On the identification and quantification of proton-initiated species in the synthesis of poly(2-alkyl-2-oxazoline)s by high resolution liquid chromatography

Abstract

Hydrophilic poly(2-oxazoline)s represent a promising alternative to replace poly(ethylene glycol) in the biomedical field. For that purpose, reliable analytical protocols to confirm identity and quantity of impurities are required. In particular, side products deriving from chain transfer reactions occurring during the cationic ring-opening polymerization and incomplete end-capping processes may be present. The analytical approach must hence be capable of separating polymers according to minor changes regarding their end group. We demonstrate that liquid chromatography, relying on a monolithic C18-modified silica column and isocratic as well as gradient elution using water / acetonitrile mixtures and varying detectors, can accomplish such demanding high resolution separations. Poly(2-ethyl-2-oxazoline)s (PEtOx) with acetyl, hydroxyl, and phthalimide ω-end groups were investigated. Identification of side products was achieved through coupling with electrospray ionization mass spectrometry. UV / Vis detection was applied to quantify chain transfer products in PEtOx comprising biphenyl moieties. In addition, gradient elution enabled the separation of PEtOx into macromolecules according to their specific degrees of polymerization in molar mass ranges around 2,000 g mol^-1.

Keywords: Cationic ring opening polymerization (CROP); Chain transfer reactions; Electrospray ionization mass spectrometry; Liquid chromatography on monolithic silica rods; Poly(2-oxazoline)s.