Origin of the Rapid Trimerization of Cyanate Ester in a Benzoxazine/Cyanate Ester Blend

Blends of cyanate ester and benzoxazine have been independently studied by several researchers, and different reaction mechanisms were reported. Recently, we unexpectedly observe that gelation occurred in a 50 wt % methyl ethyl ketone solution of P-oda/BACY (1/1 mol/mol) blend after 24 h at 30 °C, in which P-oda is a 4,4′-oxyaniline/phenol-based benzoxazine and BACY is a dicyanate ester of bisphenol A. Previous studies suggest that the rapid trimerization of cyanate ester in the blend is related to the ring-opened structure of benzoxazine. However, the possibility of ring-opening polymerization for benzoxazine at 30 °C is rare. Therefore, it is highly likely that the catalytic effect results from the benzoxazine itself, not from the ring-opened structure of benzoxazine. Through IR and DSC analyses, we conclude that the tertiary amine of benzoxazine catalyzes the trimerization of cyanate ester, and we propose a three-step catalytic mechanism of benzoxazine for the trimerization of cyanate ester.