Sustainable Polymers from Native Silicon

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With funding from the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division, Professor Stephen A. Miller of University of Florida is building sustainable polymers from silica (silicon oxide and sand) and organic molecules derived from renewable biomass feedstocks. Commodity plastics in today's society are largely derived from fossil fuels and contain as much as 80 percent of carbon. This is inconsistent with the elemental composition of earth's crust which contains only 0.02 percent carbon. This work focuses on the use of silica, which comprises nearly 60 percent of the terrestrial environment, to prepare large-scale polymers. Targeted polymerization strategies include multiple additions of silicon/oxygen based building blocks to form extended polymer chains with the content of silicon and oxygen ranging between 38 and 60 percent. This novel class of polymers yields materials with new properties and provides opportunities for tuning these polymers toward easy environmental degradation. The research associated with this award provides a solid training ground for the next generation of scientists. The project prepares the students for a world which follows the principles of sustainability. This research embraces various sustainability metrics. The inclusion of inexpensive silica into commodity plastics could accelerate the growth of sustainable polymers and widen the variety of materials applications. Professor Miller and his research team continue their strong commitment to training and mentoring of undergraduate students, many of whom are members of underrepresented groups in chemistry. This research focuses on the direct conversion of native silica and biobased diols to discrete alkylorthosilicate monomers. The silica depolymerization strategy specifically avoids costly redox chemistry and is optimized with improved phase transfer catalysts, in combination with elevated temperature, microwaves, or sonication. Direct monomer polymerization or alkylorthosilicate metathesis polymerization (ASMP) renders polyesters, polycarbonates, polyacetals, polyamides, and polyimides with a silicon+oxygen content ranging between 38 and 60 percent. Preparation and characterization of these novel polyalkylorthosilicates reveal insight into fundamental polymer structure/property relationships, along with improved understanding of how to manipulate the silicon-oxygen bond, which is enigmatic despite its ubiquity in the earth's crust. The synthesized polyalkylorthosilicates possess new material properties, including those impacted by the very strong silicon-oxygen bond. Additionally, confining the silicon-oxygen bond to a ring is expected to improve the glass transition temperature into the range of high-temperature packaging plastics. While molecular alkylorthosilicates are hydrolyzed over the course of hours, the hydrolysis of this functional group embedded in relatively hydrophobic polymers is predicted to be slower and tunable. Systematic studies associated with rates, pathways, and degradation products during polyalkylorthosilicate hydrolysis could enable facile environmental degradation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.