EXCITED-STATE PROCESSES IN POLYMERIZATION PHOTOINITIATORS

The present paper is devoted to the investigation of excited state processes efficiency/reactivity reactivity relationships of photoinitiators of polymerization. The reactivity of a substituted benzophenone depends upon the nature and position of the substituent. For 4-methoxybenzophenone, the lifetime of the triplet state is 800 ns; replacement of methoxy by methylthio changes the lifetime to 5 mu s. The rate constant for deactivation by methyl methacrylate (MMA) falls by a factor of 60; the rate of polymerization of a solution of MMA (7 mol l(-1)) irradiated in the presence of these compounds increases by a factor of 1.5. The behavior of some hydroxyalkylphenylketones bearing an ether substituent at the para position of the phenyl ring is special: their triplet states are short lived (8.5-15 ns in deaerated solutions) but are longer lived than the parent unsubstituted compound (1.4 ns). Introduction of thio and dialkyl amino groups leads to an increase of the lifetimes (3-4 mu s). These substituents change the reactivity of the trip]et states in the presence of oxygen, monomer and amine but have a weak influence on the rate of polymerization of MMA in organic solvent initiated by light irradiation of these compounds. Irradiation of morpholino- and amino-ketones substituted by ether, thioalkyl and dialkylamino groups leads to the generation of triplet states with main characteristics (spectra, lifetimes, rate constants of interaction with O-2, amines and monomers) strongly dependent on the substitution (e.g. the introduction of a para dimethylamino group on a morpholino ketone increases the triplet state lifetime to 2 mu s, from 1 ns for the parent compound). Interaction between these compounds and thioxanthones as photosensitizers is indicated by using a dye laser operating at lambda = 430 nm. Results on the behavior of these compounds in solution photopolymerization experiments are compared to those obtained through laser spectroscopy. Polymers obtained from photosensitive copolymerizable systems show outstanding reactivity and durability. Excited state properties of photoinitiators containing unsaturated ethylenic groups have been studied, with respect to triplet state lifetimes, rate constants of interaction with the environment (oxygen, amines, monomers). Measurements of rates of polymerization (R(p)) in solution (at various monomer concentrations [M]) allowed comparison of the theoretical dependence of R(p) on [M] with the experimental results.