Polymerization of HEMA photoinitiated by the Safranine/diphenylborinate system

The photoinitiator system Safranine/2-ethylaminodiphenylborinate (Sf/2APB) was used to polymerize HEMA and the kinetics of the reaction were followed by isothermal photocalorimetry. The active initiating radical is 2APB−, arising from the reaction of triplet Safranine with 2APB. The formation of a semioxidized form of Sf leads to photobleaching of the dye, which occurs with the same rate as polymerization, indicating that both species are formed simultaneously. When adding diphenyliodonium chloride (DPIC) to the photoinitiating mixture the polymerization rate is accelerated due to the reaction of the 2APB− radical ion with DPIC to form Ph radicals. These radicals are about 7 times more efficient for polymerization than the boron centered radicals. Photobleaching of the dye is also inhibited by the addition of DPIC.