Liquid-crystalline (LC) thermosets were prepared by the thermal cyclotrimerization of dicyanate compounds of ring-substituted bis(4-hydroxyphenyl) terephthalates. The resulting triazine networks formed a mesophase as the curing reaction proceeded, thereby enabling alignment in applied fields. Alignment of the LC phase during the curing process under the influence of a magnetic field produced oriented triazine networks with a smecticlike molecular organization. The effect of this anisotropy on the properties of these networks was investigated by both wide-angle X-ray diffraction and by measuring the coefficient of thermal expansion parallel and perpendicular to the direction of alignment. Remarkably well-oriented triazine networks were obtained (f ~0.50) with low values of the coefficient of thermal expansion observed (17 ppm/°C) in the direction of the applied field. The triazine networks have glass transition temperatures of ~190 °C and show little decomposition until ~440 °C. The alignment of the LC triazine networks induced by curing in magnetic fields was found to be thermally stable until at least 100 °C above the glass transition temperature.

The possible cross reactions indicated by solid‐state NMR between cyanate functionalized resin and epoxy functionalized resin have been investigated by using both natural abundance and labeled monofunctional model compounds. These soluble products were isolated and purified by silica gel adsorption chromatography and gel permeation chromatography. They were fully characterized by high resolution 1H‐, 13C‐, 15N‐NMR spectroscopy and by mass spectrometry. The major cross‐reaction product is a racemic mixture of enantiomers, which contain an oxazolidinone ring formed by one cyanate molecule and two epoxy molecules. However, epoxy consumption lags cyanate consumption in the overall reaction as triazine formation from the cyanate is much faster than the two competing reactions, the cross reaction between cyanate and epoxy, and the self‐polymerization of epoxy, under the conditions investigated. The cross reaction between cyanate and epoxy is limited. Approximately 12% of cross reaction between cyanate and epoxy was found in the overall reaction. In addition to the cross reactions of epoxy and cyanate, the reactions of epoxy and the carbamate, which is the major side product for the curing reaction of cyanate resin in solution, have also been investigated, and the mechanism of these reactions discussed. From the reactions of epoxy and carbamate, several products related to cross reaction between epoxy and cyanate have been isolated and identified. It is suggested that the reaction of epoxy and carbamate is one of the pathways in the overall cross reaction between epoxy and cranate resins. Finally, the mechanism of the overall cross‐curing reaction between the diepoxy and dicyanate mixed resins is discussed. © 1994 John Wiley & Sons, Inc.

SYNOPSISThe preparation of cyanate ester thermosetting resins by coreacting a fluorinated bisphenol A dicyanate (6F-BADCY) monomer with a monofunctional reactive cyanate, namely, the dinonyl phenol cyanate (DNPC) , is described in this paper. Our objective in this study was to modify the 6F-BADCY system with reactive diluents of a nonpolar nature and produce matrix resins processable at FR-4 epoxylike conditions. By introducing the monofunctional cyanate as a reactive diluent, it is expected that the molar crosslink density of the resulting network will be reduced. Highly branched triazine polymers will result from this approach. Dinonyl phenol cyanate was quantitatively synthesized by reacting dinonyl phenol with cyanogen bromide in the presence of triethylamine.