First, we aimed to establish a safe and reliable protocol to form a soluble pre-polymer through the reaction of sulfur and DCPD. For safety, it was important to run the reaction at lower temperatures than those typically used in inverse vulcanisation (160 to 185 °C), including the reaction of sulfur and DCPD. At these elevated temperatures, dicyclopentadiene can undergo a retro Diels-Alder reaction, generating cyclopentadiene (S7). In such a situation, the amount of alkenes in the reaction system increases which could result in an uncontrolled increase in the reaction rate of this exothermic reaction. Cyclopentadiene is also a gas at these temperatures, which could lead to reagent loss and fluctuations in pressure. Indeed, the direct reaction of DCPD and sulfur at these elevated temperatures is known to result in unsafe, runaway reactions. And while accelerators and catalysts can be used to run this reaction at a lower temperature, 11 these additives remain in the polymer at the end of the synthesis. We therefore investigated the catalyst-free reaction of DCPD in molten sulfur at 140 °C. At this temperature, there is minimal conversion of DCPD to cyclopentadiene (3% based on 1 H NMR analysis) (S8). And while ringopening polymerisation of sulfur does not occur at this temperature (potentially distinguishing this reaction mechanism from those proposed for other inverse vulcanisation reactions), it is known that DCPD and sulfur react at 140 °C, first forming cyclic polysulfides across the norbornene alkene, with oligomerisation upon prolonged heating. Importantly, however, the amount of sulfur in these earlier reports was typically far more than the report here which resulted in substantial free sulfur which ultimately crystallises from the product and causes embrittlement. Please do not adjust margins