Polyester urethanes with multiblock sequence structure and alkynyl-functionalized side chains


Thermoplastic polyester urethanes (TPU) are typically synthesized by copolymerization of dihydroxy-terminated polyester-based “macrodiols” and diisocyanate components. A second diol component of low molar mass (so called "chain extenders") may be added to the polymerization to increase the overall molecular weight of the TPU and to form oligourethane segments. In absence of functionalized side chains, means for post-functionalization of the TPU backbone are limited and often non-specific. An efficient way to introduce defined motifs for specific covalent functionalization of the TPU backbone is utilizing chain extenders that bear non-nucleophilic functional side chains. A structurally simple alkynyl-functionalized diol, 3-(prop-2-yn-1-yloxy)propane-1,2-diol (YPD), is presented as component for the synthesis of covalently functionalizable polyester urethanes (PEU) with multiblock sequence structure. YPD can be synthesized in two steps from commercially available materials on multi-gram scale. PEUs with Mn of 22 to 87 kgꞏmol-1 (GPC, universal calibration) were successfully synthesized in one pot from dihydroxyterminated poly(ε-caprolactone) (PCL) with different molecular weights, L-lysine ethyl ester diisocyanate (LDI) or trimethyl(hexamethylene)diisocyanate (TMDI), and YPD. 1H/13C-NMR spectroscopy confirmed the chemical composition of the materials. Model reactions to couple azides to the alkyne moieties of the PEUs via copper(I)- catalyzed azide-alkyne-cycloaddition reactions (CuAAC, “click chemistry”) using benzyl azide under various conditions were investigated, allowing conversions of >95% (1H-NMR) of the alkyne moieties with yields of up to 94% for the purified functionalized PEUs. The optimized conditions (40 °C, THF/t-BuOH/H2O [2.5:2.5:1] as solvent mixture, acetic acid as additive) were employed to successfully couple a GRGDS-peptide bearing an azide-functionalized aliphatic linker with 50% degree of functionalization of the polymer. The obtained results show that YPD can serve as versatile addition to the pool of available diol-components for the synthesis of functionalized multiblock PEUs, as the alkynyl moieties are highly suitable for further functionalization, e.g. the coupling of biomolecules under aqueous and fairly mild conditions.
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