Investigation of Barton esters as radical photoinitiators

Abstract

Different photoinitiators based on Barton thiohydroxamic esters, O-acyl-N-hydroxy-pyridine-2(1H)-thione and O-acyl-N-hydroxy-thiazole-2(3H)-thione derivatives, were tested in photopolymerization reactions through time-resolved FTIR spectroscopy. Good rates of polymerization and final monomer conversion were obtained for some compounds. The excited state processes, investigated by time resolved absorption spectroscopy, lie on a fast singlet state cleavage leading to thiyl and alkyl radicals. Both the pyridine-2(1H)-thiyl and alkyl radicals are able to initiate a polymerization, in contrast with the thiazole-2(3H)-thiyl radical. A triplet state is observed for some derivatives. Computational studies help to describe the excited state properties and show a strong difference in the spin localization in the formed thiyl initiating radicals.

Fragmentation of a dioxolanyl radical via nonstatistical reaction dynamics: characterization of the vinyloxy radical by ns time-resolved laser flash photolysis

“…As far as the quenching reactions of the short-lived transient C are concerned, we note that both the high reactivity towards electron-rich arenes and the increase in reactivity upon addition of TFA would also be consistent with an assignment to the vinyloxy radical (22). A hydrogen-bridged complex of 22 with TFA would be very similar to a free ethenol radical cation in both structure and likely also oxidative power.…”

“…8), which would normally make a fragmentation on the ps timescale completely impossible. Our observation of the vinyloxy radical (22), however, necessitates such a rapid fragmentation. The BDE of the N-O bond in Barton esters is small, estimates range between 16 and 28 kcal mol À1 .…”

Quantifying Photoinitiation Efficiencies in a Multiphotoinitiated Free‐Radical Polymerization

“…In combination with two known photoinitiators [benzoin (A) and 2,4,6-trimethylbenzoin (D)], we employ them in a multiphotoinitiated free-radical pulsed laser polymerization in bulk for quantifying the radical fragments (1), (2), (3), (4), (5), (6) in their initiation effi ciency toward MMA employing SEC/ESI-MS as analytical quantifi cation method. 28.2% of all disproportionation products are initiated with fragment (1), 26.6% with (2), 22.0% with (3), 10.7% with (4), 3.9% with (5), and 8.6% with (6).…”

“…A free-radical polymerization of MMA in bulk in the presence of a mixture of all six photoinitiators [(A)-(F), see Scheme 1 ]-all in a 1:1 ratio to each other-leads to a multiphotoinitiated pMMA sample. The fi nal values for the absolute incorporation propensities (or net/overall initiation abilities) of the individual radical fragments toward MMA are 28.2% for (1), 26.6% for (2), 22.0% for (3), 10.7% for (4), 3.9% for (5), and 8.6% for (6). This implies that 28.2% of all unsaturated disproportionation products were initiated with fragment (1), 26.6% with fragment (2), 22.0% with fragment (3), 10.7% with fragment (4), 3.9% with fragment (5), and 8.6% with fragment (6) (see also Figure S2 and Table S2, Supporting charged repeating units of the SEC/ESI-MS overview spectra (9 repeating units in sum) of the multiphotoinitiated pMMA sample (Figure 1 c) in comparison to all disproportionation signals (D 1, = ,MMA -D 6, = ,MMA ) was calculated and multiplied by 100% ( Equation 1 ) to achieve the absolute initiation ability percentage of the substituted benzoyl-type fragments [(1)- (6)].…”

“…Thus, each peak is labeled with D v, = ,MMA or D v,H,MMA where v denotes the radical fragment that has initiated the polymerization [(1), (2), (3), (4), (5), (6) or (7), see Scheme 1 ]. Combination products (labeled with C v,w,MMA , where v and w denotes the radical fragment as end group (see Scheme 1 )) are mentioned for the sake of completeness of the assignments and can be found in the Supporting Information section (see Figure S1 and Schemes S1-S4), yet they are of limited use in the present study as it is not possible to differentiate whether they have been generated by initiation or termination of the macromolecular growth.…”