Reversible Redox Crosslinking of Thiopropylsilicones

Abstract

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 °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; HSiMe2OSiMe3 is identified as a convenient reducing agent. The initially formed silicone‐(CH2)3S‐SiMe2OSiMe3 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 1H NMR spectra of the uncrosslinked fluids after reduction. Further oxidation of disulfide groups on the elastomer surface permanently and significantly improved water wettability.

Elastomeric Silicone Sponges for Bleach Delivery

“…Note that overoxidation of thiol was also possible, as is discussed separately below (Figure 2). 18,23 Several parameters can be used to control the structure of the sponge, including total water/silicone ratio, concentration of bleach in the water, and mixing protocol. Structural features, including bubble density and bubble size are moderated by cross-link density and the presence of any functional groups (e.g., sulfonate, see below), are manifested in physical properties such as Young's modulus (Table 1).…”

“…Sodium hypochlorite solution (Sigma-Aldrich, available chlorine 10−15%) was titrated with standardized thiosulfate solution (Supporting Information, SI). 18 The average mass % of NaOCl in the solution was measured to be 11.51 ± 0.46%. Potassium iodide (Fisher Scientific), hydrochloric acid (Caledon), sodium thiosulfate (Chem-Impex), and starch (EMD) were used as received.…”

“…17 The conversion of thiols to disulfides can be induced by organosoluble oxidants to give elastomers (Figure 1). 18 These, in turn, can be reduced back to thiols using a reductive silylation process; this powerful process was previously used to convert sulfur-cross-linked automobile rubber back to linear polymers. 19 We wondered if a sulfur-based cross-linking/de-cross-linking process could be developed that used more benign oxidants than (diacetoxyiodo)benzene, 18 such as commercial bleach.…”

“…18 These, in turn, can be reduced back to thiols using a reductive silylation process; this powerful process was previously used to convert sulfur-cross-linked automobile rubber back to linear polymers. 19 We wondered if a sulfur-based cross-linking/de-cross-linking process could be developed that used more benign oxidants than (diacetoxyiodo)benzene, 18 such as commercial bleach. While bleach has its own challenges as noted above, the reagent is inexpensive, easy to obtain in a variety of concentrations, and is aqueous-based.…”

Polysiloxanes Grafted with Mono(alkenyl)Silsesquioxanes—Particular Concept for Their Connection

“…It may be performed at the stage of polysiloxanes chain formation, i.a., anionic ring-opening polymerization of cyclic siloxanes, or hydrolytic polycondensation of two chloro/alkoxysilanes varying in functional groups [23,[31][32][33]. On the other hand, it may be achieved by modification of functional group present on the existing polysiloxane chain, mostly via catalytic reactions [34][35][36][37][38][39][40][41][42][43][44], to gain co-polymers with precise and tailored properties [45][46][47][48][49][50][51][52][53][54][55]. Silsesquioxanes represent a versatile group of organosilicon species, with distinct and defined structures, but the most popular are cage silsesquioxanes, especially cubic T 8 [56,57].…”