Fig. 3. (a) Vinylogous urethane vitrimers exhibiting a dual temperature response based on two concomitant exchange mechanisms: iminium-type mechanism in lower temperature range with E_a of 60–70 kJ mol⁻¹ and Michael-type addition at higher temperature with higher energy barrier of 130–170 kJ mol⁻¹. Reproduced with permission from ref. 38. Copyright 2018 American Chemical Society. (b) Arrhenius plot of poly(propylene glycol) and vinylogous urethane based vitrimers showing a control of the bond exchange rate by playing with: (b₁) the concentration of active nucleophilic moieties (primary amine groups), (b₂) the concentration of acid catalyst (pTsOH), (b₃) the concentration of both active nucleophilic moieties and acid catalyst. Adapted from ref. 39 with permission from the Royal Society of Chemistry.