Improved circularity in elastomers and composites requires better opportunities for reusing/repurposing before ultimate disposal, and better degradability once placed in the environment. We report that tannins—commonly used for enhancing fermentation and flavors in wines—are excellent crosslinkers for aminosilicones. Simply mixing a solution of the tannin in alcohol/water with aminosilicones provided an elastomeric composite after evaporation. The physical properties could be tuned simply by varying the quantity of food grade tannin and the molar mass of the telechelic aminosilicones. The products were thermoplastic and could be (re)processed at 140 °C or completely depolymerized by the addition of butylamine to regenerate both the aminosilicone and, after evaporation of butylamine, the tannin, consistent with an enhanced life cycle for these materials. An alternative phenolic filler, lignin, did not lead to composites with useful physical properties.

Highly reticulated silsequioxanes from sol-gel processes of coupling agents bind waste organic rubber to make resilient composites.

Covalent grafting of sugars onto silicones imparts viscoelasticity via hydrogen-bonding among sugar moieties. Catalyst-free caramelization enables programmed, permanent adjustment of these properties by first reducing the OH content on the sugar moieties, which initially lowers viscosity, followed by oligomerization to form viscoelastic materials.

Silicones underpin an enormous range of simple and advanced technologies. Often, only small quantities of silicone are used to enable a technology such that, on a "per use" basis, one might suppose the environmental impact is low. However, silicone preparation processes have a very high carbon footprint, and billions of kg are produced each year. To provide context to the consideration of new strategies to improve silicone sustainability, we first outline traditional silicone chemistry and then describe strategies to improve the degree to which silicones are green, sustainable and circular. One strategy involves dilution of the silicone oil or elastomer by tethering organic entities, particularly natural products, that may provide new properties including facilitated degradation in nature at end-of-life. A greater focus is given to strategies that permit extensive reuse and repurposing of oils and elastomers (e.g., with thermoplastic elastomers), before the silicone undergoes recycling. Each reuse, repurposing or recycling step reduces the net carbon footprint. These mostly involve straightforward, high-yielding organic chemical processes that work efficiently in a silicone milieu. Silicones will eventually end up in the environment, where linear oils are known to rapidly degrade, particularly when compared to organic polymers. Alternative strategies that permit triggered or biological degradation of oils and, more importantly elastomers, are described, including enzymatic degradation and composting.

Silicone elastomers are valued for their resilience, particularly in stressful environments. Commercial cross-linking processes lead to covalent bonds based on Si-C or Si-O links, which, like all conventional rubbers, make degradation of the elastomer difficult at the end of life. We show that hydrated gelatin, a natural protein, can be covalently incorporated into silicone elastomers to produce hydrogels; the protein serves as both a diluent for silicone and a handle for devulcanization. Diacrylated cross-linkers based on poly(ethylene glycol) or triglycerol led to homo- and heteropolymer cross-linking between silicone and gelatin; the homogeneity of the elastomer products was dependent upon mixing prior to cure. Reactions occurred over a few hours at 80 degrees C to give hydrogels initially or, after drying, elastomers that exhibited the properties of both constituents. The cross-linked materials were stable during use, dry or after swelling in water, but very susceptible to enzymatic degradation to give silicone oils when treated with the enzymes bromelain and lipase.

The incorporation of natural amino acids onto a silicone framework is efficiently and simply achieved, without protecting groups or solvents, by using aza-Michael addition to acrylate-modified silicones.

In commerce, silicone elastomers are typically cured by platinum-catalyzed hydrosilylation or radical induced crosslinking (high temperature vulcanization (HTV)), both of which lead to crosslinks comprised of 2 or 3 carbon fragments. Alternatively, room temperature vulcanization (RTV) utilizes catalyzed nucleophilic substitution to give elastomers with Si-O-Si crosslinks. At end of life, the best current recycling protocols use aggressive acid/base conditions to regenerate cyclic monomers and mixtures of crosslinkers/fillers, etc. Over longer time periods all three types of elastomers undergo acid-catalyzed depolymerization initiated by HBr derived from bromine. However, we report that oxidative de-crosslinking with bromine of the Pt-cured elastomers occurs in minutes to hours to regenerate long chain silicone polymers, terminated with SiOH groups. It is therefore possible to devulcanize Pt-cured elastomers in the presence of RTV elastomers and reuse the high molar mass materials directly in new RTV elastomers, avoiding complete depolymerization, and thereby increasing the circularity of silicones. Oxidative bromination of hydrosilylation cured silicone elastomers occurs much more rapidly than acid-catalyzed depolymerization. The beta-effect facilitates SiC cleavage permitting decrosslinking to oils that can be reprocessed into RTV elastomers.

