The oxidation of alcohols to aldehydes and ketones by air or oxygen under mild conditions (room temperature and atmospheric pressure), catalysed by persistent and non-persistent nitroxyl radicals in combination with transition metal salts, appears to be the most convenient of the numerous processes developed for these purposes. The thermochemistry, the kinetics, and the Hammett correlations have allowed us to establish, on a quantitative basis, the fundamental difference
A systematic study on the synthesis and properties of aza-
[5]helicenes bearing one or two nitrogen atoms in selected
ring positions is reported for the first time. This photochemical
approach can be conveniently applied to the preparation
of either mono- or diaza[5]helicenes. The aza[5]helicenes
were characterized by NMR spectroscopy, X-ray crystallography, emission spectroscopy, and luminescence lifetime.
The extremely long triplet lifetime observed (in the range
of seconds) makes these molecules promising candidates for
practical applications in photo- and optoelectronics
The aim of this work is the investigation of the effect of ultrasound treatment on the structural characteristics of carbon nanotubes (CNTs) and the consequent influence that the shortening induced by sonication exerts on the morphology, rheological behaviour and thermo-oxidative resistance of ultra-high molecular weight polyethylene (UHMWPE)-based nanocomposites. First, CNTs have been subjected to sonication for different time intervals and the performed spectroscopic and morphological analyses reveal that a dramatic decrease of the CNT’s original length occurs with increased sonication time. The reduction of the initial length of CNTs strongly affects the nanocomposite rheological behaviour, which progressively changes from solid-like to liquid-like as the CNT sonication time increases. The study of the thermo-oxidative behaviour of the investigated nanocomposites reveals that the CNT sonication has a detrimental effect on the thermo-oxidative stability of nanocomposites, especially for long exposure times. The worsening of the thermo-oxidative resistance of sonicated CNT-containing nanocomposites could be attributed to the lower thermal conductivity of low-aspect-ratio CNTs, which causes the increase of the local temperature at the polymer/nanofillers interphase, with the consequent acceleration of the degradative phenomena.
Poly(lactic acid) (PLA) is melt mixed with polyamide 11 (PA11) to obtain a heat-resistant fully bio-based blend with PLA as the dominant component. The goal is achieved by adding small amounts of organoclay (OMMT), which is used to manipulate the blend microstructure. The selective positioning of the OMMT inside the PA11 and at the PLA/PA11 interface turns the blend morphology from drop/matrix into co-continuous at high PLA content (70 wt%). The OMMT-rich PA11 framework that interpenetrates the major PLA phase effectively contributes to bear stresses, and the nanocomposite blend keeps its structural integrity up to 160 °C, i.e., about 100 °C above the PLA glass transition
Functionalized poly(butylene succinate) (PBS) samples were prepared by a post-polymerization method
based on the coupling reaction between TEMPO derivatives bearing different functionalities and
PBS macroradicals generated by H-abstraction using a peroxide. 4-Benzoyloxy-2,2,6,6-
tetramethylpiperidine-1-oxyl (BzO-TEMPO) and 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl
(NfO-TEMPO), a pro-fluorescent nitroxide, were successfully grafted on PBS, as revealed by MALDI TOF
MS and UV-Vis spectroscopy. The functionalization degrees were accurately determined by UV-Vis
analysis and confirmed by 1H-NMR spectroscopy. The grafting site was identified by combining
theoretical calculations with experimental evidence. This evidence was collected by both EPR analysis of
a functionalized sample subjected to controlled heating in the EPR cavity, and by 1H-NMR spectroscopy.
Our functionalization method, which was also tested for poly(lactic acid) (PLA), preserves the original
polymer structure. This avoids the crosslinking-branching side reaction, which generally affects the free
radical treatment of biodegradable aliphatic polyesters. In addition, using a pro-fluorescent nitroxide to
form functionalized samples is a significant step towards unambiguously demonstrating the radical
grafting on these types of polymer. It also proves that well-defined fluorescently labeled biodegradable
polyesters can be tailored
The aerobic oxidation of 4,4′-diisopropyldiphenyl and 2,6diisopropylnaphthalene, catalysed by N-hydroxyphthalimide and Co(II) salts, leads to the corresponding tertiary benzyl alcohols with high conversion and selectivity under mild conditions (temperature 30-60 °C and atmospheric pressure). Solvent and temperature effects, as resulting from the pioneering work of C. Walling, and more recently from the conclusive resolution of K. U. Ingold and co-workers on a quantitative kinetic basis, strongly affect the selectivity of the aerobic oxidation. This is related to the ratio between the rate of β-scission of the alkoxyl radical, which leads to acetophenone derivatives, and the rate of hydrogen atom abstraction, leading to tertiary benzyl alcohols. These latter are efficiently converted either to diphenols for the production of liquid crystals, by reaction with H 2 O 2 , or to dienes, useful as cross-linking agents, by dehydration. The aerobic oxidation of p-hydroxyacetophenone catalysed by Mn(NO 3 ) 2 and Co(NO 3 ) 2 leads with high selectivity to p-hydroxybenzoic acid, a useful monomer for liquid crystals.
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