Kinetics of multifunctional thiol-epoxy click reactions studied by differential scanning calorimetry: Effects of catalysis and functionality

Isothermal reaction kinetics of thiol-epoxy (multifunctional thiol-difunctional epoxy) reactions with and without external catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were studied using differential scanning calorimetry (DSC). Autocatalytic behavior is observed both without DBU and with DBU, which is attributed to autocatalysis effects of hydroxyl products of the reaction. The isothermal curing kinetics are well described by the Kamal autocatalytic model, which is expressed as dα/dt = (k₁ + k₂α^m)(1 - α)ⁿ (α: fractional conversion; dα/dt: apparent rate of reaction). Parameters of the reaction, including reaction orders m and n, apparent rate parameters k₁ and k₂, and activation energies are determined by fitting the experimental data to the Kamal model. For the thiol-epoxy reaction without DBU, the overall reaction order (m + n) is approximately 3 (with m ≈ 2 and n ≈ 1) but decreases to 2 (with m ≈ 1 and n ≈ 1) upon addition of DBU. Based on the most commonly described mechanism for nucleophilic thiol-epoxy reactions in the literature, new reaction schemes are proposed by incorporating the hydroxyl autocatalysis effect as well as the effect of external catalyst DBU on apparent reaction order. The proposed reaction schemes allow for excellent fits to reaction kinetics for thiol-epoxy reactions with and without DBU. In contrast to the strong effect of external DBU catalyst on reaction kinetics, changing the thiol monomer functionality leads to relatively small changes in apparent rate parameters and no change in the order of reaction or reaction mechanism.