Increasing demand for fully recyclable polymers has prompted an interest in materials crosslinked via non-covalent interactions. Aggressive depolymerization can cleave backbone SiO bonds to generate linear oils or cyclic monomers from silicone rubbers, but one would rather recover the starting material oils simply by selectively breaking crosslinks. We demonstrate that ligand binding to metals can be used to reversibly crosslink silicone chains. Aminopropylsilicones were crosslinked via complexation with copper (II) acetate to form blue silicone oils. These oils slowly cured in air at room temperature to form soft, green elastomers over a month, or overnight at 50–55 °C in air to give robust, hard red elastomers. Potential explanations for the observed color changes are discussed. Elastomers prepared by either pathway underwent ready degradation by removal of the copper ions via competitive ligand binding using ethylenediamine, allowing for recovery of the unmodified silicone oil; the recovered amine could be reused with more copper to form a (softer) elastomer. This process provides a method for the synthesis of stable elastomeric silicones that are degradable on demand.

Elastomers, including thermoset silicone rubbers, are known for exceptional stability, which compromises their circularity; these materials are not expected to undergo facile degradation in the environment. Crosslinkers based on the protein gelatin were examined as a strategy to facilitate the biological degradation of silicone elastomers at end of life. Silicone hydrogels were prepared from aminopropylsilicones in ratios of gelatin : silicone from 33 : 67 -> 67 : 33 and fixed using formaldehyde. The physical properties of the copolymers depend on the degree of hydration, crosslink density and ratio of constituents, and follow an expected correlation with crosslink density. The products undergo degradation catalyzed by the enzyme bromelain to permit the recovery of protein fragments in the aqueous phase and, separately, silicone oils modified by protein constituents, suggesting that environmental decomposition may also be possible. Silicone-protein elastomers form with crosslinking provided by HCHO. The resulting product undergoes enzymatic degradation in 1-2 weeks to give silicone oils and protein fragments.

Steel from used automobile tires can be rendered suitable for recycling by stripping rubber contamination using reductive silylation with silicones.

Many strategies have been adopted to prepare silica materials with highly controlled structures, typically using sol–gel chemistry. Frequently, the alkoxysilanes used in sol–gel chemistry are based on monoalcohols, e.g., Si(OEt)4. The structural control over silica synthesis achieved by these precursors is highly sensitive to pH and solvency. Alkoxysilanes derived from the sugar alcohol glycerol (diglycerylsilane) react more slowly and with much less sensitivity to pH. We report that, in the presence of cooled aqueous starch solutions, glyceroxysilanes undergo transesterification with the sugars on starch, leading to (hollow) microtubules resembling worms of about 400 nm in diameter. The tubes arise from the pre-assembly of starch bundles, which occurs only well below room temperature. It is straightforward to treat the first-formed starch/silica composite with the enzyme amylase to, in a programmed fashion, increasingly expose porosity, including the worm morphology, while washing away untethered silica and digested starch to leave an open, highly porous materials. Sintering at 600 °C completely removes the starch silane moieties.

While the beneficial physical properties of silicone polymers are exploited in many sustainable applications, the high energy requirement for their synthesis compromises to a degree their sustainability. We report a strategy to mitigate this issue by filling the silicone with inexpensive and renewable starch. Elastomeric materials with covalently grafted starch, utilizing anhydride-modified silicones, permits loading of up to about 75% starch while maintaining many of the properties of the silicone. Alternatively, 50 wt.% starch-filled silicone foams can be prepared simply by mixing powdered starch with a mixture of HSi-functional silicone fluids in the presence of B(C6F5)3. The physical properties of the resulting foams are determined by the quantity of SiH, which controls the final density of the foams (ranging from 0.258–0.875 g mL−1), their Young’s modulus, and their degree of elasticity; both rigid and flexible foams were prepared. Materials with a high natural and renewable material content better adhere to green chemistry principle 7, should enhance the ease of degradation at end of life, and augment the sustainability of these silicone composites.

