Dissociation of 2,2′-substituted thioindigo whites,and recombination of their radicals in solution and in polymer matrix

The dissociation of 2,2′-disubstituted thioindigo whites into captodative radicals and the recombination of these radicals have been examined in solution and in polymeric matrix. For the methyl- and ethyl-substituted TIW the quantum yield of dissociation in solution is equal to 4 × 10^?4 and 5 × 10^?4, respectively. Using the iodine scavenging method the quantum yield of formation of free radicals was almost 3 to 4 × 10^?4, i.e. 20–25% radicals recombine in cage. For the isopropyl and aryl TIWs partial dissociation occurs already at room temperature. Their dissociation enthalpies have been evaluated in solution on the basis of the ESR signal surfaces at different temperatures. For the aromatic substituted Δ H_diss amounts to 88–95 kJ mol^?1, while for i-PrTIW ΔH_diss is 150 kJ mol^?1. The behavior of these compounds was studied in three different matrices: polymethyl methacrylate (PMMA), polypropyl methacrylate (PPMA), and polystyrene (PST). In the cases of the aryl substituted derivatives, cage dimensions of the TIW-radical pairs were measured at low temperature, and their apparent dissociation enthalpies above their glass transition temperatures were evaluated using ESR spectrometry; they are equal to 118 ± 3 kJ mol^?1 in PPMA and around 155 kJ in PMMA and PST. The decay kinetics of the radicals after photolysis of the TIWs below T_g were interpreted on the basis of Waite's equations for diffused controlled reactions. The great influence of steric effects in the cases of the 2-orthochlorophenyl and the 2,4-dichlorophenyl TIWs is stressed as well as the importance of the merostabilization of the captodative radicals on these kinetic characteristics. The nature of the polymeric matrix is also underlined; below T_g the diffusion rate constant is only one-third in PST compared to PMMA. The radicals generated from Ph TIW and its para-nitro and p-methoxy derivatives show a strong inhibiting effect toward the polymerization of methyl methacrylate, while the alkyl TIWs behave as free radical initiators.