Kinetic pathway investigations of three‐component photoinitiator systems for visible‐light activated free radical polymerizations

Abstract

Three‐component photoinitiator systems generally include a light‐absorbing photosensitizer (PS), an electron donor, and an electron acceptor. To investigate the key factors involved with visible‐light activated free radical polymerizations involving three‐component photoinitiators and 2‐hydroxyethyl methacrylate, we used thermodynamic feasibility and kinetic considerations to study photopolymerizations initiated with either rose bengal or fluorescein as the PS. The Rehm–Weller equation was used to verify the thermodynamic feasibility for the photo‐induced electron transfer reaction. It was concluded that key kinetic factors for efficient visible‐light activated initiation process are summarized in two ways: (1) to retard back electron transfer and recombination reaction steps and (2) to use a secondary reaction step for consuming dye‐based radical and regenerating the original PS (dye). Using the thermodynamic feasibility and kinetic data, we suggest three different kinetic mechanisms, which are (i) photo‐reducible series mechanism, (ii) photo‐oxidizable series mechanism, and (iii) parallel‐series mechanism. Because the photo‐oxidizable series mechanisms most efficiently allow the key kinetic factors, this kinetic pathway showed the highest conversion and rate of polymerization. The kinetic data measured by near‐IR and photo‐differential scanning calorimeter verified that the photo‐oxidizable series mechanism provides the most efficient kinetic pathway in the visible‐light activated free radical polymerizations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 887–898, 2009

The Comparison of the Photoinitiating Ability of the Dyeing Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism

“…On the other hand, under conditions where photosensitizer (dye) has both reduction and oxidation potentials, the photoexcited dye may act as both an electron donor and an electron acceptor, resulting in a parallel-series mechanism [3]. In this kinetic pathway, the electron transfer between the excited dye molecule and an electron donor competes with an electron transfer between the excited dye and an electron acceptor, as the primary photochemical reaction.…”

“…In this type of photoinitiating system, the photo-excited dye may act as either an electron acceptor (for example, if an amine is used as the second component), or an electron donor (for example, when an iodonium salt is used as the second component) [3]. Athouugh both reaction pathways are known, electron transfer from an electron donor to the photo-excited dye and the generation of radicals followed by either proton transfer from radical cation of electron donor or bond cleavage in electron donor is more common [3]. The intrinsic characteristics of two-component initiator systems leads to numerous kinetic limitations.…”

“…Figures 1-4 show the corresponding kinetic observed for N,N'-diethylthiacarbocyanine dye, and Table 1 shows the final conversions, polymerization rates and inhibition times for all runs after 5 min of irradiation. It has been reported that to enhance the kinetics of a visible-light activated initiation process, it is important to: (1) retard the back electron transfer and recombination reactions and (2) use the secondary reaction step to consume the nonproductive dye-based radical and thereby regenerate the original photosensitizer (dye) [3]. Figures 1-4 Figures 1-3 demonstrate that the photoreducible series mechanism (Cy/B2/second coinitiator) produces the highest conversion and the fastest rates of polymerization.…”

“…This process leads to an increase of the free radical polymerization rate. Therefore, certain additives improve the polymerization efficiency, leading to the development of the so-called three-component photoinitiating systems [3][4][5][6][7][8][9][10][11][12]. Three-component initiator systems have consistently been found to be faster, more efficient, and more sensitive than their twocomponent counterparts [3].…”

“…Many of the same photosensitizers used for two-component electron-transfer initiating systems may also be used in threecomponent ones [3]. Examples include coumarin dyes, xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, p-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines, squarylium dyes, and pyridinium dyes [3,[13][14][15][16][17].…”