Cross-linking polybutadiene rubber via a thiol-ene reaction with polycysteine as a degradable cross-linker

Covalently cross-linked rubber materials are widely used in elastic applications due to their excellent mechanical properties. However, the irreversibility of covalent cross-linking suffers from poor material recyclability. As a degradable polypeptide-based cross-linker for polybutadiene, poly(l-cysteine) (polyCys) was synthesized via papain-catalyzed chemoenzymatic polymerization. The resulting polyCys had intact thiol groups that cross-linked polybutadiene via the thiol-ene reaction. The cross-linking reaction of polybutadiene was performed in the presence of polyCys and a radical initiator and resulted in insoluble polybutadiene gel formation. Based on Raman spectroscopy analysis, the cross-linking reaction was confirmed by the consumption of thiol groups of polyCys. From the dynamic viscoelastic analyses of the cross-linked polybutadienes, the viscoelasticity drastically changed from that of raw polybutadiene, with the disappearance of the slow relaxation mode at low frequencies. The complete network formation was confirmed by E ' and E '' showing the power law over the whole frequency range, as determined by the time-course experiments of the dynamic viscoelastic properties. The cross-linked polybutadiene was degradable by acid hydrolysis, resulting in the regeneration of soluble polybutadiene. Polypeptide cross-linked polybutadiene materials are promising candidates for the application of polybutadiene-based rubber materials with the requirements of both material integrity and reusability. A degradable cross-linked polybutadiene was synthesized using a peptide-based cross-linker. As a degradable cross-linker, we synthesized poly(L-cysteine) (polyCys) by papain-catalyzed chemoenzymatic polymerization. The thiol-ene reaction between polybutadiene and polyCys was conducted in the presence of a radical initiator, resulting in the formation of an insoluble polybutadiene gel. The successful cross-linking and network formation was confirmed by Raman spectroscopy and viscoelastic analysis. Dynamic viscoelastic measurements of the cross-linked polybutadiene revealed the disappearance of slow relaxation mode. The polyCys-linked polybutadiene underwent degradation through acidic hydrolysis, allowing the recovery of soluble polybutadiene.