CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201301386
Low-Temperature Depolymerization of Polysiloxanes with Iron Catalysis Stephan Enthaler*[a] and Robert Kretschmer[b] The easy accessibility and high adjustability of polymers mainly accounts for the great impact of such materials on modern society. Besides this great success, an important matter is the accumulation of large amounts of end-of-life polymers, which are mainly deposited in landfills or converted by thermal recycling or down-cycling to low-quality materials. In contrast to that, the depolymerization of end-of-life polymers to monomers, which can be applied as feedstock in polymerization chemistry for high-quality polymers, is only carried out for a small fraction of waste. Polysiloxanes are extensively used in a diverse array of technological applications. Based on intrinsic properties of polymers, depolymerization is challenging and only
a few high-temperature or less environment-friendly processes have been reported. In this regard, we have set up a capable low-temperature protocol for the depolymerization of poly(dimethylsiloxane) in the presence of catalytic amounts of simple iron salts in combination with different depolymerization reagents. The application of benzoyl fluoride, benzoyl chloride/ potassium fluoride, or benzoic anhydride/potassium fluoride as depolymerization reagents affords difluorodimethylsilane or 1,3-difluoro-1,1,3,3-tetramethyldisilxanes as products, which are interesting building blocks for the synthesis of new polymers and allow an overall recycling of polysiloxanes.
Introduction Every year enormous amounts of end-of-life materials are generated on a multiton scale by our consumer society. Currently, the management of end-of-life plastics, which are one of the major components of waste, is dominated by landfill storage, thermal recycling (thermal decomposition for energy purposes), and down-cycling to produce low-quality materials. In contrast to that, the selective degradation of end-of-life plastics to monomers or useful synthons is only carried out for a small fraction of waste.[1–5] Notably, low-molecular-weight depolymerization products can be applied as feedstock for new highperformance polymers, which allows an overall recycling process. Moreover, the realization of recycling technologies can be an option to save resources because most materials are based on steadily decreasing natural resources (e.g., crude oil, natural gas). Although the advantages of feedstock recycling are obvious, several issues hamper its application; for example, high energy demand for depolymerization or mixed end-of-life plastics originating from different monomers (copolymers). In this regard, silicones (e.g., silicone oil, silicone rubber, silicone grease, silicone resin) represent a currently widely used class of polymers and copolymers. Interestingly, these silicones are easily available by the Mller–Rochow synthesis, and sub[a] Dr. S. Enthaler Technische Universitt Berlin, Department of Chemistry Cluster of Excellence “Unifying Concepts in Catalysis” Str. des 17. Juni 115/C2, 10623 Berlin (Germany) E-mail: [email protected] [b] Dr. R. Kretschmer Technische Universitt Berlin Department of Chemistry Str. des 17. Juni 135/C4, 10623 Berlin (Germany)
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
sequent hydrolysis allows a broad range of properties.[6] Conversely, to access the starting materials (e.g., chloromethane and elementary silicon) high energy input and consumption of natural resources are required. On the other hand, a major treatment for end-of-life silicones is the thermal decomposition to generate silica, which is an unproblematic product. However, at this point the integrated efforts (e.g., methyl functions) are irretrievably lost. In this regard, low-temperature depolymerization methodologies can be an option to overcome these limitations because a smaller amount of energy should be necessary to allow recycling. Unfortunately, the intrinsic properties of silicones constrain the application of depolymerization processes, hence only a few high-temperature (> 200 8C) or less environment-friendly processes have been reported.[7] Recently we have established a low-temperature method for the cleavage of Si O bonds of polysiloxanes to produce interesting monomers (Figure 1).[8, 9] Herein, the Si O bond is activated by catalytic amounts of zinc salts and cleaved by benzoyl fluoride as depolymerization reagent, yielding Si F bonds.[10, 11] By repeating this process the polymer is steadily converted to low-molecular-weight compounds, such as R2SiF2,, which are potential building blocks for new polymers.[12] Fluoride-containing compounds are formed as side products during polymerization, which can be applied for the regeneration of the depolymerization reagent.[13] However, long reaction times, high reaction temperatures, and high catalyst loadings are required to perform the reaction. Based on that, we present herein an improved depolymerization of end-of-life polysiloxanes in the presence of straightforward iron catalysts.
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Figure 1. Depolymerization concept for polysiloxanes.
Results and Discussion Initially, the depolymerization of polysiloxanes was studied in the presence of different iron salts (Table 1). The model substrate, hydroxy-terminated poly(dimethylsiloxane) (1, Mn 550 g mol 1), reacted with iron(III) chloride (5.0 mol %) and benzoyl fluoride (2) as depolymerization reagent (2.0 equiv). Interestingly, a good yield (80 %) of the products (3[14] and 4[15])
Table 1. Depolymerization of polysiloxane 1 with iron catalysis.
