Biodegradability of Acrylate-Lipoic Acid Copolymers

Lipoic acid has attracted a great deal of attention for its ability to impart chemical degradability into vinyl polymer chains. This work investigates the biodegradability of acrylate-lipoic acid copolymers to understand how sulfide bonds along the backbone of these copolymers impact biodegradation. To achieve this objective, several acrylate monomers (methyl, ethyl, and n-butyl acrylates) were polymerized with lipoic acid (1, 3, 5, 10, and 15 mol %) via reversible addition–fragmentation chain transfer (RAFT) polymerization, and biodegradation was assessed via a high-throughput clear-zone polymer biodegradation method using Paucimonas lemoignei (P. lemoignei) and three other bacterial species. Mass loss and hydrolysis tests were also conducted in liquid culture with liquid chromatography-mass spectrometry for degradation product analysis. The acrylate homopolymers did not show any biodegradation, whereas poly(methyl acrylate-co-lipoic acid) and poly(ethyl acrylate-co-lipoic acid) copolymers showed biodegradation by P. lemoignei above a critical lipoic acid mole fraction that increased with increasing hydrophobicity of the acrylate monomer. While degradation is clearly accelerated by the formation of labile linkages in the polymer backbone resulting from the incorporation of sulfur atoms from lipoic acid, experiments with methyl acrylate oligomers show that small acrylate fragments are also degraded by P. lemoignei and that small molecule products are produced below the molar mass of the acrylate fragments. Therefore, this work illustrates that the incorporation of lipoic acid provides a route toward fully biodegradable polyacrylates.