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-oflife 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(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) (OSiMe 2 ) m OC( --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.

Full Circle Recycling of Polysiloxanes via Room-Temperature Fluoride-Catalyzed Depolymerization to Repolymerizable Cyclics

“…More recently, Enthaler et al have developed an entire series of depolymerization strategies for linear, branched, and cross-linked silicones and have been pioneers in silicone recycling. ,,, Their primary mode of action uses Lewis Acids including FeCl 3 , Zn, and BF 3 (OEt) 2 to activate the silicone backbone toward the attack by either fluoride or chloride fed by various organic sources and transfer agents (i.e., benzyl chloride, benzoylfluoride, and acetic anhydride) at temperatures above 100 °C. These methods result in the complete polymer breakdown in either dimethyldichlorosilanes or dimethyldifluorosilanes, with up to 86% yields as the primary products depending on the chloride or fluoride methods.…”

Illustrating Plastic Production and End-of-Life Plastic Treatment with Interlocking Building Blocks

“…Moreover, the degree of difficulty can be easily increased by illustration of, e.g., chain, branched, and cross-linked polymers and copolymers, depending on the precognition of the audience. For instance, the illustration of the current treatment of end-of-life plastics with interlocking building blocks was practiced with an interested public audience during the "Lange Nacht der Wissenschaft Berlin" (∼90 participants, Technische Universitaẗ Berlin), in the frame of a chemistry project 39 within the "Jugend Forscht e.V." ("Youth Research") competition for 15−21-year-old pupils/students, and in a minor chemistry lecture (general and inorganic chemistry) at Universitaẗ Hamburg (∼300 participants) applying an audience/learnercentered/interactive teaching model.…”

Solvothermal Alcoholysis Method for Recycling High-Consistency Silicone Rubber Waste

“…Enthaler and colleagues reported synthetic protocols for the depolymerization of various silicone oils (with M n from 0.5 to 110 kDa). They used benzoyl fluoride, boron trifluoride dietherate, a fatty acid anhydride, or combinations of benzoyl chloride or benzoic anhydride with KF to synthesize fluorosilane and fluorodisiloxane derivatives of Me 2 SiF 2 and (Me 2 SiF) 2 O for linear polymers, and Me 2 SiF 2 and SiF 4 for branched polymers. − The obtained fluorinated compounds are attractive starting materials and can be easily polymerized in the presence of H 2 O and NaOH to produce new silicones and NaF . Depolymerization methods were also developed in which catalytic amounts of FeCl 3 or ZnX 2 (X = acac, OAc, or OTf) were used. , The same group also reported the use of poly­(methylhydrosiloxane) in H 2 evolution by methanolysis, deoxygenation of sulfoxides to sulfides, and hydrodeoxygenation of fatty esters to hydrocarbons, prior to the depolymerization procedure described above .…”