Long-lived polymer radicals. VIII. Easy formation of long-lived propagating radicals of vinyl monomers at room temperature

Poly[acryloyl-L-valine (ALV)] microspheres containing peroxy ester groups were prepared by radical copolymerization of ALV with a small amount of di-tert-butyl peroxyfumarate. When the microspheres were irradiated in the presence of second vinyl monomers, long-lived propagating radicals of the second monomers were formed in the microspheres by the reaction of microsphere polymer radicals with the monomers. The presence of a minute quantity of ethyl alcohol served to soften the microspheres and made the polymer radicals more mobile in the microspheres. As a result, sharper ESR spectra of the propagating radicals were observed although their lifetimes became shorter. This microsphere method also yielded easily the stable propagating radicals of a-methylstyrene and 1,1-diphenylethylene which have no homopolymerizability in usual radical polymerization. When N-n-propyldimethacroylamide and N,N′-dimethyl-N,N′-dimethacroylhydrazine, which undergo cyclopolymerization, were used as second monomer, uncyclized polymer radicals were only observed. Some discussions were given on the propagation mechanism of the cyclopolymerization.     

Spin trapping of propagating radicals derived from dialkyl fumarates

Propagating radicals of dialkyl fumarates (DRFs) and deuterated fumarate were trapped by admitting a 2-methyl-2-introsopropane (BNO) solution to the polymerization mixture containing the active radicals or by the polymerizations initiated with di-t-butyl hyponitrite in the presence of BNO. Although ESR spectra of the propagating radicals were appreciably changed with the size of the ester alkyl groups, all the nitroxyl radicals resulting from the spin trapping exhibited similar six-line spectra. The hyperfine splitting constant for the α-hydrogen of the radical moiety showed a slight dependence on the chain length, and the bulkiness of the ester alkyl group did not affect splitting of the spectra. These findings indicate that a substituted methylene radical is produced by addition of the primary radical to DRF followed by propagation throughout the polymerization and that poly(DRF) radical does not encounter severe hindrance in the reaction with BNO.     

Reactions of tert-butoxy radicals with vinyl monomers

Tert-butoxy radicals generated in the photolysis of di-tert-butyl peroxide in benzene at 25°C react with vinyl monomers by double-bond addition and, in most cases, also by competitive hydrogen abstraction. The rate constant for the double-bond addition changes by a factor of 17 between isoprene, which shows the highest reactivity, and the methacrylate derivatives, which are the least reactive of the monomers considered. The fraction of tert-butoxy radicals that react by hydrogen abstraction varies considerably with the monomer structure, ranging from 0.9 (cyclohexyl methacrylate) to less than 0.05 (styrene and conjugated diolefins). In the methacrylate derivatives, most of the hydrogen abstraction takes place, not in the α-methyl group, but in the alkyl chain.     

Generation of N-tert-butyl-α-phenylnitrone radical adducts in iron breakdown of tert-butyl-hydroperoxide

ESR spectroscopy coupled to the spin trapping technique was used to evaluate the generation of radical species arising from the ferrous ion induced decomposition of tert-butyl hydroperoxide (’BuOOH) in methylene chloride. We report here that N-tert-butyl-α-phenylinitrone (PBN) can trap peroxyl radicals generated in the ferrous ion induced breakdown of high concentration of ’BuOOH (IM) at room temperature, the radical adduct being stable under the light. The peroxyl radical formation was demonstrated by direct ESR measurements at 77K. In contrast, alkoxyl and methyl radicals were trapped only in the presence of low hydroperoxide concentration (ImM). In order to measure the hyperfine splitting constants (hfsc) of the PBN-methyl adduct spectra were obtained in the presence of diphenylamine (DPA) or 2,6-di-tert-butyl-4-methylphenol (BHT), which quenched the alkoxyl radical. For this latter radical, the hfsc were calculated by computer simulation. A mechanism for a direct interaction between DPA and the alkoxyl radical is presented. DPA quenched the peroxyl radical in the reaction of high hydroperoxide concentrations, with the concomitant generation of a DPA nitrogen-based radical.     

