Tuning material properties of covalent adaptable networks containing boronate-TetraAzaADamantane bonds through systematic variation in electron density of ring substituents

An outstanding challenge in modern society remains how to make crosslinked polymers (thermosets) more recyclable. A breakthrough solution to this challenge has been the introduction of dynamic covalent bonds in polymer networks, yielding covalent adaptable networks (CANs). Ongoing research is focused on finding new suitable dynamic covalent chemistries and on how to tune the material properties of CANs derived from these new chemistries. Here, we first compare two different dynamic boronic acid based covalent adaptable networks, namely, a conventional boronate-diol and a novel boronate-TetraAzaADamantane (TAAD) system. We show that incorporating boronate-TAAD bonds in networks results in stiffer materials, as seen in a slower relaxation and higher shear and storage moduli. This offers access to more mechanically robust boronate-based materials, compared to conventional boronate-based gels. Next, we investigate the effect of molecular tuning via the electron density of meta-positioned ring substituents on the macroscopic material properties for the boronate-TAAD network. By comparing relaxation experiments on materials with different substituents, we show that the macroscopic network relaxation can be tuned through the Hammett parameter of the meta-substituent and the activation energy of the boronate-TAAD exchange. This enables subtle control over the (dynamic) material properties of these novel, robust boronate-based networks.