Article pubs.acs.org/Macromolecules
Wavelength-Dependent Photochemistry of Oxime Ester Photoinitiators David E. Fast,† Andrea Lauer,‡,§,∥ Jan P. Menzel,‡,§,∥ Anne-Marie Kelterer,† Georg Gescheidt,*,† and Christopher Barner-Kowollik*,‡,§,∥ †
Institute of Physical and Theoretical Chemistry, NAWI Graz, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany § Institut für Biologische Grenzflächen (IBG), Karlsruhe Institute of Technology (KIT), Herrmann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia ‡
S Supporting Information *
ABSTRACT: The design of efficient radical photoinitiating systems requires a systematic and detailed evaluation of their photochemical characteristics. Correlating absorbance and the corresponding electronic transitions of a photoinitiator is critical for understanding its photoinduced reaction pathways. In the current contribution, we provide an in-depth investigation into the photochemistry and photophysics of two oxime ester derivatives (O-benzoyl-α-oxooxime, OXE01, and O-acetyloxime, OXE02), known for their excellent performance in pigmented formulations. In particular, we shed light on their wavelength-dependent photopolymerization properties. We utilized a combination of UV−vis spectroscopy, density functional theory (DFT) calculations, photochemically induced dynamic nuclear polarization spectroscopy (photo-CIDNP), and pulsed-laser polymerization with a wavelength-tunable laser with subsequent size exclusion chromatography coupled to high-resolution electrospray ionization mass spectrometry (PLP-SEC-ESI-MS) for obtaining detailed insights. Both photoinitiators have high molar extinction coefficients (ε) of greater than 1.75 × 104 L mol−1 cm−1 at close to 330 nm, with the n−π* and π−π* transitions, relevant for cleavage of the N−O bond, at approximately 335 nm according to DFT calculations. We have probed the wavelength-dependent initiation behavior of both OXE01 and OXE02 in the presence of methyl methacrylate (MMA) via PLP with a wavelength-tunable laser between 285 and 435 nm at constant photon counts. Surprisingly, the highest conversions of MMA were found at a wavelength of 405 nm, even though the molar extinction coefficients of the photoinitiators are low (ε405 of 45 and 2 L mol−1 cm−1 for OXE01 and OXE02, respectively) compared with shorter wavelengths. Accordingly, the absorption spectrum of a photoinitiator is not a straightforward guide for selecting the most efficient excitation wavelength.
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INTRODUCTION Photoinitiated radical polymerization has a wide variety of applications including coatings,1,2 dental fillings,3,4 and tissue engineering.5 Oxime esters have long been known for their photoreactivity,6 which was later exploited for initiating radical photopolymerizations.7 Generally, two competing photoinduced pathways (depending on the substituents R) exist: E/Z isomerization and fragmentation yielding radicals (Scheme 1).8−10 The latter occurs via the cleavage of the N−O bond leading to iminyl and acyloxy radicals, which can undergo further fragmentation or decarboxylation reactions.11−13 Two oxime ester photoinitiatorsO-benzoyl-α-oxooxime, OXE01, and O-acetyloxime, OXE02 (commercialized as Irgacure OXE01 and OXE02; Scheme 2)have been shown to be especially powerful for the curing of highly pigmented thick films in color filter resists1 and have also been utilized for radical polymerization of acrylic monomers.13−15 © XXXX American Chemical Society
In general, photoinitiation starts with the excitation of the photoinitiator molecule from the ground state (S0) to the first (S1) or higher excited singlet states (Sn), depending on the irradiation wavelength/energy. The excited singlet states can be depopulated in many ways, e.g., by fluorescence and radiationless internal conversion (IC), and by intersystem crossing (ISC) into an excited triplet state (Tn). Generation of radicals via the triplet state by Norrish type I (α-cleavage) or Norrish type II (hydrogen abstraction) reactions are the dominant processes for most photoinitiators. Fragmentation of O-acyloximes (R1−3 = alkyl, aryl; e.g., OXE02) proceeds via an excited triplet state,16,17 whereas for O-acyl-α-oxooximes (R1 = Received: January 15, 2017 Revised: February 15, 2017
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DOI: 10.1021/acs.macromol.7b00089 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
photons at individual wavelengths. We submit that the constructions of such photoinitiator reactivity plots are crucial to fully understand a photoinitiator’s ability to perform in radical polymerizations.
Scheme 1. Photoinduced Reaction Pathways of Oxime Esters
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EXPERIMENTAL SECTION
Materials. Irgacure OXE01 and OXE02 were kindly provided by BASF and used as received. Methyl methacrylate (MMA, SigmaAldrich, 99%, stabilized) was freed from inhibitor by passing through a column of activated basic alumina (Sigma-Aldrich). Acetonitrile-d3 (Euriso-top) for the NMR and photo-CIDNP measurements was used as received. Sodium iodide (Sigma-Aldrich, 99%), tetrahydrofuran (THF, Scharlau, multisolvent GPC grade, 250 ppm BHT), and methanol (Roth, HPLC ultra gradient grade) for the SEC-ESI-MS measurements were employed as received. PMMA samples were precipitated in hexane (VWR, p.a.) prior to the postirradiation experiments, which were carried out in methyl isobutyrate (MIB, Acros Organics, 99%). Acetonitrile (Sigma-Aldrich, CHROMASOLV, for HPLC, gradient grade, ≥99.9%) for UV−vis measurements was used as received. UV−Vis Spectroscopy. UV−vis spectra were recorded on a Shimadzu UV-2102 PC UV−vis scanning spectrophotometer in acetonitrile. Molar extinction coefficients (ε) were determined over a range of five concentrations by plotting the absorbance (A) versus the concentration (A was