Studying the Fundamentals of Radical Polymerization Using ESR in Combination with Controlled Radical Polymerization Methods

Electron spin resonance (ESR) spectroscopy can contribute to understanding both the kinetics and mechanism of radical polymerizations. A series of oligo/poly(meth)acrylates were prepared by atom transfer radical polymerization (ATRP) and purified to provide well defined radical precursors. Model radicals, with given chain lengths, were generated by reaction of the terminal halogens with an organotin compound and the radicals were observed by ESR spectroscopy. This combination of ESR with ATRPs ability to prepare well defined radical precursors provided significant new information on the properties of radicals in radical polymerizations. ESR spectra of the model radicals generated from tert-butyl methacrylate precursors, with various chain lengths, showed clear chain length dependent changes and a possibility of differentiating between the chain lengths of observed propagating radicals by ESR. The ESR spectrum of each dimeric, trimeric, tetrameric, and pentameric tert-butyl acrylate model radicals, observed at various temperatures, provided clear experimental evidence of a 1,5-hydrogen shift.     

Enhanced Recognition of Spin Trapped Radicals In Complex Mixtures: Deuterated Nitronyl Adducts Provide A Gas Chromatographic/Mass Spectrometric Marker

Abstract

The use of deuterated analogues of α-phenyl-N-tert-butyl nitrone (PBN) are reported for GC/MS analysis of spin trapping products. PBN-d₉ and PBN-d₄₁, deuterated in the tert-butyl group (the latter also deuterated in the phenyl ring), result in a more diagnostic fragment ion, C₄D₉ ⁺ (m/z = 66) which improves the recognition of PBN adducts in mixtures. This feature has helped identify a new 1,3-radical addition product of PBN-d₉ formed during azobis(isobutyronitrile) thermolysis (addition of 2-cyano-2-propyloxyl followed by 2-cyano-2-propyl radicals). the ¹³C-trichloromethyl radical adduct of PBN-d₁₄ produced during ¹³C-carbon tetrachloride incubation with rat liver micrcsomes was identified by GC/MS in a complicated biological extract mixture.     

ESR study of the thermal decomposition of di-tert-butoxy-tert-butyl alumotrioxide formed in the reaction of tri-tert-butoxyaluminum with tert-butyl hydroperoxide

Tri-tert-butoxyaluminum reacts with tert-butyl hydroperoxide to produce di-tert-butoxy-tert-butyl alumotrioxide, which decomposes heterolytically to form singlet dioxygen and homolytically with the O—O bond cleavage. The Bu^tOO^·, (Bu^tO)₂AlOO^·, Bu^tO^·, and (Bu^tO)₂AlO^· radicals were identified by ESR using spin traps. These findings confirm the formation of aluminum-containing trioxide. The above radicals initiate alkylarene oxidation by the tri-tert-butoxyaluminum—tert-butyl hydroperoxide system. The carbon-centered and alkylperoxy radicals originated from the oxidized substrates were identified.     

Unusual reactions of isolable dialkylgermylene and -stannylene with galvinoxyl radical

The reactions of isolable dialkylgermylene and -stannylene with a galvinoxyl radical were found to give rather unusual cyclic compounds in high yields via intermediate adduct radicals. The structures of the cyclic compounds were determined by X-ray crystallography. We propose a concerted cyclization mechanism in which the abstraction of a hydrogen atom from the tert-butyl group by the galvinoxyl radical is accompanied by the simultaneous attack of the germyl radical center to the tert-butyl carbon atom.     

Electron spin resonance studies of propagating radicals in polymerization. II. Acrylate propagating radicals

Ferric chloride-photosensitized free-radical initiation was used to generate propagating radicals in polymerization of acrylic acid (AA), methyl acrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), acrylamide (A), and diacetone acrylamide (DAA) in rigid glasses of methanol, ethanol, n-propanol, isopropanol, or acetone at near liquid nitrogen temperatures. When the temperatures of the glasses were increased, primary radicals derived from the solvents reacted with the monomers to yield propagating radicals. Formation and conformational changes of the propagating radicals at different temperatures were studied by electron spin resonance (ESR) spectroscopy. It was concluded that one type of propagating radical was formed in all cases. However, when the temperature of the rigid glass was increased, the structural conformation of the radical that initially allowed the near-equivalent interaction of the α-hydrogen and one of the β-hydrogens with the unpaired electron generated a three-line spectrum.     

Homolytic reductive alkylation of methylvinyl ketone with ethers by Ti(III) decomposition of t-butyl hydroperoxide

Reductive alkylatin of methylvinylketone has been accomplished by hydrogen abstraction from cyclic ethers with the redox couple: t-butyl hydroperoxide-titanous chloride. A redox radical mechanism is proposed and the selectivity of the hydrogen abstraction by t-butoxy radicals and reduction of α-ketoalkyl radical by titanous ions is discussed.     

