Beyond the Diffusion Limit: Pre-Associated Ion-Pair Photoinitiating Systems for Radical Photopolymerization

The pre-association mechanism of an ion-pair Type II photoinitiating system (PIS) acting in the visible range was studied. The choice of a dye as photoinitiator (crystal violet) with an excited state lifetime of 200 ps ensured the absence of dynamical quenching by the borate salts used as coinitiator. In spite of the fact that no diffusional bimolecular quenching can take place, excellent polymerization efficiency was found, underlining the high reactivity of the PIS. A saturation effect is observed when increasing the concentration of borate as a consequence of the pre-association mechanism arising from coulombic interaction between the cationic dye and the anionic coinitiator. The association equilibria of the system were described and the corresponding association constant values were calculated, allowing to quantify the pre-association efficiency. This was further successfully correlated to the observed maximal rates of polymerization. The efficiency of this super-stoichiometric sample with an excess of borate was finally compared to the stoichiometric 1 : 1 dye:borate ion-pair. It is shown that the pre-association mechanism could lead to much higher efficiency than the stoichiometric ion-pair.