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The photoinitiator (P1) plays a key role in UV-curable systems by generating the reactive species, free radicals, or ions, which initiate the polymerization of the multifunctional monomers and2 This component has a great influence on. the curing rate and curing depth, the double-bond conversion (DBC), and the storage life and surface cure of such formulations. However, the performance of all PIs strongly depends on the type of formulation. Among the large group of P1 structures for free radical polymerization, benzophenone (BP) (Figure 1) is one of the most widely used PIs because of its low costs, universal applicability, and good reactivity. One of the major drawbacks of this system is that coinitiators, commonly tertiary amines, are, used, and they give rise to odor, discoloration, and migration. Recently is has been shown by differential scanning photocalorimetry experiments using lauryl acrylate (LA) as monomer that 1 ,5-diphenylpenta- 1 ,4-diyn-3-one (DPD) has given surprisingly high activity for the DBC and rate of polymerization (Rn) at low P1 concentrations (0.025 M; 0.34 wt%) and even without any coinitiator. By the application of monomers with abstractable hydrogens, significant improvement in the photoreactivity has been observed (Figure 2). Ultraviolet—visible spectroscopy has revealed strong absorption up to 350 nm (Figure 3) which could be explained by a cross-conjugation effect. Steady-state photolysis experiments (SSP) have proven that the photochemistry of this compound is significantly faster than that of.
