Mechanistic and Kinetic Study of Free Radical Polymerization Photoinitiated by Xanthene Dye-n-Phenylglycine Derivatives Photoredox Pairs

The mechanistic and free radical photopolymerization kinetic studies for the selected xanthene dyes in the presence of N-phenylglycine derivatives are described. The examined transient phenomena, using nanosecond laser flash photolysis, confirm that the oxidation of N-phenylglycine derivatives is a one electron transfer process. The laser flash measurements as well as the steady-state experiments allowed to establish the structure of a free radical, which in ethyl acetate-DMF (9:1) solution, is mostly formed after the electron transfer, e.g. N-phenylglycine _-aminoalkyl radical (Ph-NH-C _ (H)-COOH). However, a careful analysis of the photolysis products indicates another path of secondary reactions that includes the decarboxylation of N-phenylglycine N-centered radical cation, yielding Ph-NH-CH2 radical. The measured electron transfer rates oscillate from 0.23_108 to 35.9_108M-1 s-1 that, in turn, suggests the possibility that the electron transfer process might control the rate of photoinitiated polymerization. The comparison of the rates of xanthene dye triplet state quenching and the measured rates of polymerization shows, however, that there is no correlation between these two variables. This observation permits to conclude that the rate of photopolymerization is probably controlled by the secondary processes such as: (i) the rate of proton transfer reaction between N-phenylglycine radical cation and xanthene dye radical anion, (ii) the participation of decarboxylation process in the secondary reactions and (iii) the reactivity of the free radicals obtained after electron transfer.