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

Abstract

The photoinitiation ability of several dyeing photoinitiating systems composed of positively charged N,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 an N-methylpicolinium derivative radical that fragments, forming an (N-methylpyridinium)methyl radical, organic acid anion and reduced dye. Finally, from a combination of either an N-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.