Gemini ionic crosslinks produced by neutralization of dicarboxylic and diamino silicones lead in a facile manner to thermoplastic silicone elastomers.

Most traditional methods for silicone elastomer preparation involve catalysts whose incomplete removal can result in compromised thermal stability of the product elastomers and/or safety concerns. Herein, we report the oxidative curing of hydrosilane (H–Si) functionalized polymers to generate silicone elastomers at 250 °C in air but in the absence of catalysts. Both oxygen and Si–H groups are required for the oxidative curing process to occur; standard Me₃Si-terminated silicone polymers exhibited no change under the same conditions. There are essentially no restrictions on molecular weights and the nature of Si–H (terminal or pendant) groups exploited in the precursor HSi-containing polymers. The product elastomers are bubble-free, transparent and colorless; they do not exhibit any yellowing after thermal aging at 300 °C in air for 7 days. The moduli can be easily tuned by curing time and [HSi] in the precursors. Chain extension arises from conversion of SiH groups to Si–O–Si linkages, and crosslinks and chain extension are produced via radically induced ether formation (first step: RR′R′′SiH + O₂ + MeSi-silicone → RR′R′′SiO-CH₂Si-silicone + oxygen species). The efficiency of the radical chain reaction and the resulting crosslinking processes increase with temperature. This process provides a facile, economic, green and catalyst-free way to manufacture silicone elastomers from inexpensive, readily available starting 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.

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.