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.     

Covalent Modification of MoS2 with Poly(N‐vinylcarbazole) for Solid‐State Broadband Optical Limiters

New soluble MoS₂ nanosheets covalently functionalized with poly(N‐vinylcarbazole) (MoS₂–PVK) were in situ synthesized for the first time. In contrast to MoS₂ and MoS₂/PVK blends, both the solution of MoS₂–PVK in DMF and MoS₂–PVK/poly(methyl methacrylate) (PMMA) film show superior nonlinear optical and optical limiting responses. The MoS₂–PVK/PMMA film shows the largest nonlinear coefficients (β_eff) of about 917 cm GW^?1 at λ=532 nm (cf. 100.69 cm GW^?1 for MoS₂/PMMA and 125.12 cm GW^?1 for MoS₂/PVK/PMMA) and about 461 cm GW^?1 at λ=1064 nm (cf. ?48.92 cm GW^?1 for MoS₂/PMMA and 147.56 cm GW^?1 for MoS₂/PVK/PMMA). A larger optical limiting effect, with thresholds of about 0.3 GW cm^?2 at λ=532 nm and about 0.5 GW cm^?2 at λ=1064 nm, was also achieved from the MoS₂–PVK/PMMA film. These values are among the highest reported for MoS₂‐based nonlinear optical materials. These results show that covalent functionalization of MoS₂ with polymers is an effective way to improve nonlinear optical responses for efficient optical limiting devices.     

Substituted propenyl end groups as reactive intermediates in radical polymerization

The addition of propagating radicals of methyl acrylate (MA) and styrene (St) to CH₂?C(CO₂CH₃)CH₂? and CH₂?C(C₆H₅)CH₂? ω‐end groups of poly(methyl methacrylate) (PMMA) and polystyrene (PSt) was investigated. The end groups were as reactive as MA and St toward the poly(methyl acrylate) (PMA) and PSt radicals, respectively. The adduct radical derived from the two types of PMMA end groups and PMA radicals underwent β fragmentation exclusively to yield PMMA radicals and end groups bound to PMA chains. The addition of PSt radicals to PMMA with CH₂?C(CO₂Me)CH₂? end groups resulted in adduct radicals that underwent β fragmentation and addition to St or coupling with PSt radicals. Adduct radicals formed by the addition of PMA radicals to both types of end groups of PSt exclusively formed C? C bond by coupling with PMA radicals to form branched structures or by addition to MA monomer to give a copolymer. The fate of the adduct radicals was highly dependent on the type of polymer chain and the substituent bound to the end group. Steric congestion of the adduct radical arising from the α‐methyl group of the PMMA chain was considered to be crucial for fragmentation to expel the PMMA radical. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 645–654, 2003     

EFFECT OF TACTICITY OF PMMA ON CRYSTALLINE STRUCTURE OF POLY VINYLIDENE FLUORIDE

Fourier transform infrared spectroscopy (FTIR) has been used to study the effect of tacticityof PMMA on β phase formation of poly vinylidene fluoride (PVF_2) during quenching process.For pure PVF_2, quenching at lower temperature results in the formation of β phase crystallites.The critical quenching temperature for β phase formation is about 30℃. Adding a given amountof PMMA (30%) results in the increase of the critical quenching temperature. For the blends ofPVF_2 with atactic PMMA (a-PMMA), the critical quenching temperature is about 45℃, whilefor the blends with syndiotactic PMMA (s-PMMA), attains to about 70℃.     

ESR study of free radicals in UHMW-PE fiber irradiated by gamma rays

ESR spectra of the trapped radicals in an ultra-high molecular weight polyethylene (UHMW-PE) fiber irradiated by gamma rays showed well-resolved hyperfine splitting at room temperature since the c-axis of the crystallites is aligned with the fiber direction and the radicals are trapped in crystallites. The alkyl radical (?CH₂?^?CH?CH₂?) was the major product after irradiation in vacuum and in air at room temperature. Some of the alkyl radicals converted to allyl radicals (?CH₂?^?CH?CH=CH?) and polyenyl radicals (?CH₂?^?CH?(CH=CH)_n?CH₂?) during storage in vacuum. Upon storage in air atmosphere, the alkyl radicals decayed by reaction with oxygen. Of particular interest is the very slow decay rate of the alkyl radical trapped in UHMW-PE fiber, the half-life is 26 days in vacuum, and 13 days in air at room temperature, which is about 1/30 and 1/100 of that reported for high density polyethylene (HDPE), respectively. The extremely long lifetime of the alkyl radical is supposed to be caused by the large size of crystallites in UHMW-PE fiber. The rate of radical decay was accelerated by annealing at elevated temperature.     