Study on the long life radicals of hexanediol diacrylate in irradiated urea canal complexes in relation to post-polymerization by irradiation

The properties of radicals formed on irradiating long chain monomers in urea canal complexes were studied by ESR and related to the results of post-polymerzation. The initiating radicals (CH₃-?HCOOR) were trapped in canal complexes as a result of γ-irradiation at relatively high temperature such as 25° and the radicals converted to propagating radicals (～CH₂-?HOOR0 on heating to 70°. The temperature dependence of radical concentration was followed and the decay curves corresponded well with the results of post-polymerization.     

Radicals in the synthesis of sulphur dioxide-based copolymers

Free radicals generated from a number of olefinic, acetylenic, and vinyl monomers in the presence of sulphur dioxide and tert-butyl hydroperoxide were investigated. In all reaction systems a sulphonyl propagating radical first appears which depropagates into the corresponding carbon radical at higher temperatures. The sulphonyl radicals were observed between ?119 and ?44°C, whereas the carbon radicals were observed from ?63 to ?1°C. The order of decreasing reactivity of the reaction system was: 3-hexyne > 1-heptene > methyl acrylate > acrylonitrile > 1-heptyne. © 1993 John Wiley & Sons, Inc.     

ESR study of spin-trapping with two glycosylated analogues of PBN able to target cell membrane lectins

The spin trapping behaviour of the two galactosylated nitrones LAMPBN and TA1PBN, both of which are able to target cells through recognition by cell membrane lectins, were widely investigated on a variety of free radicals in aqueous media. The ESR spectra of the more amphiphilic nitrone, TA1PBN, were interpreted in the light of the LAMPBN trapping results. The spin adducts of nitrone TA1PBN, which may be better distributed inside the cell, were surrounded by two distinct environments due to the tensioactive organisation of the trap and gave two different ESR signals for each radical trapped.     

Introduction

Abstract

The initiation mechanism for spontaneous copolymerizations of vinyl sulfides (VS) with electron-accepting monomers such as maleic anhydride (MAn), diethyl fumarate (DEF), acrylonitrile (AN), and methyl acrylate (MA) was investigated by means of spin trapping technique using 2-methyl-2-nitrosopropane as a spin trapping agent. From the ESR spectrum observed from the system ethyl VS-DEF, two types of radicals, a vinyl radical (I; RSCH=CH) and an alkyl (1, 2-dicarboethoxyethyl) radical (II; C₂H₅OCOCHCH₂COOC₂H₅) which derived from VS and DEF, respectively, were detected as their nitroxides. Similar radicals, I and III (NCCH₂CH₂), were also observed from the system VS-AN, but in the system VS-MA, three types of radicals, I, IV (CH₃OCOCH₂CH₂), and V (CH₃OCOCHCH₃) were trapped as their nitroxides. In the system isopropyl VS-MAn, I and a propagating radical (VI) ～～CH₂CHSR, were detected. The system isobutyl vinyl ether-MAn also showed a weak ESR spectrum due to the nitroxide from Radical I. These initiating radicals were assumed to be produced from the charge transfer complex formed between both donor and acceptor monomers. Based on these results, the cross-initiation mechanism for spontaneous alternating copolymerization is discussed.     

Long-lived polymer radicals. IV. Reactions of poly(N-methylacrylamide) and poly(N-methylmethacrylamide) radicals with binary mixtures of vinyl monomers

N-methylacrylamide (NMAAm) and N-methylmethacrylamide (NMMAm) were polymerized to give polymer microspheres containing living propagating radicals. The microsphere polymer radicals were allowed to react with some binary mixtures of vinyl monomers including alternating copolymerization combinations. The reaction processes were investigated by ESR spectroscopy. In the poly(NMMAm) radical/methyl methacrylate (MMA)/styrene (St) system, the propagating radical from MMA was mainly observed at the higher MMA concentration, while polySt radical prevailed at the lower MMA concentration. In the poly(NMMAm) radical/α-methylstyrene (α-MeSt)/diethyl fumarate system, the α-MeSt radical was exclusively observed, while the maleic anhydride (MAn) radical was predominantly observed in the α-MeSt/MAn system. In the MAn/diphenylethylene system, the propagating radicals from both monomers were observed at comparable concentrations. The poly(NMAAm) microsphere radical behaved differently in the reaction with the MMA/St mixture. The poly(NMAAm) microsphere was found to incorporate preferentially St, leading to formation of the St radical. The St preference was enhanced in the St/cyclohexyl methacrylate (CHMA) system. These results were in agreement with those of block copolymerization via the reaction of poly(NMAAm) radical with the MMA/St or CHMA/St mixture, where the compositions of the resulting polymers were analyzed by pyrolysis gas chromatography.     

