Macromolecules, Vol.53, No.17, 7487-7495, 2020
Lewis Acids As Highly Active Silanol Polycondensation Catalysts Affording Low Levels of Cyclosiloxanes
Silanol polycondensation is an important industrial process that is used to produce high-molecular-weight silicones for a wide array of applications. This technology is practiced commercially using strong Bronsted acids or bases that produce cyclosiloxane byproducts that can compromise performance properties and are also under increased regulatory scrutiny. We have found that Lewis acids are active silanol polycondensation catalysts that limit the formation of cyclosiloxane byproducts. Ligand donicity was found to have a dramatic effect on the activity of metal salts, as polycondensation catalysts with more weakly coordinating ligands-i.e., trifluoromethanesulfonate-displaying higher activity. Similarly, the Lewis acidities of the metal cations contribute strongly to the activity of these catalysts. Finally, the steric environment around the metal center can be tuned by coordination to a bis imine ligand to further suppress the formation of cyclosiloxanes.