New sparse polymethylsiloxane dendrimers

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Synthesis of polymethylsiloxane molecular brushes

Dense purely methylsiloxane molecular brushes with high molecular masses were obtained by the ‘grafting to’ method. Vinyl groups of poly(vinyldimethylsiloxy)methylsiloxane were converted into (chlorodimethylsilyl)ethyl ones whose active chlorine atoms were replaced by monofunctional dimethylsiloxane oligomers having OLi terminal group to arrange side arms of the product. The molecular brushes thus prepared were characterized using physicochemical methods of analysis.     

Photodecomposition of ketene to form methylene

Correlation curves for a least motion departure path of methylene in the photocomposition of ketene are interpreted in terms of least energy paths. It is concluded that the first excited triplet state of ketene can probably form ³CH₂(³B₂) and ¹CH₂(¹ A ₁) relatively rapidly and the first excited singlet state can give ³CH₂(³ B ₂) easily in a near least motion path. However, the formation of ¹CH₂(¹ A ₁) from the first excited singlet state of ketene by a near-least motion path appears to be highly improbable.

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Zusammenfassung Die Korrelationskurven des Reaktionswegs mit dem geringsten anfänglichen Bewegungsaufwand des Methylens, das durch Photozersetzung des Ketens gebildet wird, werden mit Hilfe des Reaktionswegs geringster Energie interpretiert. Es wird gefolgert, daß sich aus dem ersten angeregten Triplettzustand des Ketens wahrscheinlich relativ schnell ³CH₂(³ B ₂) und ¹CH₂(¹ A ₁) bilden können und daß die Bildung von ³CH₂(³ B ₁) aus dem ersten angeregten Singulettzustand auf einem Weg, der näherungsweise dem eingangs genannten entspricht, erfolgt. Die Bildung von ¹CH₂(¹ A ₁) nach diesem Reaktionsweg ist dagegen sehr unwahrscheinlich.

     

Photodecomposition of poly(styrene peroxide)

Photodecomposition of poly(styrene peroxide) (PSP) has been carried out in detail by irradiating (300-360 nm) the PSP solution in CHCl₃ and the products were analyzed by gas chromatography-mass spectrometry (GC-MS) method. In addition to benzaldehyde, α-hydroxy acetophenone and phenyl glycol reported in earlier studies, two new products, viz., α-methoxy acetophenone and α-benzyloxy acetophenone were also observed, the mechanism of their formation discussed. The plausible reasons for the higher percentage of disproportionation products in PSP photolysis compared to that reported in its thermolysis are also discussed.     

Structure and Medium-Dependent Photodecomposition of Fluoroquinolone Antibiotics

The photochemical reactivity of four fluoroquinolone antibiotics is examined. For norfloxacin (NOR), enoxacin (ENX) and lomefloxacin (LOM), the only process occurring is defluorination (from position 6 for the first two drugs, from position 8 for the last one). The quantum efficiency is both structure and medium dependent (4 close to 0.5 both in water and in 0.1 M phosphate buffer for LOM,- 0.01 for ENX and 0.004 for NOR in buffer, but more than an order of magnitude higher in neat water). Ofloxacin (OFL) is less light sensitive φ 0.001) and undergoes, in part, reactions different from defluorination. The photoreaction involves heterolytic C-F bond fragmentation and its efficiency is determined by the internal charge-transfer character of the excited state (increasing in the series OFL < NOR < ENX < LOM according to the electronegativity of the substituent in position 8) and by the stabilization of the resulting aryl cation (larger for the 8-cation than for the 6-cation). The relevance of these data for the rationalization of the known phototoxicity of these drugs is discussed.     

Photodecomposition of several compounds commonly used as sunscreen agents

The photolysis reactions of three compounds commonly used as a sunscreen agents, Parsol 1789 (1-[4-(1,1-dimethylethyl)phenyl]-3-(4-methoxyphenyl)-1,3- propanedione), Oxybenzone ((2-hydroxy-4-methoxyphenyl)phenyl-methanone) and Padimate O (2-ethylhexyl-4-(dimethylamino)benzoate), were investigated to provide a chemical background to aid in the understanding of the photosensitization of the sunscreen agents. Photolysis was carried out in cyclohexane for 70–140 h using a mercury vapor lamp (450W) without excluding oxygen.

Irradation of Parsol 1789 in cyclohexane yielded tert-butylbenzene, p-tert-butylbenzoic acid and p-methoxybenzoic acid; products obtained from the combination of the sunscreen with the solvent included the cyclohexyl esters of p-methoxybenzoic acid, p-tert-butylbenzoic acid and methanoic acid; products obtained from the solvent included cyclohexanol, cyclohexanone and dicyclohexyl ether.

