Low-temperature depolymerization of polysiloxanes with iron catalysis

Chemical Recycling of Silicones-Current State of Play (Building and Construction Focus)

As the demand for silicone polymers continues to grow across various industries, the need for effective recycling methods has become increasingly important, because recycling silicone products reduces landfill waste, conserves resources, and uses less energy. Chemical recycling involves the depolymerization of silicone waste into oligomers, which can then be used to produce virgin-grade silicone. While this sector of the recycling industry is still in its infancy-we estimate that 35,000 to 45,000 metric tons of silicone waste will be chemically recycled worldwide in 2024-an increasing number of companies are beginning to explore the implementation of closed-loop systems to recycle silicones. This article examines the technical options and challenges for recycling silicone polymers, the major degradation chemistries available for depolymerizing silicones, and the current industrial reality of chemical recycling of silicones.

A robust depolymerization route for polysiloxanes

A versatile, robust, and stable tetrabutylammonium difluorotriphenylsilicate (TBAT) catalyst has been deployed for efficient depolymerization of silicones. This catalyst is soluble in a variety of organic solvents and is stable up to 170 °C, enabling a wide range of reaction conditions under which F--catalysed siloxane bond cleavage can be initiated. This effort offers significant advancement overcoming the traditional limitations of silicone depolymerization, such as high catalyst loading, storage and handling, and few viable reaction media.

Iron(II)-Catalyzed Activation of Si-N and Si-O Bonds Using Hydroboranes

We report the activation and functionalization of Si-N bonds with pinacol borane catalyzed by a three-coordinate iron(II) β-diketiminate complex. The reactions proceed via the mild activation of silazanes to yield useful hydrosilanes and aminoboranes. The reaction is studied by kinetic analysis, along with a detailed investigation of decomposition pathways using catecholborane as an analogue of the pinacol borane used in catalysis. We have extended the methodology to develop a polycarbosilazane depolymerization strategy, which generates hydrosilane quantitatively along with complete conversion to the Bpin-protected diamine. The analogous Si-O bond cleavage can also be achieved with heating, using silyl ether starting materials to generate hydrosilane and alkoxyborane products. Depolymerization of poly(silyl ether)s using our strategy successfully converts the polymer to 90% Bpin-protected alcohols.

Molecular Scissors for Tailor-Made Modification of Siloxane Scaffolds

The controlled design of functional oligosiloxanes is an important topic in current research. A consecutive Si-O-Si bond cleavage/formation using siloxanes that are substituted with 1,2-diaminobenzene derivatives acting as molecular scissors is presented. The method allows to cut at certain positions of a siloxane scaffold forming a cyclic diaminosilane or -siloxane intermediate and then to introduce new functional siloxy units. The procedure could be extended to a direct one-step cleavage of chlorooligosiloxanes. Both siloxane formation and cleavage proceed with good to excellent yields, high regioselectivity, and great variability of the siloxy units. Control of the selectivity is achieved by the choice of the amino substituent. Insight into the mechanism was provided by low temperature NMR studies and the isolation of a lithiated intermediate.

Facile Approach to Recycling Highly Cross-Linked Thermoset Silicone Resins under Ambient Conditions

Silicone resins are traditional thermoset polymers with an inorganic backbone, affording chemical inertness and high thermal stability, which also makes them inherently difficult to recycle by traditional methods. Here, we demonstrate that catalytic amounts of fluoride ion at room temperature solubilize highly cross-linked silicone resins initially cured up to 250 °C. After solubilization equilibria are achieved, solvent is removed to reform the polymer network. Coatings on aluminum substrates and monoliths of virgin and recycled silicone resins were evaluated for hydrophobicity, wear resistance, substrate adhesion, and thermal stability. Silicones recycled under optimized conditions retained nearly 100% wear resistance, thermal stability, and adhesion properties. In some instances, the recycled coatings offer properties superior to the initial materials.