In Situ NMR to Monitor Bulk Photopolymerization Kinetics

The ability to precisely measure photopolymerization kinetics is paramount to controlling curing characteristics and material properties in photocurable systems. Traditional methods used to measure kinetics, such as real-time Fourier-transform infrared spectroscopy (RT-IR), are often limited by broadening mechanisms intrinsic to the system and poor spectral resolution. In this work, we present an in situ NMR spectroscopy technique to monitor bulk photopolymerization reactions wherein the polymer system is separated from the locking solvent via a capillary insert and photoexcited using an LED-coupled fiber optic. This technique allows for the isolated observation of the bulk reaction system while also benefiting from the high spectral resolution and rich chemical information offered by NMR. Relative rate constants (k_p') and ultimate monomer conversion were determined for four systems: neat hexyl acrylate, isobornyl methacrylate, N,N-dimethylacrylamide, and hexyl acrylate with cross-linker. By observing kinetic data of simple photopolymer systems, this work demonstrates the utility of in situ NMR photopolymerization as a complementary technique to conventional RT-IR and other methods for the kinetic monitoring of bulk photopolymer materials.