Photo-stabilisation of polypropylene by a hindered piperidine compound: Influence of hydroperoxide and carbonyl groups

The influence of prior thermally generated hydroperoxides and added benzophenone on the photo-stabilising action of a hindered piperidine compound, bis[2,2,6,6-tetramethyl-4-piperidinyl]-sebacate, in polypropylene film has been examined. The hindered piperidine compound was found to effectively inhibit both the hydroperoxide and carbonyl group sensitised photo-oxidation of the polymer. Using ESR spectroscopy, evidence is presented to show that the hindered piperidine compound reacts stoichiometrically with the hydroperoxide groups generated by thermal oxidation to give a stable nitroxyl radical. The photo-stabilising effects observed are discussed in relation to our understanding of the ultraviolet anti-oxidant action of the hindered piperidine systems.     

Redox-switching of intramolecular magnetic interaction through π-conjugation mode change of 1,2-bis(4-dianisylamino)-1,2-bis(3-N-oxylamino)-substituted tetraarylethylene

To develop the redox-switching system of intramolecular magnetic interaction, 1,2-bis[3-(N-tert-butyl-N-oxylamino)phenyl]-1,2-bis[4-{N,N-bis(4-methoxyphenyl)amino}phenyl]ethylene, tetraarylethylene with two nitroxide radical groups at the meta-position, was synthesized, and characterized by the electrochemical method and ESR spectroscopy. Cyclic voltammetry showed the tetraarylethylene core has the lower oxidation potential than the substituted nitroxide radical moiety. ESR spectroscopy in frozen solution revealed that the neutral form shows the fine-structured spectrum characteristic of the spin triplet species, while the dicationic form shows the anisotropic hyperfine-structured spectrum characteristic of the randomly-oriented nitroxide radical, indicating the drastic change of intramolecular magnetic interaction.     

Synthesis of Poly （styrene-b-isoprene-b-styrene） via Nitroxide-mediated Radical Polymerization by a Novel Alkoxyamine

Bifunctional alkoxyamine bis-TIPNO derived from 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxyl （TIPNO） and α, ω-alkyl bromide by atom transfer radical addition（ATRA） was employed as “biradical initiator” for nitroxide-mediated radical polymerization（NMRP） of isoprene and styrene. The kinetics study for the polymerization of styrene at different time showed living features. The poly（styrene-b-isoprene-b-styrene） （SIS） copolymers have two glass transition temperatures, indicating the immiscibility of the corresponding blocks.     

STUDIES OF THE INITIATION MECHANISM OF VINYL POLYMERIZATION WITH THE SYSTEM PERSULFATE/N-ALKYL SUBSTITUTED ETHYLENEDIAMINE DERIVATIVES

Effects of N-alkyi substituted ethylenediamine derivatives on vinyl polymerization using persulfate as initiator were studied. The apparent kinetic equations and overall activation energies of acrylamide polymerization were determined using the above mentioned system as initiator. The promoting activities of different diamine derivatives on vinyl polymerization are in the order of tertiary diamine>secondary diamine>primary diamine. Diamines having methyl groups as the substituent on their nitrogen atom possess higher promoting activity than that of having larger alkyl groups. The initial free radicals produced through the redox reaction of persuifate and diamines were studied by spin strapping technique and ESR spectroscopy. The results obtained confirm the fact that the initial free radicals of the diamine species can initiate vinyl polymerization and become the amino end group of the resulting polymers.     

The variety of reactions of radical cations derived from 2-diphenylaminothiophene oligomers

ESR spectroelectrochemical measurements of 2-diphenylamino-substituted oligothiophenes 8_m proved the existence of radical cations upon oxidation. Their stability and dimerization depend significantly on the number m of thiophene units. The radical cations and are very reactive and dimerize spontaneously to yield either 2,5-bis(diphenylamino)-2,2′-bithiophene 10₂ or 2,5-bis(diphenylamino)-5,5′-bis(2-thienyl)-3,3′-bithiopene 11₂, respectively. In contrast, the radical cations are highly stable and do not dimerize at all.     

An efficient synthesis of 3-(N-piperidinemethyl)-2,2,5,5-tetramethyl-1-oxy-3-pyrroline,a promising radioprotector for cancer radiotherapy

Nitroxides can ameliorate the toxic effects of radiation during cancer therapy. Nitroxides are paramagnetic and can be used in magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI) to monitor in vivo oxidative stress status. Compound 5 (3-(N-piperidinemethyl)-2,2,5,5-tetramethyl-1-oxy-3-pyrroline) was found to be the most effective nitroxide radioprotector. An efficient synthesis for this promising radioprotector was developed.     

Synthesis of unsymmetrical spin-labelled bolaamphiphiles

This work presents the efficient synthesis of a novel spin-labelled bolaamphiphile bearing sugar and cationic polar heads. Preliminary ESR studies were conduced both in CH₂Cl₂ and water and have confirmed that this methodology can be used to study the transversal rotation (flip-flop) dynamic of membrane involving such bolaamphiphiles.     

