The invention relates to a catalyst system for the polymerization of olefins comprising a metal complex of formula CyLMD and an activating cocatalyst, wherein M is titanium, Cy is a cyclopentadienyl-type ligand, D is a diene, L is a guanidinate-containing ligand of the formula (I) wherein each A is independently selected from nitrogen or phosphorous and R, R1, R2 and R3 are independently selected from the group consisting of hydrogen, hydrocarbyl, silyl and germyl residues, substituted or not with one or more halogen, amido, phosphido, alkoxy, or aryloxy radicals, and Cy is a mono- or polysubstituted cyclopentadienyl-type ligand, wherein the one or more substituents of Cy are selected from the group consisting of halogen, hydrocarbyl, silyl and germyl residues, optionally substituted with one or more halogen, amido, phosphido, alkoxy, or aryloxy residues. The invention further relates to a process for the preparation of a polymer comprising at least one aliphatic or aromatic hydrocarbyl C2-20 olefin wherein the at least one aliphatic or aromatic olefin is contacted with the catalyst system of the present invention.