A process for preparing 1-butene polymers, comprising polymerizing 1-butene or copolymerizing 1-butene with ethylene, propylene or an alpha-olefin of formula CHT2=CHT wherein T is a C3-C10 alkyl group, in the presence of a catalyst system obtainable by contacting: A) a metallocene compound belonging to formula (I): wherein M is zirconium titanium or hafnium; X, equal to or different from each other, is a hydrogen atom, a halogen atom, a hydrocarbon radical optionally containing heteroatoms; R4, R5, R6, R7, R8, R9, R10, R11, R12 and R13 are hydrogen atoms, or C1-C4O hydrocarbon radicals optionally containing heteroatoms; R1, R2 and R3, are linear or branched, C1-C2o-alkyl radicals, optionally containing heteroatoms: B) a Jumoxane or a compound capable of forming an alkyl metallocene cation; and optionally C) organo aluminum compound.