A combined computational and experimental study of metathesis and nucleophile-mediated exchange mechanisms in boronic ester-containing vitrimers

The rheological properties of vitrimer materials are largely controlled by the kinetics of covalent bond exchange and therefore a mechanistic understanding of bond exchange can provide insights into how the performance of vitrimers can be improved. In this work, density functional theory (DFT) calculations are combined with experimental measurements in order to understand the nature of the exchange reaction between boronic esters. Quantum chemical calculations show that both a metathesis and a nucleophile-mediated pathway are possible, although a significant increase in the rate of exchange in the presence of a nucleophile is observed. The DFT studies are supported by kinetic measurements of the exchange of small molecule dioxaborinanes in the presence and absence of nucleophile. Finally, the accelerating effect of nucleophiles is demonstrated in vitrimeric materials synthesized using a dioxaborinane-based crosslinker and a hydroxyl functional monomer as internal catalyst to produce vitrimers with significantly reduced relaxation times. The mechanism of bond exchange in boronic-ester containing vitrimers is shown to occur by either direct metathesis or a nucleophile mediated mechanism, which can be used advantageously to give orders of magnitude increase in exchange dynamics.