Iron‐catalyzed depolymerization of polysiloxanes to produce dichlorodimethylsilane, diacetoxydimethylsilane, or dimethoxydimethylsilane

Abstract

Polymers are one of the important pillars of our current society. Besides the great success a matter is the accumulation of huge amounts of end-of-life polymers. Current waste management bases primarily on landfills, thermal recycling, and down-cycling. Noteworthy, only a small part of the end-of-life materials is recycled by depolymerization, means low-molecular weight synthons are created, which can be polymerized to new polymers to close the cycle. Widely used polymers in modern life times are silicones (polysiloxanes). Based on the intrinsic properties the depolymerization is challenging and only a few high temperature or less environmental-friendly processes have been reported. In this regard, we have set up a capable low-temperature protocol for the depolymerization of silicones with acid chlorides, acetic acid, or methanol in the presence of cheap iron salts as precatalysts to yield dichlorodimethylsilane, diacetoxydimethylsilane, or dimethoxydimethylsilane as well-defined products. Notably, dichlorodimethylsilane, diacetoxydimethylsilane, and dimethoxydimethylsilane can be useful starting materials for synthesizing new polymers; overall a recycling is feasible.

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

“…In more detail, iron(III) chloride, silicone 14 and hexanoic anhydride were heated for 24 h at 200°C. Investigation of the reaction mixture by GC-MS revealed the formation of the depolymerization products 3:4:5:6 in a ratio of 31:36:24:9 after 24 h. Subsequently the polymerization was performed by simple addition of water and heating for 24 h at 100°C [26]. A GC-MS analysis revealed the formation of cyclic oligosiloxanes.…”

“…In terms of selectivity and yield no obvious relation between, e.g., Lewis acidity or oxidation state of the iron center was observed. Importantly, a better performance was found applying hexanoic anhydride compared to acetic anhydride, which was recently used as depolymerization reagent [26]. Moreover, iron oxides were tested as precatalysts in the depolymerization reaction ( Table 1, entries [12][13][14][15][16].…”

“…Moreover, the fatty acid anhydride was hydrolyzed to hexanoic acid. Afterwards, the fatty acid layer was easily separated from the aqueous layer, which is advantageously in contrast to the acetic anhydride approach [26] and the oligosiloxanes were removed by distillation. As major silicon containing compounds octamethylcyclotetrasiloxane 18 [27] and decamethylcyclopentasiloxane 19 [27] were isolated in a ratio of 2.7:1.0.…”

Insights on toxicity, safe handling and disposal of silica aerogels and amorphous nanoparticles

“…A methodology to achieve that has been devised by Enthaler and co-workers by using metal-based catalysts. [149][150][151] Based on previous works on depolymerization of, e.g., polyethers, 152,153 by using zinc and iron salts as catalysts, a comprehensive and systematic study was carried out by testing the performance of those catalysts and different reagents on the depolymerization of polysiloxanes. In this respect, different polysiloxanes have been depolymerized by using zinc trifluoromethanesulfonate as catalyst and benzoyl fluoride as depolymerization agent, showing that, in general, the reaction yields are very high (>80%), independently of the molecular weight and pending groups of the polysiloxane.…”

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

“…[9][10][11] In this regard, we recently established a feedstock recycling concept for linear poly(dimethyl siloxane)s (4s) with the aid of zinc or iron catalysis. [12][13][14][15] In more detail, the depolymerization of 4s 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 1,3difluoro-1,1,3,3-tetramethyl disiloxane (6) as products. Interestingly, 5 and 6 are useful starting materials for new silicones.…”