Polymeric rotaxanes, consisting of crown ether macrocycles threaded onto a polyurethane backbone, were synthesized by allowing the crown ether to equilibrate with tetrakis(ethylene glycol) prior to its reaction with bis(p-isocyanatophenyl)methane (MDI). Use of 18-crown-6, which is too small to be threaded, demonstrated that (1) free macrocycles were easily removed by reprecipitation and (2) no ring opening or other side reactions of the crown ether took place. The threading efficiency, as measured by x/n, the mole ratio of the cyclics per repeat unit, increased linearly with ring size from 36- to 60-membered crown ethers and at constant ring size nonlinearly with the ratio of the cyclic to linear components in neat reactions. Up to 63% by mass of macrocycle was incorporated. The rotaxane structures were demonstrated after repeated precipitation to constant composition as determined by proton NMR using GPC analyses to establish the absence of free macrocycles. The polyrotaxanes have solution and solid-state behaviors that differ from those of the model backbone polyurethane. In solution the interactions of the crown ethers with solvent alter the hydrodynamic volume of the polymers, leading to changes that depend on the size of the crown and the proportion incorporated. In the solid state the glass transition temperature varies with crown content, and at sufficiently high loadings, crystallization of the macrocycle occurs without dethreading. These physically linked analogs of conventional covalent copolymers thus offer new avenues for control of the behavior and properties of polymeric materials.