Irradiation of Oxybenzone in the cyclohexane for 100 h produced no detectable products by either gas or liquid chromatographic analysis. Oxybenzone was recovered unchanged and no products were observed from the photoinitiated reaction of oxygen with the solvent.

Irradiation of Padimate O in cyclohexane yielded the ethylhexyl esters of p-aminobenzoic acid, p-monomethylaminobenzoic acid and p-dimethylamino (o/m)-methylbenzoic acid, as well as products from the photoinitiated reaction of oxygen with the solvent.     

Photodecomposition reaction of ethyl bromide at 147 nm

A vacuum ultraviolet photolysis of C₂H₅Br at 147 nm was studied over a pressure range of 0.5–50 torr at 298 K. The effects of additives He and NO were also investigated. The principal reaction products were found to be C₂H₄ and C₂H₆, with lesser yields of CH₄ and C₂H₂. With increasing pressure the product quantum yields Φ_i of C₂H₄, CH₄, and CH₂H₆ remained constant, while that of C₂H₂ decreased from 0.03 to almost 0. The effect of He as an additive was found to be extremely small on the quantum yields of the major products. Addition of NO completely suppresses the formation of CH₄, C₂H₂, and C₂H₆, and reduces partially the production of C₂H₄. The primary processes appear to involve two electronically excited states. One state mainly yields C₂H₄ by molecular elimination of HBr and is thought to be due to a Rydberg transition. The other state decomposes to C₂H₅ and Br radicals by C? Br bond fission. These two competitive reaction modes contribute to the photodecomposition in proportions of 50% and 50%. The extinction coefficient for C₂H₅Br at 147 nm and at 298 K has been determined as ? = (1/PL) In(I_o/I_t) = 712 ± 7 atm^?1 · cm^?1.     

Photodecomposition of selenosulfonates and their facile photoaddition to alkenes 1

Phenyl areneselenosulfonates ($\text{1}$) are very photosensitive and easily undergo photodecomposition via initial homolysis of the SeS bond. In the presence alkenes this facile photodissociation of $\text{1}$ can be used to initiate a free radical chain reaction (eq 6) that leads to addition of $\text{1}$ to the alkene to form β-phenylselenosulfones ($\text{2}$). The photoaddition requires much shorter reaction times than the non-photolytic addition of $\text{1}$ to alkenes described recently.²     

Comparative characterization of polymethylsiloxane hydrogel and silylated fumed silica and silica gel

Polymethylsiloxane (PMS) hydrogel (C(PMS)=10 wt%, soft paste-like hydrogel), diluted aqueous suspensions, and dried/wetted xerogel (powder) were studied in comparison with suspensions and dry powders of unmodified and silylated nanosilicas and silica gels using (1)H NMR, thermally stimulated depolarization current (TSDC), quasielastic light scattering (QELS), rheometry, and adsorption methods. Nanosized primary PMS particles, which are softer and less dense than silica ones because of the presence of CH(3) groups attached to each Si atom and residual silanols, form soft secondary particles (soft paste-like hydrogel) that can be completely decomposed to nanoparticles with sizes smaller than 10 nm on sonication of the aqueous suspensions. Despite the soft character of the secondary particles, the aqueous suspensions of PMS are characterized by a higher viscosity (at concentration C(PMS)=3-5 wt%) than the suspension of fumed silica at a higher concentration. Three types of structured water are observed in dry PMS xerogel (adsorbed water of 3 wt%). These structures, characterized by the chemical shift of the proton resonance at delta(H) approximately 1.7,3.7, and 5 ppm, correspond to weakly associated but strongly bound water and to strongly associated but weakly or strongly bound waters, respectively. NMR cryoporometry and QELS results suggest that PMS is a mesoporous-macroporous material with the textural porosity caused by voids between primary particles forming aggregates and agglomerates of aggregates. PMS is characterized by a much smaller adsorption capacity with respect to proteins (gelatin, ovalbumin) than unmodified fumed silica A-300.     

Photodecomposition of di(B-carboranyl)mercury in solution

Conclusions ESR spectroscopy was used to record the radical adducts of the boron-centered and mercury-centered radicals formed upon the photodecomposition of di(B-carboranyl) mercury compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2644–2646, November, 1985.     

