The separation of hydrocarbon mixtures comprising a polymerizable vinyl containing polyester of the formula I: ##STR00001## Each R and R.sup.2 is independently a C.sub.6-C.sub.20 aromatic residue or a C.sub.1-C.sub.20 aliphatic residue. Each R.sup.1 is independently hydrogen or C.sub.1-C.sub.5 alkyl; "j" is an integer in the range of 1-1000. Z is a heteroatom or two hydrogen atoms. The invention is also directed to the method of preparing the separation membrane.