(790c) Investigation of Diffusion-Controlled Kinetics in Free-Radical Phototpolymerizations Initiated Via Photoredox Catalysis

Photopolymerization reactions are ubiquitous in the production of materials such as dental restorative composites, contact lenses, photolithographic resists and optoelectronic coatings[1]. Initiation, propagation and termination kinetics have been characterized, and models have been developed to provide accurate predictions for polymerization rates, and conversion profiles[2],[3],[4]. Nonetheless, Type I photoinitiators, which generate radicals through homolytic cleavage upon light absorption, have been almost exclusively utilized. However, in many of the currently expanding applications of photopolymerized materials, more complex radical-generating reactions are required or preferred, e.g. Type II photoinitiators that produce radicals through hydrogen abstraction are dominant in dentistry, which relies on visible light activation[5]. This leads to an even more challenging kinetic system since the complex polymerization mechanism is intertwined with bimolecular reactions that produce the primary radicals. Thus, initiation is also affected by the diffusion constraints imposed by the developing polymer matrix; and ultimately, the polymerization profiles can change significantly with varying types of crosslinking monomers. There is a relative absence of comprehensive studies to elucidate and predict polymerization kinetics initiated by two- or multi-component photoinitiators, which is the focus of this investigation. 

            More specifically, three-component photoinitiators that generate radicals via photoredox catalysis have recently gained attention as alternatives to expand the photoinitiators portfolio into the visible light region[6]. Photoredox catalysis essentially involves the reduction and oxidation of a chromophore. Since these two occur in series, the chromophore acts as a photocatalyst, as it is regenerated. In several cases it has been reported that the catalytic nature of these photoinitiators actually leads to an acceleration of the radical production rate[7]. For example, methylene blue chromophore can be photoreduced by a tertiary aliphatic amine, and then oxidized back to its original state by an onium salt[8]. Radicals are generated via this cycle, efficiently initiating chain-growth polymerization of acrylates and methacrylates. Nonetheless, the complex series of reactions is very diffusion-sensitive and varies significantly in different monomers. It becomes important then to look into the convolution between dramatically changing polymerization-based mobility restrictions and photoredox initiation since efficient utilization of the photoinitiators is paramount to the application of photopolymerization, i.e. unreacted initiator concentration is required to be minimum, and fast polymerization rates are preferred.

            In this work, we first obtained the kinetic constants and reaction orders for the photoreduction and ground-state oxidation reaction of the methylene blue-sensitized photoredox catalysis photoinitiator. The latter was achieved utilizing an in-house made ‘batch micro-reactor’ set-up, in which we can simultaneously monitor chromophore and monomer consumption. We then use these parameters to develop a comprehensive mathematical model that couples ‘Marcus’ type kinetics for the photoreduction by the amine, and redox kinetics for the ground-state oxidation by the onium salt, to the changing diffusion via a screening parameter for the bimolecular reactions. We show modified profiles of how radical production does not remain necessarily constant during photopolymerization. Also we find optimum irradiation conditions to achieve maximum phoinitiator consumption, as well as rate of polymerization in a series of mono- and di-vinyl monomers by modifying photoinitiator concentrations and light intensities. Ultimately, this work serves as a guide to understand how to better take advantage of the photocatalytic nature of this type of photointiator to achieve fast photopolymerizations with the mildest irradiation regimes possible.

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