Ionic species in γ-irradiated poly(methyl methacrylate) at 77°K

Ionic species in poly(methyl methacrylate) (PMMA) have been studied by optical absorption and ESR spectrometry. γ-Irradiation of PMMA containing aromatic solutes gave rise to the absorption spectra of corresponding cation radicals. The G value for the formation of cation radicals was determined to be 1.5. Anionic radicals of the solutes were not detected for the aromatic solutes studied. Anionic species of pure PMMA were identified by both absorption spectra and ESR spectra. It has an absorption maximum at 440 nm. A sharp singlet with a line width of about 5 G which was found by ESR spectrometry was tentatively assigned to the trapped electron of PMMA. The results show that a large number of electron traps in PMMA may exist on the ester side chain themselves.     

Photo-stability of photo-cured organic coatings: Part I—Secondary reactions in thioxanthone/amine photo-cured organic coatings

Luminescence measurements on 2-iso-propylthioxanthone (ITX) in a poly(methyl methacrylate) (PMMA) film as a model system were carried out to study the secondary reactions of ITX during and after photocrosslinking of organic coatings. The energy of the lowest excited triplet state (T₁) of ITX was calculated to be E₁₁ = 62·2 kcal/mol. The ITX triplet lifetime in a PMMA film was calculated from the phosphorescence decay to be τ = 1·0 ms. Irradiation of a PMMA film containing 4 mm ITX resulted in rapid photoreduction of the aromatic ketone. In a PMMA-benzene solution, studied for comparison, the photo-reduction rate was lower than in film. Triplet excited ITX may decompose peroxides by an energy transfer process. As the model system a mixture of tert-butylhydroperoxide and di-tert-butylperoxide was irradiated with uv light in the presence of ITX in benzene solution. This was found to give tert-butoxy radicals. The same radicals were formed by irradiation of a PMMA sample containing peroxides. The radicals formed were identified and measured using ESR spectroscopy combined with the spin trapping technique. The investigations reported here have established evidence for two important secondary reactions in the photo-curing of organic coatings using thioxanthone/amine as the initiating system: photo-reduction of ITX by hydrogen abstraction from the polymer and photo-decomposition of peroxides and hydroperoxides sensitized by thioxanthone.     

Polymer radicals trapped in radical-initiated polytetrafluoroethylene

The electron spin resonance (ESR) spectra of polymer radicals found to be trapped in polytetrafluoroethylene (PTFE) polymerized with radical initiators were comparatively examined under various conditions and assigned. They are identified as the primary (propagating) radicals RCF₂CF₂·, which are transformed to primary peroxy radicals RCF₂CF₂OO· in the atmosphere. Studies of the rates of polymerization and postpolymerization and ESR measurements indicate that the radical content steadily increases during polymerization. The results are discussed in connection with the mechanism of polymerization of tetrafluoroethylene (TFE) and the unusual thermal stability of these radicals in PTFE prepared with initiator.     

Redoxreaktionen an nickelorganokomplexen durch nitrosoverbindungen und aliphatische aldehyde: Möglichkeiten des einsatzes der spintrapmethode

ESR investigations of the reaction between (bipy)Ni(C₂H₅)₂ and aromatic nitroso compounds (RNO) show the formation of paramagnetic, unstable complexes containing the radical RNO^. which is followed by elimination of nitroxide radicals C₂H₅(R)NO^..(bipy)Ni(COD) is oxidized by RNO to give nickel(I) species and several trapped radicals derived from COD. In the presence of aldehydes no paramagnetic nickel species, but ethyl radicals and spin adducts of the aldehydes can be observed. The mechanism of the reaction is discussed.     

