Introducing Students to Feedstock Recycling of End-of-Life Silicones via a Low-Temperature, Iron-Catalyzed Depolymerization Process

The straightforward large-scale synthesis and the ability to adjust the properties of polymers make polymers very attractive materials. Polymers have been used in numerous applications and an increased demand is foreseeable. However, a serious issue is the accumulation of enormous amounts of end-of-life polymers, which are currently recycled by thermal degradation, undergo downcycling, or buried in landfills. In contrast, only a minor fraction of polymers is recycled by selective depolymerization processes to produce low molecular weight chemicals that can be polymerized to new polymers. Polysiloxanes (silicones) are widely used polymers, and recycling is challenging due to their intrinsic properties. A few high temperature or less environmentally friendly protocols have been reported for recycling silicones. To circumvent these problems, a low-temperature process was developed for the depolymerization of polysiloxanes using catalytic amounts of cheap, iron salts as a precatalyst and benzoyl fluoride as a depolymerization reagent. Low molecular weight products (difluorodimethylsilane and 1,3-difluoro-1,1,3,3-tetramethyldisiloxane) are used for the synthesis of new polysiloxanes; hence, overall a recycling process is feasible. This inorganic chemistry experiment introduces second-year undergraduate students to the concept of feedstock recycling via depolymerization/polymerization processes and exemplifies modern advances in sustainable chemistry.