Photodecomposition of Liquid Water with TiO2-Supported Noble Metal Clusters

Rh- and Ru-loaded TiO₂-particles were produced from cluster precursors (Rh₆(CO)₁₆, Ru₃(CO)₁₂) and their activity in mediating H₂O-decomposition through band-gap excitation was investigated. Activity increases in the order Ru<RuO₂<Rh?Rh₂O₃. Bifunctional Rh/RuO₂-loaded TiO₂ exhibits optimal performance with overall light-to-chemical-energy conversion efficiency of 0.13%. Lack of O₂-appearance in the gas phase during photlysis observed with closed systems is due to photo-uptake of O₂ by the TiO₂-particles. In alkaline solution the capacity for O₂-uptake is surprisingly high and the nature of the stored O₂ is tentatively identified as a μ-peroxo-bridged Ti-species.     

氯水光照分解实验改进

新编高级中学化学课本(必修)第一册[实验1-6]介绍的氯水光照分解实验(同以前各版本相同,是将无支管的盛满氯水的圆底烧翻倒立在盛氯水的水槽内。)由于氯水与空气大贰积接触,氯气逸出污染实验环境且氯水易失效;不便于验证分解产物,使用新制氯水进行实验时即使在日光直射下也不能在45分钟内有明显现象。实验效果不够理想。     

Photodecomposition of phenol in a flow reactor: Adsorption and kinetics

Summary In this study, a new air lift loop photoreactor was used for both continuous and non-continuous water photodetoxification. Also, the kinetics of phenol photodegradation were measured under flow conditions. Both the kinetics of phenol adsorption on TiO₂/SiO₂ and of the total photo-mineralization of phenol were measured in order to gain more knowledge on the photocatalytic oxidation of phenol on illuminated titania.Adsorption of phenol on TiO₂/SiO₂ proceeds even from diluted solutions. However, based on our data, it is impossible to state precisely the influence of the adsorption on the overall rate of phenol photodecomposition. The photoreaction yields stable intermediates which subsequently undergo total mineralization to CO₂ and H₂O. It is postulated that the intermediates consist mainly of polymeric compounds, difficult to determine by conventional analytical methods like GC, HPLC, or UV/Vis spectroscopy.

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Photolyse von Phenol in einem Durchflußreaktor: Adsorption und Kinetik

Zusammenfassung Ein neuartiger Photoreaktor wurde auf seine Verwendbarkeit zur kontinuierlichen und nichtkontinuierlichen Wasseraufbereitung getestet. Die Kinetik des photolytischen Abbaus von Phenol wurde unter Durchflußbedingungen untersucht. Zum besseren Verständnis der photokatalysierten Oxidation von Phenol auf belichtetem Titanoxid wurde außerdem die Kinetik sowohl der Adsorption von Phenol an TiO₂/SiO₂ als auch seiner Gesamtphotomineralisierung erforscht.Adsorption von Phenol an TiO₂/SiO₂ erfolgt bereits aus verdünnten Lösungen. Nach unseren Ergebnissen ist es allerdings unmöglich, den Einfluß der Adsorption auf die Photolyse genau zu bestimmen. Die Photoreaktion ergibt stabile Zwischenprodukte, die sich im weiteren Verlauf zu CO₂ und H₂O umsetzen. Es wird postuliert, daß es sich bei den Zwischenprodukten hauptsächlich um Polymere handelt, die mit konventionellen analytischen Techniken wie GC, HPLC oder UV/Vis-Spektroskopie schwer nachzuweisen sind.

     

Photodecomposition of organic peroxides containing coumarin chromophore: spectroscopic studies

The photodecomposition of coumarin-3-t-Bu peroxyester (1) and coumarin-3-carbonyl-m-chlorobenzoylperoxide (2) has been studied using nanosecond and femtosecond spectroscopy to elucidate the nature of transient species involved. Excitation of the coumarin chromophore leads to its singlet excited-state decaying with the rate 9 x 10(9) s(-1) that results from a composite of emission, intersystem crossing, thermal relaxation, and -O-O- bond homolysis. Dissociation of the weak oxygen-oxygen bond proceeds from both triplet and singlet excited states. The nature of this combination of states is predissociative rather than dissociative as demonstrated by the relatively slow rates of oxygen-oxygen bond rupture. The decomposition of 1 and 2 leads to the formation of coumarin-3-carbonyloxyl radical (R1). The later was observed spectroscopically on the nanosecond time scale using both time-resolved FTIR and UV-vis transient techniques. R1 is consumed in two competitive processes: unimolecular decarboxylation and bi-molecular hydrogen atom transfer. The rates of these reactions are 4.3 x 10(5) s(-1) and 1 x 10(6) M(-1) s(-1) respectively. The transition state geometries and energies of decarboxylation of R1 have been determined using DFT calculations and are compared with values for the benzoyloxyl radical. The decarboxylation of R1 proceeds via a transition state in which the carboxyl group is almost perpendicular (dihedral angle 114 degrees) to the plane of the coumarin chromophore. The transition state of the benzoyloxyl radical, in contrast, is flat (0 degrees). The varied transition state energies of the radicals (13.6 kcal/mol for coumarin carboxyl radical vs 8 kcal/mol for benzoyloxyl radical) correlate with different decarboxylation rates of these two species.     

