Insights into Melting Behavior of Propyl-Bridged Di(cyanate ester) Monomers through Crystal Packing, Thermal Characterization, and Computational Analysis

Four isomeric propyl-bridged di(cyanate ester) monomers having an unexpectedly wide range of melting points were analyzed using X-ray crystallography, thermal analysis, and both empirical and semiempirical modeling, in order to examine the structure property relationships that determine the melting characteristics. The four monomers' crystal structures were determined, and intermolecular contacts and packing were examined. Enthalpies and entropies of melting were determined experimentally via differential scanning calorimetry and compared against two empirical models. Computational insights were provided by examining the monomers' energetic rotational barriers. Significant effects that could not be accounted for within the framework of either the empirical or semiempirical models altered the entropy of melting over a range spanning about 40% of the average value, while the enthalpy of melting varied over a range equivalent to 50% of the average value. These large variations, even within an isomeric series exhibiting a high similarity in chemical structure, combined with an apparent correlation between the two parameters, complicate the prediction of melting phenomena for technologically important molecules, even when data for close structural analogues are available.