Synthesis and characterization of new phosphorus and other heteroatom containing aryl cyanate ester monomers and networks

Abstract

Several aromatic dicyanate monomers have been synthesized bearing para-linked strong electron withdrawing groups, such as phenylphosphine oxide, sulfone, and carbonyl. These groups increased the reactivity of the cyanate functional groups and eliminated the need for curing catalysts. However, an undesirable decrease in the processing window between the monomer melting point and the onset of cure was also generally observed. An arylene ether phenyl phosphine oxide system was designed that displayed several attractive characteristics such as a low softening point, a wide processing window, cure with no catalyst, high Tg and high char yield in air, suggesting that these new thermosets might show good fire resistance. The dicyanate ester monomers were synthesized in high yield by reacting various bisphenols with cyanogen bromide in the presence of triethylamine. The high reactivity of the cyanate functional groups required that the cyanation reaction be conducted at temperature below 0°C in order to prevent imidocarbonate side reactions. Proton NMR and FT-IR were used to characterize these monomers. The cyclotrimerization curing process was monitored by the disappearance of the carbon-nitrogen triple bond stretch (2270 cm−1). An optimal cure schedule was determined and the cured polycyanurate networks were characterized by DSC, DMTA, and TGA. Tg values were typically > 250°C and 5% weight loss values were observed by TGA in air above 400°C. Several of the dicyanate monomers with sufficiently large processing windows were cured into single lap shear adhesive bonds onto titanium 6/4 and the measurements are reported herein. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 977–987, 1997