Topochemical polymerization in microparticles of crystalline triazine-based monomers: Study by conventional and ultra-fast chip calorimetry

The work addresses complex thermal behavior of two triazine-based azide-alkyne monomers: 2-azido-4,6-bis (prop-2-yn-1-yloxy)-1,3,5-triazine (ABPOT) and 2,4-diazido-6-(prop-2-yn-1-yloxy)-1,3,5-triazine (DAPOT). Despite the similarity of the two AB2-type structures, the isothermal aging kinetics of DAPOT occurs significantly faster than that of ABPOT. During storage at room temperature, the chemical transformation starts in the crystalline state via topochemical reactions. Calorimetry, along with X-ray diffraction, makes it possible to probe transformations in isolated microcrystals including melting, oligomerization, polymerization and decomposition. The thermal behavior of microparticles can strongly differ from that of the same material in bulk due to enhanced contribution of the monomer evaporation at the particle surface at extremely high heating rates. The isothermal melting caused by the topochemical reaction and subsequent polymerization during long-term storage can be modeled using the correlated two-stage Avrami equation. The Avrami parameter corresponding to the amorphization process is close to unity while that of the polymerization is approximately 0.5. Consequently, both processes can likely be viewed as one-dimensional, with diffusion control dominating the polymerization process. The observed impact of long-term storage on the polymerization capability of the monomers may prove beneficial in synthesizing hyperbranched polymers through "green" topochemical methods.