Latent free radical polymerization of bulk methacrylates: Organic visible-light photocatalysis and supramolecular effects

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

On the role of -vinylpyrrolidone in the aqueous radical-initiated copolymerization with PEGDA mediated by eosin Y in the presence of O[subscript 2]

The photochemistry of eosin Y has attracted attention for its role in visible-light induced polymerization reactions that proceed in the presence of ambient oxygen to form various macromolecular architectures that are useful for a wide range of applications, including biosensing and drug delivery. N-Vinylpyrrolidone (NVP) has been employed as a comonomer in the eosin-mediated synthesis of hydrogels with polyethylene glycol (PEG) based multifunctional monomers to aid in reducing oxygen inhibition and enhancing the rate of radical polymerization and the final conversion. However, the mechanism by which NVP reduces the oxygen inhibition time (t[subscript inh]) remains unclear. Additionally, no investigations were found on the integration of NVP into the radical-generating photocatalytic mechanism of eosin Y. Towards a better understanding of eosin-mediated synthesis of PEG-based hydrogels, we analyzed the effect of NVP on the steady-state kinetics of the aqueous NVP/PEG-diacrylate copolymerization reaction. In this case, the reduction in t[subscript inh] is lower than that reported for copolymerization with neat (meth)acrylate monomers. We propose the formation of a ground-state complex between eosin and NVP as the main reason for the reduction in oxygen inhibition and contrast it with previous theories. In addition, we discuss the role of this eosin/NVP complex and cross-propagation kinetics to explain the ∼70% increase in the initial rate of polymerization upon addition of NVP. The effect of cross-propagation kinetics is enhanced at the later stages, leading to a 10% increase in final vinyl conversion in this relatively mobile network. By analyzing the change in the scaling of the eosin decay coefficient as a function of light intensity during and after oxygen inhibition, we then link eosin inactivation to radical termination kinetics. Finally, we discuss the role of radical recombination between semireduced eosin and the propagating radicals as an additional eosin inactivation route by which leuco-eosin ends tethered to the network. These insights contribute to a thorough understanding of visible-light activated polymerization in the presence of oxygen and of the role of NVP in eosin-mediated radical production.Consejo Nacional de Ciencia y Tecnología (Mexico) (Award I0010-2015-01-263622/I0010-2016-02-275449)Kwanjeong Educational Foundation (Korea) (Scholarship)National Science Foundation (U.S.). Graduate Research Fellowship ProgramBurroughs Wellcome Fund (Career Award

UV-Vis/FT-NIR in situ monitoring of visible-light induced polymerization of PEGDA hydrogels initiated by eosin/triethanolamine/O₂

In conjunction with a tertiary amine coinitiator, eosin, a photoreducible dye, has been shown to successfully circumvent oxygen inhibition in radical photopolymerization reactions. However, the role of O₂ in the initiation and polymerization processes remains inconclusive. Here, we employ a UV-Vis/FT-NIR analytical tool for real-time, simultaneous monitoring of chromophore and monomer reactive group concentrations to investigate the eosin-activated photopolymerization of PEGDA-based hydrogels under ambient conditions. First, we address the challenges associated with spectroscopic monitoring of the polymerization of hydrogels using UV-Vis and FT-NIR, proposing metrics for quantifying the extent of signal loss from reflection and scattering, and showing their relation to microgelation and network formation. Second, having established a method for extracting kinetic information by eliminating the effects of changing refractive index and scattering, the coupled UV-Vis/FT-NIR system is applied to the study of eosin-activated photopolymerization of PEGDA in the presence of O₂. Analysis of the inhibition time, rate of polymerization, and rate of eosin consumption under ambient and purged conditions indicates that regeneration of eosin in the presence of oxygen and consumption of oxygen occur via a nonchain process. This suggests that the uniquely high O₂ resilience is due to alternative processes such as energy transfer from photo-activated eosin to oxygen. Uncovering the intricacies of the role of O₂ in eosin-mediated initiation aids the design of O₂ resistant free radical polymerization systems relevant to photonics, optoelectronics, biomaterials, and biosensing.United States. Department of Defense (W81XWH-13-1-0272