We report the development of an extrinsic, self-healing coating system that shows no fluorescence from intact coating, yellowish fluorescence in cracked regions, and greenish fluorescence in healed regions, thus allowing separate monitoring of cracking and healing of coatings. This fluorescence-monitoring self-healing system consisted of a top coating and an epoxy matrix resin containing mixed dye loaded in a single microcapsule. The dye-loaded microcapsules consisted of a poly(urea-formaldehyde) shell encapsulating a healing agent containing methacryloxypropyl-terminated polydimethylsiloxane (MAT-PDMS), styrene, a photo-initiator, and a mixture of two dyes: one that fluoresced only in the solid state (DCM) and a second that fluoresced dramatically in the solid than in the solution state (4-TPAE). A mixture of the healing agent, photo-initiator, and the two dyes was yellow due to fluorescence from DCM. On UV curing of this mixture, however, the color changed from yellow to green, and the fluorescence intensity increased due to fluorescence from 4-TPAE in the solid state. When a self-healing coating embedded with microcapsules containing the DCM/4-TPAE dye mixture was scratched, the damaged region exhibited a yellowish color that changed to green after healing. Thus, the self-healing system reported here allows separate monitoring of cracking and healing based on changes in fluorescence color.

A recyclable catalytic system for ionic polymerization under ambient conditions is still undoubtedly a challenging issue that needs to be addressed for industrial production. In this study, a metal-free cationic ring-opening polymerization of glycidol (GD) using tris(pentafluorophenyl)borane (B(C₆F₅)₃, BCF) as a catalyst affords a well-controlled branched cyclic polyglycidol (BC-PGD) structure and a recycling polymerization process was achieved using unpurified reagents and ambient conditions. Although homogeneous catalysts cannot usually be readily recycled during polymerization, the growing PGD chains in nonpolar solvents induces self-precipitation in catalyst solutions with increasing molecular weight and hydrophilicity, causing a phase separation of PGD with a uniform molecular weight distribution. Specifically, the recycling polymerization process is successfully performed by repeating the simple sequence of decantation and addition of the unpurified monomer. The unique structure of the obtained PGDs was confirmed by ¹H NMR, inverse-gated ¹³C NMR analyses, SEC, and MALDI-ToF-MS. Based on the green and recyclable BCF-catalyzed cationic ring-opening polymerization, more intriguing examples with simple and well-reproducible polymerization techniques are anticipated for challenging industrial applications.