Push–pull (thio)barbituric acid derivatives in dye photosensitized radical and cationic polymerization reactions under 457/473 nm laser beams or blue LEDs

A series of new dyes based on (thio)barbituric acid derivatives were synthesized and used as photoinitiators of polymerization upon very soft irradiation (laser diodes at 457 nm and 473 nm; blue LED bulbs or even a halogen lamp). These dyes can work according to an oxidative cycle (dye/diphenyl iodonium salt/N-vinylcarbazole system) or a reductive cycle (dye/amine/alkyl halide system) and partly behave as organic photocatalysts. Ring-opening polymerization (ROP) of epoxides and radical polymerization (RP) of acrylates are easily promoted. Excellent RP and/or ROP polymerization profiles are obtained. The contrasting behavior of the thiobarbituric vs. barbituric acid derivative is discussed. The initiation mechanisms, analyzed by Electron Spin Resonance (ESR) and steady state photolysis experiments, are detailed.