Quantifying Photoinitiation Efficiencies in a Multiphotoinitiated Free‐Radical Polymerization

Abstract

Online size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI-MS) is employed for quantifying the overall initiation efficiencies of photolytically generated radical fragments. In a unique experiment, we present the first quantitative and systematic study of methyl-substituted acetophenone-type photoinitiators being employed in a single cocktail to initiate the free-radical polymerization of methyl methacrylate (MMA) in bulk. The photoinitiators are constituted of a set of two known and four new molecules, which represent an increasing number of methyl substituents on their benzoyl fragment, that is, benzoin, 4-methylbenzoin, 2,4-dimethylbenzoin, 2,4,6-trimethylbenzoin, 2,3,5,6-tetramethylbenzoin, and 2,3,4,5,6-pentamethylbenzoin. The absolute quantitative evaluation of the mass spectra shows a clear difference in the initiation ability of the differently substituted benzoyl-type radical fragments: Increasing the number of methyl substituents leads to a decrease in incorporation of the radical fragments.

Studying the polymerization initiation efficiency of acetophenone-type initiators via PLP-ESI-MS and femtosecond spectroscopy

“…For instance, by comparing 2,4,6-trimethyl benzoin with mesitil, a distinct difference of the initiation ability of the 2,4,6-trimethyl benzoyl fragment can be detected, depending on its original photoinitiator molecule. A further study carried out by Voll et al 5 demonstrates that in addition to its ISC behavior, the incorporation efficiency of a particular radical fragment is also inu-enced by steric effects. By increasing the number of methyl substituents on the benzoyl fragment a decrease of the initiation ability was found.…”

“…5(A)). In our earlier studies, where we analyzed the quantitative initiation efficiency of benzoin-like derivatives, 5,6 such a clear chain length-dependent ionization behavior, as it appears for MMMP, was not observed. In order to exclude any unusual ionization behavior inherent to MMA -however unlikely it may be -ethyl methacrylate (EMA) was polymerized with MMMP via PLP and the resulting polymer was assessed via ESI-MS. EMA was chosen due to its structural similarity to MMA, bearing one additional carbon unit.…”

“…Therefore, in the current study we compare the initiation efficiency of the radical fragments of the type I photoinitiators MMMP, benzoin (Bz), and 4-methyl benzoin (4MB) (Scheme 1) derived from a post-mortem analysis of their polymerization products via PLP-ESI-MS. In order to achieve an exact comparison of the initiation efficiencies of the aforementioned initiators, the already established PLP-ESI-MS technique [4][5][6] was chosen, providing quantitative insight into the incorporation propensity of radical fragments at the polymer termini during free radical polymerization. For this purpose -in a rst steppolymerization reactions are carried out via pulsed laser polymerization (PLP) with a high-frequency pulsed laser system at 351 nm wavelength, employing mixtures containing the monomer methyl methacrylate (MMA) and two or three photoinitiators simultaneously present at different molar ratios.…”

Mehrdimensionale Massenspektrometrie von synthetischen Polymeren und modernen Materialien

“…Es sei jedoch darauf hingewiesen, dass durch die geringere Effizienz der ESI bei sehr schweren Oligomeren das Molekulargewicht von Polymeren mit einer Polydispersität ( M w / M n ) über etwa 1.4 zu niedrig geschätzt werden kann, was das maximal bestimmbare Molekulargewicht begrenzt . Ungeachtet solcher Aspekte wird durch die Fraktionierung vor der MS‐Analyse der Dynamikbereich vergrößert, was die Detektion von Nebenprodukten ermöglicht, die mechanistische Daten zu neuen Polymerisationen liefern …”

Benzoin type photoinitiator for free radical polymerization

“…Benzoin after photoexcitation and efficient intersystem crossing populates the triplet state, which undergoes efficient α‐cleavage (Norrish Type I reaction) to generate benzoyl and α‐hydroxy radicals (eq 1). 4–10 In the presence of vinyl monomers, these radicals can initiate free radical polymerization. The major disadvantage of benzoin and its ether derivatives is the poor light absorption in the near‐UV, a spectral region which is most attractive for industrial photopolymerizations.…”