Entry[a]
Iron source ([mol %])
2 [equiv]
Yield [%][b]
Yield [%][c]
1 2[d]
– FeCl3 (5)
2 2
Low-Temperature Depolymerization of Polysiloxanes with Iron Catalysis Stephan Enthaler*[a] and Robert Kretschmer[b] The easy accessibility and high adjustability of polymers mainly accounts for the great impact of such materials on modern society. Besides this great success, an important matter is the accumulation of large amounts of end-of-life polymers, which are mainly deposited in landfills or converted by thermal recycling or down-cycling to low-quality materials. In contrast to that, the depolymerization of end-of-life polymers to monomers, which can be applied as feedstock in polymerization chemistry for high-quality polymers, is only carried out for a small fraction of waste. Polysiloxanes are extensively used in a diverse array of technological applications. Based on intrinsic properties of polymers, depolymerization is challenging and only
a few high-temperature or less environment-friendly processes have been reported. In this regard, we have set up a capable low-temperature protocol for the depolymerization of poly(dimethylsiloxane) in the presence of catalytic amounts of simple iron salts in combination with different depolymerization reagents. The application of benzoyl fluoride, benzoyl chloride/ potassium fluoride, or benzoic anhydride/potassium fluoride as depolymerization reagents affords difluorodimethylsilane or 1,3-difluoro-1,1,3,3-tetramethyldisilxanes as products, which are interesting building blocks for the synthesis of new polymers and allow an overall recycling of polysiloxanes.
Introduction Every year enormous amounts of end-of-life materials are generated on a multiton scale by our consumer society. Currently, the management of end-of-life plastics, which are one of the major components of waste, is dominated by landfill storage, thermal recycling (thermal decomposition for energy purposes), and down-cycling to produce low-quality materials. In contrast to that, the selective degradation of end-of-life plastics to monomers or useful synthons is only carried out for a small fraction of waste.[1–5] Notably, low-molecular-weight depolymerization products can be applied as feedstock for new highperformance polymers, which allows an overall recycling process. Moreover, the realization of recycling technologies can be an option to save resources because most materials are based on steadily decreasing natural resources (e.g., crude oil, natural gas). Although the advantages of feedstock recycling are obvious, several issues hamper its application; for example, high energy demand for depolymerization or mixed end-of-life plastics originating from different monomers (copolymers). In this regard, silicones (e.g., silicone oil, silicone rubber, silicone grease, silicone resin) represent a currently widely used class of polymers and copolymers. Interestingly, these silicones are easily available by the Mller–Rochow synthesis, and sub[a] Dr. S. Enthaler Technische Universitt Berlin, Department of Chemistry Cluster of Excellence “Unifying Concepts in Catalysis” Str. des 17. Juni 115/C2, 10623 Berlin (Germany) E-mail: [email protected] [b] Dr. R. Kretschmer Technische Universitt Berlin Department of Chemistry Str. des 17. Juni 135/C4, 10623 Berlin (Germany)
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
sequent hydrolysis allows a broad range of properties.[6] Conversely, to access the starting materials (e.g., chloromethane and elementary silicon) high energy input and consumption of natural resources are required. On the other hand, a major treatment for end-of-life silicones is the thermal decomposition to generate silica, which is an unproblematic product. However, at this point the integrated efforts (e.g., methyl functions) are irretrievably lost. In this regard, low-temperature depolymerization methodologies can be an option to overcome these limitations because a smaller amount of energy should be necessary to allow recycling. Unfortunately, the intrinsic properties of silicones constrain the application of depolymerization processes, hence only a few high-temperature (> 200 8C) or less environment-friendly processes have been reported.[7] Recently we have established a low-temperature method for the cleavage of Si O bonds of polysiloxanes to produce interesting monomers (Figure 1).[8, 9] Herein, the Si O bond is activated by catalytic amounts of zinc salts and cleaved by benzoyl fluoride as depolymerization reagent, yielding Si F bonds.[10, 11] By repeating this process the polymer is steadily converted to low-molecular-weight compounds, such as R2SiF2,, which are potential building blocks for new polymers.[12] Fluoride-containing compounds are formed as side products during polymerization, which can be applied for the regeneration of the depolymerization reagent.[13] However, long reaction times, high reaction temperatures, and high catalyst loadings are required to perform the reaction. Based on that, we present herein an improved depolymerization of end-of-life polysiloxanes in the presence of straightforward iron catalysts.
ChemSusChem 0000, 00, 1 – 8
&1&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
Figure 1. Depolymerization concept for polysiloxanes.
Results and Discussion Initially, the depolymerization of polysiloxanes was studied in the presence of different iron salts (Table 1). The model substrate, hydroxy-terminated poly(dimethylsiloxane) (1, Mn 550 g mol 1), reacted with iron(III) chloride (5.0 mol %) and benzoyl fluoride (2) as depolymerization reagent (2.0 equiv). Interestingly, a good yield (80 %) of the products (3[14] and 4[15])
Table 1. Depolymerization of polysiloxane 1 with iron catalysis.
Entry[a]
Iron source ([mol %])
2 [equiv]
Yield [%][b]
Yield [%][c]
1 2[d]
– FeCl3 (5)
2 2
