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 redox 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 IV-phenylglycine (NPG) (electron donor) and (2) the Rose Bengal
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.
A series of sulfur-containing carboxylic acids (SCCA) was investigated as electron donors in photoinduced free-radical polymerizations, in conjunction with 4-carboxybenzophenone (CB) as sensitizer. These carboxylic acids included (phenylthio)acetic acid, S-benzylthioglycolic acid, 4-(methylthio)phenylacetic acid, 2-(methylthio)ethanoic acid, 4-(methylthio)benzoic acid, 2,2′-thiodiethanoic acid, and 3,3′-thiodipropionic acid. The results were compared to the simplest thioether, dimethyl sulfide. The mechanism of the radicals' formation was established using nanosecond laser flash photolysis and photochemical steady-state measurements of carbon dioxide formation. Photopolymerizations were carried out in aqueous solutions of acrylamide. Several conclusions follow from the experimental data: (1) The rates of polymerization of the CB/SCCA/acrylamide systems are affected by the yields of secondary processes that follow photoinduced electron transfer, e.g. decarboxylation or deprotonation. This is in addition to the reactivity of the free radicals themselves. (2) The highest initiation yield is observed for the system where there is efficient diffusion apart of the radical-ion pairs, followed by efficient decarboxylation. The resulting radicals are localized on carbons adjacent to the sulfur atom. (3) Free radicals with aromatic moieties are better initiators than are the analogous aliphatic radicals. (4) Radicals that retain the carboxylate moiety are inefficient initiators. (5) Photopolymerization of these systems appears to proceed by the conventional mechanism where termination is bimolecular.
We report a new method which uses fluorescence probe methodology to follow the kinetics of polymerization and postirradiation processes of multifunctional acrylates in real time.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.