Generalization of the Kinetic Scheme for a Dye-Photosensitized Free-Radical Polymerization Initiating System via an Intermolecular Electron-Transfer Process. Application of Marcus Theory

Abstract

We present a theoretical description of the kinetics for dye-initiated photopolymerization via an intermolecular electron-transfer process which considers the properties of the organic redow pair forming initiating radicals. An application of the Marcus theory yields a kinetic scheme, which considers both the thermodynamical and kinetic aspects of the electron-transfer process. The intermolecular electron transfer is the limiting step in the polymerization initiation. The theory is supported by experimental data. Two organic redox pairs forming free radicals have been tested, (1) a series of pyrazolone azomethine dyes (PAD) (electron acceptors) and N-phenylglycine (NPG) (electron donor) and (2) the Rose Bengal derivative (RBAX) (C2' benzyl ester, sodium salt), serving as an electron acceptor and series of tertiary aromatic amine (TAA) electron donors. The following conclusions are reached: (i) The experimental data demonstrate the inverted region or inverted-region-like kinetic behavior, e.g., the rate of polymerization decreases with an increasing thermodynamic driving force (-ΔG o ) for electron transfer. This behavior allows the use of the Marcus theory for analyzing or predicting the ability of organic redox systems for light-induced free-radical polymerization. (ii) The dependence of the rate of polymerization on ΔG o suggests that the dark stability of the monomer-initiating system mixture may be due to the excited-state activation energy (E 00 ). (iii) Considering the reorganization energy factor.L (for the reacting molecules and the monomer), one may suspect that the molecular geometry and structure effect the photoinitiation efficiency