Butene-1 terpolymers having a content of propylene derived units of 0.5-13% mol, and a content of ethylene derived units of 1-3% mol, a ratio C3/C2 of the content of propylene and ethylene derived units is of from 1 to 10, said butene-1 terpolymers having a melt flow rate MIE, measured at 190° C./2.16 Kg of from 0.3 to 3 g/10 min, and a molecular weight distribution curve determined by GPC with a ratio Mw/Mn of from 4 to 10, and the portion of molecular weights of 1×105 or lower, accounting for 22% or larger of the total area.