Polymerization Reactions of Polysilazane-Catalyzed Cyanate Esters

Two types of polysilazane polymers, polydimethylsilazane (PMS) and polymethylsilazane (PHS), were synthesized through ammonolysis. PMS and PHS were used to catalyze the polymerization of cyanate ester resin (BADCy), resulting in the prepolymers, PMS-BADCy and PHS-BADCy, respectively. The structures of polysilazanes and prepolymers were characterized using Fourier ransform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). Catalytic polymerization mechanism and curing process of prepolymers were examined, and the properties of the resulting cured resin products were also verified. Results indicated that polysilazanes effectively catalyzed the polymerization of cyanate ester resins at 80 ℃, and catalyzed the formation of triazine ring structures in BADCy resin. Research into the prepolymerization mechanism showed that during the catalytic polymerization of cyanate ester resins by polysilazanes, highly reactive intermediates containing ―O―Si―NH―CN structures were formed. These intermediates subsequently reacted with ―C≡N to form dimers or oligomers containing ―C=NH structures, followed by cyclotrimerization to form symmetric or asymmetric triazine ring structures. The cured resin exhibited excellent thermal mechanical properties, superior thermal stability, and improved moisture resistance. The cured product exhibited a dense, homogeneous structure. Polysilazane-catalyzed cyanate ester resins showed significantly enhanced mechanical properties compared to pure cyanate ester resins. It is very promising that the reaction between ―OCN and polysilazane may be used to reduce the prepolymerization temperature and achieve controllable prepolymerization. The prepolymers have a wide processing window and exhibit a lower curing temperature and curing exothermic enthalpy when compared to BADCy.