A propylene, ethylene, 1-butene terpolymer containing from 0.5 wt % to 2.2 wt % of ethylene derived units and from 6.0 wt % to 20.0 wt % of 1 butene derived units; wherein: i) the ratio C2 wt %/C4 wt % ranges from 0.09 to 0.06; wherein C2 wt % is the weight per cent of ethylene derived units and C4 wt % is the weight per cent of 1-butene derived units; ii) the Melt flow rate ranges from 0.4 to 54 g/10 min; iii) the xylene soluble fraction at 25° C. is lower than 15.0 wt % the minimum value being 5.0 wt %.