Reversible Cross-Linking via Covalent Bonds

Significance

The authors exploit the pH-sensitive reversible reaction between phenylboronic acid (PBA) and salicylhydroxamic acid (SHA) to develop a new method for covalently cross-linking polymers. Two different systems are described, one with 2-hydroxypropylmethacrylamide (HPMA) and one with acrylic acid (AA) as the co-monomers for each of the polymers. The different backbones result in different pH-dependant behavior for the gel systems. The HPMA-based gel is a self-healing viscoelastic liquid at pH 4.2, while it is a brittle solid at pH 7.6. The AA-based gel, which features a carboxylic acid in the backbone, is a slow-flowing gel at pH 7.6. In general, more basic conditions result in the equilibrium lying strongly towards the covalently cross-linked system, and the diminished reversibility of the cross-linking reaction results in a more brittle solid.