Polydimethylsiloxane (PDMS) is found in many different products ranging from greases and lubricants to elastomers and biomedical devices. The silicon in PDMS originates in silica and must undergo three energy- and pollution-intensive transformations during its journey from silica to PDMS. Silica is first subjected to carbothermal reduction during which Si(IV) is reduced to Si(0), then oxidized back to Si(IV) to form SiMexCl4-x in the Müller-Rochow process, and finally polymerized via hydrolysis. There are two pathways that can be explored to avoid this pollution-intensive synthesis: 1) using PDMS as a feedstock to fully exploit the transformation and 2) directly depolymerizing silica to avoid the transformation altogether. PDMS and silica can be depolymerized in one step using inexpensive catalysts and chelating diols. Chelating diols provide an entropic driving force for extracting silicon, allowing for moderate temperatures and ambient pressure. Depolymerization of silica using chelating diols to form spirocyclic alkylorthosilicates has been demonstrated by others. We’ve shown that these spirocycles fully hydrolyze to silicic acid and diol within six weeks under neutral aqueous conditions. Copolymerization with polyethylene glycols results in flexible networks possessing weak links for degradation and good thermal properties (T95 &gt; 300 °C). We developed an analogous method for depolymerizing PDMS; our method gives good yields (&gt;70%) for pure samples of PDMS with variable end-groups and molecular weights. Commercial products, including grease, caulk, and elastomers, were successfully depolymerized to pure silicon acetal product, indicating the method is highly selective for PDMS in complex polymeric mixtures. PDMS (Mn ~ 49,000) was reconstituted via cationic emulsion polymerization.

One step forward or three steps back: Avoiding the pollution-intensive synthesis of polydimethylsiloxane

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