An ESR study of trapped radicals from the gas-phase thermolysis of dialkyl peroxides

The low-pressure gas-phase thermolysis of several dialkyl peroxides has been investigated in the 250–350°C range. The free radicals were trapped at 77 K and identified by ESR. No primary alkoxyl radicals have been detected. There is evidence for the decomposition of methoxyl and ethoxyl radicals with formation of H and CH₃ radicals, respectively, which readily combine with traces of O₂ to yield HO₂ and CH₃O₂. For higher homologues only RCH₂O₂ radicals have been detected and identified from the hyperfine structure.     

Main-chain scissions of poly(methyl methacrylate) induced by chlorinated nitrosobenzenes as radical generators

The spin trap agent, 2,6-di-chloronitrosobenzene (DCNB), which is decomposed into a nitrogen monoxide and a chlorinated phenyl radical on slight warming, was known to act as a radical generator through hydrogen abstraction by the chlorinated phenyl radical from an adjacent molecule. An ESR spectrum was observed at room temperature from a poly(methyl methacrylate) (PMMA)-benzene solution after the addition of DCNB followed by a warming to ca 30°C. The radical concentration increased with time. The spectrum was assigned to the spin adducts of PMMA radicals generated and trapped by DCNB. Analyses of the spectra observed from normal PMMA and partially deuterated PMMA's indicated that the majority of the PMMA radicals were the chain-scission species

and a minority were

. It was concluded that the main-chain scissions in PMMA were caused by the radicals (D), which had been primarily produced by the chlorinated phenyl radicals. These ESR data are supported by the fact that a decrease in molecular weight of PMMA was observed after addition of DCNB, and further reinforced by the fact that a molecular weight estimated from the number of the scission radicals agreed fairly well with the measured molecular weight. Similar results were obtained when both tri-chlorinated nitrosobenzene and tetra-chlorinated nitrosobenzene were used instead of DCNB.     

Nonlinear optical properties of polyvinylcarbazole composites with graphene

The study has focused on polyvinylcarbazole (PVK) composites with graphene. It has been shown that there is a noticeable nonadditive shoulder on the long-wavelength edge of the optical absorption of PVK in these samples, which can be attributed to the formation of a charge-transfer complex between PVK as a donor and graphene as an acceptor. The formation of the complex causes a significant nonlinear optical effect in the PVK/graphene composite. The revealed increase in both the nonlinearity coefficient with increasing laser intensity and the cross section with increasing incident energy density is due to the formation of the graphene^?· radical anion, an additional species contributing to nonlinear absorption, with an increase in the radiation energy density. Nonlinear optical properties of PVK composites with graphene isolated from a solution in tetrachloroethane after 1.5-h centrifugation (sample 1) have been considered. It has been suggested that a significant decrease in optical transmission of laser radiation by the composite T _OA = 0.4 at an energy density at focus of 502 J/cm² is due to the formation of the PVK/graphene charge-transfer complex responsible for the nonadditive shoulder on the long-wavelength optical absorption edge of PVK. During photoexcitation of graphene in the PVK/graphene composites at a laser wavelength of 1064 nm, mobile holes are generated in PVK, indicating the formation of graphene^?· radical anions as a result of charge transfer from PVK to photoexcited graphene. The observed increase in both β with an increase in the laser radiation intensity and the cross section (σ_exc — σ₀) with an increase in the incident energy density may be due to either the contribution of nonlinear transitions (S ₀ → S ₂, S ₀ → S ₁ → S ₂, T ₁ → T ₂) or the formation of the additional species, the graphene-· radical anions, participating in nonlinear absorption by increasing the energy density at the focus (F _foc, J/cm²).     

On the emitters of sulfuric acid sonoluminescence

A comparative study of the sonoluminescence spectra of water and argon-saturated aqueous H₂SO₄ solutions was carried out. At an H₂SO₄ concentration of 18 mol L⁻¹, the sulfuric acid sonoluminescence is fifty times more intense than water sonoluminescence. The sulfuric acid luminescence spectrum differs from the water sonoluminescence spectrum caused by the emission of excited water molecules and OH radicals from the gas phase of cavitation bubbles. The sulfuric acid sonoluminescence spectrum exhibits maxima at 330, 420, 500, and 630 nm. Emitters of sonoluminescence of sulfuric acid are the singlet (330–340 nm) and triplet (∼420 nm) excited SO₂ molecules formed by sonolysis of H₂SO₄ molecules. Another product of sonolysis of H₂SO₄, atomic oxygen, is assumed to be responsible for the luminescence at λ = 630 nm. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1742–1745, August, 2005.     

