Tuning the Viscoelastic Properties of Dioxazaborocane Vitrimers via Chemical Design of the Functional Groups

Vitrimers are polymer networks that can rearrange their topology via thermally triggered dynamic covalent bonds. While their versatility offers interesting paths for developing new applications, one needs first to understand how their dynamics depends on the topology of the network to exploit their large potential. These past few years, boronic esters, especially dioxaborolanes, have been widely studied as moieties to create reversible covalent bonds. Recently, dioxazaborocanes, moieties featuring a labile N–B bond, have also been proposed, showing much faster dynamics at high temperatures compared to dioxaborolanes when incorporated in vitrimers. Herein, we study the viscoelastic properties of vitrimers obtained from linear precursors bearing complementary dioxazaborocane and dioxaborolane moieties. By playing with the density of these two different moieties, we can accurately control the proportion of free functional groups present in the vitrimers and therefore study their influence on the dynamics of the reversible network. It is found that the network dynamics can be either slowed down or accelerated in the presence of dioxaborolane or dioxazaborocane free functional groups, respectively, regardless of the total density of functional groups. The results also show a nonmonotonic dependence of the network relaxation time as a function of the cross-linking density, which we attribute to the antagonistic effects of the subdiffusion process of the dynamic moieties and the restricted mobility of the polymer chains within the network.