Patent ID: 6300434
Filing Date: 2001-10-09
Classification: C08F

Abstract:
A process for preparing a propylene-ethylene copolymer which is conducted in three distinct polymerization steps in the gas phase in an agitated fixed bed by means of a Ziegler-Natta catalyst system which, in addition to a titanium-containing solid component based on a finely divided silicia as a carrier, also contains, as cocatalyst, an aluminum compound, which process comprisesin a first polymerization step, polymerizing propylene at from 60 to 90.degree. C. and at from 20 to 40 bar and at a mean residence time of the reaction mixture of from 0.5 to 5 hours, then,in a second polymerization step, polymerizing a mixture of propylene and ethylene onto the polymer obtained from the first polymerization step at from 40 to 110.degree. C. and from 5 to 30 bar, this pressure being at least 7 bar below the pressure in the first polymerization step, and at a mean residence time of the reaction mixture of from 0.2 hours to 4 hours, and then,in a third polymerization step, polymerizing ethylene and a mixture of ethylene a propylene onto the polymer obtained from the second polymerization step at from 50 to 110.degree. C. and from 5 to 30 bar and at a mean residence time of the reaction mixture of from 0.1 to 5 hours,the weight ratio between the monomers reacted in the first and second polymerization steps being from 1:1 to 20:1 and the weight ratio between the monomers reacted in the first two polymerization steps and those reacted in the third polymerization step being from 1:2 to 20:1.