Molecular weight distribution and endgroup functionality of poly(2-ethyl-2-oxazoline) prepolymers

Cryogels Based on Poly(2-oxazoline)s through Development of Bi- and Trifunctional Cross-Linkers Incorporating End Groups with Adjustable Stability

1,4-Bis(iodomethyl)benzene and 1,3,5-tris(iodomethyl)benzene were used as initiators for the cationic ring-opening polymerization (CROP) of 2-ethyl-2-oxazoline (EtOx) and its copolymerization with tert-butyl (3-(4,5-dihydrooxazol-2-yl)propyl)carbamate (BocOx) or methyl 3-(4,5-dihydrooxazol-2-yl)propanoate (MestOx). Kinetic studies confirmed the applicability of these initiators. Termination with suitable nucleophiles resulted in two- and three-armed cross-linkers featuring acrylate, methacrylate, piperazine-acrylamide, and piperazine-methacrylamide as polymerizable ω-end groups. Matrix-assisted laser desorption/ionization mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy confirmed the successful attachment of the respective ω-end groups at all initiation sites for every prepared cross-linkers. Except for acrylate, each ω-end group remained stable during deprotection of BocOx containing cross-linkers. The cryogels were prepared using EtOx-based cross-linkers, as confirmed by solid-state NMR spectroscopy, scanning electron microscopy, and thermogravimetric analysis. Stability tests revealed a complete dissolution of the acrylate-containing gels at pH = 14, whereas the piperazine-acrylamide-based cryogels featured excellent hydrolytic stability.

Poly(2-ethyl-2-oxazoline) Featuring a Central Amino Moiety

The incorporation of an amino group into a bifunctional initiator for the cationic ring-opening polymerization (CROP) is achieved in a two-step reaction. Detailed kinetic studies using 2-ethyl-2-oxazoline demonstrate the initiators' eligibility for the CROP yielding well-defined polymers featuring molar masses of about 2000 g mol^-1 . Deprotection of the phthalimide moiety subsequent to polymerization enables the introduction of a cyclooctyne group in central position of the polymer which is further exploited in a strain-promoted alkyne-azide click reaction (SpAAC) with a Fmoc-protected azido lysine representing a commonly used binding motif for site specific polymer-protein/peptide conjugation. In-depth characterization via electrospray ionization mass spectrometry (ESI) confirms the success of all post polymerization modification steps.

Copolymer chain formation of 2-oxazolines by in situ 1H-NMR spectroscopy: dependence of sequential composition on substituent structure and monomer ratios

In situ 1H NMR characterization of copolymerization reactions of various 2-oxazoline monomers at different molar ratios offers detailed insight into the build-up and composition of the polymer chains. Various 2-oxazolines were copolymerized in one single solvent, butyronitrile, with 2-dec-9'-enyl-2-oxazoline, where the double bond allows for post-polymerization modification and can function as a crosslinking unit to form polymer networks. The types of the monomers and their molar ratios in the feed have a strong effect on the microstructure of the forming copolymer chains. Copolymers comprising 2-dec-9'-enyl-2-oxazoline and either 2-ethyl-, 2-isopropyl-, 2-butyl-, 2-heptyl, 2-nonyl- or 2-phenyl-2-oxazoline, show significant differences in sequential structure of copolymers ranging from block to gradient and random ordering of the monomer units. 1H NMR was found to be a powerful tool to uncover detailed oxazoline copolymerization kinetics and evolution of chain composition.