Achieving a Rapid and Selective Ring Opening Polymerization of Dimethylcyclosiloxanes via Potassium Silanolate Catalysis: Understanding the Role of Potassium Chelation on Polymerization Rate and Backbiting

Silicones are a unique class of polymers based on the Si–O repeat unit, with differentiated properties and numerous applications. Ring opening polymerization (ROP) of cyclosiloxanes is commonly industrially practiced; however, this process is plagued by high (∼13%) levels of residual cyclosiloxanes formed by competitive backbiting reactions. To meet customer specifications and regulatory requirements, these cyclics often need to be removed by vacuum stripping, an inefficient process. We demonstrate that commercially produced macrocyclic siloxanes (including tetradecamethyl cycloheptasiloxane, D7) can act as both a monomer and a ligand for traditionally practiced ROP catalysts and result in a significant rate enhancement as well as a suppression of the backbiting reaction which leads to cyclosiloxane formation. We show that this method can be applied to other macrocyclic siloxanes, but the full benefit is not observed when using nonsiloxane-based ligands. Modeling of the interaction between the propagating siloxane chain and the macrocyclic siloxane rings provides insights into the mechanism of backbiting suppression.