Fast synthesis of high molecular weights polydiethylsiloxanes and random poly(dimethylsiloxane-co-diethylsiloxane) copolysiloxanes via cyclic trimeric phosphazene base catalyzed ring-opening (co)polymerization

Abstract

Polysiloxanes containing diethylsiloxane units with unique low temperature flexibility have been widely applied under extreme conditions in many fields. However, the synthesis of polydiethylsiloxane (PDES) and its copolymers was challenging due to the easy accessible polymerization-depolymerization equilibrium between linear propagating chains and cyclosiloxane byproducts in the ring-opening polymerization (ROP) of hexaethylcyclotrisiloxane (D3Et) and the large reactivity gaps among different cyclosiloxanes in the copolymerization. In this work, PDESs were successfully prepared via cyclic trimeric phosphazene base (CTPB) catalyzed ROP at room temperature in high yields. The ROP of D3Et proceeded efficiently with low CTPB loading (low to 0.01 mol%) and the resulting PDES homopolymers had high molecular weights (up to 404.0 kg/mol). Moreover, poly(dimethylsiloxane-co-diethylsiloxane) (PMES) random copolymers with tunable diethylsiloxane contents (20 ∼ 87 mol%) can be directly prepared via ring-opening copolymerization (ROCP) of D3Et and octamethylcyclotetrasiloxane (D4) using CTPB as catalyst within 5 min at ambient temperature. The reactivity ratio of D3Et (r = 1.04) and D4 (r = 0.89) suggested the comparable reactivity between D3Et and D4 to ensure their random copolymerization characteristics, thus proving the unique high efficiency of CTPB catalyst applied here. Microstructure characterizations of the PMES copolymers by 1H and 29Si NMR verified the controlled copolymer composition and the random distribution of diethylsiloxane and dimethylsiloxane units. The thermal property investigations by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) indicated the good low-temperature flexibility and thermal stability of PMES copolymers, displaying great potential in extreme environmental applications.