However, both quantities require tedious compositional measurements and are restricted to low degrees of monomer conversions. The authors demonstrated that in situ photopolymerization enables the direct measurement of both f1 and F1 in real time even to high degrees of monomer conversion, thus allowing for the determination of reactivity ratios through high conversions. On the other hand, since kinetic constants are generally affected by solventmonomer interactions, accurate predictions of kinetic behavior of bulk polymerization reactions (e.g., additive manufacturing) are challenging on the basis of data collected in solution. Furthermore, as part of a broader move to sustainable materials and environmentally friendly processing techniques, bulk photopolymerizations avoiding the use of VOC solvents have become increasingly popular in recent years. The inherent need for a deuterated locking solvent in NMR spectroscopy thus poses a challenge to obtaining kinetic data of bulk photopolymer systems. In this work, we employed in situ NMR to monitor the kinetic evolution of the bulk photopolymerization of linear and crosslinked hexyl acrylate (HA), N,N-dimethylacrylamide (AAm), and isobornyl methacrylate (IBOMA) systems using a fiber optic cable to direct UV-light into the spectrometer (Figure ). Monomer conversion was monitored and polymerization rate constants (kp') were obtained from linearized conversion data using the steady state assumption. By performing a broad range of bulk photopolymerizations using this technique, in situ NMR spectroscopy has provided unique insight into the kinetic nature of photopolymer reactions and has demonstrated the potential as a powerful and complementary tool to examine photo-polymerization kinetics.