1-butene/ethylene/propylene terpolymer containing: c) from 4 to 10 wt % of ethylene derived units, and d) from 2 to 10 wt % of propylene derived units; the said terpolymer having the following properties: distribution of molecular weights (Mw/Mn) measured by GPC lower than 3; no melting point detectable at the DSC; melting enthalpy, measured after 10 days of aging at room temperature, from 12 to 20 J/g.