A process for preparing heterophasic propylene copolymers by polymerizing
propylene in the presence of a polymerization catalyst and hydrogen as a
molecular weight regulator, the process comprising the following steps:
a) polymerizing in gas- or liquid-phase propylene to prepare a
crystalline polymer fraction; b) copolymerizing ethylene with propylene
and/or 1-butene, and optionally one or more alpha-olefin comonomers
C.sub.5-C.sub.12, in a gas-phase reactor having interconnected
polymerization zones, wherein the growing polymer particles flow upward
through a first polymerization zone (riser) under fast fluidization or
transport conditions, leave said riser and enter a second polymerization
zone (downcomer) through which they flow downward under the action of
gravity.