Modifications of Poly(vinylphosphonates) toward Dynamic Covalent Networks

Dynamic covalent networks (DCNs) are an attractive class of materials combining the beneficial properties of chemically and physically cross-linked networks. In this study, poly(vinylphosphonates) were functionalized toward precursors capable of forming DCNs with boronic acid derivatives. Statistical copolymers from diethyl vinylphosphonate (DEVP) and diallyl vinylphosphonate (DAlVP) were obtained with high control over the polymer microstructure and narrow dispersities applying catalytic rare earth metal-mediated group-transfer polymerization (REM-GTP). Subsequent functionalization with 1-thioglycerol through photochemical thiol-ene click chemistry, confirmed via ¹H DOSY NMR spectroscopy, yielded water-soluble, diol-containing polymers that could undergo network formation with benzene-1,4-diboronic acid (DBA) in dimethyl sulfoxide and sodium tetraborate decahydrate (borax) in water. The aqueous system displayed tunable viscoelastic properties by varying the polymer functionalization degree and concentration, as evidenced by oscillatory rheology monitoring the plateau storage moduli G′ and the crossover frequencies ω_c. The self-healing capability of the novel DCNs was demonstrated on the macroscopic level and rheologically by performing ramping strain sweeps between 1% and 500% deformation. Additionally, the poly(vinylphosphonate)-based hydrogels exhibited pronounced pH-responsive behavior, allowing network formation and dissolution upon alternations of the pH value. Finally, the shear-thinning behavior of the DCNs was confirmed via rotational rheology. The extrudability of this type of hydrogel allowed for initial additive manufacturing experiments applying extrusion-based direct ink writing to yield simple printed objects.