To benchmark the reaction at lower temperature, equal masses of DCPD (>95% endo) and sulfur (3.0 g each) were placed in a 20 mL glass vial and incubated, uncapped, in an oven at 140 °C for up to 24 hours (S10). This feed ratio corresponds to an average of 2 sulfur atoms per alkene, which was expected to provide a greater thermal and chemical stability than materials with longer and more labile polysulfide crosslinks. At least 3 hours were required for the twophase reaction to darken and appear homogenous, even without stirring. Incubating longer than 10 hours resulted in a substantial amount of material that was insoluble in chloroform, which is undesirable for subsequent solvent processing. 1 H NMR analysis indicated consumption of the norbornene alkene over 8 hours, and then slower consumption of remaining alkenes over the next 24 h at 140 °C (S11). Continued heating after 8 hours resulted in a linear increase in Tg with incubation time, consistent with crosslinking (S12). Importantly, this protocol never caused a runaway reaction.