Reaction of zirconium alkoxides with tert-butyl hydroperoxide. Oxidative ability of the Zr(OBu-t)4-t-BuOOH system

Oxidation of the isopropoxy group in the Zr(i-PrO)₄·i-PrOH complex involves both direct reaction with tert-butyl hydroperoxide and intermediate formation of zirconium peroxy compound. Zirconium tetra-tert-butoxide reacts with tert-bytyl hydroperoxide to form metal-containing peroxide and trioxide. Decomposition of the latter leads to oxygen evolution and is accompanied by radical formation. The alkoxyl and peroxyl radicals formed were identified by ESR spectroscopy. The nature of the oxidant (oxygen, zirconium-containing peroxide and-trioxide) in the Zr(OBu-t)₄-t-BuOOH system is determined by the structure of the substrate molecule.     

Mechanistic Studies on the Photogenerated Dienol with α-Phenyl-N-tert-Butylnitrones

The mechanism for the photochemical reactions of o-methyl-benzaldehyde ( 1 ), o-methyl-acetophenone ( 2 ) and o-methyl-benzophenone ( 3 ) in the presence of α-phenyl-N-tert-butylnitrone (PBN) to the formation of stable nitroxyl radicals 4–6 is studied. The nitroxyl radical product 6 can also be obtained by the thermolysis of benzocyclobutenol with PBN. Thus, the radical products were derived from a novel and regioselective 4+2 cycloaddition of the photogenerated dienol intermediate with PBN.     

Application of spin trapping to redox-initiated vinyl polymerization with thiourea as the reductant

An electron spin resonance (ESR) study confirms a free radical mechanism for aqueous vinyl polymerization initiated by three redox systems involving thiourea. The primary radical, NH₂? C (?NH)S^., could not be detected by ESR spectroscopy, perhaps because of its large g anisotropy and/or very short relaxation time. Propagating radicals are trapped from methyl-methacrylate by 2-methyl-2-nitroso propane (MNP) and their structures are found to be the same as those produced by either azo-bis-isobutyrnitrile or di-t-butyl peroxylate. The latter monomer, however, reacts with MNP in the absence of any initiator, giving different ESR lines.     

有机锡化合物光解活泼自由基的ESR研究

本文用自旋捕捉技术与ESR相结合的方法,研究了六种有机锡化合物的光解反应历程。结果表明:有机锡化合物紫外光照时,碳-锡键发生均裂,产生碳中心和锡中心自由基。其中碳中心自由基易被捕捉剂α-苯基-N-特丁基氮氧化物(PBN)或2,3,5,6-四甲基亚硝基苯(ND)所捕获;而锡中心自由基可与菲醌形成稳定的环状加合物。由于环状加合物中配体的不同,产生的空间效应也不同,从而导致加合物中的未偶电子云密度发生变化,给出相异的ESR信号。     

Spin trapping studies of poly(methyl methacrylate) degradation in solution

Free radicals produced either by γ or ultrasonic irradiation of poly(methyl methacrylate) (PMMA) in benzene solution were stabilized by spin trapping; they were identified by analysis of ESR spectra of the trapped radicals (the spin adducts). The radical species identified after γ-irradiation were methyl, ester (COOCH₃), a pair of the chain scission radicals, ～CH₂C(CH₃)(COOCH₃) and CH₂C(CH₃)(COOCH₃)～, and phenyl radical originating from the solvent. The chain scission radicals were also detected by spin trapping after ultrasonic irradiation of the benzene solution. Taking account of the difference in the trapping rate for two spin trapping agents, 2,4,6-tri-t-butylnitrosobenzene (BNB) and penta-methyl-nitrosobenzene (PMNB) the radical species trapped by PMNB are assumed to be precursors of those trapped by BNB. Based on the radical species found by the spin trapping method, plausible degradation processes for PMMA in benzene solution are proposed.