Synthesis and properties of a series of cyanate resins based on phenolphthalein and its derivatives

Two new cyanate monomers simultaneously containing phthalide group and different alkyl substituents on the phenylene rings (o-PCY and t-PCY) and the Phenolphthalein-based cyanate (p-PCY) were successfully synthesized by the reaction of o-cresolphthalein, thymolphthalein and phenolphthalein with cyanogen bromide in the presence of triethylamine, respectively. Their chemical structures were confirmed by means of FTIR, NMR and elemental analysis methods. All these monomers owned sufficiently wide processing temperature windows and could be readily cured without the addition of catalyst. The dynamic mechanical analysis (DMA) results showed that the introduction of phthalide structure into the polycyanurate network could effectively improve the thermal properties of the cyanate ester (CE) resin. Especially, the Tg values of the fully cured p-PCY, o-PCY and t-PCY resins are 362 °C, 328 °C and 298 °C, respectively, which are apparently higher than that of most bisphenol-based cyanate resins reported in the literatures (190–290 °C). The thermal and thermo-oxidative properties as well as the water absorptions of the cured products were compared with those of the bisphenol A cyanate resin (BACY), and the structure–property relationships were explained according to the chemical structures and crosslinking densities of the formed polymer networks. The high-Tg thermosetting materials thus prepared are expected to expand the usage of CEs to areas where higher temperature requirements are encountered.