Abstract: The invention relates to a process for the production of ethylene polymers by the polymerization of ethylene or the copolymerization of ethylene with one or more C3 to C8 olefins in contact with a chromium-based catalyst deposited on a support essentially consisting of silica. According to the invention, the chromium catalyst is subjected in succession to a heat treatment in nitrogen and then to a heat treatment in air under defined temperature conditions before bringing it into contact with ethylene and, optionally, one or more olefins under polymerizing conditions. The process of the invention makes it possible to manufacture ethylene polymers having an improved compromise between processability, stiffness and resistance to slow crack growth.

Abstract: In a first stage, a lactone is polymerized by an ionic route in vinyl chloride with the aid of a catalyst consisting of the product, optionally complexed with tetrahydrofuran, of the reaction of an alkylmetal with ethylenically unsaturated alcohol, optionally in the presence of water and, in a second stage, the said vinyl chloride is polymerized by a radical route in an aqueous suspension and in the presence of the polymerization medium originating from the first stage, to which other unsaturated monomers are optionally added.The invention also relates to new modified vinyl chloride polymers consisting of lactone polymers whose number-average molecular weight is less than 5,000, grafted with vinyl chloride polymers.