A polymer film obtainable by curing a composition comprising a compound of Formula (I) wherein: R is vinyl, epoxy C.sub.1-3-alkylenethiol: n has a value of 1 or 2; m has a value of 1, 2 or 3; M.sup.+ is a cation; wherein X is as defined in the claims; and wherein the molar fraction of the compound of Formula (I) in relation to all curable compounds in the composition is greater than 0.25. Also claimed are compositions, processes membranes and their uses. ##STR00001##