METHOD FOR PRODUCING PROPYLENE-BASED BLOCK COPOLYMER

PROBLEM TO BE SOLVED: To provide a method for stably producing a propylene-based block copolymer having high fluidity suitable for injection molding and sufficient heat resistance, and further having excellent flexibility together.SOLUTION: A method for producing a propylene-based block copolymer with MFR of 20-1,000 g/10 min employs a multi-step vapor phase polymerization process including a first step and a second step using a metallocene-based catalyst. In the first step, 30-70 wt.% of a propylene-ethylene random copolymer component (I) is produced. The melting peak temperature (Tm) of the component (I) and the polymerization temperature (T1) in the step satisfy the relation of 45°C≤(Tm-T1)≤80°C. In the second step, 70-30 wt.% of a propylene-ethylene random copolymer component (II) with higher ethylene content than that of the component (I) and higher MFR than that of the component (I) is produced. The polymerization temperature (T2) in the step lies in a range of 40-62°C, and the temperature of circulating gas to be fed to the reactor of the second step is higher than the polymerization temperature (T2).