| 摘要 |
Propylene-ethylene copolymers obtainable by two-step polymerisation from the gas phase in an agitated fixed bed by means of a Ziegler-Natta catalyst system containing an aluminium compound as cocatalyst in addition to a titanium-containing solid component, in which, in a first polymerisation step, propylene is polymerised at a pressure of from 20 to 40 bar, a temperature of from 60 to 90 DEG C and a mean residence time of the reaction mixture of from 1 to 5 hours, giving from 0.05 to 2 kg of polypropylene per mmol of the aluminium compound, and subsequently, in a second polymerisation step, a mixture of propylene and ethylene is polymerised onto the polymer discharged from the first polymerisation step, at a pressure of from 5 to 30 bar, this pressure being at least 7 bar below the pressure in the first polymerisation step, a temperature of from 30 to 100 DEG C and a mean residence time of the reaction mixture of from 1 to 5 hours, the ratio between the partial pressures of propylene and ethylene being set in the range from 0.5 : 1 to 5 : 1, and a) the weight ratio between the monomers reacted in the first and second polymerisation steps is set so that it is in the range from 2.5 : 1 to 20 : 1, or b) the polymerisation in the second polymerisation step is carried out in the presence of a C1-C8 alkanol, the weight ratio between the monomers reacted in the first and the second polymerisation steps being from 1 : 1 to 20 : 1. s |
