Kinetic and Mechanistic Insights into Thermally Initiated Polymerization of Cyanate Esters with Different Bridging Groups

This study investigates the kinetics of thermally initiated polymerization of aryl cyanates with different bridging fragments ((CH3)(2)C, S, and O). To minimize the effect of impurities on the kinetics, the aryl cyanates are synthesized in a highly pure form. The rates of polymerization are measured by differential scanning calorimetry (DSC) and treated by an advanced isoconversional method. The DSC measurements indicate that the ((CH3)(2)C- and S-bridging monomers have, respectively, the lowest and highest reactivities. The observed variation of the activation energy with conversion and temperature is determined to be in agreement with the two-step kinetic model by Kamal. Estimating the parameters of the Kamal model has provided mechanistic insights into the polymerization process. The enhanced reactivity of the S-bridged monomer is found to be associated with acceleration of the first step that demonstrates an increased value of the preexponential factor. The acceleration is linked to higher polarizability of this monomer that can promote stronger intermolecular interaction and preferred orientation of the reacting species.