Highly conductive solid polymer electrolytes were generated by blending linear poly(ethyleneimine)-graft-poly(ethylene glycol) with linear poly(ethyleneimine) bearing lithium N-propylsulfonate groups as the lithium source. The effect of polymer backbone structure on Li+ conductivity was determined by comparing a series of blends made from the PEI-based materials with those from polymethacrylate backbone analogues. The use of PEI backbones promoted ion-pair dissociation, stabilized the macromolecular mix and generated blends with ionic conductivities up to 2 orders of magnitude higher than those of the polymethacrylate-based systems. Blends containing the PEI-bound lithium sulfonates exhibited lithium conductivities higher than those measured for PEG doped with lithium bis(trifluoromethyl)sulfonimide. Shifts in the v(s)(SO3) IR absorption band suggest that the solvation environment for the lithium sulfonates changes with polymer structure. The PEI-based blends are thermally stable up to 200 degrees C , electrochemically stable in the +/- 5 V range, and showed unparalleled ionic conductivities (0.4 mS/cm at room temperature and 5 mS/cm at 80 degrees C) for solvent-free systems with polymer-bound anions.