UV-Triggered End Group Conversion of Photo-Initiated Poly(methyl methacrylate)

The analysis of photo-initiated poly­(methyl methacrylate) via electrospray ionization-mass spectrometry (ESI–MS) (synthesized by pulsed laser polymerization (PLP, at λ = 351 nm) of methyl methacrylate (MMA) and benzoin as photoinitiator at 6 mJ/pulse laser energy) evidences the presence of unidentified species. The determination of the origin of these species requires a detailed investigation via size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI–MS) and chemically induced dynamic nuclear polarization-nuclear magnetic resonance spectroscopy (CIDNP–NMR). It was found that post-irradiation of benzoin-initiated poly­(methyl methacrylate) leads to α-cleavage of the benzoyl fragment leading to a sequence of cascade reactions, including the formation of an additional double bond within the polymer chain as evidenced via ESI–MS. Furthermore, the reaction products of the benzoyl radical post α-cleavage (e.g., benzaldehyde, phenyl methyl ketone, methyl formate, or methane) as well as the formed macroradical can be followed by CIDNP–NMR, which allows establishing a reaction mechanism for the UV-induced cleavage process. The study thus evidence thatif the integrity of UV initiated polymers is to be kept intact during their synthesisvery low irradiation energies need to be employed

Elucidating the Early Steps in Photoinitiated Radical Polymerization via Femtosecond Pump–Probe Experiments and DFT Calculations

The excited states and dynamics of the three triplet radical photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone, Bz), 2,4,6-trimethylbenzoin (2-hydroxy-1-mesityl-2-phenylethanone, TMB), and mesitil (1,2-bis­(2,4,6-trimethylphenyl)-1,2-ethanedione, Me)employed in our previous studies for quantifying net initiation efficiencies in pulsed laser polymerization with methacrylate monomers [Voll, D.; Junkers, T.; Barner-Kowollik, C. <i>Macromolecules</i> <b>2011</b>, <i>44</i>, 2542–2551]are investigated via both femtosecond transient absorption (TA) spectroscopy and density functional theory (DFT) methods to elucidate the underlying mechanisms causing different initiation efficiencies when excited at 351 nm. Bz and TMB are found to have very similar properties in the calculated excited states as well as in the experimentally observed dynamics. After excitation into the first excited singlet state (S₁) Bz and TMB undergo rapid intersystem crossing (ISC). The ISC can compete with ultrafast internal conversion (IC) processes because an excited triplet state (T_<i>n</i>) of nearly the same energy is present in both cases. ISC is therefore the dominating depopulation channel of S₁, and subsequent α-cleavage to produce radicals takes place on the picosecond time scale. In contrast, Me is excited into the second excited singlet state (S₂). In this case no isoenergetic triplet state is available, which inhibits ISC from competing with ultrafast deactivation processes. ISC is therefore assigned to be a minor deactivation channel in Me. Employing these findings, quantitative photoinitiation efficiency relations of Bz, TMB, and Me obtained by pulsed laser polymerization can be directly correlated with the relative TA intensities found in the femtosecond experiments. The ISC efficiency is thus a critical parameter for evaluating the overall photoinitiation efficiency and demonstrates that the employment of the herein presented method represents a powerful tool for attaining a quantitative picture on the suitability of a photoinitiator