Reversible Redox Crosslinking of Thiopropylsilicones

Most silicone elastomers are thermosets. As a response to the new paradigm of polymer recyclability, the development of silicone elastomers that can be reversibly and repeatedly cured and uncrosslinked using redox conditions is reported. Thiopropyl-modified silicones are oxidized to elastomers with disulfide crosslinks using the organosoluble oxidant PhI(OAc)(2). As with any elastomer, mechanical properties can be tuned by varying crosslink density. Thermal stabilities in air show that the products are comparable to traditional silicone thermosets, with degradation only starting over 300 degrees C. Uncrosslinking back to the same thiopropyl-modified silicones involves reductive S-S bridge cleavage using a Piers-Rubinsztajn reaction with hydrosilanes catalyzed by B(C6F5)(3); HSiMe(2)OSiMe(3)is identified as a convenient reducing agent. The initially formed silicone-(CH2)(3)S-SiMe(2)OSiMe(3)products need deprotection with water in isopropanol/water to completely regenerate the thiopropylsilicones. This oxidation/reduction crosslinking/uncrosslinking cycle is practiced thrice, with a yield of 89% per cycle, with essentially no change in the Young's moduli of the elastomers, or(1)H NMR spectra of the uncrosslinked fluids after reduction. Further oxidation of disulfide groups on the elastomer surface permanently and significantly improved water wettability.

Automated summary

Most silicone elastomers are thermosets, but this study reports on the development of silicone elastomers that can be reversibly cured and uncrosslinked using redox conditions. The mechanical properties of these elastomers can be adjusted by varying crosslink density. The oxidation and reduction cycle results in comparable thermal stabilities and significantly improved water wettability on the elastomer surface.