Process for preparation of a particulate ultra high molecular weight polyethylene (pUHMWPE), comprising the steps of a) preparing a magnesium containing carrier by interaction of a solution of an organomagnesium compound having composition MgR 1 2.nMgCl 2.mR 2 2 O where n = 0.37-0.7, m = 1.1-3.5, R 1 is each an aromatic or aliphatic hydrocarbyl residue and R 2 2 O is an aliphatic ether, with a chlorinating agent at a molar ratio Mg/CI of at most 0.5, wherein Mg represents the Mg of the organomagnesium compound and Cl the Cl of the chlorinating agent; b) loading the magnesium containing carrier with a organometallic compound forming a supported catalyst, c) contacting the supported catalyst with at least ethylene under polymerization conditions, wherein the organometallic compound is a compound of the formula R 3 3 P=N-TiCpX n, wherein each R 3 is independently selected from the group consisting of a hydrogen atom, a halogen atom, a C 1-20 hydrocarbyl radicals optionally substituted by at least one halogen atom, a C 1-8 alkoxy radical, a C 6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula -Si-(R 4) 3 and a germanyl radical of the formula -Ge-(R 4) 3 wherein each R 4 is independently selected from the group consisting of hydrogen, a C 1-8 alkyl or alkoxy radical, C 6-10 aryl or aryloxy radicals, Cp is a cyclopentadienyl ligand; X is an activatable ligand and n is 1 or 2, depending upon the valence of Ti and the valence of X, preferably, X is selected from the group consisting of Cl, Br, Me and Et if n = 2 or X is a substituted or unsubstituted butadiene if n = 1. a) preparing a magnesium containing carrier by interaction of a solution of an organomagnesium compound having composition MgR 1 2.nMgCl 2.mR 2 2 O where n = 0.37-0.7, m = 1.1-3.5, R 1 is each an aromatic or aliphatic hydrocarbyl residue and R 2 2 O is an aliphatic ether, with a chlorinating agent at a molar ratio Mg/CI of at most 0.5, wherein Mg represents the Mg of the organomagnesium compound and Cl the Cl of the chlorinating agent; b) loading the magnesium containing carrier with a organometallic compound forming a supported catalyst, c) contacting the supported catalyst with at least ethylene under polymerization conditions, each R 3 is independently selected from the group consisting of a hydrogen atom, a halogen atom, a C 1-20 hydrocarbyl radicals optionally substituted by at least one halogen atom, a C 1-8 alkoxy radical, a C 6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula -Si-(R 4) 3 and a germanyl radical of the formula -Ge-(R 4) 3 a hydrogen atom, a halogen atom, a C 1-20 hydrocarbyl radicals optionally substituted by at least one halogen atom, a C 1-8 alkoxy radical, a C 6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula -Si-(R 4) 3 and a germanyl radical of the formula -Ge-(R 4) 3 wherein each R 4 is independently selected from the group consisting of hydrogen, a C 1-8 alkyl or alkoxy radical, C 6-10 aryl or aryloxy radicals, Cp is a cyclopentadienyl ligand; X is an activatable ligand and n is 1 or 2, depending upon the valence of Ti and the valence of X, preferably, X is selected from the group consisting of Cl, Br, Me and Et if n = 2 or X is a substituted or unsubstituted butadiene if n = 1.