1,428 research outputs found
Il recupero del candelabro pasquale della cattedrale di Amalfi
No full textNella cattedrale di Sant’Andrea ad Amalfi, ai due lati dell’altare maggiore, si conserva una coppia di colonne portacandelabro gemelle. L'analisi approfondita dei manufatti e delle relative fonti documentarie, con ausilio di analisi archeometriche e di strumenti digitali di rilievo fotogrammerico e modellazione 3D, hanno consentito di individuare l'origine moderna dei due brani marmorei e di ricostruire le vicende conservative che hanno determinato la trasformazione e la scomparsa dell'antico candelabro per il cero pasquale della cattedrale amalfitana
Aqueous Emulsion Homo- and Co-polymerization of 1,3-Dienes and Styrene using Cp2TiCl2 as catalyst
No full text1,3-Butadiene and isoprene are tested in aqueous emulsion homo-polymerization
by using bis cyclopentadienyl titanium dichloride (Cp2TiCl2) as active catalytic precursor. Copolymerizations
of the same monomers with styrene are also performed. Cp2TiCl2, that have
been already used for the production of atactic polystyrene, shows low activity in
homopolymerization of conjugated dienes, while better performance are instead obtained in
co-polymerization. Both homopolymers, polybutadiene and polyisoprene, present similar
microstructures. Polymer chains are formed by the majority of 1,4-trans units and, in minor
amount, by 1,4-cis and 1,2 (3,4 in the case of isoprene) units. All co-polymers present a
statistic distribution with atactic polystyrene sequences and, 1,4-trans, 1,4-cis, and 1,2 (3,4 in
the case of isoprene) diene units. Co-polymerization activity values and the percentage of
diene inserted units are strictly correlated to reaction mixture composition. Co-polymer
molecular weights are substantially lower than those detected for styrene homopolymer and
similar to those detected for polybutadiene and polyisoprene. Depending on co-polymer
composition, glass transition temperatures present values between those polystyrene and
polydienes
Copolymerization of ethylene and styrene to a nearly-alternating crystalline copolymer
No full textAn almost alternating stereoregular copolymer can be obtained by copolymerizing ethylene and styrene with the catalyst ethylenebis(1-indenyl)zirconium dichloride activated by methylaluminoxane at -25 degrees C. The regular microstructure pointed out by the NMR spectrum allows the copolymer to crystallize as shown by calorimetric and X-ray diffraction measurements
Alkyne Hydroamination Promoted by NHC-Gold(I) Complexes: Activity and Mechanistic Insight
No full textThe pharmaceutical and chemical industries heavily rely on the production of N-containing
compounds, as these molecules serve as scaffolds for the synthesis of both fine chemicals and
polymers of interest. The hydroamination reaction can be hailed as a paradigm of a
contemporary, sustainable, catalytically promoted chemical process, since it can form C-N
bonds by adding amines to multiple carbon bonds with a 100% atom economy.1 By reducing
the high energy barriers connected to alkyne activation, use of metal complexes can facilitate
this process; lately, gold(I) N-heterocyclic (NHC) complexes were demonstrated to be
promising in this regard.2 Herein, we present a library of gold(I)-NHC complexes which were
identified as promising catalysts for regioselective Markovnikov addition of aromatic and
aliphatic alkynes to anilines, with high yields.3 Accurate catalyst’ design, experimental reaction
scope and computational mechanistic insight will be presented, by means of which the nonmonotonic
impact of substrate substituents on the reaction course was elucidated.
References
(1) Müller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795–3892.
(2) Mariconda, A.; Sirignano, M.; Troiano, R.; Russo, S.; Longo, P. Catalysts 2022, 12(8), 836.
(3) Sirignano, M.; D’Amato, A.; Costabile, C.; Mariconda, A.; Crispini, A.; Scarpelli, F.; Longo, P. Front. Chem.
2023, 11, 1260726
Ethylene-1,2-cyclopentane random copolymers from cyclocopolymerization of ethylene/1,3-butadiene
No full textCrystalline methylene-1,2-cyclopentane (E-1,2CP) random copolymers, containing up to 15% of cyclic
units, are obtained in high yields by copolymerization of ethylene and 1,3-butadiene in the presence of
rac-[CH2(3-methyl-1-indenyl)2]ZrCl2/MAO catalyst. The highly selective formation of trans methylene-
1,2-cyclopentane units occurs by an intramolecular insertion of the double bond pendant from polymeril
chain after an unusual primary vinyl insertion of 1,3-butadiene. Thermal properties and structure of
E-1,2CP copolymers, as function of 1,2-cyclopentane molar content, are analyzed and compared with
those of other ethylene copolymers. The variation in melting temperature and crystallinity observed in
E-1,2CP copolymers is consistent with that observed for ethylene copolymers with comonomers bulkier
than propylene. Moreover, the 1,2-cyclopentane units, as well as comonomers bulkier than propylene,
cause only slight deformations of polyethylene orthorhombic unit cell parameters. These data are
rationalized by assuming that 1,2-cyclopentane units, are confined in amorphous phase, while the
polyethylene crystalline phase remains pure
Cyclocopolymerization of 1,4-Pentadiene with Ethenein the Presence of Group-4 Metallocenes
No full textThe behaviors of rac-[CH2(3-tert-butyl-1-indenyl)2]ZrCl2 (1) and Cp2ZrCl2 (2) activated by methylaluminoxane in ethene/1,4-pentadiene copolymerization are compared. In the presence of 1, inserted methylene-1,3-cyclobutane units, a large number of crosslinks, and a small number of methylene-1,3-cyclohexane units are obtained. Differently, a polyethene containing only 1,3-cyclohexane rings is achieved with 2 as the catalytic precursor. Polymer microstructures are compared with those obtained
with 1 and 2 in ethene/1,6-heptadiene copolymerization, which leads only to polyethene containing cyclohexane rings. A tentative rationalization of the experimental data is reported
Synthesis,characterization and cytotoxic activity on breast cancer cellsof new half-titanocenes derivatives
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