Chemistry of 2-oxazolines is involved in the polymer synthesis fields of cationic ring-opening polymerization (CROP) and enzymatic ring-opening polyaddition (EROPA), although both polymerizations look like a quite different class of reaction. The key for the polymerization to proceed is combination of the catalyst (initiator) and the design of monomers. This article describes recent developments in polymer synthesis via these two kinds of polymerizations to afford various functional polymers having completely different structures, poly(N-acylethylenimine)s via CROP and 2-amino-2-deoxy sugar unit-containing oligo and polysaccharides via EROPA, respectively. From the viewpoint of reaction mode, an acid-catalyzed ring-opening polyaddition (ROPA) is considered to be a crossing where CROP and EROPA meet. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:1251-1270, 2010