Photodecomposition and charge transfer complexes of acetylferrocene in binary solvents

Photoreactions of acetylferrocene in equimolar binary solvents: CCl₄ — cyclohexane and C₂Cl₄ — cyclohexane were investigated. Photoproducts were identified and the relations between the quantity of photoproducts, their quantum yield, the amount of decomposed acetylferrocene and exposure time and wavelength were determined. Equilibrium constantsK and molar extinction coefficients _c of the CT complexes formed between acetylferrocene and halogen derivative were determined.

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Photochemische Zersetzung und Charge-Transfer-Komplexe von Acetylferrocen in binären Lösungsmitteln

Zusammenfassung Die Photoreaktionen von Acetylferrocen in binären (äquimolaren) Lösungsmittelgemischen — CCl₄/Cyclohexan und C₂Cl₄/Cyclohexan — wurden untersucht. Die Photoprodukte wurden identifiziert und die Abhängigkeit ihrer Menge, der Quantenausbeute und der Menge des zersetzten Acetylferrocens von der Belichtungszeit und der Wellenlänge bestimmt. Die Gleichgewichtskonstanten und die molaren Extinktionskoeffizienten der Charge-Transfer-Komplexe zwischen Acetylferrocen und den Halogenverbindungen wurden ermittelt.

     

Photodecomposition of vitamin A and photobiological implications for the skin

Vitamin A (retinol), an essential human nutrient, plays an important role in cellular differentiation, regulation of epidermal cell growth and normal cell maintenance. In addition to these physiological roles, vitamin A has a rich photochemistry. Photoisomerization of vitamin A, involved in signal transduction for vision, has been extensively investigated. The biological effects of light-induced degradation of vitamin A and formation of reactive species are less understood and may be important for light-exposed tissues, such as the skin. Photochemical studies have demonstrated that excitation of retinol or its esters with UV light generates a number of reactive species including singlet oxygen and superoxide radical anion. These reactive oxygen species have been shown to damage a number of cellular targets, including lipids and DNA. Consistent with the potential for damaging DNA, retinyl palmitate has been shown to be photomutagenic in an in vitro test system. The results of mechanistic studies were consistent with mutagenesis through oxidative damage. Vitamin A in the skin resides in a complex environment that in many ways is very different from the chemical environment in solution and in in vitro test systems. Relevant clinical studies or studies in animal models are therefore needed to establish whether the pro-oxidant activity of photoexcited vitamin A is observed in vivo, and to assess the related risks.     

Crystal growth of CaCO3 induced by monomethylitaconate grafted polymethylsiloxane

We report the preparation of a new monomethylitaconate grafted polymethylsiloxane (CO₂H-PMS) copolymer and its effect as template for crystal growth of CaCO₃. The in vitro crystallization of CaCO₃ was carried out using the gas diffusion method at different pH values at room temperature for 24 h. The CO₂H-PMS was prepared using polydimethylsiloxane-co-methylhydrogensiloxane (PDMS-co-PHMS), obtained through cationic ring opening polymerization, from cyclic monomers and monomethyltaconate (MMI) via hydrosilylation reactions with platinum complex as catalyst. FTIR results are in an agreement with the proposed template structure and confirmed that the hydrosilylation was complete. Experimental results from pH values and SEM analysis showed that the carboxylate groups of CO₂H-PMS alter the nucleation, growth and morphology of CaCO₃ crystals. SEM revealed single-truncated (ca. 5 μm) modified at pH 7-9, aggregated-modified (ca. 20 μm) at pH 10-11, and donut-shaped crystals at pH 12. These morphologies reflect the electrostatic interaction of carboxylic moieties with Ca²⁺ modulated by CO₂H-PMS adsorbed onto the CaCO₃ particles. EDS confirmed the presence of Si atoms on the crystals surface. XRD analysis showed the existence of only two polymorphs: calcite and vaterite revealing a selective control of CaCO₃ polymorphisms. In summary, the use of grafted polymethylsiloxane template offer a good alternative for polymer controlled crystallization and a convenient approach for understanding the biomineralization process useful for the design of novel materials.