Lipoic acid/ethyl lipoate as cleavable comonomers for synthesis of degradable polymer networks

Abstract

α-Lipoic acid, and its derived ester ethyl lipoate, can copolymerise with n-butyl acrylate to install labile disulfide bonds within the polymer backbone. Covalently crosslinked gel networks containing these comonomers were synthesised by conventional radical polymerisation (FRP) and reversible addition fragmentation chain transfer (RAFT) polymerisation. Gels synthesised by both methods and using both comonomers could be degraded by thiol-disulfide exchange or heating in DMF to form soluble polymer fragments. The critical comonomer loading for degradation was lower for RAFT-synthesised gels due to their more homogenous network structure. As these fragments were thiol functional, they could be oxidised in air with a base catalyst to reform a solid network. However, the presence of the carboxylic acid and the relatively low dispersity of the fragments act to prevent regelation. Therefore, only the gels containing the minimum amount of ethyl lipoate synthesised by RAFT could successfully regel as these fragments had no acid functionality and the highest dispersity value. Furthermore, cleavage of the trithiocarbonate end-groups aids reformation due to the presence of additional thiols. We suggest that uniform comonomer incorporation leading to lower dispersity of the degraded fragments can be detrimental for the efficient reformation of the degraded network. However, the large amounts of the lipoate comonomer allow the dynamic exchange properties of the disulfide bonds within the polymer backbone, in the presence of DBU catalyst, to impart the networks with self-healing ability with no external pressure or heat.