Vitrimerization of crosslinked poly(ethylene-vinyl acetate): the effect of catalysts

Over the past decade, research interest has grown rapidly in covalent adaptable networks, called vitrimers, which can balance the processability and recyclability of thermoplastics with the performance properties of thermosets, including elastomers. While most vitrimer research focuses on generating new materials, several studies have demonstrated that permanently crosslinked networks can be transformed into covalent adaptable networks through a mechanochemical process. This finding points to an effective and efficient technical approach for upcycling waste thermoset plastics. Recent studies have demonstrated that crosslinked ethylene vinyl acetate (EVA) thermosets can be converted to vitrimers using a mechanochemical process employing a zinc-catalyzed transesterification reaction. The concept has been applied successfully to vitrimerize crosslinked EVA elastomers and foams, including shoe midsole foam, which is otherwise difficult to recycle. To investigate whether catalyst selection could be used to control the crosslink exchange kinetics and network properties of the vitrimers produced, we compare the effects of different zinc catalysts on the vitrimerization of crosslinked EVA elastomers. We use a computational chemistry approach to select zinc catalysts with different small molecule activation energies and then apply these catalysts to vitrimerize crosslinked EVA. We find that the flow activation energies measured by experiments for the dynamic network exchanges are markedly different from the activation energies predicted by simulation for small molecule exchange. Our results suggest that the dynamic exchange rates of vitrimerized crosslinked EVA elastomers depend not only on the activation barrier for small molecule exchange but also on catalyst physical properties such as the molecular size and stability at the processing temperature. DFT calculations and experiments with four zinc salts as transesterification catalysts show discrepancies between computational and experimental flow activation energies, highlighting the importance of catalyst thermal stability and molecular size.