Abstract: Copolymerization of ethylene and a C.sub.3 -C.sub.20 alpha-monoolefin at high temperature gives a low density polyethylene having a density of between 0.91 and 0.93 g/cc. The copolymerizations are accomplished with a catalyst made by co-comminuting an anhydrous magnesium halide, an anhydrous aluminum halide, a titanium compound, and at least one electron donor, and a cocatalyst comprising a synergistic mixture of an alkyl aluminum chloride and an oxygen-containing aluminum compound having oxygen attached to at least one aluminum valence.

Abstract: Catalysts for the polymerization of ethylene or its copolymerization with C.sub.3 -C.sub.2 alpha-monoolefins are often pre-activated by polymerizing a small amount of C.sub.4 -C.sub.12 alpha-monoolefin onto the catalyst in the presence of an alkyl aluminum compound. The resultant pre-activated catalyst component is used in the form of a dispersion in an inert solvent. The dispersions have a tendency to separate into two phases rapidly and are thus difficult to add uniformly to a polymerization reaction. When the alkyl aluminum compound is replaced by a synergistic mixture of an alkyl aluminum choride and an oxygen-containing aluminum compound having oxygen attached to at least one aluminum valence, a stable dispersion is formed which will not separate into two phases for long periods of time.