Generalization of the Kinetic Scheme for Photoinduced Polymerization via an Intermolecular Electron Transfer Process. 2. Application of the Marcus Theory

Abstract

This paper presents a theoretical description of the kinetics of free radical dye initiated photopolymerization via an intermolecular electron transfer process. Analysis considers the properties of organic redox pair forming initiating radicals. An application of the Marcus theory gives the kinetic scheme, which considers both the theromodynamic and kinetic aspects of the electron transfer process. The analysis shows that both the reactivity of free radicals resulting from the photoinduced intermolecular electron transfer (PET) process and the rate of the PET process can limit the rate of the polymerization initiation process. The theory is supported by experimental data. Several organic redox pairs forming free radicals have been tested. As the electron-accepting molecules, xanthene dyes and camphorquinone have been tested. As the electron donors, tertiary aromatic amines (TAAs) and N-phenylglycines (NPGs) were used. Several important conclusions are drawn from the theoretical and experimental data: (i) For the process with the rate of PET much lower than the rate of the diffusion-controlled process, the Marcus theory can be used for analyzing or predicting the ability of organic redox systems for light-induced free radical polymerization. (ii) For the process controlled by the diffusion, the reactivity of radicals formed as a result of the PET process limits the rate of the polymerization initiation. It is shown that this relationship can also be presented as a function of thermodynamic driving forces of the photoredox reaction (−ΔG°). (iii) Experimental results show that one can describe the rate of photoinitiated polymerization as a function of thermodynamic driving forces of the photoredox reaction (−ΔG°; described by the Rehm−Weller equation) of the organic donor−acceptor pair. For the photoinduced electron transfer occurring much slower than diffusion-controlled processes, the relationship between the rate of polymerization and −ΔG° presents a classical Marcus parabolic relationship. For PET controlled by diffusion, the relationship between the rate of polymerization and −ΔG° is dependent on the reactivity of free radicals resulting from the PET process and gives a linear relationship indicating the “inverted region-like” kinetic behavior.