The myriad benefits of silicone polymers can be made more sustainable by replacing much of the elastomer body with acrylated soybean oil. The crosslinked copolymers are simply made without catalysts using an aza-Michael reaction.

Lipoamide formation, from lipoic acid and aminopropylsilicones, is accompanied by ring-opening polymerization to generate thermoplastic, silicone lipoamide copolymers. The materials are readily degraded by reduction of the disulfide linkages.

Improved sustainability is associated with elastomers that readily breakdown in the environment at end of life and, as importantly, that can be reprocessed/reused long before end of life arises. We report the preparation of silicone elastomers that possess both thermoplasticity-reprocessability-and antioxidant activity. A combination of ionic and H-bonding links natural phenolic antioxidants, including catechol, pyrogallol, tannic acid, and others, to telechelic aminoalkylsilicones. The mechanical properties of the elastomers, including their processability, are intimately linked to the ratio of [ArOH]/[H2NR] that was found to be optimal when the ratio exceeded 1:1.

Silicones are mostly utilized for their stability to a range of vigorous environmental conditions, which arises, in part, from the lack of functionality in finished products. The commonly used functional groups in silicones, e.g., SiH, SiCHCH2, are mostly consumed during final product synthesis. Organic functional groups may also be found in silicone products, including organic alcohols, amines, polyethers, etc., that deliver functionality not achieved by traditional organic polymers (e.g., aminosilicones, softening of fabrics; silicone polyethers, superwetting agricultural adjuvants). However, relatively little organic chemistry is practiced in commercial silicones, limiting the types of desirable functionality that can be attained. We report the utilization of a series of simple-to-practice organic reactions that take place efficiently on silicone oils to allow the preparation of a wide variety of functional silicones. The silicone oil starting materials typically act as both solvent and educt to allow many of the newer reactions, such as Click processes, to be used to tune the properties of both silicone oil and elastomer products. The review considers the concept of 'functionality' to include: the reactive groups used to enable synthesis of more complicated structures; and separately, the functional properties of the product silicones. One such property that is considered throughout is degradability at end-of-life, which is related to the sustainability of silicones.

The current paradigm in polymer chemistry increasingly involves controlled syntheses with better structural control and, consequently, products with narrow ranges of properties. Traditionally, silicone polymers are prepared under equilibrating conditions to give broad mixtures. The alternative, kinetically controlled ring opening polymerization at low temperature using organometallic initiators with promoters under inert atmospheres, can provide precise products. We report that fluoride, normally associated with desilylation reactions, may be productively used as a catalyst to prepare low dispersity polydimethylsiloxane polymers from D 3 (hexamethylcyclotrisiloxane); solvency is key to the process. Polymerization at 0 °C in water-depleted, but not completely anhydrous, solutions leads to polymers with different groups at their α,ω-termini, including F and, optionally, a functional silane at the termini, respectively. The Si-F group can be converted to an Si-OH group using base to give difunctional linear silicones with two different functional termini.

Natural antioxidants, such as vitamin E and eugenol, once grafted to silicone oils maintain their antioxidant activity and dilute the quantity of silicone needed for a given application.

The high refractive index aromatic compound, binaphthol (BINOL), is readily incorporated into silicone polymer chains using the Piers-Rubinsztajn (PR) reaction; alternating and random linear copolymers, and elastomers are available. The highest refractive index (RI) materials are BINOL rich. It is not possible to directly make high refractive index linear polymers with very short HSi-capped, telechelic silicone chains, as they do not react cleanly. However, chain extending short vinyl-capped BINOL macromers with simple arylsilanes using hydrosilylation leads to polymers with a molar mass of up to 8000 and refractive indices of up to 1.58. Elastomers are prepared using similar processes. The reactions are facile to practice and suggest BINOL can be harnessed in these and other processes to augment RI.