The invention relates to a process for the preparation of a polymer comprising at least one aliphatic or aromatic hydrocarbyl C2-20 olefin in the presence of an ionic catalyst, comprising an organometallic compound and an activator, and optionally a scavenger, characterized in that the organometallic compound is a compound according to formula (1): where: M is a metal of group 3, 4, 5, 6 or 7, orthe lanthanide series, and p is the valency of the metal M, A represents a neutral or anionic spectator ligand whose valency v is 0, 1 or 2, and q is an integer denoting the number of spectator ligands A, Z is an optional bridging moiety, n is the integer number of parallel bridging moieties Z, Y is an amidine-containing spectator ligand represented by formula (2): wherein the amidine-containing ligand is covalently bonded to the metal M via the imine nitrogen atom, Sub1 is a substituent, which comprises a group 14 atom through which Sub1 is bonded to the imine carbon atom. Sub2 is a substituent, which comprises a heteroatom of group 15-16, through which Sub2 is bonded to the imine carbon atom, r is an integer > 0, L is an optional neutral Lewis basic ligand, j is an integer denoting the number of neutral ligands L, and X is an anionic ligand. The invention further relates to a organometallic compound according to formula (1) and a process for the preparation of this compound as well as to a UHMWPE having a weight average molecular weight of at least 4.000 kg/mol and a molecular weight distribution Mw/Mn of less than 2.6.