【Frontiers in HaiChuan Chemical Technology】Pirelli and AIST have jointly developed a two-stage chemical recycling process that can decompose vulcanized tire rubber at room temperature

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Pirelli and AIST have jointly developed a two-stage chemical recycling process that can decompose vulcanized tire rubber at room temperature. When announcing the results on December 4, the two parties stated that this new technology enables the \"rearrangement\" of carbon-carbon double bonds through a \"double-displacement reaction,\" thereby facilitating the chemical decomposition of vulcanized polyisoprene, followed by thermal decomposition. The products obtained from this process include isoprene, carbon black, and BTX (benzene, toluene, xylene) which can be used as chemical raw materials. It is reported that polyisoprene, such as natural rubber, can be easily decomposed through catalytic metathesis reactions, but Bridgestone states that vulcanized polyisoprene rubber in used tires is difficult to decompose. The company explained that this is because the sulfur component in vulcanized rubber inhibits the progression of the double-displacement reaction. Researchers say that by selecting the \"appropriate catalysts and solvents\" for tire rubber, it is possible to achieve chemical decomposition of the cross-linked polyisoprene under \"mild conditions near room temperature\". In explaining the principle of the process, Bridgestone mentioned that when an intermolecular double-displacement reaction occurs, the carbon-carbon double bonds between polyisoprene molecules are rearranged. However, the polyisoprene molecular chains formed as a result of the reaction have varying lengths, making it impossible to obtain only the short-chain molecules. Therefore, the research team focused on achieving the chemical degradation of polyisoprene through \"intermolecular metathesis between polyisoprene and low-molecular-weight reactants containing carbon-carbon double bonds\". On the other hand, Bridgestone states that when polyisoprene undergoes an intramolecular double displacement reaction, cyclic polyisoprene is formed, thereby shortening the polyisoprene molecular chains. Through experiments in their research, the two parties confirmed that by adding appropriate catalysts and solvents to vulcanized polyisoprene rubber, it is possible to significantly shorten the polyisoprene molecular chains within a few hours at room temperature. To elucidate this mechanism, they conducted a detailed structural analysis of the reaction products and found that the main component of the liquid polymer was \"cyclic compounds with cyclic isoprene tetramers as their core.\" Furthermore, by isolating and purifying the cyclic isoprene tetramer and conducting single-crystal X-ray structure analysis, the researchers \"for the first time elucidated its molecular structure, including its three-dimensional structure.\" Pirelli states that when the resulting liquid polymer is subjected to thermal decomposition, isoprene is the main product. The National Institute of Advanced Industrial Science and Technology in Japan and Bridgestone stated that this research achievement lays a scientific foundation for the chemical recycling of used tires, addressing the long-standing challenges associated with the complex sulfur cross-linking structure of vulcanized rubber. Both parties plan to extend this technology to other types of rubber such as butadiene rubber, while also exploring potential applications for the newly isolated cyclic isoprene tetramers. They aim to advance research on scale-up processes with the goal of achieving commercial application by the 2030s.