Catalyst system for producing ethylene copolymers in a high temperature solution process, the catalyst system comprising: (i) a metallocene complex of formula (I) wherein M is Hf or a mixture with Zr, provided that more than 50% by moles of the complex of Formula I has M=Hf; X is a sigma ligand; R are the same or different from each other and can be saturated linear or branched C1-C10 alkyl, C6-C10 aryl, C4-C10 heteroaryl, C6-C20 alkylaryl or C6-C20 arylalkyl groups, which can optionally contain up to 2 heteroatoms or silicon atoms; R1 is a C6-C10 aryl or C6-C20 alkylaryl group optionally containing up to 2 heteroatoms or silicon atoms or a C4-C10 heteroaryl group; R2 is a C4-C20 cycloalkyl group, optionally carrying alkyl substituents in beta-positions, of formula (II) in which R′ can be the same or can be different from each other and can be hydrogen or is defined as R and n is 1 to 17; and (ii) a boron containing cocatalyst.