Monomer quenching

The high rate constants for chemical quenching of triplet ketones by amines provide two sidelights of considerable importance. Photoinitiation of polymerization has received widespread and varied industrial applications. One problem is that many vinyl monomers quench triplet ketones very rapidly either by charge transfer or energy transfer mechanisms, without forming any radicals. Most solvents cannot compete with the olefins for the triplet ketone. However, tri-ethylamine quenches at rates close to diffusion-controlled, so that radical formation and polymerization initiation are quite efficient 158>. 

A similarly detailed study of the polymerization of methyl methacrylate by diphenyliodonium chloride in aq. AN using various aromatic ketone sensitizers has recently been carried out by Timpe s group [103a]. On the one hand the results were not complicated by monomer quenching of excited ketone sensitizer, but were complicated by operation of simultaneous singlet and triplet pathways for the electron-transfer sensitization. In this study the data were analyzed without reference to the possibility of triplet energy transfer sensitization of the iodonium salt photolysis [22], or the possible involvement of charge transfer complexes. 

FG/tolerated by catalyst Monomer Quenching Post functionalization Chain end Side chain... 

The effect of excimer kinetics on fluorescence decays of monomers and excimers upon excitation with a short pulse was studied first by Birks et al. [119]. They took into account all the relevant processes that proceed after the excitation of a low fraction of monomers by an ultrashort pulse and derived the rate equations describing the monomer and excimer decays. Most processes involved in the Birks scheme are monomolecular and depend only on the concentration of the excited species and on the first-order rate constant one of them is a bimolecular process and depends on the concentrations of both the excited and ground-state molecules. They include (1) monomer fluorescence, (rate constant fM), (2) internal monomer quenching, M —>M, ( iM). (3) excimer formation, M - -M D (bimolecular reaction, i.e., the rate depends on the product of the rate constant and concentration of the ground-state... 

MacCallum [66,67], addressing the particular case of styrene containing polymers, has suggested that kinetic discrimination within the microcomposition might be consequent upon the dependence of excited monomer quenching through formation of excimer as a result of the location of styrenes in triads of the type -MS M-, -MS S- and -SS S-. 

This premise is incorrect because kqj simply describes the quenching of an isolated excited phenyl ring while kqjj always describes the direct quenching of the isolated excited complex. The possible contribution to excimer quenching caused by monomer quenching before the excitation can reach an EFS is already included in kqx. Thus, kqj for the copolymer monomer should be approximately equal to kqj) for the PS excimer. MacCallum and Rudkin s conclusion shows that it is often easy to misinterpret quenching experiments. 

Rate Constants of Cleavage, Electron Transfer and Monomer Quenching in Radical Photoinitiators 11-170... 

TABLE 1. RATE CONSTANTS OF CLEAVAGE, ELECTRON TRANSFER AND MONOMER QUENCHING IN RADICAL PHOTOINITIATORS ... 

TABLE 1. cant d Rate constants of cleavage, electron transfer and monomer quenching In radical photolnitlators II / 171... 

The photopolymerization threshold is determined by the production efficiency of initiating species from excited triplet states, which is characterized by the quantum yield of polymerization. The reactions that produce radicals should compete with monomer quenching, oxygen quenching and other pathways of deactivation of the excited states like phosphorescence emission. The threshold is also determined by the reactivity of radicals and monomers. 

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