Eosin termination rate

The decrease in the termination rate is attributed to the efficient capture of semireduced Eosin radical by diphenyliodonium. The rate constant of the reaction shown in Eq. (30) was determined by laser flash photolysis as 1 x 1010 M-1s-1. 

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

We, as well as Chesneau and Fouassier, find that the photospeed increases linearly with light intensity. From this observation one can conclude that chain termination reaction is not the usual interaction between two macroradicals. We have measured the initial rate of photopolymerization using thin foil calorimetry and find a linear relationship between the rate of photopolymerization at low conversions (less than 15%) and the absorbed light intensity. Using the same monomer but with a different photoinitiator (to be discussed in detail later) we observe an order of one half with respect to light intensity both by thin foil calorimetry and by measuring the polymer spike. Therefore we conclude that the linear dependence observed for the Eosin-triethanolamine system is real and not an artifact of the technique employed to determine the photospeed. 

To explain the reduced rate of termination observed in the presence of PDO, the authors propose that one of the termination reactions involves the protonated form of the semireduced Eosin radical which, by reaction with PDO, is reduced to the leuco dye (Eqs. 21 and 22). 

Fouassier and Chesneau [219] is not consistent with the experimental observations. From the values of the rate constants of triplet decay presented in Table 8, and taking into account that k3/k2 = 0.23 (as determined by Kasche and Lindqvist), we calculate the quantum yield of D + under the polymerization conditions. For Eosin (3 x 10 5 M) and MDEA (0.1 M) the yield of semioxidized Eosin radical is 4 x 10 3 M in the presence or in the absence of 2 x 10 3 M PDO. From the values for the quantum yield of photopolymerization and the molecular weight in the absence of PDO we calculate a quantum yield of initiation between 0.086 and 0.17, the actual value depending on the mode of termination. Therefore, we conclude that formation of a-amino radicals according to Scheme 10 represents only a minor contribution to the quantum yield of initiation observed in the presence of PDO. 

The photoinduced polymerisation of methacrylic acid by sodium peroxide has been determined to have an activation energy of 17.7 kJ/molel03. From this the macroradical lifetimes were determined to be 3.9 secs. In the intermittent photoinitiation of methyl methacrylate-styrene the dependence of the rate of propagation on monomer feed composition was understood in terms of a penultimate model and not a terminal model as previously assumed 4 with some evidence for cross-termination. An eosin-periodate combination has been found to induce the photopolymerisation of acrylonitrile where the dye was found to act as both the sensitiser and reducing agent. Hydroxyl radicals are assumed to initiate polymerisation while the N-bromosuccinimide induced photopolymerisation of N,N -methylenebisacrylamide is faster in 2-propanol solution O . 

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