Radical ring‐opening polymerization of lipoates: Kinetic and thermodynamic aspects

Abstract

The radical ring‐opening polymerization of a lipoate‐based monomer, ethyl lipoate, in bulk and in solution was studied at various temperatures and it was found that in all cases, only limited (plateau) conversions were reached, which were lower at higher temperatures and/or at higher dilutions. It was established that a monomer‐polymer equilibrium exists with a corresponding ceiling temperature of 139°C. Due to the reversibility of the lipoate polymerization, when poly(ethyl lipoate) was heated to 150°C, it degraded and within 3 h, the molecular weight decreased to less than 15% of the initial value. Likewise, when the polymer was dissolved in anisole and a radical initiator was added, degradation was observed even at 60°C and it became increasingly pronounced at higher concentrations of the radical source. Due to the presence of multiple disulfide groups in the backbone, poly(ethyl lipoate) also degraded in the presence of reducing agents, such as tributylphosphine, yielding the reduced (dithiol) form of the monomer, ethyl dihydrolipoate.

Light-Mediated Synthesis and Reprocessing of Dynamic Bottlebrush Elastomers under Ambient Conditions

“…The depolymerized mixture can be recured under UV light resulting in a solid material, although the resulting modulus is lower than the original sample (Figure S41). This reactive nature of disulfides can be exploited at lower temperatures to intentionally degrade the backbone of LA-derived bottlebrush elastomers under basic or reducing conditions (Figure a). , To test the degradability of these materials, a cured sample was placed in tetrahydrofuran (THF) containing sodium borohydride (NaBH 4 ) or in an aqueous solution of potassium hydroxide (KOH). See the SI for details.…”

“…In contrast to UV light, heating samples to 180 °C results in liquification (Figure S36) that is related to the relatively low ceiling temperature of polymer chains based on LA repeat units (Figure S37). 47 After thermal depolymerization, ∼30−40% of the sample transforms back into the original macromonomer (Figures S38−S40). The depolymerized mixture can be recured under UV light resulting in a solid material, although the resulting modulus is lower than the original sample (Figure S41).…”

Renewable and recyclable covalent adaptable networks based on bio-derived lipoic acid

“…Thioctic acid, or lipoic acid, is a naturally occurring carboxylic acid that contains a 5-membered cyclic disulfide which can be polymerised into a polydisulfide by ring-opening polymerisation (ROP). 24 The ROP of various strained cyclic disulfides to afford linear polydisulfides is well established and versatile as exemplified by numerous reaction pathways -such as thermal 25 , radical 26,27 or anionic (via thiolate) [28][29][30][31][32][33][34] . An exemplary feature of strained cyclic disulfides is their dynamic nature that results from facile thiol-disulfide exchange [35][36][37][38] and has been exploited to reversibly depolymerise (or chemically recycle) some of the aforementioned polydisulfides 27,[29][30][31]33 or to create functional supramolecular cyclic structures 39 .…”

Recent advances in the ring-opening polymerization of sulfur-containing monomers

“…In this regard, Raeisi and Tsarevsky identified ethyl lipoate ( M91 ), a 5-membered cyclic disulfide derived from naturally occurring lipoic acid ( M90 ), as a promising monomer to synthesize degradable polymers by radical polymerization (Scheme 32). 128 The rROP homopolymerization of M90 was studied at various temperatures in both bulk and solution, but monomer conversion was observed to plateau in all cases, with the plateau being lower at higher temperatures and/or higher dilutions. At higher temperatures, the molecular weight distributions of the synthesized polymers ( polyM90 ) were reported to be in a narrow range due to the fast thiyl radical-disulfide exchange reactions.…”