Critical role of the steric factor in the viscoelasticity of vitrimers

Dynamic covalent networks (DCNs) have been seen as a promising solution to address plastic waste, but the mechanisms governing their viscoelasticity, especially in vitrimers, is still not well understood. In this study, we investigated the bond exchange mechanism and bond rearrangement time in boronic-ester based vitrimers. We found that while the bond rearrangement process slows down significantly upon drying, the activation energy for this process remains unchanged. This suggests that transesterification reactions are mainly responsible for bond exchange in these materials. A surprising discovery emerged from analyzing the temperature dependence of bond rearrangement times, revealing unusually large pre-factors from 10^-4 to 1s in studied samples. We ascribe it to the often-overlooked chemical steric factor that slows down bond exchange by many orders despite the low energy barrier for the bond exchange. Our view is supported by the comparable large pre-factor of boronic ester exchange reactions between small molecules. Consequently, we propose a general approach for describing viscoelastic properties of vitrimers that emphasizes the crucial role of steric factor in dynamic bond rearrangements, in addition to the traditionally considered activation energy barrier and matrix properties.