Process for the polymerization or copolymerization of alpha-olefins in a fluidized bed, in the presence of a Ziegler-Natta catalyst system

Claim 1

Process for the polymerisation or copolymerisation of alpha-olefins comprising from 2 to 8 carbon atoms in a fluidized bed, by means of a catalyst system of the Ziegler-Natta type consisting of a catalyst (a) comprising atoms of magnesium, halogen titanium and/or vanadium, and of a cocatalyst (b) consisting of an organometallic compound selected from the group consisting of organoaluminium and organozinc compounds, the catalyst system being converted into a prepolymer by bringing one or several alpha-olefins into contact with the catalyst system in such quantities that the prepolymer contains per gram from 2×10 -3 to 10 -1 milligram of atoms of titanium and/or vanadium, and the atomic ratio of the quantity of metal of the cocatalyst (b) to the quantity of titanium and/or vanadium is comprised between 0.5 and 50, the process being characterized in that the prepolymer is (i) previously subjected to a treatment by carbon monoxide or carbon dioxide, in the absence of alpha-olefin, in such a quantity that the ratio of the number of grams of molecules of carbon monoxide or dioxide to the number of grams of atoms of titanium and/or vanadium is comprised between 0.001 and 0.1, and (ii) then brought into contact with one or several alpha-olefin(s) under polymerisation or copolymerisation conditions in a fluidized bed, in the presence of a cocatalyst (c); said cocatalyst (c) consisting of an organometallic compound selected amongst organoaluminum and organozinc compounds, which is identical to or different from cocatalyst (b), in a quantity such that the atomic ratio of the quantity of metal in the said cocatalyst (c) to the quantity of titanium and/or vanadium is comprised between 0.5 and 100.