Creating and Using More Sustainable Silicones

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Silicone polymers (typically based on (Me2OSi)n) monomers) are frequently key constituents in low tech - bathtub sealant - to high tech applications - wound dressings, LED lights, etc., because of their unusual and highly valuable properties. They are often used at much lower quantities/application than other materials and they exhibit efficient breakdown in the environment to CO2, H2O and sand. However, silicones cost more than organic polymers because of the very high energy input required for their synthesis (via silicon metal). Like other polymers and plastics there is an imperative to make silicones more sustainable by: using greener processes for the synthesis of their starting materials; the development of new green reactions, including cure chemistries (e.g., avoiding toxic metal catalysts) to create useful products; and producing less waste. This research program will improve the sustainability of silicone polymers by: i) using silicones as reagents for enhanced recovery of polymers, ii) preparing precise silicones with narrow ranges of desired properties so that less silicone is needed for given applications, iii) dilute the silicones with renewable, environmentally acceptable materials to give composites with interesting properties. Precise silicones will be prepared using the Piers-Rubinsztajn reaction; functional groups like SH, NH will be arrayed in precise locations on linear, branched and network polymers. Simple, high yielding reactions will introduce natural products at selected locations. These constituents include metal ion binding (chelating) moieties, sugars, and proteins that will convey to the silicones antibacterial activity and/or the ability to swell in water for wound dressings, or for energy damping. Silicone rubbers will also be reinforced with natural biopolymers including cellulose, gums, starch, and proteins like waste wool. High quality rubbers will result but, when composted, the materials will undergo fast breakdown of both the natural material and silicone. Bringing natural materials into silicones in a controlled way will require new synthetic strategies for polymer modification. An important outcome of this work is the development of effective methods to link water repellent silicones to water loving natural materials. These will lead to libraries of new silicones with new properties that can be exploited in evolving applications. The natural product will imbue enhanced sustainability of silicone polymers per kg, as judged by total energy input, feedstocks used, reusability/recyclability/compostability. As importantly, we will develop predictable rules relating silicone structure to properties, with respect to the variables: type of silicone, natural product, linker; and the physical and chemical ways that they have been combined. These rules and the compounds prepared, have potential for application by Canadian silicone industries. The larger impact will be the HQP trained during the research period.