Can lewis acids initiate the polymerization of cyclic siloxanes via direct addition?

Following a preliminary report stating that ethylboron sesquitriflate initiates the polymerization of hexamethylcyclotrisiloxane (D 3 ), a more rigorous investigation was carried out to ascertain that cyclic siloxanes can indeed be polymerized by Lewis acids without the intervention (cocatalysis) of proton‐donor impurities. The evidence gathered with this strong boron Lewis acid suggested that direct initiation does occur, although its efficiency is much lower than that of an equivalent amount of a strong Brønsted acid. The use of a sterically hindered pyridine was a key factor in discriminating between monomer protonation and direct initiation.