Development of new dyeing photoinitiators for free radical polymerization based on the 1H-pyrazolo[3,4-b]quinoxaline skeleton. Part 2

Abstract

Several dyes containing a pyrazoloquinoxaline moiety have been synthesized and evaluated as novel photoinitiators for free radical polymerization induced with the argon-ion laser. The kinetic study of photoinitiated polymerization, performed for viscous monomeric formulations with the use of the most effective dye–N-phenylglycine derivative photoinitiating systems has shown unusual kinetic properties. The experimental data show the presence of “Marcus inverted region”-like kinetic behavior. Analysis of possible reasons for this specific feature suggests that one of the processes that may be responsible for such a specific property is the back electron transfer process. Study of photoinitiated polymerization has shown that there is a linear relationship between the rate of polymerization and the square root of the efficiency of singlet oxygen formation. This finding clearly indicates that the electron transfer process between the tested dyes and electron donors occurs via a triplet state. The observed relationship is also in good agreement with the general equation describing the rate of polymerization, which is (among other parameters) a function of the square root of the quantum yield of triplet state formation. In the paper, it is also shown that the structure of the dye has a strong effect on its polymerization photoinitiation ability. The dyes tested were prepared in such a way as to, (i) restrict the rotational freedom of selected parts of the molecule and (ii) increase the quantum yield of the triplet state formation. The latter was achieved by introducing to the molecule a heavy atom (ZCl2, CI) or different heavy atoms (CICl2).