Mild Synthesis of a Dimethoxy-Terminated Siloxane through a Ring-Opening Reaction

Alkoxy-terminated siloxanes can be used as coupling agents to improve the properties of materials, and they are also widely used in organic synthesis. Herein, a method to synthesize a dimethoxy-terminated siloxane by the ring-opening reaction of 2,4,6-trimethyl-2,4,6-trivinyl-1,3,5,2,4,6-trioxatrisilinane (D₃-Vi) and methanol under mild condition was reported. The reaction does not require strong acids, strong bases or transition metal catalysts. The structure of the product was confirmed by ¹H-NMR, ²⁹Si-NMR and mass spectrometry (MS). The Gibbs free energy change of the reaction under the implicit solvent model was calculated by density functional theory (DFT), which proved the feasibility of the reaction. The calculation discussed the two-step mechanism of the ring-opening reaction. One methanol attacked D₃-Vi to form a monomethoxy-terminated product, and then another methanol attacked from the other side to form a dimethoxy-terminated product. The calculated reaction rate constants of the reaction at different temperatures indicate that the reaction has a tunneling effect, especially at low temperatures. Considering the complex reaction of D₃-Vi and methanol explained why the calculated rate constants at low temperatures were large but the ring-opening reaction could hardly occur. The mild synthesis can promote the application of alkoxy-terminated silicones.