GENERALIZATION OF THE KINETIC SCHEME FOR PHOTOINDUCED POLYMERIZATION VIA AN INTERMOLECULAR ELECTRON-TRANSFER PROCESS .2. APPLICATION OF THEMARCUS THEORY

This paper presents a theoretical description of the kinetics of free radical dye initiated photopolymerization via an intermolecular electr on transfer process. Analysis considers the properties of organic redo x pair forming initiating radicals. An application of the Marcus theor y gives the kinetic scheme, which considers both the theromodynamic an d kinetic aspects of the electron transfer process. The analysis shows that both the reactivity of free radicals resulting from the photoind uced intermolecular electron transfer (PET) process and the rate of th e PET process can limit the rate of the polymerization initiation proc ess. The theory is supported by experimental data. Several organic red ox pairs forming free radicals have been tested. As the electron-accep ting molecules, xanthene dyes and camphorquinone have been tested. As the electron donors, tertiary aromatic amines (TAAs) and N-phenylglyci nes (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, t he Marcus theory can be used for analyzing or predicting the ability o f organic redox systems for light-induced free radical polymerization. (ii) For the process controlled by the diffusion, the reactivity of r adicals formed as a result of the PET process limits the rate of the p olymerization initiation. It is shown that this relationship can also be presented as a function of thermodynamic driving forces of the phot oredox reaction (-Delta G degrees). (iii) Experimental results show th at one can describe the rate of photoinitiated polymerization as a fun ction of thermodynamic driving forces of the photoredox reaction (-Del ta G degrees); described by the Rehm-Weller equation) of the organic d onor-acceptor pair. For the photoinduced electron transfer occurring m uch slower than diffusion-controlled processes, the relationship betwe en the rate of polymerization and -Delta G degrees presents a classica l Marcus parabolic relationship. For PET controlled by diffusion, the relationship between the rate of polymerization and -Delta G degrees i s dependent on the reactivity of free radicals resulting from the PET process and gives a linear relationship indicating the ''inverted regi on-like'' kinetic behavior.