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.