Recycling Concept for End-of-Life Silicones: Boron Trifluoride Diethyl Etherate as Depolymerization Reagent to Produce Difluorodimethylsilane as Useful Commodity

Abstract

The current waste management system is based primarily on landfills, thermal recycling and down-cycling. On the other hand, only a small portion of the end-of-life materials is recycled by depolymerization. In more detail, low-molecular weight commodities/monomers are produced by this process, which can be polymerized in a second step to produce new high quality polymers. Importantly, by applying these depolymerization/polymerization methodologies, contributions to more a sustainable, resource-conserving and environmental-benign society can be made. In this regard, we have set up a capable low-temperature (100°C) protocol for the depolymerization of poly(dimethylsiloxane)s. In more detail, boron trifluoride diethyl etherate was applied as a depolymerization reagent in the conversion of numerous silicones to obtain difluorodimethylsilane and 1,3-difluoro-1,1,3,3-tetramethyldisiloxane as well-defined products under solvent-free conditions in yields up to 85%. Moreover, it was demonstrated that difluorodimethylsilane and 1,3-difluoro-1,1,3,3-tetramethyldisiloxane are useful starting materials for the synthesis of new silicones (polymerization). In consequence a recycling of silicones is feasible. The principle of operation of the recycling concept was demonstrated in the depolymerization/polymerization of 500 g of a polysiloxane.

Depolymerization protocol for linear, branched, and crosslinked end‐of‐life silicones with boron trifluoride diethyl etherate as the depolymerization reagent

“…Recently, we studied the conversion of hexamethyl disiloxane ( 1 ) with 2 as a model for the depolymerization of dimethyl siloxane units in linear silicones (Scheme ) . We have found that with 2 equiv of 2 , the SiO bonds were cleaved and transformed to SiF bonds in excellent yields ( 3 : > 99%); this demonstrated the potential for depolymerization reactions.…”

“…By this approach, the generated low‐boiling products 5 and 6 were continuously distilled off and collected at low temperature (−196°C) in 86% yield ( 5 ) and 7% yield ( 6 ) along with diethyl ether. Interestingly, the depolymerization products 5 and 6 are interesting building blocks for new silicones . In the reaction flask, an off‐white solid [B x (O y R z )] was formed; it could be reconverted to BF 3 by NaF/H 2 SO 4 .…”

“…5 : 1 H‐NMR (200 MHz, CDCl 3 , 25°C, δ, ppm): 0.31 (t, 3 J HF = 6.2 Hz). 19 F‐NMR (188 MHz, CDCl 3 , 25°C, δ, ppm): −131.4 (sept, 3 J HF = 6.2 Hz, ddec, 1 J SiF = 289.9 Hz).…”

“…To overcome these issues, the depolymerization of silicones can be an interesting option (see previous) . In this regard, we recently established a feedstock recycling concept for linear poly(dimethyl siloxane)s ( 4 s) with the aid of zinc or iron catalysis . In more detail, the depolymerization of 4 s can be realized in the presence of depolymerization reagents, for example, benzoyl fluoride, benzoyl chloride/potassium fluoride, benzoic anhydride/potassium fluoride, or boron trifluoride dietherate, to obtain difluorodimethyl silane ( 5 ) and 1,3‐difluoro‐1,1,3,3‐tetramethyl disiloxane ( 6 ) as products.…”

Spent coffee ground as source for hydrocarbon fuels

“…In this regard, depolymerization methods can be valuable to convert the silicon-waste to useful commodities, therefore increasing the sustainability of the hydrodeoxygenation protocol. Recently, we established several methods for the recycling of poly(dimethylsiloxanes) waste [15,16]. For instance applying boron trifluoride diethyl etherate (BF 3 OEt 2 ) as depolymerization Scheme 1.…”