1,720,994 research outputs found
Le mille bolle rosa: piccola storia del chewing-gum
No full textSappiamo veramente cosa finisce fra i denti quando mastichiamo un chewing-gum? Se non si tratta di cibo, di solito non prestiamo molta attenzione a quello che ci mettiamo in
bocca, gomma da masticare compresa. Ed è un peccato, perché le origini di questo interessante prodotto si perdono nella notte dei tempi
Sogni di celluloide: i polimeri al cinema
No full textI derivati della cellulosa occupano un posto speciale nella storia dei polimeri, perché la loro invenzione diede l’avvio all’era dei polimeri sintetici. La celluloide può essere a buon diritto considerata la prima materia plastica industriale,
definita anche “il materiale che la natura aveva dimenticato di creare”
I metalloceni: una scatola a sorpresa?
No full textUn recente articolo, dal titolo “Group 4 Transition Metal Sandwich Complexes: Still Fresh after Almost 60 Years”, ha fatto il punto sull’attualità della ricerca su questa famiglia di composti metallorganici sottolineando quanto essa sia più che mai vitale e di interesse per tutti i chimici che si occupano di sintesi e catalisi. in particolare ci occuperemo delle relazioni fra metalloceni e polimerizzazion
From zirconium to titanium: The effect of the metal in t-butylacrylate photoinitiated polymerisation
No full textWe report here the synthesis, photochemistry and photoinitiator activity of some titanocenes (1-4) and compare the results with those obtained for the corresponding zirconocenes (5-8). Analysis of the electronic spectra showed that the energy modulation of the lowest electronic transition, which appears to be LMCT in character, is driven both by the substituent on the cyclopentadienyl moiety and by the metal centre. Furthermore, the excited state resulting from irradiation of the complexes at the wavelength of the LMCT transition undergoes ligand-metal bond dissociation with formation of a radical pair, as evidenced by EPR spectroscopy coupled with spin trapping techniques. All the complexes were very active, compared with known organometallic photoinitiators, for the free-radical photopolymerisation of t-butylacrylate. The titanium complexes 1-4, which can be used with visible light, were more active than the zirconium derivatives 5-8. The better yields in photopolymerisation can be interpreted on the basis of the combined effect of two factors: (i) the photoreactivity of the complexes in solution and (ii) the high persistence of the Ti(III) radical species in solution, which guarantees high concentrations of initiators in the polymerisation process
Intramolecular Charge Shift Following Bimolecular Reductive Quenching of a Rh(III)polypyridine-Diquat Dyad
No full textThe Rh(NN)33+-DQ2+ dyad, which contains a Rh(III) polypyridine moiety (Rh(NN)33+) and a N,N′-bridged diquaternarized 4,4′-dimethyl-2,2′-bipyridine (DQ2+) as covalently linked components, has been synthesized and used in the study of intramolecular electron transfer. The study, performed by laser flash photolysis, makes use of a relatively unconventional reaction scheme. First, bimolecular electron-transfer quenching of the Rh(NN)32+-localized excited state of the dyad (using 1,2,3-trimethoxybenzene as external reductant) is used to generate the reduced dyad in the thermodynamically unfavored Rh(NN)32+-DQ2+ form. Then, this species is observed to relax by a fast (k = (3 ± 1) × 107 s-1) intercomponent charge-shift process to the stable (ΔG°, ca. 0.2 eV) Rh(NN)33+-DQ+ form. A slower bimolecular back-electron-transfer reaction with the radical cation of the external quencher (k = 3.3 × 109 M-1 s-1) finally brings back the dyad to its original oxidation state. © 1991 American Chemical Society
Crystal Structure and Spectroscopic Analyses of Guanylurea Hydrochloride. Evidence of the influence of Hydrogen Bonding on the pi-Electron Delocalization
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Alternative Insight into the “Self-immobilization” Mechanism by New Tethered Zirconocene Catalyst
No full textNew -alkenyl-substituted ansa-bridged bisindenyl zirconium complexes are prepared and tested as self-immobilized catalysts for ethene polymerization. But, even at very high concentration of the tethered complexes and low pressure of ethene, there is no evidence of their insertion into the polyethene chain. A cross polymerization test, performed by copolymerizing the tethered complexes with ethene using rac-Me2Si(2-MeBenzInd)2ZrCl2 (MBI), does not lead to their incorporation into the polyethene chain. However, the corresponding ligand proves to be a suitable comonomer for ethene, and, through copolymerization promoted by MBI, innovative poly(ethene-co-2,2-bis[(1H-inden-3-yl)-hex-5-ene) copolymers are prepared and characterized by 13C NM
Photocatalytic and catalytic activity of heterogenized W10O324- in the bromide-assisted bromination of arenes and alkenes in the presence of oxygen
No full textPhotochemical excitation (l > 300 nm) of the decatungstate (nBu4N)4W10O32 heterogenized with Amberlite IRA-900 and dispersed in a CH3CN/H2O mixture causes the reductive activation of O2 to alkyl hydroperoxides. The light-assisted formation represents a new approach for inducing the bromination of activated arenes and cycloalkenes at atmospheric pressure and room temperature. The active species “Br+” is formed as a consequence of the two-electron oxidation of Br- by the photogenerated hydroperoxides. This process is catalyzed by the decatungstate, which, therefore, plays also a catalytic role in addition to the photocatalytic one. Phenol and anisole can be converted to the corresponding mono-brominated derivatives and a wide range of cycloalkanes to the corresponding bromohydrins and dibromides, with bromohydrins as intermediates for the formation of epoxides. The anionic exchange resin plays a crucial role in fostering the enrichment of bromide ions close to the surface and consequently, their reaction with the photogenerated hydroperoxides. As a matter of fact, the efficiency of the bromination photocatalytic processes may benefit by the Heterogenization of the decatungstate.
As to the chemoselectivity of the photocatalytic process, we have found that the solid matrix plays the important function of increasing the yields of epoxides and bromohydrins from cyclohexene, 1-methyl-1-cyclohexene, and styrene, upon heterogenization of the decatungstate. The solid matrix can also control the chemoselectivity in anisole bromination, by favoring the functionalization of the para-position
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