Kinetics of polycyclotrimerization of aromatic dicyanates

Polycyclotrimerization of 4,4′-thiodiphenylcyanate (TDPC) and bisphenol-A dicyanate (BADC) were studied and compared by means of differential scanning calorimetry. Samples of different impurity levels (most likely residual moisture) or different catalyst loadings (n-nonylphenol) were studied. Moisture absorbed may indeed function as a catalyst. In the absence of residual phenols or added catalyst, the polycyclotrimerization of identical species shared the same apparent activation energy and all followed an autocatalyzed first-order rate expression. The autocatalytic effect was justified by the cure behavior of partially cured monofunctional cyanate (p-diphenyl cyanate). The rate expression obtained by the dynamic method is still available for the isothermal method. In the presence of an externally added catalyst (n-nonylphenol, NP), the polycyclotrimerization proceeded at obviously lower temperature as compared with the uncatalyzed case. In addition to the expected lowering of activation energy (independent of the catalyst concentration), the cure reaction exhibited an autocatalyzed higher order kinetics (second order for TDPC system and 1.7-th power for BADC system). These kinetic observations were explained in terms of proposed mechanistic scheme in which the competition between hydroxyl-catalyzed and autocatalytic paths is considered. In particular, formation of NP aggregates ((NP)_m, where m = 7 for TDPC system and m = 5 for BADC system, respectively) is proposed to explain the less-than-additive catalytic capacity with increasing added NP level.