Fig. 7. (a) Stress relaxation of poly(butadiene)-based vitrimers cross-linked with different ratio of dioxaborolane cross-linker (V3, V5 and V7 in PB-BE-VX is standing for the number of cross-linkers per chain). Adapted with permission from ref. 59. Copyright 2019 American Chemical Society. (b) Arrhenius plots of PTHF-based vitrimers composed of different molecular weights with high (PTH₂₅₀) to lower crosslinking density (PTHF₂₉₀₀). Adapted from ref. 51 with permission from the Royal Society of Chemistry. (c) Arrhenius plots of polyester vitrimers with different cross-linking density but same OH concentration (low crosslink density for CL-PE-0.5 to high cross-linking density for CL-PE-1, with X standing in CL-PE-X to the mole fraction of the epoxy groups derived from the di-epoxy ratio to the total epoxy groups: for example, 1 = all di-epoxy). Adapted with permission from ref. 65. Copyright 2020 American Chemical Society.