Abstract: The present invention relates to a process for the polymerisation or copolymerisation of alpha-olefins in the presence of a Ziegler-Natta catalyst system comprising on the one hand a catalyst solid comprising compounds of halogens, transition metals of Groups IV, V, VI of the Periodic Table of Elements, magnesium and optionally aluminum, free from a metal/carbon bond and on the other hand a catalyst consisting of organometallic compounds of metals of Group II or III of the said Table. This process is characterised in that before its use in polymerisation or copolymerisation, the catalyst solid is treated by water in a given quantity, this treatment making it possible to improve the conditions for obtaining polymers, particularly in a process of polymerisation or copolymerisation in the gas phase.

Abstract: The invention concerns a process for the production of polyolefins with a wide range of distribution in respect of molecular masses.The process comprises polymerizing ethylene, which is optionally accompanied by one or more other olefins, in contact with a catalytic system comprising:(a) a catalyst comprising a solid compound of titanium, magnesium and a halogen such as chlorine or bormine(b) one or more organometallic compounds of a metal of groups II and III of the periodic table of elements(c) one or more halogenated ethylenic hydrocarbons.The resulting polyolefins are particularly suitable for being shaped by means of extrusion-blowing.