Iron‐catalyzed depolymerizations of silicones with hexanoic anhydride provide a potential recycling method for end‐of‐life polymers

Abstract

One of the great impacts of polymeric materials is the matter of the accumulation of huge amounts of end‐of‐life polymers. At the present date, waste management is based primarily on landfills, thermal recycling, and down‐cycling, while only a small part of the waste is subjected to feedstock recycling. In more detail, polymers are converted to low‐molecular weight synthons by depolymerization and a subsequent polymerization process creates new high‐quality polymers. Widely‐used polymers in modern society are polysiloxanes (silicones) and recycling is still an issue. In this regard, we present herein the depolymerization of polysiloxanes, applying a fatty acid anhydride as the depolymerization reagent. In the presence of catalytic amounts of iron salts, low‐molecular weight products with the motif RC(O)(OSiMe2)mOC(O)R (R = n‐pentyl, m = 1–4) were observed. Notably, the reaction of RC(O)(OSiMe2)mOC(O)R with water revealed the formation of octamethylcyclotetra‐ and decamethylcyclopentasiloxanes and the corresponding fatty acid as side product, which can potentially be reconverted to the fatty acid anhydride. Interestingly, a recycling of the polymer and the depolymerization reagent is feasible.Practical applications: Huge amounts of end‐of‐life polymers are produced by our society and several methodologies for waste treatment have been established so far. One interesting recycling approach can be the conversion of end‐of‐life polymers, via depolymerization, into useful commodities, which can be applied as feedstock for new materials. Taking these aspects into account we studied the depolymerization of end‐of‐life polysiloxanes, applying cheap and abundant iron salts as a precatalyst and a fatty acid anhydride as a depolymerization reagent, which can be easily accessed from renewable resources, hence avoiding the use of fossil fuels‐based chemicals. As major products RC(O)(OSiMe2)mOC(O)R were obtained, these can be utilized as starting material for new polysiloxanes. Moreover, the simple operation of the reaction (e.g., no protection atmosphere, solvent free) makes this recycling method attractive for large scale applications.In the present work, the straightforward iron‐catalyzed depolymerization of end‐of‐life polysiloxanes with a fatty acid anhydride has been studied. Depolymerization products RC(O)O(SiMe2)mOC(O)R have been produced, which can be appropriate starting materials for new polymers.