wt. of the powder is mixed with about 0.001-1.0 pt.wt. organic peroxide such as methyl ethyl peroxide and, if necessary, various additives such as an antioxidant, a light stabilizer and a lubricant, and the mixture is melt compounded through, e.g., an extruder at about 170-300°C to form pellets, thus giving the objective flexible polypropylene.