Plasma for Modification of Polymers

The effect of nonpolymer-forming plasma (e.g., plasma of hydrogen, helium, argon, nitrogen) can be viewed as the following two reactions: 1) reaction of active species with polymer, and 2) formation of free radicals in polymer which is mainly due to the UV emitted by the plasma. The incorporation of nitrogen into the polymer surface by N₂ plasma and the surface oxidation by O₂ plasma are typical examples of the first effect. The latter effect generally leads to incorporation of oxygen in the form of carbonyl and hydroxyl and to some degree of cross-linking depending on the type of substrate; however, the degradation of polymer at the surface manifested by weight loss occurs in nearly all cases when polymers are exposed to plasma for a prolonged period of time. The effects of polymer-forming plasma is predominated by the deposition of polymer (plasma polymer); however, with some plasma-susceptible polymer substrates the effect of UV emission from polymer-forming plasma cannot be neglected. The mechanism of polymer formation can be explained by the stepwise reaction of active species and/or of an active specie with a molecule, and the chain addition polymerization of some organic compounds (e.g., vinyl monomers) is not the main route of polymer formation.

Plasma polymers contain appreciable amount of trapped free radicals; however, the concentration is highly dependent on the chemical structure of the monomer. In plasma polymerization, 1) triple bond and/or aromatic structure, 2) double bond and/or cyclic structure, and 3) saturated structure are three major functions which determine the rate of polymer formation and the properties of plasma polymers. The changes of some properties of plasma polymers with time are directly related to the concentration of trapped free radicals in plasma polymers. The amount of trapped free radicals in a plasma polymer is also influenced by the conditions of discharge; however, the UV irradiation from the polymer-forming plasma is not the main cause of these free radicals. Excess amount of free radicals are trapped during the process of polymer formation (rather than forming free radicals in the deposited polymer by UV irradiation). The properties of a plasma polymer is generally different from what one might expect from the chemical structure of the monomer, due to the fragmentation of atoms and/or functions during the polymerization process. This is another important factor to be considered for the modification of polymer surfaces by plasma polymerization.     

Radiolysis of Aqueous Solutions of Poly(ethylene oxide) at 77 K

Low-temperature (77 K) γ-radiolysis of aqueous solutions of poly(ethylene oxide) was studied by ESR spectroscopy. Trapped electrons, hydroxyl radicals, and -CH₂-ĊH-O- radicals were identified as principal paramagnetic products of radiolysis. It was shown that an increase in the polymer concentration in solution led to a growth in number of trapped electrons and -CH₂-ĊH-O- macroradicals. The decay of hydroxyl radicals occurring at 110–115 K was accompanied by the formation of -CH₂-ĊH-O- macroradicals and partial recombination of hydroxyl radicals. Action of visible light resulted in virtually quantitative transformation of trapped electrons into -CH₂-ĊH-O- macroradicals. The radiation-chemical yields of the species trapped at 77 K were estimated and the scheme of radiation-induced chemical processes was suggested.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 4, 2005, pp. 243–249.Original Russian Text Copyright © 2005 by Zakurdaeva, Nesterov, Feldman.     

Photoinduced recombination reaction of trapped electrons and macroradicals in radiolyzed cellulose

Light-induced reaction of photorecombination of trapped electrons and free radicals (mainly, the C₁-centered radical) has been detected in cellulose γ-irradiated at 77 K. The dark process of annealing of trapped electrons in γ-irradiated cellulose occurs in the range of 77–170 K without participation of alkyl radicals and includes only the recombination of electrons with cations. The action of light on the radiolyzed cellulose increases the yield of carbon dioxide (upon heating up photo-bleached samples) by a factor of 2–2.5 as compared to the dark process. It is supposed that the reaction of photorecombination of trapped electrons and free radicals follows the chain mechanism: at doses of preliminary irradiation up to 100 kGy, the decay of one electron accompanied by disappearance of 4 to 20 alkyl radicals and evolution of up to 25 CO₂ molecules.     

