New photoiniferters:: Respective role of the initiating and persistent radicals

Three new structures (S-benzoyl O-ethyl xanthate, benzoyl diethyldithiocarbamate, and benzyl 9H-carbazole-9-carbodithioate) are proposed as photoiniferters for controlled photopolymerization reactions. They are more efficient than the reference compound-benzyl dimethyldithiocarbamate-for both the polymerization rates of an acrylic difunctional monomer and the final control properties of the MMA photopolymerization (polydispersity PDI = M-w/M-n and/or number-average molecular weight M-n ranges). The best results are obtained with benzyl 9H-carbazole-9-carbodithioate (PDI = 1.14 at 10% conversion and 1.5 at 40%; M-n = 16 000 at 20% conversion). Transient radical absorptions were recorded and the interaction rate constants with MMA and oxygen measured by laser flash photolysis. Quantum mechanical calculations show that the singly occupied molecular orbital delocalization is higher for the carbazole derivative. The significant improvement of the control mechanism by benzyl 9H-carbazole-9-carbodithioate is due to both the increase of the light absorption of the dormant species and the higher persistent character of the associated radical.