Abstract
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.
