Assessing reactivity of cyanate esters in [2+2+2] cyclotrimerization: Meeting the challenge by augmenting differential scanning calorimetry with kinetic computations

Abstract

[2+2+2] cyclotrimerization of cyanate groups is a key to understanding the cyanate esters polymerization. A proper comparison of the monomers reactivity requires kinetic measurements to be made under the same tempertaure conditions, which is rarely possible due to drastically different physical properties of cyanate esters. A case in point is structurally similar cyanate esters differing by a number of p-phenylene units considered in the present study. Their melting temperatures differ by more than 170 °C. The challenge of assessing the pertinent reactivities is proposed to be addressed by augmenting calorimetric data collected for each monomer in its reaction temperature range with a special computational procedure that reduces such data to the same set of isoconversional-isothermal conditions. Isoconversional kinetic analysis of the cyclotrimerization of the three studied cyanate esters demonstrates that the corresponding activation energies are markedly different and span the range 50-100 kJ mol−1. The evolution of the activation energy with conversion is significantly different for all studied monomers because of the reaction mechanism sensitivity to residual parental phenols and water impurities, whose concentrations differ by an order of magnitude as evaluated by HPLC and 1H NMR spectroscopy. Despite such drastic differences the approach applied permits accomplishing a quantitative assessment of the reactivity of the studied cyanate esters. The resulting reactivity is discovered to depend on conversion and temperature. Mechanistic insights into this phenomenon are provided.