N‐methylpicolinium esters as co‐initiators in dye photosensitiser systems for the polymerisation of acrylate monomers

The photoinitiation ability of several dyeing photoinitiating systems composed of positively chargedN,N’‐diethylthiacarbocyanine iodide and 3‐ethyl‐2‐(p‐N,N‐dimethylaminostyryl)benzothiazolium iodide acting as light absorbers,n‐butyltriphenylborate anion being the electron donor, and diphenylacetic acid (N‐methylpyridinium)methyl ester cation acting as a ground state electron acceptor are compared. Both co‐initiators in the polymerising formulations were present, either as tetrabutylammonium salt and diphenylacetic acid (N‐methylpyridinium)methyl ester perchlorate, respectively, or as an ion pair. The best photoinitiating ability was observed when the borate anion and diphenylacetic acid (N‐methylpyridinium)methyl ester cation formed an ion pair. Modification of the ability of the photoinitiating system was concerned with the possibility of elimination of a diffusion process. This was achieved by synthesis of a properly designed dye‐possessing diphenylacetic acid (N‐methylpyridinium)methyl ester group. This dye, after electron transfer, forms anN‐methylpicolinium derivative radical that fragments, forming an (N‐methylpyridinium)methyl radical, organic acid anion and reduced dye. Finally, from a combination of either anN‐methylpicolinium derivative or alkyltriphenylborate salts with a suitable sensitiser or a properly designed dye and alkyltriphenylborate salt, two radicals can be generated per one absorbed photon, thus enhancing the overall efficiency of the polymerisation.