Upgrade Traditional KOH Catalyzed Ring-Opening Polymerization of Cyclosiloxanes to More Efficient Process by Adding Catalytic Phosphazenium Salt as Cocatalyst

The development of efficient catalyst to overcome the limitation of equilibrium ring-opening polymerization (ROP) of cyclosiloxanes represents the most appealing solution to resolve the back-biting side reaction and high energy consumption during the traditional polysiloxane production. In this contribution, equilibrium polymerization of octamethylcyclotetrasiloxane (D₄) catalyzed by KOH at high temperature was shifted to more efficient polymerization at ambient conditions via fast kinetics and back-biting suppressing by adding a catalytic amount of phosphazenium salt as cocatalyst. Polydimethylsiloxanes (PDMSs) were synthesized in high conversion (≥90%) by bulky phosphazenium salt P₅Cl/KOH catalyzed ROP of D₄ at room temperature in THF solution. The ratio of P₅Cl could be lowered to 0.001 mol % of D₄, and the back-biting reactions were negligible as evaluated by in situ ¹H NMR characterization of the polymerization system. Kinetics investigations suggested that the fast and controlled manner of the ROP was promoted by the P₅Cl/KOH combination. Furthermore, bulk ROP of D₄ and octaphenylcyclotetrasiloxane (P₄) catalyzed by activated phosphazenium salt P₅OMe conveniently afforded PDMSs with different molecular weights (up to 1616 kg mol^–1) and poly(dimethylsiloxane-co-diphenylsiloxane) (PMPS) copolymers with different diphenylsiloxane contents (8.4–63.8 mol %) at 100 °C. The good random characteristic of PMPS copolymers were thoroughly verified by ¹H/²⁹Si NMR and thermodynamic measurements.