Linear Viscoelasticity of ABA-type Vitrimer Based on Dioxaborolane Metathesis

In this study, we synthesized a series of ABA-type vitrimers by crosslinking the short A moieties of precursors with a bifunctional crosslinker and evaporating the small molecular byproduct. The vitrimer samples thus prepared exhibit linear viscoelasticity dependent on the length of A moiety as well as the content of the crosslinks. When the average number of A monomers per end moiety m =1.1, the crosslinker can only extend the chain but not crosslink the chain. When m becomes 2.8 or higher, introducing a crosslinker first leads to the gelation, whereas excess in crosslinker molecules leads opening of the crosslinking sites and accordingly reentry into the sol regime. Surprisingly, a further increase in the length of the A moieties increases the relaxation time much weaker than the exponential increase seen for the physically crosslinked ABA-type ionomers. We attribute this difference to the distinct relaxation mechanisms: the relaxation of the vitrimer samples is based on relatively independent exchange reactions, which contrasts with the ABA-type ionomers that relax through the collective hopping of connected ionic groups from one ion aggregate to another.