4 research outputs found
Time-Resolved EPR as a Tool to Investigate Oxygen Quenching in Photoinitiated Radical Polymerizations
It
is challenging to obtain absolute rate constants for the quenching
of organic radicals by molecular oxygen because they often do not
present absorbance in the UV–vis range. Here, it is shown that
time-resolved EPR (chemically induced dynamic electron polarization,
or CIDEP) spectroscopy is useful in establishing rate constants for
the addition of benzoyl radicals to molecular oxygen. It was found
that benzoyl radicals are particularly reactive toward O<sub>2</sub> and can, therefore, act as oxygen scavengers in the initiating phase
of radical polymerizations. Kinetic simulations underpin this reactivity
UV-Triggered End Group Conversion of Photo-Initiated Poly(methyl methacrylate)
The analysis of photo-initiated polyÂ(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 polyÂ(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 thatî—¸if the integrity
of UV initiated polymers is to be kept intact during their synthesisî—¸very
low irradiation energies need to be employed
Photoinduced Reactivity of the Soft Hydrotris(6-<i>tert</i>-butyl-3-thiopyridazinyl)borate Scorpionate Ligand in Sodium, Potassium, and Thallium Salts
The soft scorpionate ligand hydrotrisÂ(6-<i>tert</i>-butyl-3-thiopyridazinyl)Âborate (<b>Tn</b>) was
found to exhibit pronounced photoreactivity. Full elucidation of this
process revealed the formation of 6-<i>tert</i>-butylpyridazine-3-thione
(<b>PnH</b>) and 4,5-dihydro-6-<i>tert</i>-butylpyridazine-3-thione
(<b>H</b><sub><b>2</b></sub><b>PnH</b>). Under exclusion
of light, no solvolytic reactions occur, allowing the development
of high-yield preparation protocols for the sodium, potassium, and
thallium salts and improving the yield for their derived copper boratrane
complex. The photoreactivity is relevant for all future studies with
electron-deficient scorpionate ligands
Acylgermanes: Photoinitiators and Sources for Ge-Centered Radicals. Insights into their Reactivity
Acylgermanes
have been shown to act as efficient photoinitiators.
In this investigation we show how dibenzoyldiethylgermane <b>1</b> reacts upon photoexcitation. Our real-time investigation utilizes
femto- and nanosecond transient absorption, time-resolved EPR (50
ns), photo-chemically induced dynamic nuclear polarization, DFT calculations,
and GC-MS analysis. The benzoyldiethylgermyl radical <b>G</b>• is formed via the triplet state of parent <b>1</b>. On the nanosecond time scale this radical can recombine or undergo
hydrogen-transfer reactions. Radical <b>G</b>• reacts
with butyl acrylate at a rate of 1.2 ± 0.1 × 10<sup>8</sup> and 3.2 ± 0.2 × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup>, in toluene and acetonitrile, respectively. This
is ∼1 order of magnitude faster than related phosphorus-based
radicals. The initial germyl and benzoyl radicals undergo follow-up
reactions leading to oligomers comprising Ge–O bonds. LC-NMR
analysis of photocured mixtures containing <b>1</b> and the
sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals
that the products formed in the course of a polymerization are consistent
with the intermediates established at short time scales