Polymerization of olefins using a ziegler-natta catalyst and two organometallic compounds

Claim 1

A process for the polymerization or copolymerization of ethylene comprising the steps (A) preparing a prepolymer-based catalyst system of the Ziegler-Natta type consisting of a catalyst comprising basically atoms of halogen, magnesium and a transition metal belonging to Groups IV, V or VI of the Periodic Table of Elements, and of a cocatalyst based on one or more organometallic compounds of a metal belonging to Groups II or III of this Table, and (B) contacting the prepolymer-based catalyst under gas phase fludized bed polymerization conditions with ethylene or ethylene and one or more alpha-olefins in the presence of organometallic cocatalyst the process being characterised in that: (1) the cocatalyst employed in step (A) is at least one low volatility organometallic compound (a) having a vapour pressure at 80° C. of less than 65 Pa, in a quantity such that the atomic ratio of the quantity of metal in the organometallic compound (a) to the quantity of transition metal in the catalyst is at least 0.5 and at most 2.5, and wherein said organometallic compound (a) is an organo-aluminium compound of the general formula AlR n X 3-n in which R represents an alkyl group comprising from 4 to 20 carbon atoms, X is a hydrogen or halogen atom, or an alcoholate group, and n is an integer or fraction which may have any value from 1 to 3, and (2) the cocatalyst employed in step (B) is at least one volatile organometallic compound (b), having a vapour pressure at 80° C. equal to or greater than 65 Pa in a quantity such that the atomic ratio of the quantity of metal in the organometallic compound (b) to the quantity of transition metal in the prepolymer-based catalyst is at least 0.5, and that the atomic ratio of the total quantity of metal in the organometallic compound (a) and (b) to the quantity of transition metal in the catalyst is at least 2.5 and at most 9, the organometallic compound (b) being introduced into the polymerisation medium separately from the prepolymer-based catalyst, and wherein the organometallic compound (b) is an organo-aluminium compound of the general formula Al R' n X 3-n in which R' represents an alkyl group comprising from 1 to 3 carbon atoms, X is a hydrogen or halogen atom or an alcoholate group and n is an integer or fraction which may have any value from 1 to 3.