Delocalized π-electrons in 2-oxazoline rings resulting in negatively charged nitrogen atoms: revealing the selectivity during the initiation of cationic ring-opening polymerizations

The inventory of the single‐crystal X‐ray structures of aliphatic and aromatic 2‐oxazolines, namely 2‐nonyl‐2‐oxazoline, 2,2′‐tetramethylenebis(2‐oxazoline) and 2‐phenyl‐2‐oxazoline, reveals significant delocalization of π‐electrons along the NCO segment. The delocalization of π‐electrons is stabilized by inductive and resonance contributions of the side‐chains; in 2‐phenyl‐2‐oxazoline, also π‐arene interactions between the benzene ring and the CN and the CO bond stabilize the crystalline phase. This delocalization gives a partial negative charge to the nitrogen atom and a partial positive charge to the oxygen atom. The partial negative charge of the nitrogen atom makes this atom the exclusive reaction partner also for highly reactive non‐selective cations, which explains the regioselectivity of electrophilic attacks in cationic ring‐opening polymerizations. Copyright © 2011 Society of Chemical Industry Aliphatic as well as aromatic 2‐oxazoline monomers, the side‐chains of which do not bear any heteroatoms, exhibit significant π‐electron delocalization along the C‐N‐O segment.