Ethylene polymer having a blow-up ratio (R.sub.B) of at least 1.4, a resistance to crazing under stress (ESCR) of at least 55 h and a melt index (MI.sub.5) of at least 0.2 g/10 min. Processes for obtaining this ethylene polymer using different catalytic systems, the first based on titanium and zirconium used in a polymerization in two reactors; the second consisting of a mixture of a titanium catalyst and a titanium and zirconium catalyst, used in a polymerization in two reactors; the third based on chromium on a support comprising at least two constituents chosen from silica, alumina and aluminium phosphates, used in a polymerization in a single or in two reactors.

Ethylene polymer having a die swell ratio (R.sub.B) of at least 1.4, a resistance to crazing under stress (ESCR) of at least 55 h and a melt index (MI.sub.5) of at least 0.2 g/10 min. Processes for obtaining this ethylene polymer using different catalytic systems, the first based on titanium and zirconium used in a polymerization in two reactors; the second consisting of a mixture of a titanium catalyst and a titanium and zirconium catalyst, used in a polymerization in two reactors; the third based on chromium on a support comprising at least two constituents chosen from silica, alumina and aluminum phosphates, used in a polymerization in a single or in two reactors.

A catalytic solid for (co)polymerization of ethylene, includes (a) at least one inorganic support; (b) at least one transition metal metallocene having a formula (C.sub.p).sub.a (C.sub.p ').sub.h MX.sub.x Z.sub.z, in which C.sub.p and C.sub.p each denote a cyclopentadienyl, indenyl, or fluorenyl radical or a substituted derivative of such a radical, coordinated to the central atom M, optionally the groups C.sub.p and C.sub.p are linked via a covalent bridge, M denotes the at lease one transition metal, which is selected from the group consisting of scandium, titanium, zirconium, hafnium and vanadium, a, b, x and z denote integers such that (a+b+x+z)=m, x>0, z.gtoreq.0 and a and/or b.noteq.0, m denotes the valency of the at least one transition metal M, X denotes a halogen, and Z denotes a hydrocarbon radical which may optionally comprise oxygen or a silyl radical of formula (--R.sub.1 --Si--R'R"R'") where R denotes an alkyl, alkenyl, aryl, alkoxy or cycloalkyl group comprising up to 20 carbon atoms, R', R", R'" are identical or different and each denote a halogen or an alkyl, alkenyl, aryl, alkoxy or cycloalkyl group comprising up to 20 carbon atoms, and t denotes 0 or 1; (c) at least one ionizing agent selected from the group consisting of triphenylcarbenium tetrakis(pentafluorophenyl)borate, N,N-dimethylanilinium tetrakis (pentafluorophenyl)borate, tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate, lithium tetrakis(pentafluorophenyl)borate, tris(pentafluorophenyl)boron, triphenylboron, trimethylboron, tris(trimethylsilyl)borate and organoboroxines, and (d) at least one chromium compound which is selected from the group consisting of chromium oxides, chromium acetate, chromium chloride, chromium sulfate, chromates, dichromates and chromium acetylacetonate, and which is present in a molar ratio of chromium/transition metal M ranging from 0.001 to 100.