Patent ID: 7196140

Claim:
A method for producing a highly flowable propy-lene block copolymer comprising 50 to 80 wt.-% of a propylene homopolymer, and 20 to 50 wt.-% of a propylene copolymer, wherein the propylene copolymer comprises from 10 to 70 wt.-% of a C 2 –C 8 1-alkene other than propylene, the highly flowable propylene block copolymer being obtained by two-stage polymerization by means of a Ziegler-Natta catalyst system from a gas phase reaction comprising in a first polymerization stage, propylene monomer is polymerized in a reaction mixture forming a propylene homopolymer at a pressure of 10 to 50 bar and a temperature of 50 to 100° C., the reaction mixture having a mean dwell time of 0.3 to 5 hours in presence of at least 5.0 vol.-% of hydrogen in proportion to a total volume of the reaction mixture; the propylene homopolymer obtained in the first polymerization stage and the Ziegler-Natta catalyst system is introduced into an intermediate container, wherein the propylene homopolymer and the Ziegler-Natta catalyst system are depressurized to less than 3.5 bar for a period of time of 0.1 to 5 minutes and are maintained at a temperature of 10 to 80° C., with pressure then being raised after the period of time of 0.1 to 5 minutes by 5 to 60 bar; and the propylene homopolymer along with the Ziegler-Natta catalyst system is transferred to a second polymerization stage, wherein a mixture of propylene monomer and C 2 –C 8 1-alkene monomer excluding propylene is added to the propylene homopolymer by polymerization at a pressure of 10 to 50 bar, a temperature of 50 to 100° C., and a mean dwell time of 0.5 to 5 hours; wherein a weight ratio between the propylene monomer and the C 2 –C 8 1-alkene monomer excluding propylene in the highly flowable propylene block copolymer is adjusted to be in the range of 4:1 to 1:1 and the highly flowable propylene block copolymer obtained has an MFR from 2 to 100 g/10 min.