A new self-healing epoxy with tungsten (VI) chloride catalyst.

Using self-healing materials in commercial applications requires healing chemistry that is cost-effective, widely available and tolerant of moderate temperature excursions. We investigate the use of tungsten (VI) chloride as a catalyst precursor for the ring-opening metathesis polymerization of exo-dicyclopentadiene (exo-DCPD) in self-healing applications as a means to achieve these goals. The environmental stability of WCl6 using three different delivery methods was evaluated and the associated healing performance was assessed following fracture toughness recovery protocols. Both as-received and recrystallized forms of the WCl6 resulted in nearly complete fracture recovery in self-activated tests, where healing agent is manually injected into the crack plane, at 12wt% WCl6 loading. In situ healing using 15wt% microcapsules of the exo-DCPD produced healing efficiencies of approximately 20%.

Self-healing kinetics and the stereoisomers of dicyclopentadiene.

While original epoxy resin-based self-healing systems used the commercially available endo-isomer of dicyclopentadiene (DCPD), the exo-stereoisomer is known to have much faster olefin metathesis reaction rates with first-generation Grubbs' catalyst. Here, we measure the energy to failure of healed specimens as a function of healing time and compare the kinetics of damage repair for endo- and exo-DCPD, and mixtures of the two isomers. Using catalyst loading levels previously reported to be effective for endo-DCPD, exo-DCPD was found to heal approximately 20 times faster than the endo-isomer, but with a lower healing efficiency. The fracture toughness of the repaired specimens decreased when the exo content of the blends was greater than 40% and, for the pure exo-DCPD, when the catalyst loadings were below 1%. Possible causes of the reduced healing efficiencies of the exo-DCPD healing agent are discussed.

Radical polymerization initiated by Bergman cyclization.

The diradical generated by the Bergman cyclization of 3,4-benzocyclodec-3-ene-1,5-diyne is shown to initiate the radical polymerization of several monomers. Methacrylates are polymerized to high molecular weight by the diradical initiator much more efficiently than other monomers. The relation between the rate of polymerization and the degree of polymerization indicates that the polymer primarily propagates as a monoradical. This monoradical growth is in agreement with established theory predicting that diradical initiators can produce high polymer only through chain transfer followed by monoradical growth due to the rapid intramolecular termination of short diradical chains. In agreement with this mechanism, the polymerization rate of acrylonitrile initiated by the diradical is shown to increase by more than 20-fold upon addition of a chain transfer agent. Small molecule products consistent with intramolecular termination of diradical oligomers were isolated, and the structures of these molecules suggest how the diradical self-terminates in the absence of chain transfer.