Controlled Synthesis of Degradable Lipoate-Vinyl Copolymers via Xanthate-Mediated Reversible Addition–Fragmentation Chain Transfer Polymerization

Incorporation of disulfide bonds into polymer backbones is an effective strategy for the synthesis of degradable polymers. In this research, controlled synthesis of degradable lipoate-vinyl copolymers by xanthate-mediated reversible addition–fragmentation chain transfer (RAFT) polymerization is investigated. Lipoates, including methyl lipoate (LpMe), butyl lipoate (LpBu), and benzyl lipoate (LpBz), are copolymerized with a series of vinyl ester monomers and N-vinyl monomers. Vinyl acetate (VAc) is used as a typical vinyl ester monomer for the RAFT copolymerization with LpMe. The polymerization kinetics study demonstrates that the xanthate-mediated RAFT system enables the controlled copolymerization of VAc and LpMe. The monomer reactivity ratios of LpMe (rLpMe) and VAc (rVAc) are calculated to be around rLpMe = 10.75 and rVAc = 0.19. Similarly, N-vinylformamide (NVF) is used as a typical N-vinyl monomer for xanthate-mediated RAFT copolymerization with LpMe. The polymerization kinetics study demonstrates the controlled synthesis of P(NVF-co-LpMe) copolymers. The monomer reactivity ratios of LpMe (rLpMe) and NVF (rNVF) are determined as rLpMe = 5.32 and rNVF = 0.19, demonstrating the ideal copolymerization behavior. Furthermore, xanthate-mediated RAFT copolymerizations are carried out between lipoates and a variety of vinyl ester monomers or N-vinyl monomers, and copolymers with low dispersities are synthesized. In addition, the xanthate-mediated RAFT strategy affords well-defined degradable diblock copolymers (DBCPs) with LpMe units in one or both block backbones. The successful synthesis and degradation of the DBCPs are confirmed by 1H NMR and size-exclusion chromatography.