Date of Award
Doctor of Philosophy (PhD)
Chemical & Biochemical Engineering
Jeffrey W. Stansbury
Christopher N. Bowman
Charles B. Musgrave
Light-activated polymerizations are important because they allow spatially and temporally controlled synthesis of polymers and polymer-based materials under ambient conditions. This capability is greatly valued in numerous applications, such as coatings, adhesives, sealants, electronics, diagnostics, dental materials, and biomaterials. Hence, the amount of precursors and synthetic routes available increases every year. However, there is limited understanding of the often intricate mechanisms via which many of these reactions work. As a consequence, this technology has not been exploited to its fullest. As a result, a need exists for the elucidation of refined mechanisms and kinetic models that aid in better understanding, predicting, and controlling such immensely valuable reactions for the production of practically relevant materials and devices. The present work delves into the refinement of the theoretical framework of free radically initiated chain growth polymerizations in solvent-free (bulk) monomer(s). This project originated to explain an unexpectedly long-lasting (>2000 s) latent polymerization observed after briefly exposing certain (meth)acrylic monomers, like 2-hydroxyethyl methacrylate, to visible-light in the presence of an organic photocatalysis composition including Methylene blue (MB+), Hünig's base and an Iodonium salt. Free radical chain growth photopolymerizations in bulk typically stop shortly (< 17 s) after irradiation is extinguished. Thus, it was clear that the available kinetic models and theories were not sufficient to account for this atypical, and potentially advantageous, phenomenon. We simultaneously monitored the photocatalyst (MB+) and monomer concentrations with UV-Vis and FT-IR spectroscopy, respectively, under several irradiation regimes. EPR spectroscopy was used to determine the nature and lifetime of the light-generated radical intermediates. Rheology confirmed that the vinyl groups consumed in the dark are in fact being polymerized. Quantum chemical calculations guided the experiments and supported the proposal of a photocatalytic mechanism via which reactive initiating free radicals can be produced long after the irradiation is extinguished. With these results, the unusually extended latent polymerization was explained by two mechanistic conclusions: 1) organic photocatalysis using MB+/Hünig's base/Iodonium salt stores energy during irradiation in the form of Leuco Methylene Blue via an e-/H+/e- transfer process instead of the typical single e- transfer; then, LMB is later used to produce radicals upon reaction with the Iodonium salts for even thousands of seconds after light cessation, and 2) hydrogen bonding exacerbates the Trommsdorff-Norrish effect via which bimolecular termination is hindered, thus resulting in the extension of the vinyl polymerization in the dark by the well-documented radical occlusion process.
Aguirre Soto, Hector Alan, "Latent free radical polymerization of bulk methacrylates: Organic visible-light photocatalysis and supramolecular effects" (2015). Chemical & Biological Engineering Graduate Theses & Dissertations. 73.