Entropy-controlled cross-linking in linker-mediated vitrimers

Recently developed linker-mediated vitrimers based on metathesis of dioxaborolanes with various commercially available polymers have show good both processability and outstanding performance, suggesting new ways of processing cross-linked polymers in industry, of which the design principle remains unknown [Science 356, 62–65 (2017)]. Here we formulate a theoretical framework to elucidate the phase behavior of the linker-mediated vitrimers, in which entropy plays a governing role. We find that, with increasing the linker concentration, vitrimers undergo a reentrant gel–sol transition, which explains a recent experiment [Macromolecules 53, 1180–1190 (2020)]. More intriguingly, at the low temperature limit, the linker concentration still determines the cross-linking degree of the vitrimers, which originates from the competition between the conformational entropy of polymers and the translational entropy of linkers. Our theoretical predictions offer guidelines in understanding and controlling the properties of this newly developed vitrimer system.