Free radical polymerization initiated via photoinduced intermolecular electron transfer process: kinetic study 3

Abstract

Various electron donors and electron acceptors have been tested in order to examine the possibility of the application of the Marcus equation to the description of the kinetics of free radical polymerization photoinitiated via photoinduced electron transfer (PET). Variations of the driving force of the electron transfer, for selected dyes (xanthene dyes, camphorquinone), were introduced by using: a series of tertiary aromatic amines (TAAs) and N-phenylglycine derivatives; a series of electron acceptors and only one type of electron donor (cyanine cations with tetraorganylborate anion); a mixed system with a series of benzophenones as electron acceptors and TAAs as electron donors. Several important conclusions follow from the experimental data. (1) For the case with the rate of PET much lower than the rate of diffusion controlled processes, the Marcus theory can be used for analysing or predicting the ability of organic redox systems for light-induced free radical polymerization. (2) For a process controlled by diffusion, the reactivity of free radicals formed as a result of PET limits the rate of initiation of polymerization. It is shown that this relationship can be also presented as a function of thermodynamic driving forces of the photoredox reaction ( − G°). For this type of process, the relationship between the rate of polymerization and − ΔG° is linear, indicating the ‘inverted-region-like’ kinetic behaviour. Additionally, it is shown that the bleaching process of the dyes competes with the polymerization photoinitiation in a way that suggests that, after free radical formation, two parallel reactions occur and that the rate of bleaching can be expressed as a fraction of the total rate of electron transfer.