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Abstract

Propylene/ethylene block copolymers are prepared by a process in which, in an agitated fixed bed comprising small polymer particles and in each case in the presence of hydrogen, first (I) propylene is homopolymerized from the gas phase, in a first polymerization zone, by feeding in a Ziegler-Natta catalyst, and then (II) the product obtained in the first polymerization zone is fed into a second polymerization zone, where a mixture of propylene and ethylene is polymerized with the propylene homopolymer present in the product from the first polymerization zone. The process has the following features: (a) in the first polymerization zone, polymerization is carried out at a particular total pressure and at a particular temperature, with the proviso that the ratio of the partial pressure of propylene to that of hydrogen (P/H-I) has a specific value, (b) in the second polymerization zone, polymerization is carried out at a particular total pressure and at a particular temperature, with the provisos that (i) the ratio of the partial pressure of propylene to that of ethylene and (ii) the ratio of the partial pressure of propylene to that of hydrogen (P/H-II) have specific values, (c) the total pressure in the first polymerization zone is kept substantially higher than that in the second polymerization zone, (d) the temperature in the second polymerization zone is chosen to be substantially lower than that in the first polymerization zone, and (e) the ratio of P/H-I to P/H-II has a specific value. Polymers having high impact strength and good morphological properties are obtained.

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