A thermoplastic polymer comprising a fluorochemical ester composition of formula I: C.sub.4F.sub.9SO.sub.2N(Q.sup.1)(CH.sub.2).sub.a--O--C(O)--(CH.sub.2).sub- .n--C(O)O(CH.sub.2).sub.bNQ.sup.2SO.sub.2C.sub.4F.sub.9 (I) wherein Q.sup.1 and Q.sup.2 which may be the same or different are each a hydrogen or an alkyl group of 1 to 4 carbon atoms; a is an integer from 2 to 11; b is an integer from 2 to 11; and n is at least 10.