A catalyst composition including: (a) a chromium compound; (b) a ligand of the general structure (A) R.sub.1R.sub.2PN(R.sub.3)P(R.sub.4)NR.sub.5R.sub.6 or (B) R.sub.1R.sub.2PN(R.sub.3)P(XR.sub.7)R.sub.8 or R.sub.1R.sub.2PN(R.sub.3)P(XR.sub.7).sub.2, with X=O or S,
wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7 and R.sub.8 are independently C.sub.1-C.sub.10-alkyl, C.sub.6-C.sub.20-aryl, C.sub.3-C.sub.10-cycloalkyl, aralkyl, alkylaryl, or trialkylsilyl, or any cyclic derivatives of (A) and (B), wherein at least one of the P or N atoms of the PNPN-unit or PNP-unit is a member of the ring system, the ring system being formed from one or more constituent compounds of structures (A) or (B) by substitution; and (c) an activator or co-catalyst; and a process for tri- and/or tetramerization.

A catalyst composition and a process for di-, tri- and/or tetramerization of ethylene, wherein the catalyst composition comprises a chromium compound, a ligand of the general structure R.sub.1R.sub.2PN(R.sub.3)P(R.sub.4)N(R.sub.5)PR.sub.6R.sub.7, or any cyclic derivatives thereof, wherein at least one of the P or N atoms of the PNPNP-unit is member of a ring system, the ring system being formed from one or more constituent compounds of the ligand by substitution, and a co-catalyst or activator.

Systems and methods for catalyst activation in ethylene reactions are described. Systems and methods may include pre-mixing at least one ligand and at least one chromium source in at least one solvent to form a pre-mixed composition; activating the pre-mixed composition with an activator to form an activated composition; and supplying the pre-activated composition to a reactor.