Controlled Synthesis of Lipoate Homopolymers via Reversible Addition-Fragmentation Chain Transfer Polymerization.

Polylipoates (PLp), derived from α-lipoic acid, are promising polymers for developing biocompatible, stimuli-responsive, and fully (closed-loop) recyclable materials. However, their synthesis is hindered by two key challenges: the high propensity of lipoate propagating radicals to undergo backbiting during polymerization, and the tendency for polymers to spontaneously depolymerize due to a low ceiling temperature. In this study, we demonstrate that reversible addition-fragmentation chain transfer (RAFT) polymerization overcomes these challenges and can be used to synthesize PLp homopolymers with a high degree of control. This was confirmed by a linear relationship between molecular weight (Mn) and monomer conversion, as well as first-order polymerization kinetics, characteristics not achievable with conventional radical polymerization. By adjusting the RAFT agent feed ratio, the Mn of homopolymer PLps was precisely controlled with an Mn ranging from 3.6 to 62.6 kg mol-1. RAFT polymerization provided stable end-groups that effectively suppressed the spontaneous depolymerization of PLp. Polymers synthesized using RAFT agents remained intact for over 2 weeks in both solution and bulk, while those prepared under traditional radical conditions showed substantial degradation. Moreover, the trithiocarbonate end-group enabled light-triggered, on-demand depolymerization back to the original monomer. RAFT was also successfully extended to the synthesis of degradable block copolymers. Together, these results demonstrate that RAFT offers a simple, accessible, and proven strategy to address key challenges in PLp synthesis and long-term stability.