Nitrile-Functionalized Polysiloxanes with Controlled End Groups for Elastomeric Networks

High-permittivity polysiloxanes are attractive for applications in dielectric actuators, sensors, energy devices, and electrolytes. A major challenge is the synthesis of polar polysiloxanes with well-defined end groups suitable for controlled cross-linking while minimizing cycle content that can compromise performance. Here, we report the synthesis of polysiloxanes bearing 3-cyanopropyl side groups and aminopropyl or vinyl end groups, with controlled molecular weights and reduced cyclic byproducts. While the reactions in chlorinated solvents predominantly give short chains, the nonchlorinated solvents favor cycle formation. In contrast, hydrolysis-condensation of (3-cyanopropyl)-methyldichlorosilane under solvent-free conditions yields high-molecular-weight polymers (≈14 kg mol^-1) with only 11% cycles, which are readily removed by toluene extraction. Additional polymer growth is achieved via anionic ring-opening polymerization of isolated cycles, yielding polymers up to 25 kg mol^-1. Finally, silanol end groups are quantitatively converted into aminopropyl (100%) or vinyl groups (92%), which are useful for cross-linking the polymers to elastic networks with improved structural control.