Cyclic di- and polysulfides readily undergo radical ring-opening polymerization, which leads to the formation of macromolecules with multiple reductively degradable disulfide groups in the backbone. Lipoic acid is a naturally occurring cyclic disulfide, which is of particular interest as a monomer for the preparation of linear, branched, and network biocompatible and biodegradable polymers. For example, the thermal radical copolymerizations of mono- and dilipoate esters (e.g., ethyl lipoate and the sugar-derived isosorbide dilipoate), conducted in the presence of radical initiators, affords gels, which degrade in the presence of reducing agents, such as thiols and phosphines. Importantly, the copolymerizations can be carried out in miniemulsion and the obtained nanoparticles can be employed as carriers of various substances (dyes, drugs), which are released upon reduction. The radical ring-opening polymerization of ethyl lipoate is reversible and is characterized by a ceiling temperature of 137 <sup>o</sup>C. Due to the reversibility of the polymerization, polylipoates degrade upon heating with formation of the parent monomers. Further, lipoic acid and its derivatives copolymerize easily with vinyl monomers, e.g., acrylates. When a lipoate and acrylate moieties are present in the same molecule, its radical polymerization affords partially biodegradable branched and eventually network polymers, containing both disulfide and thioether bonds in the backbone. Systematic kinetic studies of the radical ring-opening polymerization of lipoates will be presented, along with some applications of the synthesized disulfide-containing polymers.

Ring-opening radical polymerization of cyclic disulfides as a tool for the synthesis of (bio)degradable polymers

ACS Spring Meeting 2021

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