Introduction of Self-Healing and Recyclable Properties into Functionalized Polyisoprene Rubber via Thiol–Ene Reaction

Abstract

Covalently cross-linked rubbers face persistent sustainability challenges due to their irreversible networks hindering recycling, while polarity mismatch complicates the incorporation of additional self-healing materials into vulcanization-free cis-1,4-polyisoprene (PI). To advance the sustainable development of functionalized PI with additional new features while promoting the elasticity and mechanical properties, our group proposes a straightforward one-step free radical-mediated thiol–ene reaction using l-cysteine (LC) as a biodegradable compound that bears three main functional groups consisting of thiol, carboxylic acid, and amine. The thiol group is covalently attached to the PI double bonds via free radical thiol–ene chemistry, while the carboxylic acid and amine groups facilitate noncovalent cross-linking through dynamic hydrogen bonds. As the LC content increases, the functionalized PI-LC-X (with X = 10, 30, and 50 denoting the percentage of LC units attached to the PI double bonds) exhibits a synergistic enhancement in the mechanical strength and elasticity. Among them, PI-LC-30 represents the optimal performance in self-healing ability, achieving 100% recovery of toughness at room temperature along with excellent recyclability through acid hydrolysis. This outstanding behavior is attributed to the well-controlled ideal radical thiol–ene reaction (anti-Markovnikov addition), which prevents unwanted chain extension or interchain cross-linking and preserves the linear structure of PI. Maintaining this structural integrity is vital for recyclability, as acid hydrolysis selectively disrupts the reversible hydrogen bonds while keeping the covalent thioether linkages intact, enabling the regeneration of PI-LC-X films with properties closely matching the original material. This strategy effectively addresses polarity mismatch and recyclability challenges, offering a sustainable pathway for functionalizing diene rubbers.