Controlled Rapid Formation of Polyurethane at 700 K: Thermodynamic and Kinetic Consequences of Extreme Photothermal Heating

The photothermal effect of nanoparticles has proven to be an effective means of substantially increasing the rate of chemical transformations, by factors of up to 10⁹. For thermally activated processes, such a large increase in rate implies a corresponding increase in temperature large enough that it would be expected to affect the steady-state concentrations of products and reactants. We test this hypothesis by following the exothermic reaction between hexamethylene diisocyanate and poly-bis(triethylol) heptanedioate to produce a cross-linked polyurethane under both ambient and photothermal conditions. We demonstrate that the photothermal effect increases the reaction rate by a factor of 7.4 × 10⁶ and decreases the effective equilibrium constant by a factor of at least 3 × 10⁴. These two changes provide kinetic and thermodynamic temperature estimates of 732 ± 21 and 683 ± 28 K, respectively. Remarkably, though both estimates of temperature are extreme, the chemical species produced under photothermal heating are the same as produced under ambient conditions.