Copolymerization of ethylene oxide and a series of glycidyl ethers with precise linear/cyclic oligo(ethylene oxide) side-chains allows access to a library of well-defined poly(ethylene oxide) (PEO)-based copolymers with 2-50 mol % side-chain functionality. The structure of the linear and cyclic oligo(ethylene oxide) glycidyl ether monomers influences the apparent reactivity ratios, which were consistent with gradient-type architectures. From these isomeric side-chain materials, the solution phase behavior, semicrystallinity, and ionic conductivity of the linear and crown ether side-chain functional copolymers were characterized and compared to conventional, linear PEO homopolymers. Incorporation of linear side-chains decreased the cloud point, whereas incorporation of the crown ether side-chain functionality did not affect the cloud point when compared to linear PEO. In contrast, an ion-selective response of the cloud point was observed for the crown ether side-chain copolymers with minimal response for the isomeric linear systems. In the solid state, the copolymer materials could be tuned from semicrystalline to completely amorphous as the incorporation of linear or cyclic comonomers was increased. Ionic conductivity was characterized by using both lithium and sodium bis(trifluoromethylsulfonyl)imide and found to be consistent with amorphous PEO-based copolymer materials. Linear side-chain functional copolymers exhibited significantly higher ionic conductivity than their crown ether side-chain analogues due to the lower T-g of corresponding salt/copolymer blends. Finally, a readily available linear oligo(ethylene glycol) monomer based on a commercial starting material was prepared and copolymerized with ethylene oxide to give a copolymer with properties comparable to the corresponding precise analogues. These new structures allow for the development of property-tailored hydrophilic PEO-based copolymers for a variety of materials applications.