Synthesis of block and star copolymers by photoinduced radical coupling process

The general design for the synthesis of AB diblock, and A₂B and AB₂ star copolymers based on the statistical coupling of poly(styrene) (PSt) and poly (methyl methacrylate) (PMMA) macromolecules containing photoreactive benzophenone is presented. For this purpose, mono- and bifunctional initiators for Atom Transfer Radical Polymerization (ATRP) bearing benzophenone group were synthesized and characterized. End- and mid-chain benzophenone functional PSt and PMMA with low molecular weights were obtained by ATRP using these initiators in the presence of CuBr/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) catalytic complex. Poly(styrene-block-methyl methacrylate) (PSt-b-PMMA) copolymers were prepared by photolysis of the solutions containing end functional PSt and PMMA in THF at λ = 350 nm for 60 min in the presence of a hydrogen donor such as N-methyldiethanolamine (NMDEA). The proposed mechanism assumes hydrogen abstraction of photoexcited benzophenone moiety by NMDEA. Ketyl radicals resulting from abstraction reaction undergo radical-radical coupling to form benzpinacol structure at the core. Formation of A₂B and AB₂ type star copolymers upon irradiation of solutions containing appropriate combinations of end- and mid-chain functional polymers was also demonstrated. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2938–2947, 2009     

The chemistry of diacyl peroxides IX. An ESR study of the reduction of polyfluorodiacyl peroxides by copper (O) in the presence of 2-nitro-2-nitrosopropane: The generation of polyfluoroalkyl nitroalkyl nitroxides

Polyfluoroalkyl radicals generated by one-electron reduction of polyfluorodiacyl peroxides by copper (O) were trapped by 2-nitro-2-nitrosopropane to yield polyfluoroalkyl nitroalkyl nitroxides, R_FN(O)CMe₂NO₂.     

Photogeneration of superoxide and decarboxylated peptide radicals by carboquone, mitomycin C and streptonigrin. An electron spin resonance and spin trapping study

Visible light (405–615 nm) excitation of carboquone, mitomycin C, and streptonigrin dissolved in dimethylsulfoxide in the presence of oxygen generates superoxide anion radicals (O₂^?). The quantum yields for these reactions range from 4.2 times 10^?2 (carboquone, λ= 615 ± 10 nm) to 7.3 times 10^?6 (streptonigrin, λ=545 ± 10 nm). O₂^? radicals were spin trapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance (ESR). The efficiency of DMPO to spin trap O₂^? in dimethylsulfoxide was determined and indicated that 91% of the O₂^? present in dimethylsulfoxide is trapped by DMPO. The oxidation of the photoexcited drug molecules occurs via a direct electron transfer to dissolved oxygen in solution. Ultraviolet irradiation (λ= 313 ± 10 nm) of the aminoquinone drug solutions (80% H₂O, 20% dimethylsulfoxide) in the presence of peptides results in the decarboxylation of the peptides. In this case the photoexcited drugs are reduced, abstracting an electron from the C-terminal carboxyl group of the peptides. The reaction is specific to the C-terminal amino acid of the peptide. The decarboxylated peptide radicals were spin trapped with 2-methyl-2-nitrosopropane (MNP) and identified by ESR.     

氢自由基捕捉剂对苯乙烯超声引发乳液聚合的影响

研究了微量CCl4对超声引发苯乙烯乳液聚合的影响.随着CCl4含量增加,聚合速率先增加后降低.在CCl4存在下H2O2产率增加,pH值与所得聚合物分子量降低和无挥发性氢自由基捕捉剂对超声引发苯乙烯乳液聚合的影响表明了CCl4使超声引发苯乙烯乳液聚合速率提高的原因在于CCl4能进入空化泡内捕捉氢自由基,使反应体系的自由基浓度增高.但在超声引发甲基丙烯酸甲酯乳液聚合体系中,甲基丙烯酸甲酯较大的蒸汽压减少CCl4对氢自由基的捕捉几率,因此CCl4的加入没能提高甲基丙烯酸甲酯的聚合反应速率.