Trapping of photogenerated group IV radicals by TEMPO: Potential new organometallic initiators for ?living? free radical polymerization

Abstract

A series of trialkyl and triaryl organometallic radicals from group IV generated by hydrogen abstraction by tert-butoxyl radical from the parent hydrides have been examined using laser flash photolysis. The rate constants for the trapping of the metal-centered radicals by the persistent radical TEMPO were measured and were found to be large and similar to those of the carbon-centered radical systems, yet below the diffusion controlled limit. The metal-centered radicals were found to be efficiently trapped by TEMPO and would appear to be candidates suitable for "living" free radical polymerization similar to carbon analogue stoichiometric initiators. The radical trapping rate constants for the trialkyl series (M ϭ Si, Ge, Sn) were found to be 8.9 ϫ 10 8 M Ϫ1 s Ϫ1 (M ϭ Si), 7.2 ϫ 10 8 M Ϫ1 s Ϫ1 (M ϭ Ge), and 6.2 ϫ 10 8 M Ϫ1 s Ϫ1 (M ϭ Sn), respectively. The triaryl (Ph 3 M•) series gave slightly slower rates of 1.6 ϫ 10 8 M Ϫ1 s Ϫ1 (M ϭ Si), 3.4 ϫ 10 8 M Ϫ1 s Ϫ1 (M ϭ Ge), and 1.9 ϫ 10 7 M Ϫ1 s

Universal Group 14 Free Radical Photoinitiators for Vinylidene Fluoride, Styrene, Methyl Methacrylate, Vinyl Acetate, and Butadiene

“…Although R 3 Sn • can be obtained by Bu 3 SnH H • abstraction (AIBN, TEMPO, Mn­(CO) 5 ), Bu 6 Sn 2 oxidation (Fe­(III), , peroxide), or R 3 Sn–SnR 3 thermolysis ( T > 200 °C for R = Bu, Ph, lower for bulkier substituents − under ultrasonication), such methods are unsuitable for VDF. Thus, Bu 3 SnH would terminate (PVDF–H) by H transfer; nitroxides would irreversibly trap the chain end, and Mn­(CO) 5 • would compete for halide (i.e., R F –I) activation, while Fe­(III) or H 2 O 2 would react with PVDF • . As such, a direct and mild photochemical R 3 Sn • generation , that does not interfere with polymerization is desirable.…”

“…It is thus conceivable that group 14 R 3 Q • (Q = C, Si, Ge, Sn, Pb) radicals could initiate an FRP by direct monomer addition, ,− hydride or halide abstraction from a suitable substrate, ,− reaction with O 2 , , etc., while a reversible P n –QR 3 bond would mediate a CRP. However, while some R 6 Q 2 or R 4 Q (Q = Si, Ge, Sn) systems were evaluated as UV co-initiators for acrylates, ,− R 3 Q-TEMPO adducts were suggested as CRP mediators, and Ph 3 C–CPh 3 poorly mediates styrene CRP, − the R 6 Sn 2 polymer photochemistry remains largely ignored, with no reports on initiations by R 3 Q • or R 3 Q • with R′X/R′H, on CRPs mediated by reversible termination with R 3 Q • , or on tin derivatives in FRP or CRP under visible light.…”

“…Another resonance set observed only for Bu 6 Sn 2 in both acetone and DMC (and later for all Bu 3 Sn–X and Oct 4 Sn, Figures S1, S4–S7, and S13), but not for Me 6 Sn 2 /acetone or Mn 2 (CO) 10 mediated initiation from n BuI or sec BuI initiation in DMC, is a small doublet centered at δ = 93.03 ppm ( b u F , J HF = 42.85 Hz). Although sec Bu–PVDF initiation is observed at b 2‑butyl F (δ = −92.14 ppm, Figure S7e), it could still originate from mixtures of solvent cage photoisomerizations/rearrangements − …”

Investigation into Effects of Instability and Reactivity of Hydride-Passivated Silicon Nanoparticles on Interband Photoluminescence

“…[137][138][139][140] With this in mind, we hypothesized that TEMPO should also be able to sequester Si radicals on the surfaces of H-Si NPs, which would be expected to both increase PL intensity and slow or prevent further reactivity and changes in PL energy.…”

“…TEMPO has previously been shown to effectively passivate Si-based radicals on planar Si and as well as within polysilanes. 129,131,[133][134][135][136][137][138][139]141 Therefore, we began this work with the proposal that TEMPO would also effectively passivate Si based radicals on the surfaces of Si NPs. However, homolytic cleavage of the relatively weak Si-H surface bond could produce more Si surface radicals over time.…”