Autoxidation: Catalyst-Free Route to Silicone Rubbers by Crosslinking Si-H Functional Groups
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 oC 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 at after thermal aging at 300 oC 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 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.