Topology transformation of as tar polymer to al inear polymer is demonstrated for the first time.Athreearmed star polymer possessing am echanical linking of two polymer chains was synthesized by the living ring-opening polymerization of d-valerolactone initiated by ap seudo [2]rotaxane having three hydroxy groups as the initiator sites on the wheel component and at both axle termini. The polymerization was followed by the propagation end-capping reaction with ab ulky isocyanate not only to prevent the wheel component deslippage but also to introduce the urethane moiety at the axle terminal. The resulting rotaxane-linked star polymer with afixedrotaxane linkage based on the ammonium/crown ether interaction was subjected to N-acetylation of the ammonium moiety,which liberated the components from the interaction to movet he wheel component to the urethane terminal as the interaction site,e ventually affording the linear polymer.T he physical property change caused by the present topology transformation was confirmed by the hydrodynamic volume and viscosity.Structure or topology transformation and the resulting property change often seen in molecular switches or machines have attracted much attention because of their potential applicability to the macroscopic change of appearance or properties.[1] Because such atransformation is mainly limited to small molecules,atopology-transformable polymer would be attractive for bulk property control. In comparison with linear polymers,c orresponding topological polymers, [2] such as star polymers, [3] cyclic polymers, [4] dendrimers, [5] and hyperbranched polymers, [6] generally provide smaller hydrodynamic volume,l ower solution viscosity,a nd less polymer entanglement in the bulk. These properties are useful in solid materials that can help not only in reducing solvent use as sustainable products but also making the molding process easy because of low melt viscosity.However,m ost topological polymers [7] reported to date are not transformable,b ecause they are constructed by covalent bonds.T here are af ew reports on topology reconstruction using weak and reversible interactions to change polymer properties.[8] Meanwhile,w eh ave recently reported [9] the synthesis of mechanically linked block/graft copolymers as as hape-transformable polymer in which the two polymer chains are tied by the rotaxane structure;o ne polymer chain is threaded into the crown ether wheel to which the other is connected. Because we have found that the urethane moiety placed in the rotaxane axle has some attractive interaction with the crown ether wheel, [9c] which is consistent with Gibsons reports, [10] crown ether based rotaxanes having both ammonium and urethane moieties can be regarded as two-station-type molecular switches.Namely,the rotaxane structure plays akey role as acontrollable movable linking unit, thereby being applicable to more sophisticated transformable systems.T his paper describes the synthesis of as tar polymer characterized by the rotaxane linking of the polymer ch...