Understanding the Behaviour of Sulphur-Centred Radicals During Polymer Self-Healing

The high-level composite ab initio G3(MP2)-RAD method has been used to study the self-healing mechanism of materials based on thiuram disulfides and their derivatives (S=C(Z)SSC(Z)=S, for Z = CH₃, NEt₂, N(Et)CH₂CH₂OH, Ph, B_z), and the effects of these Z-substituents on their efficacy. The relative contributions of cross-over and reversible addition fragmentation chain transfer reactions were ascertained, and the likelihood of chain-breaking side reactions was assessed. To rationalize the results, the various stabilisation energies of the radicals and closed-shell species were also evaluated. The study revealed that the self-healing mechanism of thiuram disulfides follows predominantly the cross-over reaction because of the high energies of intermediate radicals in the chain transfer mechanism. Based on the study, the most effective self- healing materials are predicted to contain amines as Z-groups, while those containing benzyl and its derivatives are most likely to undergo side reactions.