Linear poly(ethylenimine) is a cationic polymer that is actively being progressed into clinical trials for gene therapy. The existing synthetic methodology produces a relatively broad distribution of molecular weights. We describe an improved method of polymerizing 2-alkyl-2-oxazolines as a route to linear poly(ethylenimine). By using an apolar noninterfering solvent (chlorobenzene) at low temperature (similar to 42 degrees C), the polymerization of 2-isopropyl-2-oxazoline proceeds much more rapidly than is observed in acetonitrile, and with far fewer side reactions. H-1 NMR observations showed close ion pairing at the propagating center (vice free ions in acetonitrile) to form a propagating complex of greater reactivity than free oxazolinium ions. Our results indicate that uniform or near uniform ("monodisperse") polymers can be synthesized with nominal deviation from the theoretically achievable Poisson distributions.