A process for preparing 1-butene polymers comprising polymerizing 1-butene and optionally ethylene, propylene or higher alpha-olefin, in the presence of a catalyst system obtainable by contacting:a) metallocene compound of formula (I): wherein: M is a transition metal; p is an integer from 0 to 3; X, same or different, is a hydrogen atom, a halogen atom, or a hydrocarbon group; L is a divalent C1-C40 hydrocarbon radical; R1 is a C1-C40 hydrocarbon radical; T1, is a moiety of formula (IIa) or (IIb): wherein R2 and R3, are C1-C40 hydrocarbon radicals or they can form together a C3-C7-membered ring; R4 is C1-C40 hydrocarbon radicals; T2 and T3, are a moiety of formula (IIIa) or (IIIb): wherein R6 and R7, equal to or different from each other, are hydrogen atoms or C1-C40 hydrocarbon radicals; R5 is a hydrogen atom or a C1-C40 hydrocarbon radicals; with the proviso that if T1 is a moiety of formula (IIa) at least one between T2 and T3 is a moiety of formula (IIIb), and if T1 is a moiety of formula (IIb) at least one between T2 and T3 is a moiety of formula (IIIa); and b) at least an alumoxane or a compound able to form an alkylmetallocenecation.