Synthesis of a well-defined polybromostyrene by living radical polymerization with a nitroxyl radical

Radical polymerization of p-bromostyrene was investigated with benzoyl peroxide (BPO) as an initiator in the presence of 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO). The polymerization was performed in bulk for 3.5 h at 95°C and then continued for another 48 h at 125°C to afford the corresponding polybromostyrene with a narrow molecular weight distribution in high yield. ¹H NMR study revealed that the polymer obtained had BPO and MTEMPO moieties at its head and tail, respectively. It was confirmed that the polymerization proceeded in accordance with living mechanism, because the molecular weight linearly increased with an increase of the conversion, and it was directly proportional to the reciprocal of the initial concentration of BPO. Furthermore, the polystyrene obtained in the present study could quantitatively act as the initiator for the polymerization of p-bromostyrene in the living radical manner to afford the corresponding block copolymer, and vice versa. © 1996 John Wiley & Sons, Inc.     

Organocatalytic intermolecular [2+2] cycloaddition of norbornadienes by a stable organic radical compound

An organocatalytic [2+2] cycloaddition reaction of norbornadienes (NBDs) using catalytic amount of TEMPO was reported. Single crystal X-ray diffraction of the product revealed its detailed multicyclic structure containing a 4-membered ring, formed in intermolecular reaction. Addition of AIBN to the current catalytic system improved the product yield. Quantitative reaction of the NBD and TEMPO gave a 2:2 adduct of NBD and TEMPO, which was confirmed by HR-MS. This catalytic [2+2] addition of NBDs has great advantage in selective intermolecular coupling in comparison with [2+2] photocycloaddition.     

Titanocene(III) mediated radical cyclization of α-bromo carbonyl compounds: synthesis of tri-substituted tetrahydrofurans

A novel and efficient methodology has been developed for the construction of synthetically important tri-substituted tetrahydrofuran derivatives from bromo-alkenes and bromo-alkynes by radical cyclization reactions using the radical initiator Cp₂TiCl, generated in situ from commercially available titanocene dichloride and Zn dust in tetrahydrofuran under argon.     

过硫酸盐和脂肪环胺体系引发机理的ESR研究

用ESR方法研究了过硫酸盐和脂肪环胺吗啉,哌啶及其N-烷基取代衍生物体系引发反应的初级自由基,结合端基分析,证实了这类脂肪环胺与过硫酸盐反应产生的初级自由基都能引发烯类单体聚合。当所用的是脂环仲胺时,其初级自由基为氮中心自由基;用脂环叔胺时,初级自由基是N-烷基的α-碳中心自由基。     

Light-fluorous TEMPO: reagent, spin trap and stable free radical

The synthesis of two light-fluorous TEMPO derivatives is reported, along with their fluorous-organic solvent partition coefficients and their ESR spectra. Applications of the fluorous-TEMPO reagents in oxidation reactions and as radical traps are discussed.     

Oxidation of alcohols to carbonyl compounds with diisopropyl azodicarboxylate catalyzed by nitroxyl radicals

A nitroxyl-radical-catalyzed oxidation of alcohols using diisopropyl azodicarboxylate (DIAD) as the terminal oxidant is reported. A variety of primary and secondary alcohols including aliphatic, benzylic, and allylic alcohols are efficiently oxidized to their corresponding aldehydes and ketones without overoxidation to carboxylic acid. 1,2-Diols are oxidized to hydroxyl ketones or diketones depending on the amount of DIAD used.     

Investigation of the structure of chitosan hybrid systems by pH-sensitive nitroxyl radical

Hybrid organo-inorganic materials based on the chitosan-SiO₂, chitosan-Al₂O₃, and chitosancellulose systems are prepared. The structure of the surface is studied by means of EPR spectroscopy using the stable pH-sensitive nitroxyl radical, 4-dimethylamino-2-ethyl-5,5-dimethyl-2-pyridine-4-yl-2,5-dihydro-1H-imidazole-1-oxyl as adsorbed probe molecules, and the processes that occur during the formation of hybrid materials are considered.     

Synthesis of amphiphilic block copolymers bearing stable nitroxyl radicals

We present here the synthesis of two kinds of amphiphilic block copolymers, a diblock copolymer MPEG‐b‐PTAm and a triblock copolymer MPEG‐b‐PLA‐b‐PTAm, which can self‐assemble into micelles with nitroxyl radicals‐containing PTAm segment in the core. The structure of the block copolymers was characterized by ¹H NMR and GPC. Dynamic laser light scattering and transmission electron microscopy were used to study the micellar behavior of the two block copolymers in aqueous solution. The micelles carrying nitroxyl radicals in the core can generate electron paramagnetic resonance, which is stable for a period of time up to 8 min even in the presence of reducing reagent such as ascorbic acid. The enhanced stability against the reducing agent was ascribed to the inaccessibility of the nitroxyl radical core placed in the interior of the micelles. Combined with the biocompatibility, these micelles were promising to be used as the EPR probes for bioimaging in vivo. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis and ESR behaviors of nitroxide monoradical based on calix[4]arene

A novel paramagnetic calix[4]arene 6 with an N-O monoradical on the upper rim was synthesized and studied by ESR spectroscopy. Its ESR behaviors were mainly induced by its intrinsic conformational flexibility. The effects of varying temperature and complexation of silver ions on its ESR behaviors were also investigated.