Wavelength-Dependent Photochemical Stability of Photoinitiator-Derived Macromolecular Chain Termini

3 years ago

Wavelength-Dependent Photochemical Stability of Photoinitiator-Derived Macromolecular Chain Termini

Georg Gescheidt, Jan Steinkoenig, Christopher Barner-Kowollik, Anne-Marie Kelterer, David E. Fast, Andrea Lauer

Herein, we report the unique—and first time—wavelength-dependent investigation with strictly monochromatic light of 305–405 nm wavelength into the stability of photoinitiator-derived chain termini of poly(methyl methacrylate) using a tunable laser system fused with pulsed-laser irradiation and size exclusion chromatography hyphenated to high-resolution electrospray mass spectrometry (PLI-SEC-ESI-MS). We assess several substitution patterns of methyl groups on the common benzoyl-type radical fragment. Critically, methyl substitution in the 2- and 6-positions of the benzoyl moiety, i.e., in both ortho-positions, resulted in stable chain ends up to approximately 350 nm. The stability can be attributed to a blue-shift of the n−π* transitions (relevant for the end group reactivity) as predicted by earlier density functional theory (DFT) calculations on model species. In sharp contrast, our experiments show a far reduced stability of the end groups commencing from 400 nm onwards, when the dual ortho-methyl substitution in the benzoyl fragment is missing. Thus, we demonstrate that the substitution pattern on the phenyl ring of the benzoyl group dictates the chain end stability as a function of wavelength in excellent agreement with the quantum chemical predictions. Our study thus provides critical insights into selecting suitable photoinitiation systems for specific wavelength regimes.

Publisher URL: http://dx.doi.org/10.1021/acsmacrolett.7b00499

DOI: 10.1021/acsmacrolett.7b00499