A process is disclosed for the polymerization of olefins, using (i) a catalyst and (ii) an organometallic activator from Groups I, II or III of the Periodic Table at temperatures between 20.degree. and 250.degree. C., and pressure up to 200 bar, in solution, or in suspension in an inert liquid hydrocarbon, at temperatures between 160.degree. and 350.degree. C., under pressures of between 400 and 3000 bar, in a continuous manner in a reactor where the mean residence time of the catalyst is between 1 and 150 seconds. The catalyst comprises at least one titanium compound, at least one halogen-containing organoaluminum compound and at least one inorganic magnesium compound in suspension in at least one .alpha.,.omega.-dihaloalkane. The titanium compound is essentially a titanium (III) compound, the total content of titanium (II) and titanium (IV) being less than or equal to 15% of the total content of titanium. The catalyst is obtained by a process comprising, in a first step, the reduction of a titanium (IV) compound in an .alpha.,.omega.-dihaloalkane by means of a molar excess of at least one halogen-containing organoaluminum compound, in the absence of an ether, and, in a second step, the addition to the reaction mixture of a compound which forms a magnesium halide in situ without supplementary reduction of the titanium compound obtained in the first step. Molar ratios of the .alpha.,.omega.-dihaloalkane, the titanium compound, halogen-containing organoaluminum, and the compound which forms a magnesium halide in situ are also described.

A new highly active olefin polymerization catalyst as disclosed. The catalyst is prepared by contacting a magnesium dihalide with a halogenated titanium alkoxide to obtain a first solid, washing this first solid until it is substantially free of soluble halogenated titanium alkoxide, then contacting the first solid with a titanium tetrachloride to yield a second solid.

A new highly active olefin polymerization catalyst as disclosed. The catalyst is prepared by contacting a magnesium dihalide with a halogenated titanium alkoxide to obtain a first solid, washing this first solid unit it is substantially free of soluble halogenated titanium alkoxide, then contacting the first solid with a titanium tetrachloride to yield a second solid.

A halogen group solvent comprising a mixed solvent consisting essentially of bromochloromethane and/or 1,2-dichloroethylene and a co-solvent, wherein the co-solvent is at least one member selected from the group consisting of carbon dioxide, sulfur hexafluoride, difluorochloromethane, 1,1,1,2-tetrafluoroethane, etc., and the content of said co-solvent in the mixed solvent is 3 to 65% by weight; a polyolefin solution using the solvent; and a process for producing a three-dimensional fiber of a polyolefin using the solution.