Described herein are copolymers derived from certain perfluorinated sulfonyl fluoride monomers, cyclic or cyclizable perfluorinated monomers, and one or both of ethylene and/or vinyl fluoride. Group 11 metal sulfonate ionomers of these copolymers, especially silver ionomers, are useful in membranes which separate olefins from alkanes.

Improved membranes which can separate alkanes and alkenes, and have at least one layer of a Group 11 metal ionomer, are made by exposing the ionomer to a strong Bronsted acid having a pKa of about 1 or less. The membranes have longer useful lives and have better initial permeances of alkenes.

Ionomers of Group 11 metals which include repeat units from vinylidene fluoride, can separate alkenes from alkanes when comprising a layer of a membrane. These membranes have excellent permeability and/or selectivity for alkenes in the separation of alkenes from alkanes.