A 1-butene/propylene copolymer composition having a content of propylene derived units from 4 to 10% by weight, wherein at least 50% of the polymer is present in the thermodynamically stable, trigonal form I after 100 hours at room temperature, said composition comprising: a) from 5% by weight to 95% by weight of an atactic 1-butene propylene copolymer having the following features: i) distribution of molecular weight Mw/Mn equal to or lower than 4; ii) no enthalpy of fusion detectable at a differential scanning calorimeter (DSC); and iii) infrared crystallinity lower than 0.5%; b) from 5% by weight to 95% by weight of an isotactic 1-butene propylene copolymer having the following features: i) isotactic pentads (mmmm) measured by 13C-NMR, higher than 80%; ii) melting point (Tm(II)) higher than 70° C.; and iii) distribution of molecular weight Mw/Mn equal to or lower than 4.