Quantification and elucidation of the UV‐light triggered initiation kinetics of TPO and BAPO in liquid acrylate monomer

ABSTRACT The initiation kinetics of two important UV‐light‐triggered initiators for the radical polymerization [diphenyl‐2,4,6‐trimethyl benzoyl phosphine oxide (TPO) and phenyl‐bis(2,4,6‐trimethyl benzoyl) phosphine oxide (BAPO)] has been quantified in dependence on the initiator concentration (0.25–2 mol %), the light intensity at 365 nm (0–2000 mW cm−2), the thickness of the sample (50–200 μm), the temperature (25–80 °C), the monomer [2‐ethyl hexyl acrylate (EHA) and 2‐ethyl hexyl methacrylate (EHMA)] and the atmosphere (oxygen free and air) directly in the liquid acrylate monomer. The determination of the kinetic parameters was done by applying two independent procedures: (1) following the initiator decay with respect to the irradiation time, evaluated by radiometric measurements of the UV‐light absorption at 365 nm and (2) via titration of the initiation process by using defined under‐stoichiometric to stoichiometric amounts of TEMPO as inhibitor, evaluated by means of FTIR‐ATR spectroscopy. The validity of the titration procedure was proven by means of 13C and 31P NMR studies of 13C‐labeled TPO and was explained by a Lewis acid/base interaction between the carbonyl carbon of the initiator and the oxygen of TEMPO. Both methods resulted in very close kinetic parameters. Thus, reliable values for the extinction coefficients ε365 at 365 nm, for the effective rate constants of the α cleavage kα′ (containing the quantum yield and the initiator efficiency) when dissolved in the liquid monomer could be provided for both initiators for the first time. The effect of dioxygen quenching in dependence of sample thickness and the temperature dependence on the initiation step were evaluated. EHA was compared with EHMA as liquid monomer, and a yet unmentioned inhibition in case of EHMA was discovered. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48357.