Photosensitized Processes in Polymer Chemistry: A Review

Abstract

Dedicated to the memory of Prof. Dr T. 1. Rabek O U T L I N E ( I ) Theoretical principles of the sensitization process. (2) Photosensitized polymerisation, graft polymerisation and cross-linking. Mercury-photosensitized reactions. Ion-pair complexes as photoinitiators. Photopolymerisation in the presence of metallic oxides. Photosensitization by chlorine, bromine and iodine compounds. Organic compounds as sensitizers. Halogenated compounds as sensitizers. Sensitizing compounds with the C=O group. Sensitizing compounds with t h e -O U g r o u p . Sensitizing compounds containing nitrogen. Sulphur containing sensitizers. Reactions photosensitized by dyes. (3) Photosensitized cis-trans isomerization of polymers. (4) Photosensitized degradation. (S) Photopolymers. (6) Photosensitizing polymers. (7) Spectral sensitization of the photoeffect in polymers. A * ( S , ) -*A(So)+hv'The time of emission decay is inversely proportional to the integrated intensity of the respective absorption band.Among various possibilities of deactivation without the participation of radiation two possibilities must be distinguished[339]:(1) Intramolecular processes.(2) Intermolecular processes. Both kinds are utilized in sensitized photochemical reactions of polymers. In the intramolecular process the energy of electron excitation is converted within the molecule into other forms of energy. When the excitation energy attains the magnitude of the energy of bond disruption, the sensitizer dissociates, thereby forming free radicals: A * ( S , ) + R 1 . + R I . + . . . + R iMany secondary reactions are promoted by radicals formed; this efl'ect depends upon the radical structure and activity [ 12,20,52 1,5321. Photosensitized processes in polymer chemistry: a reviewIn some cases a metastable triplet state is produced by the absorbed light quanta. This metastable triplet state is essential in phosphorescence and in certain processes of photosensitization, photoreduction, photodimerisation, etc. 7 A ( S 0 ) + h V + A * ( S I ) + A * ( T , )The probability of triplet state formation by direct radiative elevation of the ground state is very small, though there is a high probability of the radiationless transitions to the triplet, from the higher singlet states in solids and rigid solutions in a number of aromatic compounds.There is an important difference between the photophysical process of molecules in the n, 7r* first excited state and those of molecules in the lowest excited state r, r*. In the case of n, 7r* states in particular the natural life-time and the degree of excitation localisation are greater, and the S1 + TI energy gaps are smaller. The photochemical reactivity which is influenced in part by these factors shows distinct differences. It appears that the reactive state corresponds to the first excited singlet and the solvent participates only to the extent that it affects the relative probabilities of the several photochemical processes involved.

Kinetics of Photodegradation of Thifensulfuron-Methyl in Aqueous Solutions

“…[38] These complexes, in turn, may undergo photoredox reactions to form reactive z OH radicals, [39] in which the quantum yield depends on the excitation wavelength and on the iron(III) species. [38,40,41] FeOH 2þ is the most photoactive species present at 3 , pH , 5 and gives z OH free radical and Fe 2þ under photolysis [40] :…”

ESR Spectroscopy in Polymer Research

“…The photosensitized decomposition of polymers is well known and was extensively reviewed by Rabek l775 , 1779,1780,[1782][1783][1784]1814). The mechanism of photosensitized generation of free radicals in polyethylene was investigated by Tsuji et a1.…”

Determination of the photocatalytic activities of titanium dioxides and other white pigments

“…The abstraction of hydrogen by ultraviolet-irradiated titanium dioxide/ oxygen has been used to initiate crosslinking of polypropylene and polymerization of vinyl monomers. 22 Photodegradation of unpigmented polypropylene has been attributed to light absorption by polymerization catalyst residues (mainly titanium and aluminum oxides) za,24; the degradation processes were dependent on the availability of oxygen and the concentration of the metallic residues.…”