The stability of enamine crosslinks formed from acetoacetate/amine in synthetic hydrogels

This paper examines the reversible enamine crosslinking reaction between poly-l-lysine (PLL) and poly (methacrylic acid-co-2[methacryloyloxy]ethyl acetoacetate) (70:30 mol%, A70), used to form crosslinked networks for cell encapsulation. Comparison of polymeric and small-molecule reactants showed that the equilibrium formation of enamine from amine and acetoacetate in water [0.02 M each] increased from 1 to 12 mol% upon going from small molecules to a polymer (A70) – oligomer (oligoPLL, 1–5 kDa) model. This increase is attributed to higher local reactant concentration and the formation of a hydrophobic microenvironment, both the result of polyelectrolyte complexation. While enamine formation reached equilibrium levels within hours, storage of both enamine-functional linear polymers, and enamine-crosslinked network polymers, in aqueous media at 40 °C led to loss of enamine with an apparent half-life of about two weeks. This slow loss of enamine is attributed to hydrolysis of acetoacetate ester linkages in the A70, which causes a corresponding decrease of the polymer-bound acetoacetates available to participate in the equilibrium crosslinking reaction. Accordingly, calcium alginate capsules reinforced with an enamine-crosslinked A70/PLL network lost their covalent reinforcement after storage under physiological conditions for several weeks. Breakdown occurred after two weeks for a covalent network made with low MW PLL (1–5 kDa) while one made with higher MW PLL (15–30 kDa) was still intact after one month. This loss of enamine crosslinks driven by hydrolysis of the acetoacetate component leads to a mechanism for decrosslinking covalent A70-PLL networks on a timescale that would allow encapsulated cells to generate their own extracellular matrix.