A strong cationic Brønsted acid, [H(OEt₂) ₂][Al{OC(CF₃)₃}₄], as an efficient initiator for the cationic ring-opening polymerization of 2-alkyl-2-oxazolines

In this contribution, the cationic ring-opening polymerization (CROP) and copolymerization of 2-ethyl-2-oxazoline and 2-tert-butyl-2-oxazoline using a strong cationic Brønsted acid, [H(OEt₂)₂] [Al{OC(CF₃)₃}₄], as an initiator are described. First, various poly(2-ethyl-2-oxazoline) (PEtOx) samples are prepared and the living/controlled character of the reaction is demonstrated. We could show that the microwave-assisted CROP of EtOx using this initiator system proceeds faster if compared to classical initiators such as methyl tosylate. These results were then extended to the CROP of poly(2-tert-butyl-2-oxazoline) (PtButOx) and to PEtOx/PtButOx random and block copolymers of different compositions. The resulting materials were characterized using spectroscopic (¹H-NMR, FT-IR), chromatographic (SEC), and thermoanalytic techniques (DSC, TGA). Although samples containing more than 13 wt% PtButOx were insoluble in common organic solvents, thermogravimetric (TGA, DSC), spectroscopic (IR), scattering methods (wide-angle X-ray scattering, WAXS), and SEC in hexafluoro-iso-propanol (HFIP) hinted at the successful formation of block copolymers. In particular, WAXS revealed increasing crystallinity for samples containing higher weight fractions of PtButOx.