Polymer compositions with an end-capped polyether-polyester copolymer, a transition metal catalyst and an active material that acts on, interacts or reacts with moisture. The polyether-polyester copolymer has (i) polyether segments where at least one polyether segment contains at least one polytetramethylene oxide segment; (ii) polyester segments; (iii) bridging elements of the structure —CO—R2-CO—, where R2 represents an optionally substituted bivalent hydrocarbon residue consisting of 1 to 100 carbon atoms; and (iv) one or two end-caps R1-O—(C2-C4-O—)e-*, where R1 is an optionally substituted hydrocarbon residue and e is an integer of from 0 to 1000. The polymer composition is capable of scavenging oxygen and can be used in compositions and articles to remove oxygen or to function as an active oxygen barrier.