The photolysis of pyrene derivatives in the presence of amines has been employed as a source of free radicals in the polymerization of l-vinyl-2-pyrrolidinone in aqueous solution. The photoinitiation efficiency strongly depends on the amine and the sensitizer employed. With regard to the amine, only tertiary amines are efficient cocatalysts, hydroxyethanolamines being particularly efficient. For a given amine the initiation efficiency is high for pyrene and its cationic derivatives, while pyrenebutyric acid is a poor photoinitiator and pyrenesulfonic acid does not initiate the polymerization. The active radicals are produced in the interaction between the singlet excited state and the additive, the photoinitiation rate being proportional to the fraction of singlets quenched by the amine. The photoinitiation efficiency at total singlet quenching correlates with the rate constant of the quenching process, the photobleaching rate, and the pyrene radical anion yield. Photobleaching yield and the lifetime of the pyrene-derived radical anion decrease in the presence of l-vinyl-2-pyrrolidinone. All these results are compatible with an initiation mechanism involving the interaction of the pyrene radical anion with the monomer.
Rate constants for the reactions of tert-butoxy radicals (generated by the thermal decomposition of di-tert-butylperoxyoxalate) with several sulfur containing compounds have been measured a t 310 K in benzene. Hexanethiol ( k = 6.5 x lO'M-'s-') reacts considerably faster than alkyl sulfides and disulfides. For these compounds the reaction rate constants are slightly dependent on the a-hydrogen type, changing (when it is expressed per hydrogen atom) only a factor 5 for sulfides and 3 for disulfides when the a-hydrogen is changed from primary (methyl) to tertiary (isopropyl). The data obtained are compared to those found for the deactivation of the benzophenone triplet. Values of k,,, ~utoxy/kbenzophenone range from ca (di-tert-butyl disulfide) to 7.5 (hexanethiol). The results obtained are rationalized in terms of bond strength, steric hindrance, and charge transfer contributions to the critical configuration energies.
SynopsisAddition of triethylamine (TEA) notably increases the efficiency of anthraquinone (AQ) as photoinitiator of methyl methacrylate (MMA) free radical polymerization in nonpolar solvents. On the other hand, when alcohols or acetonitrile are employed as solvents the addition of TEA decreases the AQ photoinitiated polymerization rate. The polymerization efficiency measured in nonpolar solvents decreases when the MMA concentration increases. These results are explained in terms of quenching of the AQ-TEA charge transfer complex by MMA. This conclusion is supported by the decrease in AQ photoconsumption and AQHz formation produced by the monomer addition.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.