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.

Polycondensation of α,ω-disilanols is a foundational technology for silicones producers. Commercially, this process is carried out with strong Brønsted acids and bases, which generates cyclosiloxane byproducts. Homoconjugated acids (a 2:1 complex of acid:base or a 1:1 complex of acid:salt), a seldom used class of silanol polycondensation catalysts, were evaluated for their ability to polymerize α,ω-disilanols while forming low levels of cyclosiloxane byproducts. Homoconjugated acid catalysts were highly active for silanol polycondensation, even when made from relatively mild acids such as acetic acid. Both the acid and base (or cation) component of the homoconjugated species was important for activity and avoiding cyclosiloxane byproduct formation. Stronger acids and bases were found to positively affect reactivity, and the pKa of the acid was found to correlate with cyclosiloxane byproduct formation. The individual components of the homoconjugated species (the acid and base) were ineffective as catalysts by themselves, and compositions with fewer than 2 mol of acid to 1 mol of base were much less reactive. Homoconjugated trifluoroacetic acid tetramethylguanidinium and tetrabutylphosphonium complexes were found to be privileged catalysts, able to give high-molecular-weight siloxanes (Mn > 60 kDa) while generating less than 100 ppm of octamethylcyclotetrasiloxane byproduct. Finally, a mechanism has been proposed where silanols are electrophilically and nucleophilically activated by the homoconjugated species, leading to silanol polycondensation.