A terpolymer containing propylene, ethylene and 1-hexene obtainable by the step of copolymerizing propylene, ethylene and 1-hexene in the presence of a catalyst system comprising the product obtained by contacting the following components: (a) a solid catalyst component comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond and at least two electron donor compounds one of which being present in an amount from 40 to 90% by mol with respect to the total amount of donors and selected from succinates and the other being selected from 1,3 diethers, (b) an aluminum hydrocarbyl compound, and (c) optionally an external electron donor compound, wherein in the terpolymer (i) the content of 1-hexene derived units ranges from 0.5 to 5.0 wt%; (ii) the content of ethylene derived units is higher than 1.4 t% and fulfils the following relation (1): C2